A Sketch of an 8 Part Plant Hormone Theory

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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}

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. 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 by Jasmonic Acid is involved in tuber formation and de-chlorophylling leaves actions we might expect from Auxin. Presumably both these events occur because of an excess of sugar. So if this is true, my new eight hormone scheme from 2007 needs rearranging again.

Furthermore on 08/12/2008 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 may be the signal. Strigolactones help mediate the interaction with symbiotic fungi that help the plant absorb minerals and inhibit branching of the shoot. We might expect that from a mineral deficiency signal.

That is mineral deficiency might cause a suppression of growth and branching of the shoot and attempt to increase the uptake of minerals through an increase in the hosting of symbiotic mineral absorbing fungi.

The table below is almost complete, however I removed Salicylic Acid as a sign of Water abundance. In 1986 it 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. This may be due to it's role in pathogen defense. Stomata are open avenues to the interior of the plant. It is known that Salicylic Acid is released.

So here's the break down:

	Deficiency	Abundance
Sugar	Gibberellin/Brassinosteroid	Jasmonic Acid
Oxygen and Carbon Dioxide	?	Auxin
Minerals	Strigolactones	Cytokinin
Water	Abscisic Acid	Ethylene?

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 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 be an attempt to restart root growth on the long term. However, on the short run it might want to change the behavior of the shoot to bring down more sugar through the stem to the roots. It might want also minimize any increase in deficiency the root might be experiencing, through the means of inhibiting it's root growth.

Another problem, is seen when I explain below in the big table below that going against common belief, I believe Jasmonic and Salicylic Acid are needed to be present in addition to Auxin and Cytokinin for cell division. However from my now rather ancient researches into the matter I never found such a reference or indication...Additionally those two hormones do not seem to be involved in crown gall formation as we would anticipate if these hormones were additional crucial indicator "green lights" for cell division.

Keeping these in mind we might postulate two other ways to organize the overall roles of the hormones. One is to think for each of the four major nutrients hormones are released when there is not enough of the nutrient, a different one is released when there are growable amounts and finally a still different hormone is released when there is too much of any nutrient. You then might end up with the following table:

Nutrient	Deficiency Hormone	Growable Amount Hormone	Excess Hormone
Sugar	Gibberellin/Brassinostreroid	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/Brassinostreroid 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 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 Environmental Conditions	Root Derived Nutrient Deficiency + Bad Environmental Conditions
Shoot Derived Nutrient Abundance + Good Environmental Conditions	Auxin & Cytokinin - produces cell division	1st Cytokinin then 2nd Gibberellin/BA - produces 1st root broadening then older root cell senescence
Shoot Derived Nutrient Deficiency + Bad Environmental Conditions	1st Auxin then 2nd Ethylene - produces 1st stem lengthening then older stem cell senescence	GA/BA & 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 Table - Under Construction

Note the information in the table is constructed to defend the speculations of the first table and hasn't yet been modified to provide the equal treatment to all the three ways of understanding hormones.

Chemical Structure	Synthesis and Transport	Proven Effects	Insight Provided by Genetic Manipulation	Speculative Explanation	Synthesis; Exogenous Treatment; Inhibition and Stimulation; Storage; Nutrient and Hormone Attraction and Repulsion; Apical Dominance; Hormone Transport; Cell Division; Senescence; Specific to Overall Role
Abscisic Acid (ABA) ABA's main role is clearly resistance to drought conditions - A Wikimedia Commons Image Speculative Overall Role: Signal of Water Deficiency ABA May Figure Prominently in Desert Plants Like Cactus - A Wikimedia Picture	Under consistent levels of desiccation, ABA levels normally peak at night.1 Closes stomata via ABA synthesized in the root.2 Induce by drought.3 ABA coming up from the root, synergizes with auxin coming down from the apex to produce apical dominance.12	Closes stomata via ABA synthesized in the root.2 At high concentrations, inhibits root growth, but after removal. stimulates greater root lengthening and branching than controls.4 Mediate adaptation to salt.5 Mediate adaptation to heat.6 Mediate adaptation to cold.7 Inhibits Kinetin nucleotide formation8 Down regulates enzymes needed for photosynthesis9 Induces bud dormancy. Lower levels of ABA is associated with dormancy termination in winterized plants.10 ABA Promotes tuberization.11 ABA coming up from the root, synergizes with auxin coming down from the apex to produce apical dominance.12		Actions of ABA attempt to mitigate drought effects. Water loss is slowed by closing of the stomata. Shoot growth does not rectify water deficiency, however, new root growth may. High salt levels may water stress plants. Heat stress may induce water stress. Cold may make water less available to plants. Transpiration cools plants, so this might be avoided by the ABA induced stomata closing. The photosynthesis process uses twice as much water as it makes and Kinetin stimulates this.	Synthesis: Dry plants should have high levels of ABA, well watered plants, low levels. Like abundance signals ABA may be mostly made in meristematic cells and much less so as cells mature. (Or for real theoretical beauty, deficiency hormones should be made mostly in mature cells and much less so in meristematic cells). ABA should be made when a cell has less than enough water to support both it any cell dependent on it for water acquisition. Thus ABA is an indication that water exists in less than enough amounts to continue the plant at its current size, thus the plant must use emergency stores of water, find new sources of the liquid and cut down on water sinks. Exogenous Treatment: High levels of exogenously applied ABA should induce SA synthesis, because many of ABA's effects may be to increase water levels within the plant, if only temporarily. This may include making dormant reactions that are normally dependent on water. Inhibition and Stimulation: ABA should encourage root and new root growth, but inhibit shoot growth and even encourage shoot and leaf senescence. Storage: ABA should cause the emptying of stored water reserves found in vacuoles or tubers. Nutrient and Hormone Attraction and Repulsion: ABA should generally push all nutrients and abundance signals/hormones out of cells. ABA should attract the deficiency signals/hormones, GA, ET and BA, leading to positive feedback and cell senescence. Apical Dominance: Should break root apical dominance because low water levels are an indication of poor performance by the currently dominant apical root. ABA may strengthen the currently dominant shoot apex in order not to encourage any new shoot growth which would be a further sink on water levels. Hormone Transport: Water deficiency, on average should be detected in the leaves first, the point furthest from the source of water. Water may be repelled from tissues high in ABA, thus ABA may be built up in the roots, in order to force water toward the shoots. Cell Division: Although it may encourage it in the roots, if it is inducing new ones, ABA should generally inhibit cell division, as a water deficient plant is in no condition to expand. Senescence: Just as I am hypothesizing that SA, JA, IAA and CK all need to be present to induce cell division, ABA, GA, ET and BA may all need to be present for cell senescence to proceed. ABA should encourage senescence, particularly of shoot tissue whose nutrients can be cannibalized and used to make more water absorbing root tissue. Specific to a Water Deficiency Signal: Since GA causes lengthening, ET broadening, BA lengthening, what's left is ABA should cause cell and tissue broadening when it induces growth if it does. (I believe I may have seen such a finding but I have to find the reference again).
Jasmonates (JA) Jasmonates are somewhat similar chemically to prostaglandins, which are chemical messengers found in animals. both are made from fatty acids. However prostaglandins always contain 20 carbons and JA only contains 12. - A Wikimedia Commons Image Speculative Overall Role: Signal of Sugar Abundance JA prompts plants to store excess sugar in tubers like the Potato - A Wikimedia Commons Image	JA exist at high levels in flowers and developing pericarps13 JA exist at high levels in the chloroplasts of illuminated plants13 JA increases in response to mechanical stress and produce tendril coiling14 Jasmonate is made in response to wounding.15 Jasmonates levels increase along with those of ABA under desiccation conditions.19	JA increases in response to mechanical stress and produce tendril coiling14 JA is involved in the tuber storage proteins system and a derivative may stimulate tuber formation.16 17 JA induces chlorosis inhibits photosynthesis gene transcription13 What JA does in flower and fruits is unknown, but it may be involved in the converting green leaf cells contents into seed storage proteins, carotenoid and the sugars.13 JA and ethylene appear to act in tandem to enact plant defense response.18		Flowers have high amounts of sugar available in order to produce nectar. Chloroplast have high amounts of sugar due to photosynthesis. Bruising and wounding ruptures cells and releases vacuole sequestered sugars. Also neighboring plant cell reaction to wounding may be to release amylases (by inducing GA?) which increases sugar levels in the inter cell spaces. Storage proteins and tubers are induced by JA in order to store excess sugar. Chlorosis induced by JA, is a negative feedback loop to cannibalize excess photosynthesis machinery. JA is an indication of excess photosynthesis capacity. Fruits and developing seeds may use JA to store necessary sugar reserves in seeds and fruit.	Synthesis: Well lit plants should have high JA levels, darkened plants should have low levels. JA should be mostly made in meristematic cells and much less so as cells mature. JA should be made when a cell has more than enough sugar to support both it any cell dependent on it for sugar acquisition. Thus JA is always an indication that growth amounts of sugar exist and if conditions warrant, that the plant has enough sugar to grow at least in the specific cell where the JA is. (Shoot cells are responsible for acquiring sugar for both it and similar size cells in the root whereas a root cell is only responsible to itself for it own sugar level). Exogenous Treatment: Should induce GA, because JA up regulates various processes limited by sugar levels. Exogenously applying JA leads the plant to falsely believe that it has high levels of sugar, thus engaging all sorts of reactions that use sugar, thus further depleting what may simply be a homeostatic level of existing sugar and moving this level into the deficiency range. Inhibition and Stimulation: Should Induces new root growth, just like Auxin. If Auxin is also present, JA should inhibit shoot growth because high JA and IAA levels are an indication of at least a short term lack of need to expand the shoots. Storage: should cause sugar to be stored in proteins and tubers for less propitious times. Nutrient and Hormone Attraction and Repulsion: Should attract all nutrients and abundance signals to a cell and repulse deficiency signals. Apical Dominance: Should induce shoot apical dominance along with Auxin, however the possibility exists for two dominant apices if one is particularly good at sugar production (in the light) and one good at oxygen harvesting (in the wind). May break root apical dominances under conditions of low CK and SA. Hormone Transport: May be expected to travel in the direction of the roots, away from the shoots and particularly the shoot meristems. Regions of a cell or tissue or plant part that contains high JA, may particularly attract sugar and transport of sugar may follow active JA transport down a plant. Cell Division: Is actually necessary for cell division along with Auxin, Cytokinin and Salicylic acid. If there are some plant callus lines that will divide with only Auxin and Cytokinin present it is because these cell lines are mutants that produce SA and JA natively, or these other hormones are unknowingly being included with the "other" nutrients/vitamins that are also added to calluses to get them to divide. Senescence: Should protect plant tissue from senescence, particularly root tissue. Specific to a Sugar Abundance Signal: Because ET and IAA show complimentary growth patterns with ET broadening and IAA lengthening and the same is true for CK (broadening) and BA (lengthening), we might expect that JA should show a complimentary growth pattern to GA's cell lengthening, thus JA should broaden cells and plant tissue.
Gibberellin (GA) GA1 - there are many different gibberellin molecules - A WIkimedia Commons Image Speculative Overall Role: Signal of Sugar Deficiency The so called "Foolish Seedling" a disease first characterized in Japan which is due to too much GA - Image from here	GA levels go up in the dark when Sugar cannot be manufactured and down in the light.20 "The highest content of GA was characteristic of leaves in the period of growth cessation."31	Promotes shoot and flower stem lengthening especially in the dark. 21 22 23 Greatly promotes bud growth.24 GA reverses ABA effects on growth inhibition and dormancy. Dissolves stored starch.26 At low concentrations GA (Gibberellin A3) and other Gibberellins promote lateral root growth but high concentrations markedly inhibit it.27 28 GA stimulates flowering some plants.29 GA inhibits tuberization.30 11 GA in concert with Auxin induce phloem differentiation.32		GA increases stem length in the dark, to move shoots out of the shade or the ground, into the light and into sugar production. GA induces shoot growth to increase sugar levels. Breaks bud dormancy to increase sugar levels. GA temporarily increase sugar levels by dissolving stored starch. GA inhibits root growth which is counterproductive to increased sugar levels.	Synthesis: Darkened plants should have high levels of GA, well lighted plants, low levels. Like abundance signals GA may be mostly made in meristematic cells and much less so as cells mature. (Or for real theoretical beauty, deficiency hormones should be made mostly in mature cells and much less so in meristematic cells). GA should be made when a cell has less than enough sugar to support both it any cell dependent on it for sugar acquisition. Thus GA is an indication that sugar exists in less than enough amounts to continue the plant at its current size, thus the plant must use emergency stores of starch, find new sources of the molecule and cut down on its sinks. Exogenous Treatment: High levels of exogenously applied GA should induce JA synthesis, because many of GA's effects may be to increase sugar levels within the plant, if only temporarily. This may include making dormant reactions that normally depend on sugar. Inhibition and Stimulation: GA should encourage shoot and new shoot growth, but inhibit root growth and even encourage root senescence. This may be a particularly apparent when ethylene levels are high and ABA and BA levels low as this is an indication that resources need to rerouted from the root to the shoot. Storage: GA should cause the emptying of stored sugar reserves found in vacuoles or tubers. Nutrient and Hormone Attraction and Repulsion: GA should generally push all nutrients and abundance signals/hormones out of cells. GA should attract the deficiency signals/hormones, ABA, ET and BA, leading to positive feedback and cell senescence. Apical Dominance: GA should break shoot apical dominance because low sugar levels are an indication of poor performance by the currently dominant apical shoot. GA may strengthen the currently dominant root apices in order not to encourage any new root growth which would be a further sink on sugar levels. Hormone Transport: Sugar deficiency, on average should be detected in the lroots first, the point furthest from the source of wsugar. Sugar may be repelled from tissues high in GA, thus GA may be built up in the shoots, in order to force sugar toward the roots. Cell Division: Although it may encourage it in the shoots, if it is inducing new ones, GA should generally inhibit cell division, as a sugar deficient plant is in no condition to expand. Senescence: Just as I am hypothesizing that SA, JA, IAA and CK all need to be present to induce cell division, ABA, GA, ET and BA may all need to be present for cell senescence to proceed. GA should encourage senescence, particularly of root tissue whose nutrients can be cannibalized and used to make more sugar producing shoot and leaf tissue.
Auxin (IAA and others) Indol-3-ylacetic Acid , the most common Auxin - A Wikimedia Commons Image Speculative Overall Role: Signal of Oxygen Abundance Crown galls are caused by Agrobacterium tumefaciens bacteria; they produce and excrete Auxin and Cytokinin and I argue Salicylic acid and jasmonic acid, which interfere with normal cell division and cause largely undifferentiated calluses of cells - A Wikimedia Image (Caption is also partially from Wikipedia - see here)	Made mostly in meristematic cells of the shoot and root decreasing as cells mature and age. 34 35 More is made in the shoot meristem than the root.34 35 Overall levels of auxin peak during the day.36 An internal gradient within the ovary effects the development of the of the embryo. 44 46 The developing seed releases auxin, stimulating fruit growth.44 47	Auxins induces new adventitious root development and growth.33 Involved in shoot and root phototropism. (The Cholodny-Went theory).37 May mediate positive root and negative shoot gravitropism.38 50 Induce xylem differentiation.39 Auxin in concert with GA induce phloem differentiation.32 Inhibits secondary buds below site of synthesis producing apical dominance.40 41 High levels of auxins induce ethylene synthesis especially in the roots.42 Induce cell lengthening.43 An internal gradient within the ovary effects the development of the of the embryo. 44 46 The developing seed releases auxin, stimulating fruit growth.44 47 Young leaves strongly attract auxin preventing new leaves from growing out of the meristem too soon.44 48 When Auxins are no longer produced by leaf, this initiates leaf senescence and abscission.49 The Shade-Avoidance Effect.44 51	Auxins is integral to flower formation. Knockout auxin mutants do not flower. 44 45	Auxin induces new root growth to compliment oxygen abundance. Since oxygen is mostly taken in by the leaves, abundance of it shifts growth away from the leaves to roots. All the abundance signals are indications that certain meristems are worth sending nutrients to, e.g. investing in, and the strongest candidate in many species wins out to the exclusion of all others. Although possibly there is one dominant shoot apex for all four nutrient groups, water, minerals, sugar and gases making a total of four apices. Perhaps most of the time, the mineral and water apex and the sugar and gas apices are the same making two dominant apices, one for the root and one for the shoot. Opposite of what is expected, unless secondary xylem remains a hollow tube for the transport of oxygen, not the up flow of water. However, maybe xylem differentiation may be to bring water and minerals to compliment the oxygen indicated by Auxin and the sugar indicated by JA. Cuts down on excess growth of the shoot. Excess oxygen already exists, so the mission is that it needs to be complimented by excess sugar, minerals and water. Perhaps producing these complimentary nutrients is not accomplished by letting secondary shoot buds grow in an uncontrolled way, but by a second shoot apex establishing itself as the best possible site of sugar producing and putting all other resources into establishing productive roots (basically your guess is as good as mine :-). Opposite what is expected if speculative overall role is true. It is known the Auxin is made in the highest levels in the shoot apex and this is often the most light exposed part of the plant. Perhaps light induces an increase in Auxin biosynthesis, due to the photosynthesis reaction producing oxygen, but that light induces a greater active transport away of Auxin than darkness such that real levels appear to fall in illuminated plant parts. Cell lengthening requires processes that require a lot of oxygen? Excess oxygen is sequestered in vacuoles, blowing a cell up like a balloon? Perhaps flower initiation normally takes a lot of respiration, so point of highest concentration of Auxin, the dominant oxygen apex, is the best place to initiate a flower. A leaf no longer producing Auxin, may no longer be "pulling its weight" in terms of oxygen harvesting and thus needs to be excised.	Synthesis: Well aerated plants should have high IAA levels, anoxia treated plants should have low levels. IAA should be mostly made in meristematic cells and much less so as cells mature. IAA should be made when a cell has more than enough oxygen to support both it any cell dependent on it for oxygen acquisition. Thus IAA is always an indication that growth amounts of oxygen are being procured by the plant and if conditions warrant, that the plant has enough oxygen to grow at least in the specific cell where the IAA is. (Shoot cells are responsible for acquiring oxygen for both it and some of the oxygen for a similar size cell in the root. Whereas a root cell is only responsible to itself for it own oxygen level and may even obtain some oxygen from spaces between soil particles). Exogenous Treatment: Should induce ET, because IAA up regulates various processes limited by oxygen. Exogenously applying IAA leads the plant to falsely believe that it has high levels of oxygen, thus engaging all sorts of reactions that use O2, thus further depleting what may simply be a homeostatic level of existing O2 and moving this level into the deficiency range. Inhibition and Stimulation: Should Induces new root growth, just like JA. Especially if JA is also present, IAA should inhibit shoot growth because high JA and IAA levels are an indication of at least a short term lack of need to expand the shoots. Storage: should cause O2 to be stored in proteins and tubers for less propitious times. Nutrient and Hormone Attraction and Repulsion: Should attract all nutrients and abundance hormones/signals to a cell and repel deficiency hormones/signals. Apical Dominance: Should induce shoot apical dominance along with JA, however the possibility exists for two dominant apices if one is particularly good at sugar production (in the light) and one good at oxygen harvesting (in the wind). May break root apical dominances under conditions of low CK and SA. Hormone Transport: May be expected to travel in the direction of the roots, away from the shoots and particularly the shoot meristems. Regions of a cell or tissue or plant part that contains high IAA, may particularly attract O2 and transport of O2 may follow active IAA transport down a plant. Cell Division: Along with Cytokinin and JA and Salicylic acid, IAA should be necessary for cell division. If there are some plant callus lines that will divide with only Auxin and Cytokinin present it is because these cell lines are mutants that produce SA and JA natively. Alternatively these latter two hormones are unknowingly being included with "other" nutrients/vitamins that are also added to calluses to get them to divide. Senescence: Should protect plant tissue from senescence, particularly root tissue.
Ethylene (ET) Ethylene is an extremely simple gas - A Wikimedia Commons Image Speculative Overall Role: ? The earliest discovery of Ethylene occurred when when lamp post near a green house caused wilting and leaf abscission. A Wikimedia Commons Image	Induced by high levels of Auxin, especially in the roots but this can be moderated by red light which characteristic of shading.55 Ethylene levels increase during flooding, probably due to entrapment rather anoxia. Most plant appear to have a net inhibition of Ethylene production under anoxic or carbon dioxide deficient conditions.56	Promotes the ripening of fruit with climacteric respiration releasing additional ethylene.52 Broadens/thickens plant parts.53 54 Inhibits leaf expansion.54 Inhibits geotropism.55 Inhibits Auxin transport55 and production? Induces leaf, fruits, and flower petal abscission.52 Stimulates seed germination.52 57 Flooding produces the epinasty reaction through Ethylene, where leaf surfaces deliberately grow from a position perpendicular to the stem to one which is more horizontal. 52 58 Induces air spaces called Aerenchyma used for gas diffusion in roots during flooding of non-water based plants.59 Carbon Dioxide inhibits Ethylene action.58 Inhibits embryogenesis of cell cultures.60 Induces root hair growth.61 Ethylene upregulates auxin biosynthesis at least in the roots.61 Flood induce ethylene sensitizes plants to the existing steady Auxin levels, inducing adventitious roots formation.62 Induces flower formation in some species.52 63 Etephon (ethylene precursor) has a dual role in tuberization. It promotes already formed tubers by inhibiting stolon growth. Differently though it inhibits the formation of new tubers.64		Epinastic leaves act like the handles of water pumps to actively pump up flooded water from roots so that it can be evaporated from the leaves and the roots move away more quickly from being water logged. Also or may be instead, leaves parallel to the stem have a greater amount of transpiration than leaves perpendicular to the stem because they flap up and down in the wind, which increases evaporation rates. Prop roots are like snorkels. They are hollow and let the water logged roots more easily get oxygen. The flooding induced air spaces in the roots also serve to increase aeration of the roots and remove excess Ethylene.	Synthesis: Exogenous Treatment: Inhibition and Stimulation: Storage: Nutrient and Hormone Attraction and Repulsion: Apical Dominance: Hormone Transport: Cell Division: Senescence: Just as I am hypothesizing that SA, JA, IAA and CK all need to be present to induce cell division, ABA, GA, ET and BA may all need to be present for cell senescence to proceed. ET should encourage senescence, particularly of root tissue whose nutrients can be cannibalized and used to make more oxygen harvesting shoot and leaf tissue.
Cytokinin ((CK) Zeatin - the most common Cytokinin - A Wikimedia Commons Image Speculative Overall Role: Signal of Mineral Abundance e.g. Nitrogen, Phosphate, Potassium etc. Cytokinin is synthesized in the greatest amounts in root tips - A Wikimedia Commons Image	Made in high amounts in dividing shoot meristematic cells. Made in the highest concentrations in the root meristematic cells.	Exogenous CK inhibits senescence of leaves.65 66 67 Induces new shoots in undifferentiated calluses in stumps whose stems have been cut down or off. Is integral to differentiation of the shoot meristem. Stimulates the development of lateral buds and branching Induces cell broadening. Integral to root differentiation Integral to leaf formation Along with Auxin, necessary to be present to induce cell division. Integral to chloroplast development		Preserves shoots in order to compliment minerals with sugar and gases. May actually inhibit root growth and induce root senescence, just as Jasmonic Acid induces chlorosis. Attempts to compliment root derived mineral abundance with shoot derived nutrients of sugars and gases. CK is a signal that certain meristems cells are good bets for sending resources too. CK is in effect an indication that these cells are producing the amounts of minerals they supposed to and enough to warrant growth.	Synthesis: Well fertilized plants should have high CK levels, plants living in poor soils should have low levels. CK should be mostly made in meristematic cells and much less so as cells mature. CK should be made when a cell has more than enough essential minerals to support both it any cell dependent on it for mineral acquisition. Thus CK is always an indication that growth amounts of minerals are being procured by the plant and if conditions warrant, that the plant has enough minerals to grow at least in the specific cell where the CK is. (Root cells are responsible for acquiring minerals for both it and similar size cells in the root. Conversely a shoot cell is only responsible to itself for it own mineral nutrition levels). Exogenous Treatment: Should induce BA, because CK up regulates various processes limited by minerals. Exogenously applying CK leads the plant to falsely believe that it has high levels of minerals, thus engaging all sorts of reactions that use or are normally limited by mineral levels, thus further depleting what may simply be a homeostatic level of the existing fertilizers and moving this level into the deficiency range. Inhibition and Stimulation: Should induces new shoot growth, just like SA. Especially if SA is also present, CK should inhibit root growth because high SA and CK levels are an indication of at least a short term lack of need to expand the roots. Storage: CK should cause excess minerals to be stored in vacuoles, storage proteins and tubers for less propitious times. Nutrient and Hormone Attraction and Repulsion: Should attract all nutrients and abundance hormones/signals to a cell and repel deficiency hormones/signals. Apical Dominance: Should induce root apical dominance along with SA, however the possibility exists for two dominant apices if one is particularly good at fertilizer absorption (in good soil) and one good at water harvesting (in the moist part of the soil). CK may break shoot apical dominances under conditions of low JA and IAA. Hormone Transport: CK may be expected to travel in the direction of the shoots, away from the roots and particularly away from root meristems. Regions of a cell or tissue or plant part that contains high CK, may particularly attract fertilizer type minerals and transport of important minerals may follow active or passive CK transport up a plant in the xylem or other tissue. Cell Division: Along with IAA and JA and Salicylic acid, CK should be necessary for cell division. If there are some plant callus lines that will divide with only Auxin and Cytokinin present it is because these cell lines are mutants that produce SA and JA natively. Alternatively these latter two hormones are unknowingly being included with "other" nutrients/vitamins that are also added to calluses to get them to divide. Senescence: Should protect plant tissue from senescence, particularly shoot tissue.
Brassinolides (BA) Brassinolide was the first Brassinosteroid discovered - A Wikimedia Commons Image Speculative Overall Role: Signal of Mineral Deficiency e.g. Nitrogen, Phosphate, Potassium etc. Brassinosteroids were first discovered in Brassinus napus, Rapeseed, from which comes Canola Oil (from a variety). Brassins are from the Mustard Family - A Wikimedia Commons Image	"One well-supported hypothesis is that all tissues produce BRs, since BR biosynthetic and signal transduction genes are expressed in a wide range of plant organs, and short-distance activity of the hormones also supports this." BA is transported acropetally (upward).	Promotes epinasty through synthesis of Ethylene.63 Inhibit leaf abscission. Plants found deficient in brassinolides suffer from dwarfism. Promotes cell expansion and cell elongation; works with A uxin to do so Promote leaf senescence and accelerates senescence in dying tissue cultured cells; delayed senescence in BR mutants supports that this action may be biologically relevant Provide protection to plants during chilling and drought stress Low levels of BA promotes root lengthening independent of Auxin and Ethylene. 51 Higher BA levels inhibits root growth. 51 Promote apical dominance Enhance seed germination Enhance gravitropism Increases the production of ethylene Prevents premature abscission of fruit Increases the yield of Wheat and Rice	Plants found deficient in brassinolides suffer from dwarfism.	BA does this to maximize the level of sugar coming from the leaves. Leaf abscission would cut down on some sugar production. Brassinolide encourages stem lengthening just like Gibberellin, because the signal could be an indication the plant's leaves are in the shade and thus the stem needs to lengthened to move it back into the sun. All the deficiency hormones should speed senescence in those plant parts that are sinks and net users of their respective nutrients, but should preserve those plant parts that are harvesters or net producers of the nutrients they represent. All the deficiency hormones may do this by causing plant cells to either senesce (if they are in bad shape) of go into hibernation. Most sugar deficiencies should occur in the roots, so this make sense.	Note BA actually appears to be involved in the GA pathway whether as the principal and GA being a secondary messenger or vice versa. From the evidence BA is like GA, actually appears be a sugar deficiency hormone. Synthesis: If BA is a sugar deficiency signal, plants grown in the shade or with a lot of darkness, should have high levels of BA. Alternatively well illuminated plants should show low levels. BA should be made when a cell has less than enough sugar to support both it any cell dependent on it for sugar acquisition. Thus BA is an indication that sugar levels exists in less than enough amounts to continue the plant at its current size, thus the plant must use emergency stores of sugar (starch), find new sources of light and cut down on the sugar sinks, by for instance inhibiting root growth. Exogenous Treatment: If BA is truly a sugar deficiency hormone, high levels of exogenously applied BA should induce JA synthesis, because many of BA's effects may be to raise sugar levels within the plant, if only temporarily. Inhibition and Stimulation: BA should inhibit root and new root growth and even encourage root senescence. It should encourage shoot growth, preserve leaves from senescing and even encourage new shoot growth initiation. This may be a particularly apparent when ABA levels are low and GA and ET levels high as this is an indication that resources need to rerouted from the shoot to the root. Storage: If BA is really a sugar deficiency signal, it should cause the emptying of stored starch reserves found in vacuoles or tubers if such things exist. Nutrient and Hormone Attraction and Repulsion: BA should generally push all nutrients and abundance signals/hormones out of cells. BA should attract the deficiency signals/hormones, ABA, GA and ET, leading to positive feedback and cell senescence. Apical Dominance: BA should break shoot apical dominance because low sugar levels are an indication of poor performance by the currently dominant apical shoot. BA may strengthen the currently dominant root apice in order not to encourage any new root growth which would be a further sink on essential sugar levels. Hormone Transport: sugar deficiency, on average should be detected in the roots first, the point furthest from the source of mineral harvesting, the shoot. Cell Division: Although it may encourage it in the shoot, if it is inducing new ones, BA should generally inhibit cell division, as a sugar deficient plant is in no condition to expand. Senescence: Just as I am hypothesizing that SA, JA, IAA and CK all need to be present to induce cell division, ABA, ET , BA and GA (with BA and GA working together) may all need to be present for cell senescence to proceed. BA should encourage senescence, particularly of root tissue whose nutrients can be cannibalized and used to make more shoots. Specific to a Sugar Deficiency Signal: Should induce C4 photosynthesis which is a more efficient but more risky form of photosynthesis/sugar making. It's more risky because it causes the buildup of poisons in the plant. GA and BA may be part of the same chemical hormone cascade pathway.
Salicylates (SAs) Salicylic Acid Speculative Overall Role: ?	Decreases with increasing salinity Found in high amounts in Willow bark	Reverses ABA induced opening of stomata. Closes stomata Induces future resistance to pathogen infection when released during infection Inhibits Ethylene Synthesis Inhibits seed germination Salicylic Acid causes "global repression of auxin-related genes... and inhibition of auxin responses" "Phenolic compounds, viz., trans-cinnamic acid, chlorogenic acid, ferulic acid, salicylic acid, tannic acid and quercetin when applied with ABA, antagonize ABA action and restore normal seedling growth."		Inhibits seed germination – by inhibiting ABA synthesis May also block the wound response and act antagonistically to ABA – preventing the wound response from spreading further than necessary	Synthesis: Cells returning from Exogenous Treatment: Inhibition and Stimulation: Storage: Nutrient and Hormone Attraction and Repulsion: Apical Dominance: Hormone Transport: Cell Division: Senescence:
Jasmonic Acid	Increase in desiccated plants.	Effect of elevated ABA levels JA-induced proteins are lacking in the roots, in bleached leaves, and in leaves of chlorophyll-deficient		Growth inhibition Senescence promotion Stimulates wound responses Germination inhibition Tuber formation promotion Fruit ripening and fruit abscission promotion Pigment formation promotion May have a role in plant defense	Synthesis: Exogenous Treatment: Inhibition and Stimulation: Storage: Nutrient and Hormone Attraction and Repulsion: Apical Dominance: Hormone Transport: Cell Division: Senescence: Specific to a Sugar Abundance Signal:

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