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Bible Physics

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Epinasty

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Plant Senescence Theory

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Scripture Evolution

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Socrates/Plato Civilization Cycle

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Speculations of Why Sexes Exist

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1986 Version

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1995 Version

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1999 Version

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2003 Version

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2007 Version a

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2007 Version b

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Auxin

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Cytokinin

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Jasmonic Acid

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Salicylic Acid

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Ethylene

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Strigolactone

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Gibberellic

Acid

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Brassinosteroid

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Abscisic Acid

Ce document a été traduit en français. Phiên bản tiếng Việt của trang đã được biên dịch bởi. "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

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.

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.

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.

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 , 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.

Abundance sugar

• Induces new root growth that are sinks of sugar? • Helps to break shoot apical and establish root apical dominance during the day? • Synthesized more in the shoots than the roots? But the roots are more sensitive to it? • Induces storage of sugar in roots. • High amounts externally applied JA induce gibberellin/brassino- steroid through the exhausting of stores of sugar? • Levels peak during the day when sugar making through photosynthesis is at its highest? • Broadens plant parts, whereas its converse, GA, lengthens plant parts at night, balancing plant growth. Partners with IAA’s lengthening of roots, balancing root growth during the day. • Attracts all nutrients and hormones (or maybe just the ones that are known to inhibit the process—CK, ETH (?), ABA (?)) to the site of its synthesis, inducing positive feedback and breaking shoot apical dominance. • Is transported from the shoot to the root through the phloem with its strong attraction of sugar causing sugar to move with it? • Helps induce cell division along with CK, IAA and SA? • Is an indicator of shoot and leaf wounding caused by high extracellular sugar levels existing after tissue damage because of cell ruptures.

Abundance of root nutrition other than water

Abundance of water

Deficiency of oxygen (and too much carbon dioxide particularly at night in the root?)

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. 

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So here's the break down:

• Induces new root formation that are sinks of oxygen. • Helps induce shoot apical dominance during the day. • Synthesized in the shoots more than roots but the roots are more sensitive to it than the shoots. • Induces the storage of oxygen in vacuoles during the day and carbon dioxide at night. • During the day, because oxygen is a sign of photosynthesis, IAA is an indirect signal of that processes’ success. • High amounts externally applied induce ETH, perhaps by exhausting the supplies of oxygen. • With CK, SA(?) and JA (?) induces cell division. • Levels peak during the day when oxygen is given off by photosynthesis? • Lengthens plant parts whereas it’s converse, ETH, broadens them at night balancing plant growth? Also it pairs with JA’s broadening, balancing out root growth during the day. • Attracts all nutrients and hormones (or maybe just the ones that are known encourgae it, SA (?), GA and strigolactones (?)) to the site of its synthesis, inducing positive feedback and shoot apical dominance. • Transported from the shoot through the phloem to the root perhaps with its super strong attraction of oxygen causing oxygen to tag along? • Helps induce cell division along with CK, JA(?) and SA(?) • Is an indicator of shoot and leaf wounding caused by high extracellular oxygen levels existing after tissue damage because of cell ruptures and exposure to the external air..

Abundance of oxygen and maybe carbon dioxide (the latter only during the day and only in the shoot)

• Stimulates new shoot growth that are sinks of root derived materials other than water. • Helps induce root apical (?) and break shoot apical dominance during the day. • Synthesized more in the roots than the shoots. The shoots however are more sensitive to the same amounts? • Induces the storage of root derived materials other than water, i.e. minerals? • High amounts externally applied CK induce strigolactone through the exhausting of stores of minerals? • Levels peak during the day when transpiration and root nutrient level intakes are at their highest. • Broadens plant parts, whereas its converse, strigolactones, lengthens them at night, balancing plant growth? Also partners with SA’s lengthening of shoot parts during the day, balancing shoot growth? • Attracts all nutrients to its site of synthesis also including hormones, (or maybe just the hormones that stimulate root growth mainly IAA, JA(?), ABA(?) and strigolactones(?)?)? • Transported through the xylem from the roots to the shoots? • With IAA, JA(?) and SA(?) induces cell division? • Is an indicator of root damage where root cell ruptures lead to high levels of extracellular minerals.

• 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 have an abundance of water available to the root system. • Synthesized more in the roots than the shoots? However shoots are more sensitive to the same amounts. • May normally induce stomata to open. • Induces new sinks of root nutrition, mainly new shoots? • Helps to break root apical dominance. • Induces excess water storage in vacuoles? • High amounts of externally applied SA leads too much evaporation of water and water storage causing the formation of ABA? • Levels peak during the day when transpiration and root nutrient level intakes are at their highest? • Lengthens plant parts balancing out its converse ABA’s broadening of plant parts at night? Also partners with CK to balance out shoot growth during the day. • Transported through the xylem from the roots to the shoots? • Attracts all nutrients to its site of synthesis also including hormones, (or maybe just the hormones that stimulate root growth mainly IAA, JA(?), ABA(?) and strigolactones(?)?)? • With IAA, CK, and JA(?) induces cell division? • Is an indicator of root damage where root cell ruptures lead to high levels of extracellular water.

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:

• 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 root through which oxygen can be obtained and maybe carbon dioxide expelled. • Induces adventitious roots during flooding, maybe also to facilitate gas exchange because adventitious roots are exposed to the air. • Causes the abscission of leaves…This one is hard to explain. Maybe it causes the abscission of leaves that don't absorb oxygen or release carbon dioxide at night anymore. • Inhibits root growth which are sinks for oxygen use. • Stimulates the use of stored O2? And the release of CO2? • Synthesized more in the roots than the shoots but the shoots are more sensitive? • High amounts of externally applied ETH leads to too much stores of oxygen releaeased as well as the generating of new O2 flows from the stem through aerenchyma and root hairs, inducing auxin synthesis in the roots? • Levels peak at night when oxygen uptake is less because photosynthesis is not producing it and the stomates are closed? • Stimulates broadening of plant parts partnering with its converse Auxin to produce balanced growth? Also partners with GA to produce balanced growth of stems and other shoot parts at night. • Partners with ABA to break shoot apical dominance at night? • Induce cell senescence acting in concert with GA, strigolactones and ABA? • Is an indicator of shoot damage when root levels of oxygen fall because reduced numbers of intact leaves.

• Jump starts seed germina- tion by dissolving stored starch in the cotyledons of the seed. • Causes the shoot to greatly lengthen (“bolting”) perhaps bringing it into the sunlight and an increasing level of sugar making. • Partners with strigolactones to maintain apical dominance at night? • Inhibits root growth which are sinks of sugar. • Stimulates the use of sugar stores. • Synthesized more in the roots than the shoots, however the shoots are more sensitive to the same amounts? • High amounts of externally applied GA/BR leads to too much stores of sugar released causing the production of Jasmonate? • Levels peak at night when stored sugar is needed to be used? • Induces plant part lengthening is contrast to it’s converse jasmonic acid that broadens? Partners with ETH which braodens plant shoot parts at night to balance out growth at night? • Induce cell senescence acting in concert with Eth, strigolactones and ABA? • Is an indicator of shoot damage when root levels of sugar fall because reduced numbers of intact leaves.

• Closes stomates, stopping transpiration. • Mediates plant responses to salt, heat and other stresses. • Inhbits shoot growth? • Stimulates the release of stored water? • Synthesized more in the shoot parts? But the roots are more sensitive to the same amount? • Stimulates root hair growth to increase the absorption of water? • High amounts of externally applied ABA lead to an excessive levels of intracellular water, triggering salicylic acid release? • Partners with ETH to inhibit apical dominance at night? • Stimulates root growth and the absorption of water? • Levels peak at night when there is less transpiration and stored water is needed? • Induce cell senescence acting in concert with Eth, strigolactones and GA? • Is an indicator to plants in shoots and leaves of root damage leads to a drop of water absorption.

• Inhibits shoot branching and growth(?) • Partners with GA to maintain shoot apical dominance at night? • Stimulates the release of stored of nutrients other than water, sugar and oxygen? • Synthesized more in the shoots (where minerals are less available)? But the roots are more sensitive to the same amount? • High amounts of externally applied strigolactones lead to an excessive release of minerals stimulating the release of CK? • Stimulates root growth and the absorption of root derived nutrients other water? • Stimulates root hair growth to increase the absorption of minerals? • Levels peak at night when stored minerals are needed to continue growth, because root absorption of them is curtailed at that time. • Induce cell senescence acting in concert with Eth, GA and ABA? • Is an indicator of root damage to plants when shoot levels of minerals fall because reduced numbers of intact roots.

Deficiency of sugar

Deficiency of water

Deficiency of root derived nutrients other than water

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.

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.

• Numbered References • Further Reading

The table below is complete, although not without reservations. In 1986 salicylic acid (SA) 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. More fitting with my theory is the idea that SA is a signal that water is in abundance and when nutrients are in abundance in a plant, it is reasonable to assume they are used, stored or disposed of. My contention as we’ll see later, and is that hormones are for coordinating the use of acquired resources, water among them, so if SA is in my scheme, a sign that water is available and not in shortage, I believe it is reasonable to think this stimulates cells and tissues to use water for growth, to store it or dispose of excesses. Externally applied SA by scientists is an artificial situation and I believe it leads the plant to mistakenly believe there is an abundance in water and act as such, however because it’s a mistaken conclusion, this creates an artificial water shortages, which causes the synthesis of ABA which in turn causes the observed closure of stomates.

Following up on the idea that disposal is a third thing a plant can do facing an abundance of nutrients, perhaps a high enough level of a nutrient abundance signal leads to a different deficiency hormone or set of difficiency hormones which would induce the disposal of the nutrient. So focusing on SA again, if levels get high enough, they induce ABA, but maybe even higher levels lead to for example ethylene which then proceeds to do what it’s known to do, cause the the abscsission or shedding of leaves. Alternatively perhaps high enough levels of SA lead to higher levels than normal of ABA and at high enough enough levels of ABA, it to causes the shedding of water gorged tissue (which would be paradoxical of course since ABA is induced by water shortages). The shedding of plant nutrients through the shedding of plant parts, won’t be covered in this article in its present form but will be left for future versions.