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New Version of General
Speculations on Plant Hormones

"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

By Paul Pruitt

 

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/BR 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/BR 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 climacteric 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/BR levels are higher at night.

  • Ethylene and GA/BR 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/BR 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/BR 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.

Paul D Pruitt - 05/13/2007