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 ▪ AuxinCytokinin Jasmonic AcidSalicylic AcidEthyleneStrigolactoneGiberellinBrassinosteroidAbscisic Acid

A Sketch of an 8 Part
Plant Hormone Theory

"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

Summary

Informally since 1986 and on the Web since 1996, I have written several fairly different versions of comprehensive speculations on the functions and behavior of plant hormones. This present page and it accompanying tables explains the commonly accepted plant hormones as each having major roles as indicators of plant nutrient abundance or deficiency. I see four classes of nutrients: the gases, minerals (root derived nutrition other than water), water and sugar. There are then two hormone groups assigned to each nutrient class, one is an indicator of abundance and one of scarcity. Building on this structure, I postulate that all four of the abundance hormones are needed for cell division, not just the commonly accepted auxin and cytokinin. Furthermore, I postulate that all four of the deficiency hormones are needed for cell senescence to be carried out.

Introduction

In early 2008 I was reading the Wikipedia articles on jasmonates, and the article made me question the role I had made for auxin as the indicator of excess sugar. This was because  jasmonic acid is involved in tuber formation and other actions I had expected from auxin.  So if this is true, my new eight hormone scheme from 2007 needed rearranging.

Additionally in 2008 I read an article on brassinosteroid, causing me to reclassify again it as working with GA as a sugar deficiency indicator and not as a mineral deficiency one. I now leave the mineral deficiency signal up for question although the newly discovered Strigolactones appears to be that signal. Strigolactones help mediate the interaction with symbiotic fungi that help the plant absorb minerals and most notably to its discoverers it inhibits the branching of the shoot. We might expect both of these from a mineral deficiency signal. That is mineral deficiency might cause a suppression of growth and branching of the shoot, which is a root derived nutrition sink, and attempt to increase the uptake of minerals through an increase in the hosting of symbiotic mineral absorbing fungi.

The table below is complete, although not without reservations. In 1986 salicylic acid was found to reverse ABA mediated closing of stomata which is why I originally placed it there in the scheme of things. More recently it has been found that when working alone, it closes stomata. However this may be explained away due to it's role in pathogen defense, rather than its role in water abundance. In other words most of the time it acts as a water abundance signal and keeps the stomate open, however if the palnt is wounded and this is sensed through other avenues, salicylic acid mediates stomata closing.

Another one of the problems with this scheme is there appears to be some question about whether brassinosteroid increases root growth or inhibits it. If it inhibits it short term and increase it long term, than this understandable in my scheme as just the behavior we would expect from a sugar shortage message. Roots don't make sugar, and should be the first place to experience sugar deficiency. The hormone may attempt to restart root growth on the long term but only if it has successfully restored a source of sugar coming from the shoot. On the short run it might want to change the behavior of the shoot to bring down more sugar to the roots. It might want also minimize any increase in deficiency the root might be experiencing, through inhibiting it's root growth.

Hormone Table

So here's the break down:

Deficiency

Abundance

Sugar

Giberellin/Brassinosteroid

Jasmonic Acid

Oxygen and Carbon Dioxide

Ethylene

Auxin

Minerals

Strigolactone

Cytokinin

Water

Abscisic Acid

Salicylic Acid

I would like to use this table to postulate that all four of the abundance signals are needed for cell division not just cytokinin and auxin. We might explain away the fact that this has not been found yet to be the case by plant scientists, by saying that the nutrients used to cause cell division in tissue culture, unknowingly provide jasmonic and salicylic acid. Another possible explanation is the cell lines successfully used in tissue culture are mutants with native un-induced production of SA and JA.

In a related way I would like to propose all four deficiency hormones are needed to be present before a plant cell senesces. This is explained in more detail in my previous "papers", however a strong reason for pushing a plant cell into a senescent sequence is positive feedback. The idea is that a cell experiencing a deficiency in one of the four classes of nutrients is no longer able to sustain itself or do so for very long. The signal first tries to address the nutrient shortfall by stimulating the plant to use stores of the nutrients. Being unsuccessful at that, and with an increase in the level or amount of the signal the cell attempts to address the shortfall by changing the behavior of nearby cells inhibiting their growth and the behavior of cells at the opposite end of the plant (if the latter are responsible normally for harvesting that deficient nutrient), to increase their nutrient harvesting. For instance gibberellin would stimulate nearby cells in the root to stop growing and far away shoot cells to change their apparently not so successful sugar harvesting activities and bolt through the shadowed canopy into light and more sugar making success. Finally if that doesn't fix the problem, the cell decides to senesce accompanied by a critically high level of deficiency hormone, a point of no return as it were. Perhaps deficiency signal levels are directly related to the size of the nutrient shortfall and second and third stages of deficiency are not reached if the amount of the deficiency stays at a low chronic level.

The positive feedback comes in because at the third stage, high levels deficiency hormones actually push nutrients out of the cells experiencing the deficiency. Also it is not just their own respective nutrient that the hormone pushes out, but it pushes out  all four classes of nutrients.  As you can imagine once one hormone is pushing out all the types of nutrients, it soon begins synthesizing other deficiency hormones, which just snowballs the process, finally leading to a condition of high level of all four nutrient deficiency hormones and little or no nutrients left except a cellulose skeleton of where the cell used to be. Whether high levels of all four nutrient deficiency signals is a requirement or just a symptom of senescence, is a question that needs to be answered with experiments.

Here are some effects of the plant hormones encouraging my speculations. References for the effects can be seen in the greater detailed tables available here: Auxin, Cytokinin Jasmonic Acid, Salicylic Acid, Ethylene, Strigolactone, Giberellin, Brassinosteroid and Abscisic Acid. In the effects section, the ones ending in question marks are speculations on my part where I'm unaware of any research that yet backs it up.

Name Speculated Role Effects
Auxin (IAA) Abundance of oxygen and maybe carbon dioxide
  • Induces new root formation that are sinks of oxygen.
  • High amounts externally applied induce ethylene perhaps by stimulating then exhausting the supplies of oxygen.
  • With cytokinin induces cell division.
  • Levels peak during the day when oxygen is given off by the process of photosynthesis.
  • Attracts all nutrients and hormones (maybe actully the ones that stimulate shoot growth, CK, SA, GA and Eth?) to the site of its synthesis, inducing postive feedback and appical dominance.
  • Transported from the shoot through the phloem to the root perhaps with its super strong attraction of oxygen causing oxygen to tag along?
Jasmonic Acid (JA) Abundance of light, sugar and maybe carbon dioxide tied togethor to be a photosynthesis level indicator
  • Induces storage of sugar in roots.
  • Is an indicator of wounding by perhaps being an indicator of high sugar levels found due to the rupture of cell contents.
  • Attracts hormones that cause shoot growth (CK, SA, GA and Eth?) to the site of its synthesis, also strengthening apical dominance?
  • Is transported for the shoot to the root through the phloem with its strong attraction of sugar causing sugar to move with it?
  • High amounts externally applied induce gibberellin/brassinosteroid through the over-stimulation of use or the storage of sugar?
  • Induces new root growth?
  • Helps induce cell division along with CK, IAA and SA?
  • Levels peak during the day when sugar making through photosynthesis is at its highest?
Cytokinin (CK sometimes Zeatin) Abundance of root nutrition other than water
  • Induces new sinks of root nutrition, mainly new shoots.
  • I would predict high levels to induce strigolactone increases.
  • Induces the storage of excess root derived nutrition other than water.
  • With auxin, induces cell division.
  • Attracts all nutrients to its site of synthesis also including hormones, but maybe just the hormones that stimulate root growth mainly IAA, JA?, ABA? and Strigolactones?
  • Levels peak during the day when transpiration and root nutrient level intakes are at their highest.
  • Transported through the xylem from the roots to the shoots?
Salicylic Acid (SA) Abundance of water
  • Found in abundance in Willow tree bark (where it was first discovered). Willow trees are often found on the banks of rivers and other bodies of water where the plants should hav an abundance of water availalbe to the root system.
  • May normally induce sotmata to open.
  • Induces new sinks of root nutrition, mainly new shoots?
  • Induces excess water storage in vacuoles?
Ethylene (ETH) Deficiency of oxygen (and too much carbon dioxide?)
  • Inhibits root growth.
  • Stimulates root hair growth maybe increasing the roots ability to directly absorb oxygen from gaps in the soil granules.
  • Induces aerenchyma during flooding which are hollow tubes from the shoot to the rot through which oxygen can be obained and maybe carbon dioxide expelled.
  • Induces adventitous roots during flooding, maybe also to facilitate gas exchange bec ause adventitous roots are exposed to the air.
  • Stimulates shoot growth and the broadening of leaves all in an attempt to increase the intake of oxygen and the diffusion out of the plant at night of carbon dioxide?
  • Stimulates the use of oxygen stores and the inhibition of processes that produce carbon dioxide?
  • Causes the abscission of leaves. . . .This one is hard to explain. Maybe it causes the absission of leaves that don't absor oxygen or release carbon dioxide at night anymore.
  • Induce cell senescence acting in concert with GA, strigolactones and ABA?
Giberellin (GA)/
Brassinosteroid (BA)
Deficiency of sugar
  • Inhibits root growth.
  • Stimulates the use of sugar stores.
  • Levels peak at night when stored sugar is needed to be used?
  • Causes the shoot to greatly lengthen perhaps bringing it into the sunlight and an increasing level of sugar making.
  • Induce cell senescence acting in concert with Eth, strigolactones and ABA?
Strigolactones Deficiency of root derived nutrients other than water
  • Inhibits shoot branching and growth.
  • Stimulates the release of store of nutrients other water, sugar and oxygen.
  • Stimulates root growth and the absorption of root derived nutrients other water.
  • Induce cell senescence acting in concert with Eth, GA and ABA?
Abscisic Acid (ABA) Deficiency of water
  • Closes stomates, stopping transpiration.
  • Levels peak at night when stored sugar is needed to be used?
  • Stimulates root growth and the absorption of water?
  • Induce cell senescence acting in concert with Eth, strigolactones and GA?

Alternate Ways of Organizing Plant Hormones

If this is a sort of comprehensive article, I should mention other possible scenarios for organizing the overall roles of hormones in order to inspire discussion and experiments. Another way to organize the plant hormones is to think there are four hormones for the four classes of nutrient when there are nutrient deficiencies, a different set of four hormones would be released when there are there are growable amounts of nutrients and finally a third set of hormones are released when there is too much of any nutrient. You then might end up with the following table:

 

Deficiency Hormone

Growable Amount Hormone

Excess Hormone

Sugar

Gibberellin/Brassinosteroid

Auxin?

Jasmonic Acid

Gases

?

Auxin

Ethylene?

Water

Abscisic Acid

?

Ethylene

Minerals

Strigolactones

Cytokinin

Abscisic Acid

A third possible scenario is to return to a very simple system I postulated some time ago. Auxin would be released when a root or shoot meristematic cell finds that it contains more than enough shoot derived nutrients mainly sugar, and all other environmental conditions are favorable for growth. Cytokinin would be made when meristematic cells are bathed in more than enough nutrients of the sort normally provided by the root, mainly water and minerals and all other conditions are favorable for growth.

Conversely gibberellin/brassinosteroid would be made when mature cells have less than enough shoot nutrients, i.e. sugar and oxygen to survive especially if environmental conditions are poor. Finally Ethylene might be released when mature cells are receiving less than enough nutrients normally received from the roots, mainly minerals and water, to support life at all, thus senescence of the cell is warranted. Again this effect may be accentuated by poor environmental conditions.

In this scheme abscisic acid might fulfill the role akin to adrenaline or cortisol in animals, signaling a need emergency action under most kinds of rapidly developing environmental stress, not just water shortages. complementarily, salicylic acid may be the hormone released when things are running normally and no special rapid response is needed from the plant. It might be the "feel good" hormone.

The problem with this scheme has been pointed out to me is that GA is made by meristematic cells not mature ones. This is not fatal to the speculations, but does kind of make them a little less symmetrical and compelling.

A third table emerges from this speculation:

Root Derived Nutrient Abundance + Good Root Environmental Conditions

Root Derived Nutrient Deficiency + Bad Root Environmental Conditions

Shoot Derived
Nutrient Abundance + Good Shoot `Environmental Conditions

Auxin & Cytokinin - produces cell division

Auxin & Ethylene - produces stem lengthening then older stem cell senescence

Shoot Derived
Nutrient Deficiency + Bad Shoot Environmental Conditions

Cytokinin & Gibberellin/BA - produces root broadening then older root cell senescence

GA/BR & Ethylene - produces cell senescence

One thing not discussed so far is that root oxygen is probably mostly obtained from the soil surrounding the roots, not from the leaves. This resolves the perplexing property of Ethylene causing the senescence of leaves because the shoot and leaves aren't the providers of O2 for the root. So the plant wouldn't be shooting itself in the foot if it were to trim older inefficient leaves and stems and the resources freed could be used for making oxygen harvesting adventitious roots under anoxia and flooding conditions. 

Hormone Tables - A Detailed Referenced Exploration of the First Table

I am most inclined to believe or at least support further exploration of the first table, so I present links to tables that I made of the findings and references that support it. Special thanks to fiverr.com researcher fitgem.

Abundance/Stimulating/Growth Hormones

Scarcity/Inhibiting/Senescence Hormones 

References