A Start at a Comprehensive

Theory of Plant Hormones

Paul Pruitt, M.A. Biology, University of Pennsylvania 1984

Cette page Web a été traduit en français.

This is second version, Version II, of my Plant Hormones ideas first written and posted on the Web in 1995. There are more recent versions also available written in 1999 and 2003. The first version of the paper was written in 1986 and had not been previously published anywhere or posted on the Web until 06/06/2003. When the this and 1999 the versions were posted on the Internet, they received considerable comment, both positive and negative.

Summary

Plant Hormones are seen here as having clear understandable roles in plant life. The role of Ethylene in stress is already well known. The role of ABA is also thought by others to be clearly defined in Water deficit. Here I add that it is involved in all deficits of root derived nutrients. The special new insights I have here are about Auxin and Cytokinin. Auxin in this "paper" is thought as being made anywhere in the plant by cells, when shoot derived nutrients are abundant . Cytokinin is here defined as made by cells under the condition of root derived nutrient abundance. GA, is beginning to be understood as a Sugar deficit hormone, here I make explicit that I think it is made by cells under the condition of all shoot derived nutrient deficits. I also postulate that Brassinosteroid is the compliment of Ethylene, being made by cells under nonstressful, growth promoting conditions. The positive hormones (Auxin, Cytokinin, and Brassinosteroid) are seen as being made at a great rate in immature cells i.e. meristematic cells, and dropping off as cells mature. The negative hormones (GA, ABA, and Ethylene) are conversely seen as being made in small amounts in immature cells and in increasing amounts as the cells mature. The response of plant cells to hormones is also contrasted here. Two kinds of plant cells are demarcated, those that are found in the body of the plant, such as the stem and root core, and those found in the peripheral parts, such as the leaves and peripheral roots. The negative hormones are seen as moving nutrients and biological activity from the peripheral parts to the core, and the positive hormones having the opposite effect. The building up of the stem and root core by the negative hormones aids in Water retention as is the case with ABA, movement toward more sunlight, as is the case with GA, and general strength of the plant as is the case with Ethylene. All the negative hormones sacrifice some biomass, in exchange for a better position vis a vis a particular stress. In a nutshell and oversimplifying, Auxin is made when things are going well shoot-wise. Cytokinin is made when things are going well root-wise. Brassinosteroid is made when things are going well for the plant in general. Gibberellin is made when things are going poorly shoot-wise. ABA is made when things are going poorly root-wise and finally Ethylene is made when things are going poorly overall, for the plant.

Any theory of Plant Hormones needs to recognize the work of K. V. Thimann, F. Went, F. Abeles, F. Skoog, G. Avery, P. F. Wareing, P. Davies, P. W. Morgan, W. P. Jacobs, A. C. Leopold, A. W. Galston, R. Cleland, and F. Addicott. Forgive me for leaving out countless names of others who have made major contributions to the field.  Special thanks goes to Mark Jacobs for getting me so interested in plants in the first place.

Disclaimer

I'm not a professional scientist, and this "paper" is considered by most plant scientists to be pure speculation. Nevertheless I stand by what I write here because I believe it summarizes and draws valid conclusion from a large body of findings, producing a theory which is simple, cohesive and powerful. This "paper" suggests bold new directions for experiments and may have no other value than this. The use of "positive" and "negative" to describe the hormones, is not meant to put a value judgment on the hormones, but is instead meant to reflect the conditions of production and the effect of the hormone. In other words "positive," Plant Hormones are made under good growing conditions and produce further growth, whereas "negative" hormones are produced under bad growing conditions, and produce a cutting back on the size of the plant. They are simply names, however unfortunate some may consider them to be, that I currently use to describe the two sets of contrasting and complimentary Plant Hormones. At a later date the names can be changed, but they certainly are vivid.

Theory

• Plants need light (Sugar), Water, oxygen, carbon dioxide, Minerals, good temperature, and freedom from external disturbances, including diseases, to survive and thrive.

• The major Plant Hormones are made by plant cells in order to transmit information, and carry out actions which are necessary in light of the scarcity or abundance of the above.

• Auxin is made by cells faced with an abundance of shoot derived nutrients (Sugar, carbon dioxide and oxygen). It causes the initiation of new roots, in order to balance out its wealth of shoot nutrients, (in the absence of an indication of healthy roots, i.e. the absence of Cytokinin.) It also stimulates the storage of shoot derived nutrients. Perhaps even the Gases are stored. The more mature a plant cell is, the more shoot derived materials it needs to stimulate it to make the same amount of Auxin.

• Cytokinin is made by cells (in the whole plant), faced with an abundance of root derived nutrients (Water and Minerals). It stimulates the initiation of new shoots in order to balance out its abundance of root nutrients, (in the absence of healthy shoots, i.e. the absence of Auxin). It also stimulates the storage of root derived materials, for future scarcity. The more mature a root cell is, the more root derived nutrients it needs to stimulate it to make the same amount of Cytokinin.

• Gibberellin is made in cells faced with a scarcity of shoot derived nutrients and causes the release of stores of shoot derived nutrients, as well as the shrinking or inhibition of (under the circumstances) unneeded root cells. It also attempts to increase shoot derived materials, by bolting. This bolting, perhaps brings the plant into better atmosphere and better light. The more mature a cell is the less shoot derived nutrient deprivation is needed to induce the same amount of GA.

• ABA is made by cells faced with a lack of root derived nutrients. ABA also adjusts root and shoot size in order to increase Water and mineral consumption. Also, ABA releases stores of needed root derived materials and may stimulate a form of root bolting except laterally not vertically (as is the case of GA), in order to increase Water absorption. As a cell matures, it needs less and less root derived nutrient deprivation to stimulate the production of the same amount of ABA.

• Brassinosteroid is made by the plant, when things are going well in general (good growth conditions). It is released when there is no significant external threat from extreme temperature, disease, physical trauma such as wind or insects, or toxins of various biological or man made sources. It promotes a generalized storing of all nutrients, balancing this with growth. As a cell gets older it makes less Brassinosteroid in response to the same good conditions.

• Ethylene is made by cells undergoing stress, and causes the release of stores of all nutrients, in order to deal with the situation. It causes the death of older leaves and roots, transferring the nutrients from these roots, back into the plant. The energy gained from these operations are transferred into fighting the source of the stress. As a cell becomes more mature it makes more and more Ethylene in response to the same amount of stress.

• The positive hormones have the effect of being magnets for all nutrients, shoot or root. This produces a positive feedback loop for instance in the shoot apical meristem. Under good conditions the shoot apical meristem makes copious amounts of Auxin which stimulates the attraction of all nutrients including shoot derived nutrients and hormones from nearby leaves. The secondary buds are therefore starved of nutrients and hormones and start producing small amounts of the negative hormones, but not enough to kill the secondary buds, but enough to send them into hibernation.

• The negative hormones have the effect of pushing all nutrients out of the tissue where they exist (although this is much more prevalent in leaves and peripheral roots, than in the stem and root core). This creates a negative feedback loop, so for example, a leaf which becomes shaded for long enough (*see below), starts producing GA in high quantities, and pushing out nutrients, which causes the production of even more negative hormones.

• The negative hormones directly inhibit the production of the positive hormones, thus the secondary buds, control the apical buds so they don't get carried away with their positive feedback loop. The positive hormones also directly inhibit the negative hormones, so they don't get carried away inappropriately with their negative feedbacks. Thus the negative hormones indirectly stimulate the positive hormones by pushing out nutrients from unneeded plant parts toward growth centers, but directly influence the positive hormones when the negative hormones reach such high levels they come into direct contact with the cells producing the positive hormones. The converse is also true.

• A small amount of positive hormones will inhibit the production of a large amount of negative hormones in mature cells, in other words a small amount of Auxin will wipe out the production of a large amount of GA in the mature cell leaves. A small amount of the negative hormones will wipe out the production of a large amount of positive hormones in immature cells.� Thus a plant is dynamic and can respond quickly to either good or stressful conditions.

• More Auxin is made in the shoots, because there is more Sugar and shoot derived nutrients there. More Cytokinin is made in the roots because there is more Water and root derived nutrients there. More GA is made overall in the roots because there is less Sugar and other shoot derived nutrients there. GA travels mostly upward in the plant. More ABA is made in the shoot than in the root, because there is less Water and other root derived nutrients there. ABA travels downward in the plant more often than upward.

• Ethylene thickens all plant parts including the leaves, but causes some downsizing to make the plant smaller but stronger. Thus the finding that Ethylene broadens shoot cells is incomplete. Brassinosteroid has the effecting of stretching out all plant parts. All plant parts become thinner, but longer or taller. The nonstressful conditions lead to lean flourishing plants. This suggest that a synthetic Brassinosteroid maybe a better herbicide than 2,4 D if the effect of excess growth is needed. ABA, withdraws root derived nutrients from the leaves and pools them in the stem. ABA also thickens the stem to provide the same biomass, but with less surface area for evaporation. ABA inhibits shoot growth overall and even causes overall shrinkage of the shoot, while promoting root "spreading." ABA also causes the plant to overall "become smaller but stronger." GA pools shoot derived nutrients found in the roots and uses it for bolting. GA inhibits root growth and causes overall root biomass shrinkage. GA also decreases overall biomass in the interest of strength. The effects of Cytokinin and Auxin on cell are already well known.

• The positive hormones are in greater abundance during the day and the negative hormones are in greater abundance at night. We can therefore predict that senescence of plant parts occurs more at night. We can also predict from above that there is some shifting of nutrients and biological activity into the stem of the shoot, at this time. This shifting of biological activity, could also conserve heat. A similar but somehow complimentary shift might also occur in the roots at night (by nick weigel). A general decrease in overall biomass is expected at night from the fact that little photosynthesis is taking place. This might be produced by the using of stored nutrients, and or the senescence of unneeded plant parts.

• In terms of response to Plant Hormones, I'd like to speculate for the purposes of this paper, that there are two types of cells: cells found in peripheral organs such as the leaves and peripheral roots, and cells found in the body of the plant, such as the stem and the root core. I believe that the two types of cells respond differently, in general to Plant Hormones. I believe stem and root core cells respond to GA by elongating, but that plant peripheral organs respond to GA by going into dormancy, then shrinking and then going into the senescence cascade. The root core elongation anchors the plant, which is needed because of the instability produced by GA stem elongation. ABA as mentioned above causes stem cells to widen as it also does to the root core. It moves nutrients and biological activity from the leaves and peripheral roots into the stem and root core where there is less surface space for evaporation .  ABA initiates first dormancy, then shrinking, and then senescence of peripheral parts. Ethylene I believe also moves nutrients and biological activity from the peripheral parts to the stem and root core, and causes general expansion of cells in the stem and root core in order to increase overall strength of the plant. It also causes first dormancy, then shrinking, then senescence of peripheral parts.

• Perhaps the converse of the response of negative hormones in cells is true for the positive hormones, that is they move nutrients from the stem and root core, to the peripheral parts. The stem and root core also respond by energizing, growing and dividing, instead of dormancy, shrinking and senescence. These last two bulleted sections were written in response to Michael Jackson's criticism.

• Cell division occurs in meristems, only when Cytokinin, Auxin, and Brassinosteroid are present. This may explain the lack of success of propagation of some cell lines, with just Auxin and Cytokinin. Some success may have been had, with some lines, because of mutations making Brassinosteroid native. Alternatively the good conditions under which the cells or callus is held may have produced Brassinosteroid.

• Cell death (in a compliment to cell division) is only initiated when GA, ABA and Ethylene are all present, but this effect occurs much more often in the most extended fully mature cells of the peripheral parts, see above. That is cell death in general is initiated in a purposeful manner first in the peripheral parts i.e. the leaves and peripheral roots. Only if the senescence of many peripheral parts does not bring the desired effect, would I propose that the negative hormones would begin to have an effect on the stem and root core. If only one or two of the negative hormones are present, perhaps the (more often) leaf or peripheral root cell, just shrinks. Perhaps, GA for example causes leaf cells to shorten but pools nutrients into the stem for lengthening. It's only after GA has pushed out enough root derived nutrients, does the leaf begin to get stressed and produce ABA and Ethylene. At this point the leaf begins a much quicker freefall into death and abscission. *If however the pooling of nutrients into the stem for bolting does allow the leaf to again see the sunlight, and again begin making Auxin, GA production drops quickly (see above �)and the leaf expands again and health is restored.

• The above is for an "ideal" plant, "evolution" may have changed some of the functioning of the above for particular circumstances. In flower growth for instance, one could imagine that genes first turn on Auxin production to provide nutrient attraction to non-chlorophyll or chlorophyll poor, growing flower parts.

• Again the above "paper" is considered by many if not most to be highly speculative, yet I believe it is a cogent useful theory that explains most findings of plat hormone biology, while suggesting unexpected new ways of looking at hormones along with many new experiments.

Qualifications, Contact Information and Guestbook

My name is Paul Pruitt. I received a BA from Swarthmore College in 1984 where I studied under Mark Jacobs. My Bachelor's thesis was an examination of all aspects of Plant Senescence, including the role of hormones. I also received an MA from the University of Pennsylvania in 1986, where I studied plants under Scott Poethig among others. I have been studying the Plant Physiological Hormone Literature and thinking about Plant Hormones for 20 years. I'm currently an unemployed but experienced IT Support Analyst who has his own small file recovery and virtual Helpdesk business.  The Website can be seen here.  If you have any questions or comments send them to socrtwo@s2services.com.

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