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 ▪ Auxin ▪ Cytokinin ▪ Jasmonic Acid ▪ Salicylic Acid ▪ Ethylene ▪ Strigolactone ▪ Gibberellic Acid ▪ Brassinosteroid ▪ Abscisic Acid ▪

A Sketch of an 8 Part
Plant Hormone Theory

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"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 stomates open, however if the plant 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:

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, gibberellin, 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.

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:

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. My speculation though is this is not correct. If looked at more carefully GA will be found to be made in older inefficient tissue at least in the root. In the shoot it might still be found in meristematic tissue however not in the shoot apical meristem as GA would be a signal that growth from that location has not been effective enough to stave off sugar starvation. Perhaps GA then in the shoots would be made in the older meristem tissue of the stem if meristematic tissue exists in the stem.

A third table emerges from this speculation:

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

• Auxin/IAA - signal of gas abundance or maybe just oxygen abundance.
• Cytokinin/Zeatin - signal of root nutrition other water abundance.
• Salicylic Acid/Salicylates/SA - signal of water abundance.
• Jasmonates/Jasmonic Acids - signal of sugar abundance.

Scarcity/Inhibiting/Senescence Hormones 

• Abscisic Acid/ABA - obviously water deficiency.
• Gibberellic Acid/GA and Brassinolide (may act as one pathway) - sugar deficiency.
• Ethylene - oxygen and maybe carbon dioxide deficiency.
• Strigolactones - root nutrition deficiency, other than water.

References

• Numbered References
• Further Reading