Speculative Plant Hormone Theory

Restating of Epinasty Ideas

noreply@blogger.com (Paul D Pruitt) — Wed, 31 Oct 2007 09:54:00 +0000

If it is true that epinasty is a plant's supplimentation of regular transpiration with a mechanical water pump of sorts including a valve (perhaps the stomata act this way) and a handle (the epinastic leaf), then what are the conditions under which a plant might be expected to need to engage this process?

To me its clear. Too much water or too little require the supplimentation of transpiration. Thus we might expect:

1. ABA should lead to epinasty. This is because with less water in the plant, the leaves might need extra help in getting water to themselves. Maybe wilting is really epinasty at first.

2. Ethylene - because of anoxia occuring during flooding. I now believe Ethylene is really primarily an anoxia hormone.

3. Salicylic Acid - I now think this is a water abundance signal. Note that Salicylic Acid was first discovered in Salix, Willow Bark. Willows live on river banks with water logged roots. Weeping Willows are in a perputual state of Epinasty.

Restating of the Speculations on the 8 Hormone System

noreply@blogger.com (Paul D Pruitt) — Thu, 18 Oct 2007 17:22:00 +0000

This is from my Website:http://www.planthormones.info/

The 8 most widely studied plant hormones are Auxin (usually IAA), Cytokinin (CK), Ethylene (ET), Gibberellin (GA), Abscisic Acid (ABA) and the lesser studied, Brassinosteroid (BA), Salicylic Acid (SA), and Jasmonic Acid (JA).

If you look through the previous versions of this theory from the links above, they seem to suggest some contradictory things but overall the suggestions have in common that there are in general two classes of hormones, one which is made under good growing conditions, and the other made when conditions do not warrant growth and in fact necessitate cutting back on size, using stored resources and changing strategy for obtaining nutrients to more risky but also higher return rate strategies.

In general also the past ideas tied specific hormones to specific nutrients. Thus Auxin was seen tied to growth warranting levels of sugar and gases, Cytokinin was seen as made when growth warranting levels of water and minerals. Gibberellin was seen as made when there were less than enough sugar and gases for growth and Ethylene was seen as made when there were less than enough minerals and water to warrant growth.

ABA and Salicylic Acid at least in some versions were left out and not tied to any specific nutrients. With ABA, this flew in the face of conventional wisdom because it has widely been seen as a water deficiency signal. Instead ABA was seen as an emergency signal of any rapidly developing adverse situation and SA (Salicylic Acid) as its all clear signal.
Growth Hormones - Auxin, Cytokinin, Salicylic Acid, and Unknown Hormone X (Jasmonic Acid?)

Upon reflection I now am ready to reconsider my ideas and instead postulate an 8 hormone system instead of a six. Note as with all my previous versions, I speculate that "growth levels" of nutrients in a meristematic cell are twice as much in the shoot than in the root if it is supposed to procure that nutrient (i.e. the shoot is supposed to procure sugar). This is because the shoot cell will eventually have to support both it and a similar sized root cell in say sugar production. Also then in the root, a meristematic cell needs to procure twice as much minerals as a similar sized meristematic cell, before it will start producing Cytokinin.

Also note, if they have growth levels of the nutrient of which they are an indicator, Cytokinin, Auxin and any other growth hormone should continue to be made by any plant cell as they mature but it much lesser amounts. Perhaps the only reason for continuing to make the hormones in mature cells, is to avoid senescence as small growth hormone production indicates productive “profitable” cells (having nutrient levels higher than needed for the survival of that cell and any cell dependent on it for production of that nutrient). Thus it is a good thing for instance, for a mature leaf making more than enough sugar and taking in more gases than are needed for the survival of both it and a similar sized root cell, that this shoot cell continue to receive a guaranteed amount of water and maybe minerals too and for the leaf not to senesce and its resources be reused for younger cell growth. This preservation for senescence is done at least in part by the Auxin and Unknown Hormone X (see below – may be Jasmonate), it makes.

Auxin – made when there are growth levels of sugar in any meristematic cell.

Cytokinin – made when there are growth levels of minerals in any meristematic cell.

Salicylic Acid – made when there are growth levels of water in meristematic cells.

Unknown Hormone X (Jasmonic Acid?) - made when there are growth levels of needed gases, Oxygen and Carbon Dioxide.

Senescent Hormones – Gibberellin, Unknown Hormone Z (Brassinosteroid?), Abscisic Acid, Ethylene There has been criticism of my work in the past in regards to which cells make of the deficiency/senescence hormones. This was because I classify Gibberellin as a deficiency signal, but the according the Plant Physiologists, GA (Gibberellin) is made more often in abundance in meristematic tissue than mature tissue. In my earlier versions, I thought it was more beautiful aesthetically to see the deficiency hormones as made in higher amounts in mature tissue and dropping off the younger the age of a cell is. I don’t want to get in a discussion about that here but suffice it to say, either way does not effect most of the content of my versions. In the 2007 version I did concede to the critics and made a version with all the hormones including those I label as deficiency hormones (except Abscisic Acid and Salicylic Acid) being made at high levels by meristematic tissue and dropping off as the cells mature.

Gibberellin – Sugar deficiency.

Unknown Hormone Z (Brassinosteroid?) - Mineral deficiency.

Abscisic Acid – Water deficiency. This is well accepted.

Ethylene – Oxygen and maybe gas deficiency. Ethylene has been accepted in the past by some as an Oxygen deficiency signal.

Discussion and Ramifications

Most of the features discussed in previous version of the paper continue to hold in a similar way as they did before.

Growth Hormone Nutrient Attraction – Because hormones also cause the attraction of nutrients (and even other hormones), growth levels of hormones ensure the continued flow of complimentary nutrients to a nutrient producer. Thus if a leaf cell is making Auxin, it draws minerals, water and gases to itself to continue production of profitable amounts of sugar.
It may turn out Auxin even causes sugar to be attracted to site of Auxin synthesis and even more strangely, sugar may relatively speaking be the nutrient which it attracts the most strongly to where the Auxin is. The reason for this are two fold. First meristematic cells need to grow quickly and may not produce enough sugar to grow at the rates the plant wants. That may be why Auxin attracts sugar to meristematic cells in the first place. Secondly Auxin is transported down the plant, so sugar follows this downward traveling of this. Jacobs and Gilbert first showed around 1984 that Auxin transport pumps are located at the bottoms of the sugar transporting phloem. So maybe Auxin is part of the engine that drives this sugar transport.
Similarly then Cytokinin is transported upward by the Xylem and it may more strongly attract minerals to it from surrounding tissue, than other nutrients.

Perhaps the pre-eminent nutrient attracting properties of the growth hormones, for the very nutrient of which they are an indicator of, occurs much more strongly in mature cells than meristematic cells. Conversely then in the younger meristematic cells, then the nutrients the the specific hormone attracts, would be more strong for the three categories of complimentary hormones, and only weakly for the nutrient of which it is an indicator. The reason for the weak attraction of the nutrient at all may be as indicated, to have a meristematic cell grow faster than it could without the supplementation of the nutrient of which it is an indicator.

Growth Hormone Level Peak During the Day, Deficiency/Senescence Peak During the Night – There is some evidence that this is true, but I saw this mostly in older journal articles. Suffice it to say, that with colder temperatures and lack of light affecting the processes that require energy, we might expect that there is less nutrient procurement at night and more of the requirement to rely on stores. Also at night perhaps housekeeping and pruning of less efficient older plant parts is more easily carried out (although it may be difficult to know if a leaf is an efficient sugar producer at night!). This is all to say, deficiency/senescence hormones may be part of the normal activity of a plant and not simply around when unusually bad environmental conditions exist.

There are other ramifications which you can read about in other versions, that should hold to be still true with this 8 hormone theory too. For instance maybe now all four growth hormones, Auxin, Cytokinin, Salicylic Acid and Unknown Hormone X are required for before a cell will divide, and all four hormones Gibberellin, Unknown Hormone Z, Abscisic Acid and Ethylene are needed before a cell will senesce.

I will finish the rest of this paper at a later date..

Speculation: 8 Hormone System is Now Possible

noreply@blogger.com (Paul D Pruitt) — Thu, 18 Oct 2007 17:22:00 +0000

Hi, I believe it's now possible to construct an 8 hormone system along the lines of previous discussion. All the hormones would be made in great amounts by meristematic cell and small amounts by mature cells:

When a meristematic cell makes or takes in more than enough sugar to support both itself and any cell dependent on it for sugar, such that growth is possible it makes Auxin.
When a meristematic cell makes or takes in less than enough sugar to support both it any cell dependent on it for sugar, thus the plant must rely on stores and cannibalizing of older tissue it makes GIbberellin in order to continue growth of the meristematic cells.
When a meristematic cell makes or takes in more than enough Oxygen and Carbon Dioxide to support both it any dependent cell in growth, it makes Brassinosteroid.
When a meristematic cell makes or takes in less than enough Oxygen and Carbon Dioxide to support growth it makes Ethylene.
When a meristematic cell has growth levels of water it makes Salicylic Acid.
When a meristematic cell has senescent levels of water it makes Abscisic Acid.
When a meristematic cell has growth levels of minerals it makes Cytokinin.
When a meristematic cell has senescent levels of minerals it makes Hormone X (Jasmonic Acid? Polyamines?).

Cells do not divide unless they have growth levels of sugar, gases, water and minerals. This is not indicated to the plant except by high enough levels of all four growth hormones Cytokinin, Salicylic Acid, Brassinosteroid and Auxin.

A (perhaps non-meristimatic cell) does not senesce unless it has senescent levels of sugar, gases, water and mineral. This is indicated to the plant cell by high enough levels of Gibberellin, Ethylene, Abscisic Acid and Hormone X).

The growth hormones are usually higher during the day, the senescent hormones are usually higher during the night.Senescent hormones not only stop growth they induce alternate ways of getting the nutrients. One way this is done is by inducing a higher yielding but more risky way of obtaining the nutrient (GA induces C4 photosynthesis - a higher yielding method of photosynthesis that has a poisonous byproduct, Ethylene induces root hairs to absorb more Oxygen in the roots - this increases root surface area but makes the roots more susceptible to disease or drying out ).

Another alternate way of getting needed resources is by freeing up stored resource of the needed variety from vacuoles.

A third way is to inhibiting growth in the plant organ (root or shoot) that is not responsible for gathering the nutrient and making sure that nutrients are shunted to that organ that does. For instance Gibberellin stops root growth, because roots don't make sugar. Also ABA probably inhibits shoot growth and maybe Ethylene stops root growth too since most of the gases come in through the leaves. Finally hormone X should inhibit shoot growth since this organ does not take in minerals.

Senescent hormones probably induce senescent in older non-efficient plant parts that are sinks for the needed nutrient thus lowering the overall need of the plant for that nutrient. Thus Gibberellin should induce root senscence, Ethylene also root senescence, ABA - shoot senescence and hormone X - older shoot leaf senescence.

Finally nutrients are extracted for the senescent older efficient plant part and sent to the part that needs it. In fact this happens every night at a regular rate favoring the new plant part over the old and recycling nutrients from old inefficient cells to new promising youngsters.
Posted by socrtwo

Epinasty Helps Transpiration? Salicylic Acid an Indication of Water Abundance?

noreply@blogger.com (Paul D Pruitt) — Thu, 11 Oct 2007 23:34:00 +0000

Christian theologians who tried to reconcile Christianity with Science or Philosophy are (at least sometimes) I think call apologists. So what follows is a little bit of an attempt to reconcile part of the previously posted theory that may seem to be inconsistent or not fitting with experimental findings.

The main finding I want to deal with is the increase in Ethylene caused by Auxin. If the speculations are correct, how does Auxin lead to an increase in a signal that indicates low minerals and water uptake when Auxin induces new roots in plants?

The answer may be that Ethylene is released by active meristematic cells in the root or shoot that have less than enough minerals and waters than is required for their location in the plants (twice as much in root cells as in shoot cells). If you thus bathed the root with essential minerals and water, exogenous Auxin should have little or no effect on Ethylene production. Ethylene is an indication of active meristematic tissue failing to get enough minerals and water. Before exogenous Auxin activates the quiescent secondary meristems in the roots, they do not produce much Auxin or Ethylene, but once active, they are "required" to take in "profitable" or growth conducive amounts of minerals and water. This would mean more than enough minerals and water to support both it and a similar sized meristematic cell in the shoot. In the shoot on the other hand as mentioned in other places in this group, Ethylene is only made when the active meristematic cell has less than enough minerals and water to support growth period, just for itself, since it is not responsible for gathering minerals and water for any cells but itself.

My contention is that Ethylene is a meristematic cells attempt to find an alternate way of getting minerals and water. Auxin production by the plant is an indication of growth conducive environmental conditions for dividing cells via shoot gathered resources. Cytokinin would be an indication of growth conducive environmental conditions or resources gathered via the root (I realize conditions and resources are two separate things but let me blur the distinction for now). In the absence of Cytokinin or with low levels of it, Auxin is given the "green light" to try to compliment the growth conducive levels of sugar and gases with minerals and water, by lengthening the roots and inducing new ones.

Only when this doesn't succeed, does a plant make Ethylene, which tries alternative methods. Ethylene broadens roots instead of lengthening. It causes root hairs to grow, increasing root surface area, rather than induce new roots. It causes Epinasty of the leaves, increasing the rate of flow of the Xylem and thus water and mineral flow through the physical pumping action induced by leaves acting as handles of a water pump and as sails as such, being blown up and down by the wind. Thus Ethylene probably engages a "valve" in the Xylem for this pump, either by inducing it's growth or reactivating it (Ethylene levels probably peak in the plant every night anyway, so "valves" that engage nightly are a distinct possibility in my mind).

Three other effects of Ethylene could also increase the minerals and water available to meristematic cells. First Ethylene causes older leaves to senesce, particularly leaves not making Auxin and thus no longer gathering "profitable" or growth conducive amounts of sugar and gases (also an absence of Cytokinin being made by the older leaves would be an indication that the leaf is a mineral water sink and is not supporting itself - although maybe older leaves don't make Cytokinin period, nor older roots Auxin).

This senescence of older leaves of course, significantly decreases the water and mineral load requirements.

Secondly though I never saw much discussion about this, just like there are starch granules stored in vacuoles in the cells probably under the direction of Auxin that are released by Gibberellin, there must be water and minerals also stored in vacuoles under the direction of Cytokinin and released by Ethylene.

Finally the last way Ethylene could help "feed the babies", i.e. get the meristematic cells the water and minerals they need to grow, is by the release of the sugars and gases that occur with the cannibalizing of the no longer efficient older leaves (probably only occurring with the added environmental condition of high levels of Gibberellin). Thus just like Cytokinin and Auxin are needed for cell division, Gibberellin and Ethylene are always needed "to pronounce judgment" on a cell and lead it on a path of senescence. The sugar and gases can be used to support the new root hairs or maybe a further retry at lengthening the roots again.

Getting into Gibberellin and Cytokinin is the other part of the theory which I'm not concentrating on here, so that is an aside more or less. However I include it to hint at interdependence of all of four major hormone pathways.

It is quite possible that Brassinosteroid, Abscisic Acid, Salicylic Acid and an unknown eighth hormone are really involved in this system instead of four hormones. In this case, each hormone would either be an indication of abundance or scarcity of one nutrient group. Perhaps roughly, Auxin is an indication of sugar abundance, Gibberellin - sugar deficiency, Cytokinin - mineral abundance, Ethylene (or Hormone X) - mineral deficiency, Brassinosteroid - gas abundance, Hormone X (or Ethylene) gas deficiency, Salicylic Acid - water abundance (thus found in high amounts in Willow Trees growing on river banks), Abscisic Acid - water deficiency.

If you feel I'm cheating by sneaking in a big scheme at the end, its probably because I just made it up and am excited enough to include it :-))). I hope you don't find this disenguous. It is true I have been kicking these schemes around in my head for 22+ years, but the idea that Ethylene induces Epinasty as another way to move the transpiration stream other than Van Der Waal forces of surface tension, is a new one. As is seeing Salicylic Acid as a water abundance signal.

New Version of the Speculations Towards a General Theory

noreply@blogger.com (Paul D Pruitt) — Mon, 14 May 2007 01:45:00 +0000

By Paul Pruitt socrtwo@s2services.com

Introduction

Taking into consideration a criticism of my previous theories on plant hormones (see here: www.planthormones.info), I'm willing to revise the speculations so they fit more with current findings. The criticism that appears most salient is that GA is not made by older cells mainly but by younger cells. I will then make this axiomatic that the two pairs of complimentary hormones Ethylene and GA/Brassinosteroid and IAA/Cytokinin are made mainly by young cells. There are further implications to this that require some changes to my speculations. This page will take the form of theorems or axioms about plant hormones as did the other "papers" available on the site above.

Theorems

The major plant hormones Auxin, Cytokinin, Gibberellin, Brassinosteroid, Ethylene, Abscisic Acid and Salicylic Acid can be split into two groups. The first group is Auxin, Cytokinin, Gibberellin, Brassinosteroid, and Ethylene. They are hormones made to correct nutrient imbalances. The second group of Abscisic Acid and Salicylic Acid are made when any general stress occurs or the relief of that stress.
The nutrient hormones (Auxin, Cytokinin, Gibberellin, Brassinosteroid, and Ethylene) are made in the highest concentrations by dividing and young plant cells and the levels fall off precipitously, but not completely as the cells age. The stress hormones (Abscisic Acid and Salicylic Acid) are perhaps made by all cells in equal amounts facing the same stress or release from stress conditions.
Auxin is made mostly by young plant cells that have more than enough shoot derived nutrients (mainly sugar, CO2 and O2) to support both them and any dependent cell, thus growth is a possibility if balanced out by an excess of water and minerals. A root cell has no cells depending on it for sugar and gases, but a shoot cell would expect to have a similarly sized root cell depending on it for its shoot derived nutrients as well as having to fulfill its own needs.
In a likewise manner Cytokinin is made mostly by young plants cell that have more than enough root derived nutrients (mainly minerals and water) to support both it and any cells depending on it for root derived nutrients. Growth is thus a possibility here too if balanced out by an excesses of sugar and gases. For a shoot cell there would be no cells depending on it for water and minerals whereas a root cell should have a counterpart shoot cell depending on it for root nutrients.
Conversely now, Ethylene is made mostly by young cells when they do not have enough minerals and water to support both them and any cell depending on it for the acquisition of minerals and water. Restating this makes similar size cells in the root producing Ethylene when the level of minerals and water drops below 2 times that needed to maintain the cell at its present size, whereas for a similar sized shoot cell it would only have to be the amount of minerals and water dropping below what the cell itself alone needs to maintain life at its present size. Thus a plant will need to cut back in size, if the deficit can't be made up.
Also GA or Brassinosteroid (I'm lumping them for now into one hormone cascade path) is made mostly by young cells when they have less than enough sugar and gases to support both it and any cell depending on it for acquisition of these nutrients. Thus again more plainly, if a root cell does not have enough sugar and minerals to maintain their present size, they make GA or Brassinosteroid. A shoot cell of the same size and maturity will do a similar thing if making less than twice the needed sugar and acquiring less than twice the needed gases to maintain itself, because it is supporting a similar sized and maturity stage root cell for those nutrients. Thus again the plant will need to cut back if the deficit can't be made up.
The nutrient hormones are made to correct imbalances. They do this by affecting 4 things: nutrient transport, nutrient storage, direction of growth, and new growth initiation or old growth senescence.
For Auxin correcting the imbalance of the perceived excess of sugar and gases is first dealt with by initiating active transport of sugar and gases away from the site of synthesis of the hormone and induction of active transport of minerals and water to the site of Auxin production. Nutrient storage is initiated for the excess sugar and gases in vacuoles of the cell and release of store minerals and water if any from vacuoles in the cell would also initiated. The direction of growth of the plant or plant cells is also changed by Auxin. If the plant, plant organs, or plant cells have been broadening (because of the presence of Cytokinin or Ethylene) they are changed to lengthening strategy of growth. Finally Auxin corrects the imbalance by initiating new roots by causing dormant root buds to grow out, thus increasing the flow of water and minerals. Auxin is also of course known to inhibit the growth of new shoot buds with shoot apical dominance.
For Cytokinin correcting the imbalance of the perceived excess of water and minerals is done by initiating or increasing the transport of water and minerals away from the site of synthesis and increasing the active transport of sugar and gases towards the site of synthesis. Likewise, Cytokinin increases the storage of water and minerals within the synthesizing cell's vacuoles and increases the release of sugar and gases from vacuoles stores if they exist within the cell. Also Cytokinin causes or influences the growth of any cell, organ or plant toward broadening, and away from lengthening. Finally Cytokinin induces new shoot growth and inhibits root bud growth with root apical dominance.
For Ethylene the correction of the deficit of water and minerals is handled by actively increasing the flow of water and minerals to the site of synthesis (and by increasing the flow of sugar and gases out of the cell too ???). Nutrient stores of water and minerals wherever they are found are encouraged to give up their storage to needy cell(s) producing Ethylene (and sugar and gases are stored in vacuoles to decrease an imbalance???). Ethylene also is known to influences the direction of growth of a plant away from lengthening to broadening. Finally Ethylene induces older leaves to senesce, sending the resulting freed up water and minerals to locally needy leaves and sending the sugar and gases to the root to make more roots. Ethylene is known to induce root hairs which increases the surface area of the root and thus increases the uptake of water and minerals. Ethylene probably inhibits new shoot growth.
For Gibberellin/Brassinosteroid the correction of the deficit of sugar and gases is handled by actively increasing the flow of sugar and gases to site of synthesis (and increasing the rate of transport of sugar and gases out of the cell to recreate the balance???). Stores of at least sugars in the form of starches are known to be made available by GA during seed germination and probably exist under all circumstances. This would be true of any stored gases too. The direction of growth of cells is also influenced by GA toward lengthening. Finally GA/BA inhibit root growth and probably cause the senescence of older roots as well as an analogous change of strategy to root hair initiation that Ethylene produces in roots that might be represented by bolting to move the plant out of the shade.
It is widely known the Auxin and Cytokinin are needed to induce cell division. This can be seen as the plant being reassured by these signals that it has excess amounts of all nutrients of both the shoot and root derived kind, and so cell division is warranted. I believe it's also been shown that a cell under the influence of both Auxin and Cytokinin will draw all nutrients to itself not sending away any excesses. Thus if a cell or plant organ is making Auxin and it comes under the influence of enough Cytokinin, it will change strategy and stop exporting sugar and gases and instead become a net importer of the resources even if they are a successful young shoot cell.
Complimentarily I'm proposing that Ethylene and GA/BA are needed for cell senescence. In fact when Ethylene is being released in the shoot, the cells or leaves that "it chooses" to senesce, may be the ones making the most GA, because these would be cells or leaves that are the least efficient at doing what the leaf should do, which is procure sugar and gases. If a cell is synthesizing GA and comes under the influence of Ethylene, I believe it will stop actively drawing sugar and gases to it, and actually start to send them out. GA and Ethylene acting together would then send out all nutrients, leading to the synthesis of more GA and Ethylene and an even higher rate of active transport of these nutrients out, with this active feedback loop leading to a climactic rise in Ethylene and GA and senescence.
Auxin and Cytokinin are highest in levels during the day when the sun is out for photosynthesis and improved transpiration. (Perhaps transpiration increases mineral absorption and water intake following osmosis and the uptake of water and minerals is higher during the day than at night for most plants). Ethylene and GA/BA levels are higher at night.
Ethylene and GA/BA levels are higher at the beginning of the life of a plant when water, minerals, sugar and gases stored in the seed must be released. Cytokinin and Auxin levels are highest during the middle active growth period of life of the plant. Ethylene and GA/BA levels increase again relative to Auxin and Cytokinin at the end of the life of the plant or growing season, when the nutrients must be withdrawn from unneeded plant organs like leaves or flower petals or moved into the fruits and seeds.
Ethylene and GA/BA move resources more toward the center of the plant and away from the periphery. Auxin and Cytokinin are more risk taking hormones moving resources to the active edges of the plants where "the action is".
As for ABA and Salicylic Acid, ABA is a "batten down the hatches" hormone that is like adrenaline and quickly potentiates a plants response to rapidly developing environmental emergencies of all kinds. Possibly it does not do anything on its own but greatly magnifies any reaction a plant is having to a threat. Salicylic Acid on the other hand, would be the "stand down" hormone to bring the plant back to normal operations. I realize that ABA is famously known for closing guard cells and Salicylic Acid is found in Willow Bark, a tree more than any other in need of having open guard cells to pump out the excess water that occurs at the roots of the willow due to it's habitat of living on river banks. However, ABA is known to be induced by heat shock, salt shock, insect damage etc. Maybe all of these have a common denominator of water loss but my thinking is that it destroys the symmetry of the theory to say that it is actually involved per say in a nutrient issue rather than more primarily as an indicator of any kind of shock to a plant. Also there have been failures in the past to securely tie it to all desiccation events.

Speculative Plant Hormone Theory - Summary

noreply@blogger.com (Paul D Pruitt) — Wed, 20 Apr 2005 03:12:00 +0000

"Fools have no interest in understanding; they only want to air their own opinions.” _{Proverbs 18:2 NLT}"Whatever exists has already been named..."_{Ecclesiastes 6:10 NIV}

In this paper plant hormones are divided into 3 groups: Growth Hormones, Stress Hormones and Shock/Synchronizer Hormones. In a nutshell, I generalize that Growth Hormones are an indication by the plant to itself that it is prospering and has excess nutrients. What the plant does with these nutrients and the signal is determined by other things. The hormones are made mostly in young and meristem cells and much less in mature cells. I hypothesize that Stress Hormones, in contrast, are made in cells that are faced with a scarcity of nutrients and again are an indication of such a condition. The Stress hormones are made mostly in mature cells and much less in immature (e.g. meristematic) cells. Finally, the Shock/Synchronizer Hormones as the name might indicate are rapid acting hormones for fast developing bad (or good) environmental condition including rapid changes in nutrient levels but in no way limited to it. The Shock/Synchronizer hormones "batten down the hatches" for the plant, or release it from this strategy. Auxin and Cytokinins (CKs) are Growth Hormones. Ethylene, Gibberellins (GAs) and Brassinosteroids are Stress Hormones. And Abscisic Acid (ABA) and Salicylic Acids (SAs) are Shock/Synchronizer Hormones.

This paper sets out theory or assumptions or postulates. It's followed by predictions if the postulates are true and then comes an addendum. The addendum gives an alternate take on the Growth and Stress Hormones. It suggests these hormones may measure overall nutrient level, not their extent of excess or scarcity . That is are hormones like temperature indicators made in proportion to a scale and thus usually present in some quantity or are they not made at all until excess/scarcity arises? This paper will concentrate on the as needed excess/scarcity model but will briefly explain the "nutrient temperature" theory at the end in the addendum.

The most glaring problem with the theory is Gibberellin is now thought to made mostly in young tissue not mature tissue. This theory also has a limited scope. I will only lightly touch on Brassinosteroid, Jasmonates and Oligosaccharins and not at all on Polyamines. It attempts to provide a backbone or generalized framework from which to understand plant hormone behavior, on which I hope others will elaborate. I want to put forth a structure of 3 pairs of complimentary hormones and suggest what the plant uses them for. This a "sketch" at best but I hope it will stimulate recognition of the simple general main functions of these hormones or hormone pathways-cascades. Again this is a view, perhaps even cartoonish, but nevertheless revealing in my estimation, of one facet of the "beautiful diamond" that is the physiology of plants.

Speculative Plant Hormone Theory - Introduction

noreply@blogger.com (Paul D Pruitt) — Wed, 20 Apr 2005 03:03:00 +0000

A little background info on Plant Hormones and this article's intentions.

Since at least Darwin’s time, it has been known that plants regulate their growth with some kind of internally secreted chemicals. Plant hormones, according to a standard definition from the Web:

Are signal molecules produced at specific locations;
Occur in low concentrations;
Cause altered processes in target cells at other locations.

Today, it is accepted that there are five major classes of plant hormones, with a few possible candidates to add in the future. The five major classes are Auxin, CKs, Ethylene, GAs, and ABA. Recently it has been suggested that BAs, JAs, SAs, Oligosaccharins and Polyamines are new major classes of hormones. This paper is not an introduction to the discovery, chemical structure, and synthesis pathways of the hormones. There are several decent introductions to these on the Web. Instead, I shall try to provide a second-level examination of the hormones and a unifying outline of a theory that explains the underlying relationships and generalized principles under when these hormones are secreted. This paper is a simplification of the findings. I want to warn any reader, that I have a strong preference for symmetry in theories and models. Inspired by the fact that plants show a strong physical symmetry, being divided into roots and shoots, my two theories will each be strongly symmetrical.

Any theory of plant hormones needs to recognize the work of K. V. Thimann, F. Went, F. Abeles, F. Skoog, G. Avery, P. F. Wareing, P. Davies, P. W. Morgan, W. P. Jacobs, A. C. Leopold, A. W. Galston, R. Cleland, and F. Addicott. Forgive me for leaving out the names of countless others who have made major contributions to the field. Special thanks go to Mark Jacobs for getting me so interested in plants in the first place.

Speculative Plant Hormone Theory - Characteristics Table

noreply@blogger.com (Paul D Pruitt) — Wed, 20 Apr 2005 02:49:00 +0000

This is a table of all the characteristics of Plant Hormones of which I am aware. They include the location Location, Characteristics and Occasions for Synthesis Induction and the effects the hormones have.

All items in bold are known scientific findings - references are in progress
All items with “?” are from papers known from my research notes whose dates I did not record
All items with “?” Present difficulties to the theory outlined below
All items in italics, are speculations on my part

Name (With Example)

Location, Characteristics and
Occasions for Synthesis Induction

Effects

Auxins

IAA

Synthesized in shoot and root meristematic tissue (Sembdner et al., 1980)
Synthesized in much greater amounts in the shoots than roots
Synthesized in young leaves (Sembdner et al., 1980)
Synthesized in mature leaves in very small amounts
IAA peaks during the day (Jahardhan et al., 1973)
Synthesized in mature root cells
Released by meristematic cells when they have more than enough Sugar ,carbon dioxide and oxygen to support both themselves and any dependent cells in peak metabolism conditions
Alternatively Auxin is released when it has any excess of sugar or gases above which it had previously and is proportional to the excess.
Third possibility is it is released when Sugar ,carbon dioxide and oxygen rise above survivable levels (a minimum level of nutrients) and is proportional to them.
Released by all cells when they are experiencing conditions which would normally cause a shoot meristematic cell to produce Auxin
Directly or indirectly induced by high levels of Ethylene

Stimulates cell elongation (Schneider, 1938)
Stimulates cell division with CK
Induces xylem and phloem (Jacobs, 1967)
Directly stimulates Ethylene synthesis
IAA inhibits Ethylene formation and transport of precursor (Wright, 1980)
Induces shoot apical dominance (Snow, 1945; Palmer & Phillips, 1963)
Inhibits abscission prior to formation of abscission layer (inhibits senescence of leaves)
Involved in phototropism, gravitropism, tropism toward moisture
Induces sugar and mineral accumulation at the site of application (Mitchell et al., 1937; Booth, ?; Davis and Wareing, ? )
Flower initiation
Sex determination
Induces xylem and phloem
Induces new root formation (Torrey, 1957; Brown et al., 1975) by breaking root apical dominance induced by CK
Inhibits root hair growth and causes them to die back

Cytokinins

(CKs)

Zeatin

Synthesized in root and shoot meristematic tissue (Chen et al., 1985)
Synthesized in much greater amounts in the roots than shoots
Synthesized in meristematic regions of roots (van Staden & Smith, 1978)
Synthesized in mature roots – small amount
Rapid transport in xylem stream
CK activity reduced in plants suffering drought (Vaadia, 1965)
Peaks during the day (Hewett & Wareing, 1973)
Synthesized in mature shoot cells
Released by meristematic cells when they have more than enough minerals and water to support both themselves and any dependent cells in peak metabolism conditions
Alternatively Cytokinin is released when it has any excess of minerals and water above which it had previously and is proportional to the excess
Third possibility is it is released when minerals and water rise above survivable levels (a minimum level of nutrients) and is proportional to them
Released by all cells when they are experiencing conditions which would normally cause a shoot meristematic cell to produce CK
Directly or indirectly induced by high levels of GA/BA

CK promotes Chlorophyll production and leaf unrolling (Beevers et al., 1970)
CK promotes photosynthesis (Adedipe et al., 1979)
Stimulates cell broadening (Egelke et al., 1973)
Also promotes shoot formation (Skoog & Miller, 1957)
Also promotes the unloading of sugar from phloem (Hayes & Patrick, 1985)
Causes the outgrowth of secondary shoot buds – breaks shoot apical dominance/ lateral bud development (Sachs & Thimann, 1967)
Delays leaf senescence (Pooviah & Leopold, 1973)
Stimulates cell division with Auxin
Involved in morphogenesis (Houck & Lamotte, 1977)
Promotes stomatal opening
Induces xylem and phloem
Directly induces GA/BA at high levels
Inhibits C4 Photosynthesis
(From Theory II) Stimulates the rate of metabolism of cells in the shoot (who are not at their peak metabolism rates) in response to an increase in the levels minerals and water

Ethylene

(ET)

Ethylene

Directly induced by high levels of Auxin (Rubinstein & Leopold, 1964)
Found in germinating seeds (Esashi & Leopold, 1970)
Induced by root flooding (Kawase, 1972; El-Beltagy et al., 1974; Imaseki, 1985)
Induced by drought (El-Beltagy et al., 1974)
Synthesized in nodes of stems
Synthesized in tissues of ripening fruits
Synthesized in response to shoot environmental, pest, or disease stress
Synthesized in senescent leaves and flowers
Rapidly diffuses
Inhibiting effects of Ethylene on shoot growth (more specifically on stem elongation) reduced in the presence of light (Wareing & Phillips, 1981). Also Ethylene levels are decreased by light (Goeschl et al., 1967)
Released by mature cells when they have less than enough minerals and water to support both themselves and any dependent cells under any conditions (less than survivable levels of minerals and water)
Alternatively Ethylene is released when it has any scarcity of minerals and water below which it had previously and is proportional to the scarcity
Third possibility is it is released when minerals and water fall below those needed for peak metabolism levels (a maximum level of nutrients beyond which growth or nutrient storage is induced) and is proportional to gap between peak metabolism levels and the current level
Released in when they do not have enough minerals and water to support both themselves and any dependent cells
Released by all cells when they are experiencing conditions which would normally cause a mature shoot cell to produce Ethylene

Stimulates leaf and flower senescence (Wareing & Phillips, 1981)
Induces leaf abscission (El-Beltagy et al., 1974) mainly in older versus younger leaves (Leopold, 1970)
Induces seed germination (Esashi & Leopold, 1969; Ketring & Morgan, 1970)
Induces root hair growth – this increases the efficiency of water and mineral absorption
Stimulates Epinasty – leaf petiole grows out, leaf hangs down and curls into itself
Stimulates fruit ripening
Induces the growth of adventitious roots during flooding
Usually inhibits growth (El-Beltagy et al., 1974) - just shoot growth
Affects neighboring individuals
Disease/wounding resistance
Triple response when applied to seedlings – root ? and shoot growth inhibition and pronounced hypocotyl hook bending
Inhibits stem swelling ? (Contradictory to the finding below – contradictory sources)
Stimulates cell broadening (Burg & Burg, 1966) (and lateral root growth)
Interference with Auxin transport (when hormone levels are increasing)
Directly or indirectly induces Auxin at high levels
(From Theory II) Inhibits the rate of metabolism of cells in the shoot (who are not already at their lowest metabolism rates) in response to an decrease in the levels minerals and/or water

Gibberellins

(GAs)

Gibberellin 452D

Synthesized in the embryo (Webb et al., 1973) and germinating seeds
Synthesized in the roots (Barrington, 1975)
Levels go up in the dark when sugar cannot be manufactured and down in the light (Brown et al, 1975)
Synthesized in apical meristems ? and young leaves ?
Produced in the stem rather than the growing tip ? (opposite finding to above – conflicting sources)
Transport is non-polar, bidirectional producing general responses
Released by meristematic cells when they have more than enough Sugar ,carbon dioxide and oxygen to support both themselves and any dependent cells in peak metabolism conditions
Released by mature cells when they have less than enough sugar and gases to support both themselves and any dependent cells under any conditions (less than survivable levels of minerals and water)
Alternatively Ethylene is released when it has any scarcity of sugar and gases below which it had previously and is proportional to the scarcity
Third possibility is it is released when sugar and gases fall below those needed for peak metabolism levels (a maximum level of nutrients beyond which growth or nutrient storage is induced) and is proportional to gap between peak metabolism levels and the current level
Released by all cells when they are experiencing conditions which would normally cause a mature root cell to produce GA or BA
Released in response to root environmental, pest, or disease stress
Directly induced by high levels of CK

Stimulates shoot and cell elongation (Engelke et al, 1973)
Delays senescence of leaves (Manos & Goldthwaite, 1975; Goldthwaite, 1972)
Inhibits root growth (Thimann, 1977; Mitsuhashi-Kato et al., 1978)
Inhibits adventitious root growth (Rossel ?)
Produces seed germination (Egley, 1980)
Antagonist promotes root growth and GA reverses this (Kefford, ?)
Promotes root initiation in low concentration in pea cuttings (Eriksen, 1970, 1971)
Stimulates bolting and flowering in biennials (Zeevaart, 1983)
Regulates production of hydrolytic enzymes for digesting starches (Varner, 1964)
Inhibits CK bud growth on calluses (Engelke et al., 1973)
Inhibits bud formation (Murashige, 1964)
Inhibits leaf formation (Bryan et al., 1955; Tronchet, 1968)
Breaking of dormancy
Induces extra Chlorophyll production or more efficient methods of photosynthesis (C4Photsynthesis). I think this reference actually exists
Stimulates root senescence
Directly or indirectly induces CK at high levels
(From Theory II) Inhibits the rate of metabolism of cells in the roots (who are not already at their lowest metabolism rates) in response to an decrease in the levels sugar and/or essential gases

Abscisic Acid (ABA)

Abscisic Acid

Released during desiccation (Wain, 1975)
Has been found to peak at night (Lecoq et al., 1983 a, b)
Synthesized in green fruit and seeds at the beginning of the wintering period
As well as moving within the leaf it can be transferred to the leaf from the roots by the transpiration stream
Rapidly translocated
Produced in response to stress
Synthesized in leaves and stems (particularly when water stressed)
Released by cells in danger of not having enough nutrients locally or good enough environmental conditions to survive
All cells capable of synthesizing

Stimulates stomatal closure (Wain 1975)
Fruit ripening inhibition
Encourages seed dormancy by inhibiting cell growth – inhibits seed germination
ABA inhibits the uptake of Kinetin (Reed, 1974)
Pathogen resistance response defense -
Induces senescence in already damaged cells and their proximate neighbors
Quickly puts a plant, organ, tissue or individual cell in a defensive posture (whatever this entails) in response to rapidly developing nutrient or environmental stress that threaten their survival
Decreases metabolism in response to a newly developing deficiency of nutrient or adverse environmental condition, such that condition becomes survivable at the new lower level of metabolism (Not true in Theory II)
Possibly induces cell dormancy or senescence by a climactic increase or sustained level stimulating the synthesis of GA and/or Ethylene (Not true in Theory II)
A climactic rise or sustained level of ABA may be a prerequisite for the synthesis of any GA and/or Ethylene in that it presence indicates unusable or unsurvivable levels of Water, Sugar, Minerals and/or essential gases (Not true in Theory II)

Brassinosteroids (BAs)

Brassinolid

Released in mature cells when they have less than enough sugar and oxygen to support both themselves and any dependent cells
Released by all cells when they are experiencing conditions which would normally cause a mature root cell to produce BA or GA
Released in response to root environmental, pest, or disease stress

Increased rate of stem elongation (Thompson et al., 1982)
Leaf senescence inhibition
Involved in gravitropism
Bending of grass leaves at the sheath/blade joints
Inhibits leaf abscission
Inhibits root growth
Resistance to stress - just in the shoot. By rerouting resources from the root to the stressed shoot
Stimulates cell elongation and division (Thompson et al., 1982) – just in the shoot
Enhanced Ethylene production ? – induced indirectly by the causation of root cell senescence
Promotion of growth - just shoot growth
Xylem differentiation promotion - in order to transfer resources from cannibalized root cells
(From Theory II): inhibits the rate of metabolism of cells in the shoot (who are not already at their lowest metabolism rates) in response to an decrease in the levels sugar and/or essential gases

Jasmonates
(JAs)

Jasmonic Acid

Desiccation
Effect of elevated ABA levels
JA-induced proteins are lacking in the roots, in bleached leaves, and in leaves of chlorophyll-deficient

Growth inhibition
Senescence promotion
Stimulates wound responses
Germination inhibition
Tuber formation promotion
Fruit ripening and fruit abscission promotion
Pigment formation promotion
May have a role in plant defense

Salicylates

(SAs)

Salicylic Acid

Cells returning from water stress
Released by cells secure in having more than enough nutrients and environmental conditions locally to survive at its current metabolic level
All cells capable of synthesizing
Has its effect or acts by rapid local increases followed by rapid decreases in levels

Retards senescence (regulatory role) – probably by inhibiting Ethylene biosynthesis
Induces flowering
Inhibits seed germination – by inhibiting ABA synthesis
May also block the wound response and act antagonistically to ABA – preventing the wound response from spreading further than necessary
After a survival threat has passed SA quickly removes a plant, organ, tissue or cell from a defensive posture and returns it to normal functioning
Increases cell metabolism rate to take advantage of new complete more advantageous nutrient and environmental conditions (Not true in Theory II)
A climactic or sustained level of SA may occur if a cell has reached its peak metabolic levels and may signal that a plant’s resources can be turned to growth (Not true in Theory II)
This climactic or sustained level of SA may be a prerequisite for the synthesis of Auxin and/or Cytokinin, because only then does a plant know that it has enough resources to turn them to growing bigger (Not true in Theory II)

Oligogalacturonides

Pectin-Derived Polymers

Flowering inducers such as long days or short days according to the plant

Stimulates flower formation - the searched for "Holy Grail" flower inducer "Florigen."
Stimulates defense responses

Xyloglucan -
e.g. Hemicellulose -
Derived Polymers

Graminaceous hemicellulose

Induced by Auxins, pathogens, pestilence and mechanical wounding

Stimulates cell elongation and growth
Stimulates defense responses
Stimulates morphogenesis (in culture)
A compounding/magnifying element of the Auxin pathway.

Table suggested by first link. Some info also pulled from all links below as well as my own research:

Speculative Plant Hormone Theory - Theory

noreply@blogger.com (Paul D Pruitt) — Wed, 20 Apr 2005 02:45:00 +0000

Here I try to put forth what seems to me to be reasonable assumptions about how Plant hormones work.

1. The goal of a plant is to germinate, survive, grow, and reproduce (and either exploit or contribute to life in general.

2. The role of the shoot is to create sugars from sunlight, water, and carbon dioxide harvested from the air. It also harvests most of the oxygen needed by the plant for respiration. The shoot may serve as a reserve store for water and minerals. This may be far fetched as a general principle but the storage of water occurs in at least the cactus. The best place for storing all nutrients may be out of harms way in the soil, in the root. The shoot also provides the structure that supports the leaves, flowers, and fruit, but this will not be important here.

3. The role of the root is to harvest water and minerals from the soil. In order to function, the root also needs to harvest some oxygen from spaces between the soil particles. The root also provides a place for storing reserves of sugar in the form of starch and may even store oxygen. It also anchors the plant in a propitious place for it to grow and prevents it from being physically uprooted by the elements or fauna. The anchoring role of the root will not be important here.

4. If they face conditions where they have to make a choice, plants will invest in promising new meristematic cells (i.e. those that are functioning well in their role, like young leaves making good amounts of sugar and successfully harvesting oxygen) and withdraw nutrients from mature cells to feed these “babies,” even if that means withdrawing nutrients from mature cells that are functioning “adequately”. There is a cost to the transfer of nutrients from mature to juvenile cells. This cost is measured in the loss of some nutrients during the process. It is true that minerals cannot be destroyed and are usually not excreted. However, they are probably not fully recoverable from a mature cell, leaf, or root, in the same way that they would be if they were merely stored in some kind internal reserve, such as a vacuole within a cell.

5. There are three general groups of plant hormones. The Growth Hormones are released under long term good growth conditions and are separated into one predominantly synthesized in the shoot and one in the root. The Stress Hormones are released under various kinds of long term stress and are separated into one synthesized predominantly in the shoot and one in the root. Lastly, there are the Shock/Synchronizer Hormones. The idea with the Shock/Synchronizer Hormones is that they are released under rapidly developing stress of any kind or return from stress good conditions, confronting the individual cells, parts of the plant, or the plant as a whole. To elaborate, these are the first hormones released when the physical survival of the cells is under threat or when the cells return to secure environmental and nutrient conditions. They quickly shut a plant or plant part down or restore it to normal functioning. They may also play a secondary role as modulators of the rate of cell metabolism slowing it down to survivable levels according to local conditions, or speeding it up so that full use may be made of current nutrient levels and environmental conditions. In fact, a final climactic high or sustained level of these hormones may be needed to kick off the synthesis of the stress hormones (GA and Ethylene) on one end, or the growth hormones (Auxin and Cytokinin) on the other.

6. All plant cells are totipotent not just under the right conditions, such as in tissue culture, but also in the way that they behave in response to environmental and nutrient conditions. That is, a shoot cell will always act somewhat like a root cell, and a root cell will always act somewhat like a shoot cell. In addition, a mature cell will act somewhat like an immature cell, and vice versa. For example, below, it is suggested that just like a shoot meristem cell, when any cell is met with good environmental conditions and more than enough sugar and oxygen to support growth, it will make Auxin or at least a tiny amount of it.

7. The Growth Hormones include Auxin and CKs. These are made by all cells when conditions are good for growth. Auxin is made when any plant cell is facing the conditions that would be propitious for the growth of a shoot meristematic cell. These include freedom from environmental stresses and the production or existence of more than enough sugar and oxygen to support it and any cells depending on it. (Root cells, except for the few that are meristematic, have no cells depending on them for their sugar and oxygen needs.) CKs are made by any cell under the conditions that would be propitious for a root meristematic cell to grow. These include freedom from environmental stresses and the uptake or existence of more than enough minerals and water to support it and any dependent cells. To go into more detail, a root cell, for example, would make a CK if it was taking in more than enough minerals and water to support both it and any dependent shoot cell of similar size in the shoot (i.e. a cell in the shoot that depends on that cell in the root for its water and minerals). A similar relationship exists for a shoot cell vis-à-vis sugar and oxygen, and the sister cell in the root that depends on it for the nutrient.

When I say more than enough nutrients for supporting cells, I mean more than enough nutrients to support cells at their peak metabolism. There are two alternatives to this view that I discuss in the addendum.

8. The Stress Hormones include the GAs, ET and the BAs. These are made by all plant cells under some kind of stress where the plant must remove resources from the site of stress and redistribute the resources to another part of the plant so that the stress is ameliorated. They also initiate the freeing of stored resources to address the particular shortfall. GAs, for instance, are made by all cells under which conditions that would be disadvantageous to a mature root cell (e.g. in any cell when there is less than enough sugar and oxygen to support both it and any dependent cells). GA removes the resources from this cell and redistributes them into some part of the shoot so that the prospect of increasing the supply of sugar and oxygen to the entire plant is increased. (The root would keep the resulting sugar and oxygen for itself and route the minerals and water to where they were needed in the shoot.) Also GA causes the release of enzymes in the root, which turn starch stored in vacuoles into sugar. This sugar also helps, at least temporarily, to resolve deficiencies. I hypothesize that oxygen is also stored in the roots and GA initiates its freeing from storage and availability to the root. Gas is a much less compact stored item than starch, but nevertheless this phenomenon is possible.

Similarly, ET is made by all cells under similar conditions and would be disadvantageous for the existence of a mature shoot cell. For instance, in any cell when there are not enough minerals and water to support both it and any dependent, similar-sized cell, ET causes the withdrawal of all nutrients and redirects the sugar and oxygen to the root, keeping the minerals and water in the shoot. This is an attempt to produce productive new root growth and an eventual surpassing of the previous levels of minerals and water. This is an attempt by the plant at a wise reinvestment of resources. It is a gamble to jump start the growth in mineral and water levels to facilitate the growth of the shoot. These jump starts always have costs. These include some use of energy and thus an overall net loss in the weight of the plant. I hazard that minerals and water are stored in either the shoot (for theory symmetry) or the root (because of the practicality of its inaccessibility to the hostile outside world). I suggest that ET initiates the freeing of stored minerals and water in addition to its resource stress role.

When I say less than enough nutrients for supporting cells, I mean less than enough nutrients to support cells at a survivable or minimum level of metabolism. There are two alternatives to this view that I discuss in the addendum.

Like many involved in the study of plant hormones, I believe that BAs may actually be part of a hormone cascade that involves GA or a parallel path with many similarities. That is, BAs may just be a step along the way in a scheme from a stress to the root cells generally and the plant’s reaction to the stress. In fact, BAs may be the primary hormone igniter of the chemical domino path that leads to the reaction of a plant to stress and GAs may be just a step along the path. Future experimentation will elucidate this. Because of the similarities, I will refer, as some scientists do, to GA and BA together as the GA/BA class of hormones.

9. The Shock/Synchronizer Hormones are the ABAs, and SAs. They fall into two categories: general metabolic inhibitors/senescence stimulators, and general metabolic stimulators/senescence blockers. They act rapidly. I am defining ABAs as general metabolic inhibitors/senescence activators. They would be made when there is any kind of nutrient or environmental stress to a cell, such as a low or high temperature, wounding, mechanical stress from wind or other plants or animals, pests, disease, or the dropping below sustenance level of minerals, water, sugar, or oxygen.

In terms of nutrient stress, the survival ABA is made when any of the resources of the cell fall below sustenance level. This hormone would not be activated if the levels of these nutrients fell below what the particular cell nutrient level was supposed to be for that type of cell if it was supposed to harvest or synthesize that nutrient for others. For instance in a root cell, ABA would not be synthesized until either minerals or water fell below what was necessary for the survival of the root cell itself. The hormone would not be made when the cell had a level of minerals and water below what was necessary to support both it and a sister cell in the shoot. Falling below that level would initiate ET emanation. The level of sugar and oxygen necessary for ABA synthesis in the root would of course be below the sustenance level, which might be a frequent occurrence because the cell has to get these nutrients from the opposite end of the plant. In an attempt to re-establish good supplies GA/BA would also be released when sugar and oxygen fell below sustenance level. Low levels of ABA would be to slow the metabolism so that less sugar and oxygen were needed. At higher ABA levels, ABAs would work with GA/BAs to senesce the cell. At low levels, the slowing down of the metabolism itself might retard the GA/BAs’ proposed initiation of that root cell’s senescence. ABA, I hypothesize, always work first at a low level to slow metabolism, so that the nutrient sustenance level is actually lowered to a “bearable level” or in the case of environmental stress of any kind, less damage ensues than would occur at a higher rate of metabolism. When and if the plant or cell “calculates” that survival is not possible or “not warranted,” it releases higher levels of ABA, initiating the senescence of the stressed cell or damaged part of the plant.

The SAs on the other hand are categorized as metabolic stimulators/senescence inhibitors that hasten all metabolic activities and oppose the action of ABA and the Stress when they are trying to inappropriately move nutrients out of an efficiently working cell. SA would be released when a cell is in no physical danger of survival and might increase with better nutrients or any kind of internal and external environmental conditions above a base line. SA might be a “foundation” indicator, indicating that a cell is in good health without regard to how it is supposed to be functioning (i.e. as a root or as a shoot cell). For example, it might be released after a good rain after a drought, when ABA and the drought action of overall metabolism inhibition, stomate closing, and progressive senescence with plant size shrinkage is no longer warranted.

These Shock/Synchronizer Hormones may also act in low quantities on the metabolism as day-to-day status quo regulators. These actions would involve neither growth nor redirection and its cost of plant shrinkage. ABA and SA levels may rise and fall many times a day. When the levels are low, they may be the equivalent of “moods” in animals or humans or a Circadian Rhythm, alternating between levels of depression and inaction and excitement and action, as the conditions and growth opportunities warrant.

ABA and SA often seem to have counteracting effects.

10. The Growth Hormones are made primarily in meristematic tissue and the Stress Hormones are made in mature tissue.

The Shock/Synchronizer Hormones are made in all tissues equally. Although the amount of most hormones made by cells may differ according to their maturity, small amounts of each of these hormone groups are made in all cells under the right conditions. The exception is that perhaps the Shock/Synchronizer Hormones are made in all cells in equal amounts under the right conditions. Alternatively, perhaps under the same conditions, the survival hormone ABA are made in larger amounts in mature cells than in juvenile cells. The suggestion is that juvenile cells can recover from stress more easily than mature cells. Therefore, they have less need of the stress-protecting effects of low levels of ABA and do not need to be sent to the “glue factory” of senescence when they are experiencing high levels of stress (i.e. the plant has “confidence” that they will recover). Perhaps we can even say that plant cells, like all living things, are most susceptible to stress at the beginning and end of their lives. Thus, under the same stress, higher amounts of ABA would be made at the beginning and end of the lives of cells, because the plant would “know” that this was when the cells were most susceptible and least likely to survive stress. Conversely, SA might be more easily made in young cells and mature cells. That is, the highest amounts of SA would be made in cells after they have moved out of the fragile juvenile stage and before they move into senility or close to it.

Getting back to the Growth and Stress Hormones, let’s take Auxin as an example. It is a Growth hormone and primarily a shoot hormone. The largest amounts are made in shoot meristematic cells. Smaller amounts are made in root meristematic cells and also in mature shoot meristematic cells, but there is still a small or very small amount in mature root cells under the right conditions (i.e. under conditions that would induce a shoot meristematic cell to produce it, involving good external conditions and a good level of sugar and oxygen in the older root cells). Looking at ET, it is a Stress hormone and primarily a “shoot hormone” too. It is made when mature shoot cells are experiencing deficiencies in water and minerals, but would also be made when mature root cells are not taking in appropriate amounts of water and minerals. It would also be made in the shoot meristems when they were experiencing deficiencies of minerals and water. Perhaps the ET levels would only rise to levels that would cause hibernation of these meristematic regions, like in the secondary buds. Finally a small or very small amount would be made in meristematic tissue of roots experiencing water and mineral shortage.

11. Growth Hormones and SA gather all nutrients and perhaps also attract proximate supplies of Growth Hormones and SA while repelling Stress Hormones and ABA. This makes meristematic tissue get involved in positive feedback loops that are responsible for apical dominance, where the better the conditions for dividing cells, the more Growth Hormones and SA are made at the site. This happens in an exponential way. Eventually one meristematic tissue wins out over all the rest, and this becomes the apically dominant meristem. The others go into hibernation. Stress Hormones and ABA repel all nutrients from the site. They may also repel Growth Hormones and SA and attract Stress and ABA.

12. The Growth Hormones and SA may both need to be at high levels for cell division to take place. The Stress Hormones and ABA may all also need to be “in attendance” before cell death is “signed off on.” If this is true of cell division, the addition of SA may greatly increase the ease of raising calluses from single cells in tissue culture. Where success has been had in the past, the cell lines may have natively synthesized unusually high levels of SA.

13. The ratio of endogenously synthesized Growth Hormones to exogenously available ones will be an important determining factor in morphology. For instance, in a shoot meristem, a cell will store up enough sugars, gases (oxygen and carbon dioxide), minerals (solute concentrations) and water (water pressure) until it “knows” it can reach a mature size. It will then poll its exterior environment and measure the levels of Auxin and CK. If there are high enough levels of these outside the cell, then the cell “knows” that it is a good bet that the supply of sugar, gases, minerals and water will increase. Instead of using the nutrients stored and the resulting hormones synthesized within to make one mature cell, it can risk dividing into two, and trying to create two mature cells.

14. Increasing levels of Growth Hormones directly inhibit the levels and/or transport of Stress Hormones until a threshold is reached when they directly induce it. This threshold is not tied directly to the Growth Hormone level but is a moving target based on the ratio of the level of the Growth Hormone to the level of the nutrient it is intending to increase. One of the most important reasons for Auxin’s existence and its movement down to the roots is to increase the supply of water and minerals. One of the most important reasons for CK's existence and movement up to the shoots is to increase the supply of sugar and oxygen to the successful new cells in the roots. For example, Auxin will kick off ET production not after Auxin reaches a certain threshold amount, but after it reaches a certain ratio in comparison with the amount of water and or minerals that exist where the Auxin is, in the root or shoot. The point of Auxin is generally to increase the amount of water and minerals with new growth in the roots. If the level of Auxin gets too high in relation to the amount of water and minerals (and the gap is increasing), the plant “knows” that the growth angle is not presently working, and so by promoting the synthesis of Auxin, it tries to “kick start” the mineral and water growth by temporarily inhibiting the synthesis and transport or activating the degradation of itself. ET also has the effect of causing senescence in less efficient mature leaves, thus diminishing the need for water and minerals. The resulting sugar and oxygen are funneled downward to induce a temporary bloom in root growth. The extra minerals and water from this leaf senescence may be sent up to where water is limited, mainly in the shoot meristematic regions that are producing the high levels of Auxin to begin with. This again is a gamble because with the stressing of any nutrients within the plant there are opportunity energy costs.

There are four behaviors that Auxin can induce in the root to increase water and mineral supplies. These are:

a. If CK, minerals, and water are high, it can induce cell division in the root meristems and thus increase the supply through new growth without any changes.

b. If CK is low, but minerals and water are high, it can induce new root growth to replace the ineffective root apical meristems and restart mineral and water supply growth.

c. If CK is high but minerals and water are low than this would indicate there is a problem with the functioning of the mature roots. This could be due to inefficient roots but as we will see below it could also be due to healthy roots malfunctioning because of a lack of sugar and oxygen. Either way, the root would want to release at least some ET in order to lower the root nutrient requirements of the shoot and free some sugar and water from cannibalized, less efficient mature leaves. In the case of inefficient roots, the sugar and water would be used to support root growth through cell division. For that, it would also need Auxin. So under this condition, Auxin levels would not be totally suppressed by the new synthesis of ET. On the other hand if the low minerals and water conditions were due to sugar and oxygen starvation of perfectly good roots, then the root would ?? ??

d. If both CK and minerals and water supplies are relatively low in the roots, this means that neither the old roots nor the meristematic roots are working. Auxin will then induce ET. This is done, as mentioned, to lower the mineral and water load, to free both sugar and oxygen for new root growth and to free minerals and water for the shoot. This is “emergency jump start” growth.

In actuality, the first two conditions can also lead to ET emanation if sugar and oxygen levels are low in the roots. When this is true, even if levels of CK and/or minerals and water are high, the roots will not risk cell division or new root initiation because the lack of extra sugar and oxygen is limiting their growth. Auxin’s inducing of ET in the root is tied to its ratio to minerals and water and sugar and oxygen.

To elaborate further, the threshold over which Auxin will induce ET is also tied to the level of GA/BA. Clearly, high levels of GA/BA are an indicator that mature root cells are starving for sugar and oxygen. It is hard to imagine that sugar and oxygen starved roots would be good harvesters of water and minerals from the surrounding soil. The root could monitor just GA/BA to get an idea of the sugar and oxygen needs of the root. Perhaps the root monitors both sugar and oxygen levels and GA/BA levels, with one being the confirmation of the other. The bottom line, however, is the sugar and oxygen level and the root may be able to safely ignore GA/BA levels. Knowing biological systems, however, and their complexity of control, I would not be surprised if Auxin threshold level that would induce ET synthesis is tied to though they would confirm In the end, low levels of sugar and oxygen may lower the threshold for Auxin’s induction of ET. ET inhibits the root senescence promoted by GA/BA. It’s obvious that if this is true it is because it wants to preserve its existing supply of water and minerals

Conversely, increasing levels of Stress Hormones directly inhibit the synthesis and/or transport of the Growth Hormones until again a threshold is reached. Then they encourage the synthesis and/or transport of the Growth Hormones. Again, this threshold is not absolute but is dependent on the ratio of the Stress to the nutrient the Stress are trying to increase. For example, if ET levels have been increasing for a while but minerals and water levels have also been increasing for a while and the ratio between ET and these nutrients has been closing, the plant “knows” the attempt at “jump starting” water and mineral supply has succeeded and ET is no longer needed.

16. The hormones can be seen as complementary pairs. Auxin’s complement is ET, CK's complement is GA/BA, and SA’s complement is ABA. This is important because Auxin transportation to the root can be seen in part as an attempt to increase water and minerals (even if it is natively synthesized in the root, it leads to cell division or new root growth). If the levels of Auxin rise too high, the plant abandons the attempt temporarily and switches to ET, trying to jump start production. Since Auxin causes cell lengthening and ET causes cell broadening, we can surmise that this kind of thing happens often and provides for balanced growth of the root. ET is perhaps a radical at least temporary change in the root’s strategy for increasing mineral and water supply. If you look back at the chart for ET, there is an entry for a well-known finding that ET induces root hair growth. Root hairs greatly increase the surface area of the root, aiding mineral and water absorption. However, this may make the plant more vulnerable to loss of water during drought. It may also make the plant more susceptible to root predation and disease, because, I hypothesize, the root hair cell is more vulnerable to all these things than the normal wall of the root. So ET represents a major change in strategy for the root. Whereas Auxin causes it to grow down and to make no special arrangements for absorption, ET may cause it to grow out laterally and to make a special of arrangement of growing root hairs. This is all to increase water and mineral levels.

Root hair growth may be a normal part of the life of a plant, or it may be a growth gamble not always taken. At any rate, I hypothesize that normally ET, GA/BA, and ABA “rule” the night, in that they are normally released during the night when the plant cannot synthesize sugar or take in as much water and minerals. It is the normal course of events. The lowest levels of Auxin, CK and SA would occur at night and the highest during the day. GA/BA and ET do cause both growth and senescence. That is, at night, when ET is high in the roots, it will be causing lateral growth of the roots, while GA/BA will be causing some senescence of older, less efficient roots. The growth in the roots at night will be balanced, because GA/BA will be lengthening the roots that they are not killing off. In the shoot, ET is doing the converse, pruning older, less efficient leaves while GA is lengthening the good young ones. The good young ones are also broadened by ET at night. I suggest that the plant does the bulk of its self-pruning at night. Also at night, the plant lives off the nutrients it has necessarily stored from the day. In fact, pruning may not be necessary if all parts remain efficient and enough nutrients have been stored from the day to allow for sustenance and even growth.

CK and GA/BA have the same relationship as Auxin and ET. An important reason for transporting CK to the shoot is to increase its sugar and oxygen supply, either by cell division in the meristem in concert with Auxin or by the outgrowth of the secondary buds out of concert with it. If this attempt is unsuccessful CK will induce GA/BA which is a completely different strategy for the shoot. With GA/BA the stem lengthens, in an attempt to move the leaves out of a possible shade and more into the sunlight where more sugar can be synthesized. One would also guess that GA/BA would induce some kind of increase in the efficiency of the leaves, just like root hairs, but again like root hairs, the changed strategy would be more risky. Of course GA/BA does not cause induction of Chlorophyll in seedlings grown in the dark, but in perhaps in low light they might.

17. When the plant needs to trim or prune parts of itself for reasons other than nutrient deficiencies, such as disease or pestilence, it can and does use the appropriate Stress Hormone as well as ABA. JAs are volatile and may induce a spreading area of senescence as a defense mechanism. For instance, if a leaf gets infected with a disease, the plant will want to limit the spread of the disease, so it will sacrifice cells surrounding the place of infection in order to quarantine the spread. This is done with both ET and ABA. Indeed, this is described in postulates 11 and 12. Even GA/BA will eventually be made as ET and ABA push out nutrients, including sugar and oxygen, from the cells that are being sacrificed. This run-away effect feeds on itself until the cell dies. The spread of self-catalysis after injury or infection is halted perhaps directly by SA and then perhaps indirectly by new Auxin and CK synthesized in response to the influx of nutrients from cannibalized cells from the quarantine area. Indeed wounding, infection, or parasitism may only initially activate ABA directly, and the ET and GA/BA synthesis may be in response to repelling of nutrients that ABA activate. ABA may directly induce SA so that the cannibalization does not go too far. There have been references in the literature to the induction of ABA and SA after wounding or disease. I hypothesize that wounding or infection may only directly activate ABA but this leads to the synthesis of SA. ET and BA/GA would be synthesized indirectly in response to the nutrient repelling action of ABA and Auxin and CK would be indirectly synthesized in response to nutrients being both attracted by SA and pushed out of the cannibalized cells by ABA, ET and GA/BA. Like most biological systems, I hypothesize, there are multiple controls and the notion here of indirect synthesis is incorrect and the plant has more control over the process. Thus, wounding or disease may start just with the release of ABA, but this induces ET and GA/BA at the site and SA, Auxin and CK at some distance from the problem area.

18. Another “raison d’être” for the growth, and possibly the Stress Hormones too, is to facilitate nutrient transport. Auxin is transported downward, and certainly attracts sugar and oxygen to itself as it travels the phloem subway down to the roots. Incidentally Auxin also attracts minerals and water, so there may be a circulation system in the plant of minerals and water. That goes up the Xylem, but some comes back down in the phloem with Auxin. Similarly, CK is transported up the Xylem, and may take water and minerals coming up with it in the root (although the Xylem is dead wood, so I am not sure about this). Again, similarly to the above, CK might attract sugar and oxygen, as it goes up the hollow tube to the stomata.

Speculative Plant Hormone Theory - Predictions

noreply@blogger.com (Paul D Pruitt) — Wed, 20 Apr 2005 02:31:00 +0000

If the assumptions or postulates are true, I believe these must follow.

1. The reasons for the existence of the “Shoot” Growth Hormone Auxin include:

a. To attract all nutrients and Growth Hormones to new shoot cells that look like good investments for the plant in that they are producing high amounts of sugar and oxygen or most probably will do so in the future.
b. To induce orderly upward growth through apical dominance, so that the young, most efficient cells reach the most sunlight and the plant is “pointed” at the top and can pierce through the forest canopy if necessary. The Christmas tree growth pattern of apical dominance is also bottom heavy, adding to the balance and stability of the plant.
c. To cause the movement of sugar and oxygen down the phloem by attracting these nutrients from the leaves as Auxin is transported down the phloem.
d. To induce an increase in the minerals and water supply by:
i) Causing more root cell division in concert with CK;
ii) Inducing new roots in the absence of CK; or
iii) Inducing a “jump start” to the water and mineral supply growth promoting ET synthesis (see below).

2. The reasons for the existence of the “Root” Growth Hormone CK similarly include:

a. To attract all nutrients and Growth Hormones to new root cells that look like good future investments to the plant, in that they are harvesting high amounts of water and minerals or most probably will do so in the future.
b. To induce orderly downward growth of the root through apical dominance, so that the young, most efficient root cells reach the deepest, most humid, soil, which is expected to be rich in minerals. The deeper the root goes, the more even the temperature the less energy is wasted on heating and cooling the root cells.
c. To cause water and minerals to move up the xylem, by attracting these nutrients from the roots into the transpiration stream.
d. To induce an increase in the sugar and oxygen supply by:
i) Causing more shoot cell division in concert with Auxin;
ii) Inducing secondary bud outgrowth in the absence of Auxin; or
iii) Inducing a “jump start” in the sugar and oxygen supply growth by promoting GA/BA synthesis (see below).

3. The reasons for the existence of the Stress "Shoot" Hormone ET include:

a. The induction of hibernation of secondary buds under water and mineral shortage produced by Auxin and apical dominance. Because they are juvenile cells, ET may not be synthesized in large amounts there normally. Perhaps only enough ET is made to induce hibernation. As above, ABA is probably involved in slowing down the metabolism to a hibernating state. Low levels of ABA may be able to effect the hibernation alone. In fact, they may prevent senescence-inducing effects of ET.
b. To induce an increase in water and minerals by:
i) Promoting the use of stored minerals and water;
ii) Inducing root hairs;
iii) Cutting back on some of the less efficient leaves that are unnecessary water and mineral sinks;
iv) Using the freed sugar, oxygen, water and minerals from the cannibalized shoots for new roots or better mature root performance;
vi) Inducing Auxin synthesis after a successful jump start of water and mineral level growth.
c. To prune wounded and diseased tissue from the shoot, possibly in concert with ABA.

4. The reasons for the existence of the Stress "Root" Hormone GA/BA include:

a. The induction of hibernation of secondary root buds under sugar and oxygen shortage produced by CK and root apical dominance. Because they are juvenile cells, perhaps GA/BA is not usually synthesized in large amounts there. Perhaps only enough GA/BA is made to induce hibernation. As above, ABA are probably involved in slowing down the metabolism to a hibernating state. Low levels of ABA may be able to effect the hibernation alone, and in fact may prevent the senescence-inducing effects of GA/BA.
b. To induce an increase in sugar and oxygen by:

i) Promoting the use of stored sugar and oxygen;
ii) Inducing an increase in the efficiency of photosynthesis although in a more risky manner;
iii) Cutting back on some of the less efficient roots that are unnecessary sugar and oxygen sinks;

iv) Using the freed sugar, oxygen, water and minerals from the cannibalized roots for new leaves or better performance;
v) Inducing CK synthesis after a successful jump start of sugar and mineral level growth.

c. To prune wounded and diseased tissue from the root, possibly in concert with ABA

5. The reasons for the existence of the “survival” hormones ABA include:

a. To slow down of metabolism of nutrient stressed cells so that their nutrient needs fall below those provided by the environment (they lower the “sustenance level” for the cell);
b. To induce senescence of nutrient stressed cells whose nutrient supply falls below the lowest possible sustenance level;
c. To slow down the metabolism of injured or diseased cells or tissue in order to limit damage;
d. To induce senescence of injured or diseased cells or tissue in order to limit the spread of the disease or of autocatalysis of tissue induced by ET.
e. To shut down the plant in case of an emergency. The most obvious condition like this is low water conditions.

6. The reasons for the existence of the “survival” hormone SA include:

a. To speed up the metabolism of healthy cells, so that all available nutrients are used up to the point of one limiting nutrient, to provide the most efficient growth or cell functioning;
b. To induce the division (in concert with Auxin and CK) of cells that have more than enough nutrients to support normal functioning and reach mature size;
c. To speed up the metabolism of healthy efficient cells near the site of injury or disease, to preserve them from autocatalysis of tissue induced by ET;
d. Possibly to re-induce cell division in tissue near cells that have senesced and abscised, in order to replace these cells.
e. To restart a plant into normal functioning after an emergency disappears. The most obvious of these conditions is the return of normal water conditions after rain.

7. In general, the following hormones inhibit senescence of the shoot and leaves: Auxin, CK, GA/BA and SA. ABA and ET are usually the only hormones that will inhibit shoot growth or induce shoot tissue senescence. In general, the following hormones inhibit the senescence of the root core and peripheral roots: Auxin, CK. ET and SA. ABA and GA/BA are normally the only hormones that inhibit root growth or initiate root senescence.

8. Growth Hormones are at their highest levels during the day when there are the most opportunities for nutrient synthesis

and harvesting. Stress hormones reach their highest levels at night, when the plant must rely on nutrient reserves built up at night or prune inefficient tissue and use the resources to sustain itself through the period when little nutrient procurement is possible See the figure at right. The plant can also slow down its metabolism at night and thus need fewer nutrients at night. This last strategy may be induced by ABA alone but is more likely to be induced by ET and GA/BA as well. The slower metabolism is also aided by the colder night temperatures.

9. Stress and ABA hormones are at their highest level at the beginning and end of the life of the plant or growing season, when a plant must rely on stored nutrients from senesced tissue and must be content with a slower rate of metabolism because of lower temperatures. Growth and SA hormones are generally at their highest level during the middle of the growth life of the plant or season.

10. Growth Hormones tend to move resources toward the edges of the plant to facilitate the procurement of more nutrients from the environment. This means they move resources to the top and bottom of the plant in the apical meristems but also perhaps move them laterally, to where the shoot and roots branch out like river deltas or capillary beds. Also, we can expect to find the highest concentration of the Growth Hormones at these edges of the plant, where the youngest cells reside. On the other hand, Stress Hormones tend to move resources inward, toward the stem and root core and closer to the soil line on both the root and shoot sides of the plant. The Stress Hormones engage the plant in a “hunkering down,” conservative, “smaller but stronger” posture. Also, we should expect to find the highest amounts of Stress Hormones near the soil line and in the shoot and root core.

11. It might seem that apical dominance could run away in growth like a cancer, but it doesn’t for seven reasons. These will be explained for Auxin and shoot apical meristems but a similar analysis could be done for CK and root apical dominance.

a. Auxin is transported down the stem through the phloem away from the shoot apex.
b. Auxin does induce xylem, so the shoot apical meristem hard wires a supply of water and minerals to itself. However, it also induces phloem, which takes the sugar and oxygen. away from the cells as they change to the mature cell morphology, so the multiplicative positive feedback effect of Auxin synthesis trails off as the meristem cells mature in the shoot.
c. Once a leaf has made xylem to itself as above, it only has to produce a small amount of Auxin to protect itself from senescence being initiated. The xylem guarantees that the leaf’s water and mineral supply cannot be hijacked by nearby meristematic tissue.
d. At any rate, as leaves increase in maturity and the plant grows, the older leaves move further and further away from the nutrient attractive forces of the Meristem (the leaves stay at the same level above the ground, and the meristem moves higher and higher off of it).
e. Relatively speaking also, the apical dominance may get weaker over time. As the older mature cells are closer to the source of the minerals and water, they are closer to the root and have “first dibs” on the supply of these nutrients. Meanwhile the apical meristem is moving further and further away from the source, the root.
f. If the apical meristem makes too much Auxin, this will induce ET both directly and indirectly. ET in turn inhibits the transport of Auxin and may inhibit its synthesis, thereby dampening the process. Auxin induces ET indirectly (and perhaps GA/BA and ABA) because the meristem’s draining of nutrients from surrounding tissue promotes the synthesis of these hormones. In addition to ET, we can expect at least ABA to inhibit Auxin synthesis.
g. There is an equally potent attractor of nutrients at the other end of the plant – the root apical meristem. These two “superpowers” probably work out a balance – an agreement to split the resources down the middle so to speak.

12. A phenomenon called Epinasty (leaf stems – called petioles – lengthen and the leaf droops down and curls in on itself) is induced by ET. When ET is released it causes the senescence and abscission of some of the leaves. Since the leaves are the primary organs for harvesting oxygen, with the roots harvesting only some of the oxygen needed, the plant may be left with a rapidly developing partial anoxia. I hypothesize that the supply of oxygen becomes a growth and metabolism limiting factor more quickly than the supply of sugar. Thus, Epinasty is the plant’s choice as the lesser of two evils. The plant loses some photosynthesis efficiency because the leaves are no longer parallel to the ground with their face to the sun, but they may be able to trap more oxygen within their curled undersides. Alternatively or additionally, the Epinastic state may act like a sail, increasing the plant’s ability to trap oxygen and blowing up like a spinnaker. This may be bad physics, in that more gas diffusion occurs when the leaf surface is parallel to the direction of the wind. The leaves may also be trapping carbon dioxide, but this is unlikely since they only need to harvest enough carbon dioxide for their own photosynthesis needs. They do not support other cells with their supply of the gas. Thus, the leaves left after a bout of ET senescence need not configure themselves in any special way in order to get the carbon dioxide they need.

13. In addition to the three behaviors high levels of Auxin can engender in the roots, there may be a fourth behavior. That is, the plant may try for quite a while to expand its minerals and water level, but if cell division, new root growth, and “jump starting” repeatedly prove unsuccessful, the plant may “decide” to store the extra sugar and oxygen until a more opportune time comes for the root. How the plant would measure repeated lack of success here, I don’t know.

14. Finally, there is one other mysterious hormone effect that I would like to explain. It is again induced by ET. It is the phenomenon of the induction of adventitious roots (roots growing out of the shoot above the soil line and back into the ground) by the hormone under flooding conditions. I argue that this is because roots need to procure some oxygen from the soil, and flooding prevents this. With the roots starting out above the flooded soil in the atmosphere, they can absorb the oxygen needed and then dip back down into the soil to get the minerals and maybe even the water.

Speculative Plant Hormone Theory - Addendum

noreply@blogger.com (Paul D Pruitt) — Wed, 20 Apr 2005 02:26:00 +0000

There are a two alternatives that I can imagine to the most important issue of what triggers the synthesis of Growth and Stress Hormones.

The first alternative trigger for Auxin and Cytokinin any time nutrients go above survivable levels (and there are good environmental conditions). Contrast this with the main theory that says growth hormones are released when the cell has more than enough nutrients to exist at peak metabolic levels. Auxin is then stimulated by above survivable levels of sugar and essential gases. Cytokinin is then stimulated by above survivable levels of minerals and water. Auxin and Cytokinin being signals of nutrients levels, might then stimulate their use by increasing the speed of metabolism or inducing growth and cell division.

Similarly, GA and Ethylene could released when nutrients fall below that which is needed for peak metabolism. GA could be released when sugar and essential gases fall below the level needed for peak metabolism. Ethylene could be released when minerals and water fall below this peak level. GA and Ethylene might then work by doing their best to attempt to address the shortfall by decreasing the speed of metabolism, inducing cell dormancy and inducing senescence.

In this second scheme whenever nutrient levels fall between peak metabolism and survivable nutrient levels, we can expect both Stress and Growth Hormones to be synthesized. Since shoots are the organs that make sugar and harvest essential gases, we can expect that those levels would rarely fall below peak metabolism nutrient levels. Thus we can expect that Gibberellin is rarely synthesized in the shoot. I know there is supposedly experimental evidence that this is untrue, but perhaps it should be looked at again more closely. This is not to say that GA is not found in the shoot. From the definition of a hormone we can expect that the effect of a hormone occurs at a distant location from its synthesis. I would expect GA to be mostly made in the root where sugar and essential gases often fall below peak metabolism levels. Synthesis may occur in the roots, but the effects GA has are in the shoot as well as the root. As in the first version of theory, I believe all of GA’s effects are meant to address shortfalls in sugar and essential gases. This would include root growth inhibition and senescence, shoot lengthening, shoot preservation from senescence, and the changing of photosynthesis to a more efficient but more risky method.

Also complimentarily, we can expect Ethylene to be rarely synthesized in the root, where levels of minerals and water seldom fall below those needed for peak metabolism. This is not to say that similar to GA and the shoot, that Ethylene does not have a big effect on roots. Again just like in the first theory, I believe everything Ethylene does is to address nonideal levels of minerals and water. Thus Ethylene inhibits shoot and leaf growth, induces leaf senescence, initiates the somewhat risky growth of root hairs, preserves roots from senescence and causes roots to branch out and to make new lateral growth in the soil.

In this second scheme we can also postulate an explanation for cell dormancy like the dormancy that exists in secondary buds. That would be that perhaps at low levels GA and Ethylene would attract all nutrients instead of pushing them out. On the other side, perhaps at low levels Auxin and Cytokinin would push nutrients out of a cell. Thus a dynamic yet stable equilibrium would exist at very low levels of nutrients. The movement into real metabolism or back toward senescence would need amounts or deficiencies in nutrients beyond which caused the synthesis of GA and Ethylene acting in their nutrient attraction mode, and Auxin and Cytokinin acting in their nutrient pushing mode. Enough nutrients would need to be around to push a cell out of a stable valley of minimum metabolism. Perhaps another similar equilibrium exists at the peak metabolism point.

A third scheme could be envisioned where Growth Hormones are made whenever an increase in nutrients is detected at all from previous conditions. In this case the first role of the hormone would be as a metabolism stimulator. This would produce negative feedback so that the excess nutrients are used up by a higher level of metabolism stimulated by the hormones. A steadily increasing level of the hormones might then be an indication to the plant that peak metabolism has been reached so the cells have nowhere to go metabolism wise, no negative feedback occurs and growth is now warranted.

A similar scheme question could also be addressed about Stress Hormones. That is are stress hormones released only when plant cells fall below the minimum level of nutrients necessary for survival or do they follow any decrease from previous levels. In the latter case the first role of Stress Hormones would be to decrease the rate of metabolism.

Speculative Plant Hormone Theory - Contact

noreply@blogger.com (Paul D Pruitt) — Wed, 20 Apr 2005 00:26:00 +0000

My about, qualifications, and contact information.

My name is Paul Pruitt. I received a BA from Swarthmore College in 1984, where I studied under Dr. Mark Jacobs. My bachelor's thesis was an examination of all aspects of plant senescence, including the role of hormones. I also received an MA from the University of Pennsylvania in 1986, where I studied plants under Dr. Scott Poethig among others. I have been studying the plant physiological hormone literature and thinking about plant hormones for 20 years. I'm currently an unemployed but experienced IT support analyst who has his own small file recovery and virtual Helpdesk business. The Website can be seen here. If you have any questions or comments, send them to socrtwo@s2services.com

" A prudent man keeps his knowledge to himself..."_{Proverbs 12:23 NIV}

" What has been will be again, what has been done will be done again; there is nothing new under the sun. Is there anything of which one can say, "Look! This is something new"? It was here already, long ago; it was here before our time. There is no remembrance of men of old, and even those who are yet to come will not be remembered by those who follow." _{Ecclesiastes 1:9-11 NIV}

Speculative Plant Hormone Theory - References

noreply@blogger.com (Paul D Pruitt) — Wed, 20 Apr 2005 00:10:00 +0000

References pulled from my own library research started in the mid 1980's (and some say limited to this period and before. Nevertheless I believe the information is relevant to today as the earliest findings are crucial to a general theory here, not the more detailed ones of later years.

Abeles, F. B., Holm, R. E., & Gahagan, H. E. Abscission: the role of aging. Plant Physiology 42, 1351-56, 1967.

Adedipe, N. O., Hunt, L. A., & Fletcher, R. A. Effects of Benzyladenine on Photosynthesis growth and senescence of the bean plant. Phys. Plant. 25, 151-53, 1979.

Addicott, F. T., Carns, H. R., Lyon, J. L., Smith, O. E., & McMeans, J. L. On the physiology of Abscisins. Recrulateurs Naturels de la Croissance Vegetale, pp. 687-703. Paris: C.N.R.S., 1964.

Barrington, E. J. W. Hormones. The New Encyclopaedia Britannica. Macropaedia v. 8, pp. 1074-88. Chicago: Encyclopaedia Britannica, Inc., 1975.

Beevers, L., Loveys, B., Pearson, J. A., & Wareing, P. F. Phytochrome and hormonal expansion and greening of etiolated wheat leaves. Planta 90, 286-94, 1970.

Black, M. Abscisic Acid in seed germination and dormancy. Abscisic Acid, ed. F. T. Addicott, pp. 331-364. New York: Praeger, 1983.

Booth ? Nature London 194-204

Brown, A. W., Reeve, D. R., & Crozier, A. The effect of light on the Gibberellin metabolism and growth of Phaesolus coccineus seedlings. Planta 126, 83-91, 1975.

Burg, S. P., & Burg, E. A. The interaction between Auxin and Ethylene and its role in plant growth. PKAS 55, 262-69, 1966.

Davis & Wareing ? Planta 65 p. 139

Engelke, A. L., Hamzi, H. Q., & Skoog. F. Cytokinin-Gibberellin regulation of shoot development and leaf form in tobacco plantlets. Amer. J. of Botany 60, 491-95, 1973.

Esashi, Y., & Leopold, A. C. Plant Physiology 44, 1470, 1970.

Goeschl, J. D., Pratt, H. K., & Bonner, B. An effect of light on the production of Ethylene and the growth of the plumula portion of the etiolated pea seedling. Plant Physiology 42, 1077-80, 1967.

Goldthwaite, J. J. Further studies of hormone regulated senescence in Rumex leaf tissue. Plant Growth Substances, 1970, ed. D. J. Carr, pp. 581-88. Berlin: Springer, 1972.

Hayes, P. M., & Patrick J. W. Photosynthate transport in stems of Phaesolus vulgaris treated with Gibberellic Acid, Indole 3-Acetic Acid or Kinetin. Effects at the14site of hormone application. Planta 166: 371-79, 1985.

Hewett, E. W., & Wareing, P. F. Cytokinins in Populus x robusta Schneid: Light effects on endogenous levels. Planta 114, 119-129, 1973.

Houck, D. H., & Lamotte, C. E. Primary phloem regeneration without concomitant xylem regeneration--its hormone control in Coleus. Amer. J. Botany 64, 799-809, 1977.

Imaseki, H. Hormonal control of wound-induced responses. Encyclopedia of Plant Physiology. v. 11, ed. R. P. Pharis & D. M. Reid, p. 504, Heidelberg: Springer Verlag, 1985.

Jacobs, W. P. Comparison of the movement and vascular differentiation effects of the endogenous Auxin and of phenoxyacetic weed killers in stems and petioles of Coleus and Phaesolus. Ann. N.Y. Acad. Sci. 144, 102-117, 1967.

Jahardhan, K. V., Vasudeva, N., & Gopel, N. H. Diurnal variation of endogenous Auxin in arabica coffee leaves. J. Plant Crops 1 (Suppl), 93-95, 1973.

Kawase, M. Effects of flooding on Ethylene concentration in horticultural plants. J. Am. Soc. Hortic. Sci. 97, 584-88, 1972.

Lecoq, C., Koukkari, W. L., & Brenner, M. L. Rhythmic changes in Abscisic Acid (Abscisic Acid) content of soybean leaves. Plant Physiology 72 (suppl.), 52, 1983.

Manos, P.J., & Goldthwaite, J. A kinetic analysis of the effects of Gibberellic acid, Zeatin, and Abscisic Acid on leaf tissue senescence in Rumex. Plant Physiology 55, 192-98, 1975.

Marre, E. Effects of fusiccocin and hormones on plant cellmembrane activities, observations and hypothesis. Regulation of Cell Membrane Activities in Plants. eds. Marre, E. & Caffer, O. Amsterdam: Elsevier/North Holland Biomedical Press, pp. 175-202, 1977.

McMichael, B. L., & Hanny, B. W. Endogenous levels of Abscisic Acid in Water stressed cotton leaves. Agron. J. 69, 979-82, 1982.

Mitsuhashi-Kato, M., Mishibaoka, H., & Shimokoriyama, M. Anatomical and physiological aspects of developmental processes of adventitious root formation. Plant and Cell Physiology 19, 393-400, 1978.

Nooden, L. D. Senescence in the whole plant. Senescence in Plants, ed. K. V. Thimann, Boca Raton, FL: CRC Press, 1980.

Palmer, J. H., & Phillips, I. D. J. The effect of the terminal bud indole acetic acid and nitrogen supply on the growth and orientation of the petiole of the Helianthus. Annus. Physiol. Plant 16, 572-84, 1963.

Pooviah, B. W., and Leopold, A. C. Deferral of leaf senescence with calcium. Plant Physiology 52, 236-39, 1973.

??, Phys. Plant v. 51 375-79

Reid, D. M., and Wample, R. L. Water relations and plant hormones. Chapter 14 in Volume 11, Hormonal Regulation of Development III, eds. A. Pirson and M. H. Zimmerman. Heidelberg: Springer Verlag, 1985.

Reinhold, L. Phytohormones and the orientation of growth. Phytohormones and Related Compounds a Comprehensive Treatise, v. II, ed. by D. S. Letham, P. B. Goodwin, and T.J. V. Higgins. Amsterdam: Elsevier/North Holland Biomedical Press, 1978.

Ross, E. L. Growth regulators and conifers: their physiology and potential uses in forestry. Plant Growth Regulating Chemicals, v. II, ed. by L. G. Nickell. Boca Raton, FL: CRC.

Sachs, T., and Thimann, K. V. The Role of Auxin and Cytokinin in the Release of Buds from Dominance. Amer. J. Bot. 54, 136-44, 1967.

Sembdner, G., Gross, D., Liebisch, H. W., and Schneidner, G. Bio-synthesis and metabolism of plant hormones. Hormonal Regulation of Development 1, ed. J. MacMillen, Heidelberg: Springer Verlag, 1980.

Skoog, F., and Miller, W. Chemical regulation of growth and organ formation in plant tissues cultured in vitro. Symp. Soc. Exp. Biol. 11, 118, 1957.

Snow, R. Plagiotropism and correlative inhibition. New Phytologist 44, 110-117, 1945.

Sutcliffe, J. F. Regulation in the whole plant. In Transport in Plants v. 2. Part B: Tissues and Organs. Encyclopedia of Plant Physiology, ed. U. Luttge and M. G. Pitman. Heidelberg: Springer Verlag, 1976.

Thimann, K. V. Cell Enlargement and Growth. Hormone Action in the Whole Life of the Plant Amherst: Univ. of Mass. Press, 1977.

Thompson, M. J., Meudt, W. J., Mardava, N. B., Dutky, S. R.1Lusby, W. R., and Spaulding, D. W. Synthesis of Brassinosteroid and relationship of structure to plant growth promoting effect. Steroid 39 #1, 89-105, 1982.

Torrey, J. G. Auxin control of vascular pattern formation in regenerating pea root meristems grown in vitro. Amer. J. Bot. 44, 859-870, 1957.

Van Staden, J., and Smith, A. R. The synthesis of Cytokinin in excised roots of maize and tomato under aseptic conditions. Annals Bot. 42, 751-753, 1978.

Varner, J. E. GA-controlled synthesis of alpha-amylase in barley endosperm. Plant Physiology 39, 413-415, 1964.

Wain, R. L. Some development in research on plant growth inhibitors. Proc. Roy, Soc. B. 191, 335-352, 1975.

Wareing, P. F., and Phillips, I. D. J. Growth and Differentiation in Plants. Great Britain: Pergamon Press, 1981.

Webb, D. P., Van Staden, J., and Wareing, T. F. Seed dormancy in Acer. In J. Exp. Bot. 24, 105-106, 1973.

Wright, S. T. C. The effect of plant growth regulator treatments on the levels of ethylene emanating from excised turgid and wilted wheat leaves. Planta 148, 381-88, 1980.

ZeevABArt, J. A. D. Giberellin and flowering. The Biochemistry and Physiology of Giberellin, v. 2, ed. A. Crozier, New York: Praeger, 1983.

Zimmerman, P. W., and Hitchcock, A. E. Initiation and stimulation of adventitious roots caused by unsaturated hydrocarbon Gases. Contributions to the Boyce Thompson Institute 5, 351-369, 1933.