▪ Bible Physics ▪ Scripture Evolution ▪ Epinasty ▪ Plant Senescence Theory ▪
A Sketch of an 8 Part
Oxygen and Carbon Dioxide
I would like to use this table to postulate that possibly 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 using 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 and cells at the opposite end of the plant if they are responsible normally for harvesting that nutrient. Finally if that doesn't fix the problem, the cell decides to senesce accompanied by 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.
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:
Growable Amount Hormone
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. Complimentarily, 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
Auxin & Cytokinin - produces cell division
Auxin & Ethylene - produces stem lengthening then older stem cell senescence
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.
I am most inclined to believe or at least support further exploration of the first table, so I present the findings and references here that support it. Special thanks to fiverr.com researcher fitgem.