The Plant hormones reference article from the English Wikipedia on 24-Apr-2004 (provided by Fixed Reference: snapshots of Wikipedia from wikipedia.org)

Plant hormones

There are five major classes of plant hormones (a.k.a. Plant growth regulators (Pgr's)) with perhaps a few others as candidates to take that place in the future. The five major hormone classes are Auxins, Cytokinins(CK’s), Ethylene, Gibberellins (GA’s), and Abscisic Acid (ABA). Recently it has been suggested that Brassinosteroids (BA’s), Jasmonates (JA’s), Salicylates (SA’s), and Polyamines are new classes of major hormones. A slight variant of the article below recently won 2nd prize for posters at the Western Plant Growth Regulator Society Conference this January. It is outside accepted scientific thinking, but clearly explains most of the phenomena.

Outline for a Comprehensive Theory of Plant Hormones

A prudent man keeps his knowledge to himself...A fool is only interested in airing his opinions.” Proverbs 12:23 and 18:2

In this Web page, I will discuss the functions, behavior and interrelationships of plant hormones. This is a further revision of the fourth and most recent version of my theories. The first version was written in 1986 and was not published or posted on the Web until recently. The second and third versions were written in 1995 and 1999. When the 1995 and 1999 versions were posted on the Internet, they attracted much comment, both positive and negative.

This paper sets out two alternative theories. The theories have elements in common but also differ fundamentally. The first theory is described in the more detail, with an introduction followed by a table of hormone characteristics. The section headed "Hormone Characteristics" includes additions that should be made to the table if the first theory is correct. Next, a section headed "Theory" sets out my basic understanding of why plant hormones are made and how they interrelate. Following this is a section headed "Predictions." This sets out the implications of the first theory. The final section of the paper is a summary of an alternative theory.

The first theory is that the five “classic” plant hormones, Auxin, Cytokinins (CKs), Gibberellins (GAs), Ethylene, and Abscisic Acid (ABA), plus the “new” plant hormones, Brassinosteroids (BAs), and Salicylates or Salicylic Acids (SAs) can be gathered into three groups. These are Growth Hormones, Stress Hormones and Shock Hormones. Auxin and CKs are Growth Hormones. Ethylene, GAs, and Brassinosteroids are Stress Hormones. And ABAs and SAs are Shock Hormones. All tree types of hormones are similar in that they fall within the classic definition of an intercellular hormone. They are made by a cell and are meant to affect the behavior of other cells, either in nearby tissue or at the opposite end of the plant.

The first assumption I make in both theories is that plants are interested in growing larger during the vegetative period of their life (the middle period, between germination and reproduction), and this growth requires both good environmental conditions and an amount of the four basic nutrient groups that exceeds that needed to keep the plant at its current size. The environmental conditions necessary for growth are optimal temperature, and the absence of environmental stresses, including high winds, pests, disease, and consumption by herbivores. The four basic nutrients required for growth are sugar, gases (Carbon Dioxide and Oxygen), water, and minerals. I generalize that Growth Hormones are made mostly in young and meristematic cells, and much less in mature cells. They are made in mature cells only when there is an excess of nutrients. I hypothesize that Stress Hormones, in contrast, are made in mature cells that are faced with a scarcity of nutrients and to a much lesser extent in young and meristematic cells faced with the same scarcity. The Shock Hormones differ from the other two groups in that they are made by all cells in equal amounts faced with the same conditions.

I suggest First they stimulate metabolism cause growth where cells have not reached physical maturity and its size limit. Second, they induce new growth (new shoots or roots from dormant shoot or root meristems) when active cells affected by the hormones have all reached their physical maturity and size limits. This really means the activation of dormant cells and the induction of growth in them. In both cases, growth is induced to use up the nutrient excess. Third, they induce new cell production through cell division when a balance of nutrients is available. I will explain more about this later on but this is real new growth and growth causation. When plant cells are dividing, I hypothesize that this usually means both the root and shoot are prospering and the plant is doing what it wants to do, mainly grow bigger. Fourth, Growth Hormones balance growth. If, for example, there is an excess of sugar and gases, the synthesized Growth Hormones will stimulate new growth or cell division in that part of the plant, for example the root, to balance out the excess of sugar and gases, with an excess of water and minerals. Fifth. they cause nutrient storage if growth is currently impossible or unwarranted. Sixth, if the plant is using more risky ways of obtaining nutrients (see below), they will make it switch to more conventional, less risky ways of obtaining them. I suggest that Auxin is made when the sugar and gases present in a cell exceed what is needed to support both it and any dependent cells at peak metabolic activity. I suggest that CK is made by the cell when the water and minerals present exceed what is needed to support both it and any dependent cells at peak metabolic activity. Auxin is found mostly in a young and meristematic shoots, but also by any cell with more sugar and gases than it needs for survival at the peak metabolic activity for both it and any dependent cell. Thus, Auxin will be made by young and mature roots, albeit in small amounts. One issue that needs to be cleared up in this scheme is whether growth hormones are ever released when cells are not at their peak metabolic activity. That is say the local cell environment is at high level of three out of the four required nutrients e.g. there is an excess of three out four nutrients, do growth hormones get released, but they trigger storage instead of growth?

Stress Hormones complement Growth Hormones. They are triggered by a deficit of nutrients that is when there are not enough nutrients to allow growth. The Stress Hormones do six things. First they cause the release of stored nutrients. Second, they inhibit growth. Third, they cause the senescence of older plant parts in that part of the plant. So, if an older leaf or root cell does not have more the minerals and water necessary for the cell and any dependent cells to survive at peak metabolic activity, it will make Stress Hormones. The Stress Hormones then inhibit the growth of the shoot and cause the senescence of older, less efficient leaves, which have no part in making up the water and mineral deficit, except in the short term by releasing their cell contents of water and minerals in a one-off burst of senescence. This growth inhibition and senescence of older, less efficient leaves both stops the deficit from worsening and actually reverses it. This reversal occurs beyond the short-term release of water and minerals originally used in the dying cell’s biological structure, because there are now fewer leaves needing water and minerals. There is now less of a biomass needing support with these nutrients. The fourth thing that a Stress Hormone does to make up these deficits is exaggerate the rate of growth of the part of the plant that normally procures the nutrient. Thus the Stress Hormone in this case would exaggerate root growth. The fifth thing a Stress Hormone does is change the nutrient procurement strategy and to use more risky strategies. For instance, the Stress Hormone synthesized by mineral and water stresses induces the growth of root hairs in the root, which greatly increases the surface area of the root and water and mineral absorption, but may make the plant more susceptible to water loss during rapidly developing drought conditions, to disease or to pestilence. The root hairs are behind the thick waxy protective cuticle of the root. As many of you will have guessed by now, I am assigning Ethylene to this role of mineral and water nutrient deficit indicator. I assign GAs as indicators of sugar and gas deficits. I will also argue later that in fact the release of Stress Hormones is a normal part of the life of the plant, at least at night, because active procurement of nutrients is much harder during this period. This leads to the sixth feature of stress hormones, that they too balance growth proportions of cells. Ethylene broadens plant cells and GA lengthens them, so at night when both these hormones are high and Auxin and CK are low, cell proportions are still balanced.

Finally we come to the Shock Hormones. I group ABAs and SAs here. It is widely accepted that ABA is rapidly released when a plant confronts stress of most kinds. It is beginning to become accepted SA is released when a plant obtains release from these survival threats. It’s not much of a stretch to see ABA as putting the plant in a defensive posture of dormancy and maximum self protection. SA would then open the plant back to normal functioning after the threat is gone. I would see all cells making ABA in similar amounts when confronted with the same stress. The same would be true of SA. Perhaps only beyond peak rates of metabolism would the Growth Hormones Cytokinin and Auxin be released. A climactic rise or sustained high level of SA may be the signal that a plant has reached this point and that resources beyond this can be turned to Growth. On the low end, a climactic or sustained high level of ABA maybe necessary prerequisite to the synthesis of GA and Ethylene in that it would be a signal that the levels of nutrients has dipped below survivable levels.

The alternative theory says that instead of Auxin and Cytokinin being released when there are more than enough nutrients for peak level metabolism, they are instead released any time nutrients get above survivable levels. Also the second theory would say that GA and Ethylene would be released any time nutrients fall below peak metabolism rates. Therefore an absence of GA and Ethylene and the presence of high levels of Cytokinin and Auxin would be a signal of passing of peak metabolism conditions and conditions warranting growth. On the low end, the presence of high levels of GA and Ethylene and the absence of Auxin and Cytokinin would be an indication that senescence is warranted and survivable levels of nutrients are not indicated. In this scheme, ABA and SA would fall to the role of a plant’s responses to rapidly developing stress or release from such a threat. SA would be used to release a plant from a defensive dormant posture. This is the way most plant scientists see ABA anyway and are beginning to see SA’s role.

Finally this theory has its limitation. Brassinosteroid is accepted by some but by no means all Plant Physiologists as being part or the primary hormone induced in the GA hormone cascade. I don’t enter this discussion except to provisionally accept that this is true. On the subject of Jasmonates, I won’t say much except that it seems obvious to me that they are induced by wounding and help coordinate the plant’s defenses to counteract such an event. This is a special case of plant stress and will not be discussed here except to say that it induces ABA as part of its process, and when ABA is induced it falls under the purview of this general theory. This theory attempts to provide a backbone or generalized framework from which to understand plant hormone behavior. Despite Galston’s great work, I too will not include a discussion of polyamines as it is still unclear as to whether these chemicals are hormones or are membrane stabilizer released by hormones under stress conditions.

Introduction

Since Darwin’s time, it has been known that plants regulate their growth with some kind of internally secreted chemicals. Plant hormones, according to a standard definition from the Web:

· Are signal molecules produced at specific locations;

· Occur in low concentrations;

· Cause altered processes in target cells at other locations.

Today, it is accepted that there are five major classes of plant hormones, with a few possible candidates to add in the future. The five major classes are Auxin, CKs, Ethylene, GAs, and ABA. Recently it has been suggested that BAs, JAs, SAs, and Polyamines are new major classes of hormones. This paper is not an introduction to the discovery, chemical structure, and synthesis pathways of the hormones. There are several decent introductions to these on the Web. Instead, I shall try to provide a second-level examination of the hormones and a unifying outline of a theory that explains the underlying relationships and generalized principles under when these hormones are secreted. This paper is a simplification of the findings. I want to warn any reader, that I have a strong preference for symmetry in theories and models. Inspired by the fact that plants show a strong physical symmetry, being divided into roots and shoots, my two theories will each be strongly symmetrical.

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 the names of countless others who have made major contributions to the field. Special thanks go to Mark Jacobs for getting me so interested in plants in the first place.

Hormone Characteristics Table

All items in bold are known scientific findings - references are in progress All items with “?” are from papers known from my research notes whose dates I did not record All items with “?” Present difficulties to the theory outlined below All items in italics, are speculations on my part

Name (With Example)

Location, Characteristics and 

Occasions for Synthesis Induction

Effects

Auxins

IAA

·         Synthesized in shoot and root meristematic tissue (Sembdner et al., 1980)

· Synthesized in young leaves (Sembdner et al., 1980)

· Synthesized in mature leaves in very small amounts

· IAA peaks during the day (Jahardhan et al., 1973)

· Synthesized in mature root cells

· Released by meristematic cells when they have enough sugar and Oxygen to support both themselves and any dependent cells and are in good growing conditions

· Released by all cells when they are experiencing conditions which would normally cause a shoot meristematic cell to produce Auxin

Directly or indirectly induced by high levels of Ethylene

·         Stimulates cell elongation (Schneider, 1938)

· Stimulates cell division with CK

· Induces xylem and phloem (Jacobs, 1967)

· Directly stimulates Ethylene synthesis

· IAA inhibits Ethylene formation and transport of precursor (Wright, 1980)

· Induces shoot apical dominance (Snow, 1945; Palmer & Phillips, 1963)

· Inhibits abscission prior to formation of abscission layer (inhibits senescence of leaves)

· Involved in phototropism, gravitropism, tropism toward moisture

· Induces sugar and mineral accumulation at the site of application (Mitchell et al., 1937; Booth, ?; Davis and Wareing, ? )

· Flower initiation

· Sex determination

· Induces xylem and phloem

Induces new root formation (Torrey, 1957; Brown et al., 1975) by breaking root apical dominance induced by CK Inhibits root hair growth and causes them to die back (From Theory II) Stimulates the rate of metabolism of cells in the root (who are not at their peak metabolism rates) in response to an increase in the levels sugar and essential gases

Cytokinins (CKs)

Zeatin

·         Synthesized in root and shoot meristematic tissue (Chen et al., 1985)

· Synthesized in meristematic regions of roots (van Staden & Smith, 1978)

· Synthesized in mature roots – small amount

· Rapid transport in xylem stream

· CK activity reduced in plants suffering drought (Vaadia, 1965)

· Peaks during the day (Hewett & Wareing, 1973)

· Synthesized in mature shoot cells

· Released by meristematic cells when they have enough minerals and water to support both themselves and any dependent cells

· Released by all cells when they are experiencing conditions which would normally cause a shoot meristematic cell to produce CK

· Directly or indirectly induced by high levels of GA/BA

·         CK promotes Chlorophyll production and leaf unrolling (Beevers et al., 1970)

· CK promotes photosynthesis (Adedipe et al., 1979)

· Stimulates cell broadening (Egelke et al., 1973)

· Also promotes shoot formation (Skoog & Miller, 1957)

· Also promotes the unloading of sugar from phloem (Hayes & Patrick, 1985)

· Causes the outgrowth of secondary shoot buds – breaks shoot apical dominance/ lateral bud development (Sachs & Thimann, 1967)

· Delays leaf senescence (Pooviah & Leopold, 1973)

· Stimulates cell division with Auxin

· Involved in morphogenesis (Houck & Lamotte, 1977)

· Promotes stomatal opening

· Induces xylem and phloem

· Directly induces GA/BA at high levels

· Inhibits C4 Photosynthesis

· (From Theory II) Stimulates the rate of metabolism of cells in the shoot (who are not at their peak metabolism rates) in response to an increase in the levels minerals and water

Ethylene (ET)

Ethylene

· Directly induced by high levels of Auxin (Rubinstein & Leopold, 1964)

· Found in germinating seeds (Esashi & Leopold, 1970)

· Induced by root flooding (Kawase, 1972; El-Beltagy et al., 1974; Imaseki, 1985)

· Induced by drought (El-Beltagy et al., 1974)

· Synthesized in nodes of stems

· Synthesized in tissues of ripening fruits

· Synthesized in response to shoot environmental, pest, or disease stress

· Synthesized in senescent leaves and flowers

· Rapidly diffuses

· Inhibiting effects of Ethylene on shoot growth (more specifically on stem elongation) reduced in the presence of light (Wareing & Phillips, 1981). Also Ethylene levels are decreased by light (Goeschl et al., 1967)

· Released in mature cells when they do not have enough minerals and water to support both themselves and any dependent cells

· Released by all cells when they are experiencing conditions which would normally cause a mature shoot cell to produce Ethylene

· Stimulates leaf and flower senescence (Wareing & Phillips, 1981)

· Induces leaf abscission (El-Beltagy et al., 1974) mainly in older versus younger leaves (Leopold, 1970)

· Induces seed germination (Esashi & Leopold, 1969; Ketring & Morgan, 1970)

· Induces root hair growth – this increases the efficiency of water and mineral absorption

· Stimulates Epinasty – leaf petiole grows out, leaf hangs down and curls into itself

· Stimulates fruit ripening

· Induces the growth of adventitious roots during flooding

· Usually inhibits growth (El-Beltagy et al., 1974) - just shoot growth

· Affects neighboring individuals

· Disease/wounding resistance

· Triple response when applied to seedlings – root ? and shoot growth inhibition and pronounced hypocotyl hook bending

· Inhibits stem swelling ? (Contradictory to the finding below – contradictory sources)

· Stimulates cell broadening (Burg & Burg, 1966) (and lateral root growth)

· Interference with Auxin transport (when hormone levels are increasing)

· Directly or indirectly induces Auxin at high levels

· (From Theory II) Inhibits the rate of metabolism of cells in the shoot (who are not already at their lowest metabolism rates) in response to an decrease in the levels minerals and/or water

Gibberellins (GAs)

Gibberellin 452D

·         Synthesized in the embryo (Webb et al., 1973) and germinating seeds

· Synthesized in the roots (Barrington, 1975)

· Levels go up in the dark when sugar cannot be manufactured and down in the light (Brown et al, 1975)

· Synthesized in apical meristems ? and young leaves ?

· Produced in the stem rather than the growing tip ? (opposite finding to above – conflicting sources)

· Transport is non-polar, bidirectional producing general responses

· Released in mature cells (particularly root) when they do not have enough sugar and Oxygen to support both themselves and any dependent cell

· Released by all cells when they are experiencing conditions which would normally cause a mature root cell to produce GA or BA

· Released in response to root environmental, pest, or disease stress

· Directly induced by high levels of CK

·         Stimulates shoot and cell elongation (Engelke et al, 1973)

· Delays senescence of leaves (Manos & Goldthwaite, 1975; Goldthwaite, 1972)

· Inhibits root growth (Thimann, 1977; Mitsuhashi-Kato et al., 1978)

· Inhibits adventitious root growth (Rossel ?)

· Produces seed germination (Egley, 1980)

· Antagonist promotes root growth and GA reverses this (Kefford, ?)

· Promotes root initiation in low concentration in pea cuttings (Eriksen, 1970, 1971)

· Stimulates bolting and flowering in biennials (Zeevaart, 1983)

· Regulates production of hydrolytic enzymes for digesting starches (Varner, 1964)

· Inhibits CK bud growth on calluses (Engelke et al., 1973)

· Inhibits bud formation (Murashige, 1964)

· Inhibits leaf formation (Bryan et al., 1955; Tronchet, 1968)

· Breaking of dormancy

· Induces extra Chlorophyll production or more efficient methods of photosynthesis (C4Photsynthesis). I think this reference actually exists

· Stimulates root senescence

· Directly or indirectly induces CK at high levels

· (From Theory II) Inhibits the rate of metabolism of cells in the roots (who are not already at their lowest metabolism rates) in response to an decrease in the levels sugar and/or essential gases

Abscisic Acid (ABA)

Abscisic Acid

·         Released during desiccation (Wain, 1975) 

· Has been found to peak at night (Lecoq et al., 1983 a, b)

· Synthesized in green fruit and seeds at the beginning of the wintering period

· As well as moving within the leaf it can be transferred to the leaf from the roots by the transpiration stream

· Rapidly translocated

· Produced in response to stress

· Synthesized in leaves and stems (particularly when water stressed)

· Released by cells in danger of not having enough nutrients locally or good enough environmental conditions to survive

· All cells capable of synthesizing

· Stimulates stomatal closure (Wain 1975)

· Fruit ripening inhibition

· Encourages seed dormancy by inhibiting cell growth – inhibits seed germination

· ABA inhibits the uptake of Kinetin (Reed, 1974)

· Pathogen resistance response defense -

· Induces senescence in already damaged cells and their proximate neighbors

· Quickly puts a plant, organ, tissue or individual cell in a defensive posture (whatever this entails) in response to rapidly developing nutrient or environmental stress that threaten their survival

· Decreases metabolism in response to a newly developing deficiency of nutrient or adverse environmental condition, such that condition becomes survivable at the new lower level of metabolism (Not true in Theory II)

· Possibly induces cell dormancy or senescence by a climactic increase or sustained level stimulating the synthesis of GA and/or Ethylene (Not true in Theory II)

· A climactic rise or sustained level of ABA may be a prerequisite for the synthesis of any GA and/or Ethylene in that it presence indicates unusable or unsurvivable levels of Water, Sugar, Minerals and/or essential gases (Not true in Theory II)

Brassinosteroids (BAs)

Brassinolid

·         Released in mature cells when they have less than enough sugar and Oxygen to support both themselves and any dependent cells

· Released by all cells when they are experiencing conditions which would normally cause a mature root cell to produce BA or GA

· Released in response to root environmental, pest, or disease stress

·         Increased rate of stem elongation (Thompson et al., 1982)

· Leaf senescence inhibition

· Involved in gravitropism

· Bending of grass leaves at the sheath/blade joints

· Inhibits leaf abscission

· Inhibits root growth

· Resistance to stress - just in the shoot. By rerouting resources from the root to the stressed shoot

· Stimulates cell elongation and division (Thompson et al., 1982) – just in the shoot

· Enhanced Ethylene production ? – induced indirectly by the causation of root cell senescence

· Promotion of growth - just shoot growth

· Xylem differentiation promotion - in order to transfer resources from cannibalized root cells

· (From Theory II): inhibits the rate of metabolism of cells in the shoot (who are not already at their lowest metabolism rates) in response to an decrease in the levels sugar and/or essential gases

Jasmonates (JAs)

Jasmonic Acid

·         Desiccation 

· 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

Salicylates (SAs)

Salicylic Acid

·         Cells returning from water stress

· Released by cells secure in having more than enough nutrients and environmental conditions locally to survive at its current metabolic level

· All cells capable of synthesizing

· Has its effect or acts by rapid local increases followed by rapid decreases in levels

·         Retards senescence (regulatory role) –  probably by inhibiting Ethylene biosynthesis 

· Induces flowering

· 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

· After a survival threat has passed SA quickly removes a plant, organ, tissue or cell from a defensive posture and returns it to normal functioning

· Increases cell metabolism rate to take advantage of new complete more advantageous nutrient and environmental conditions (Not true in Theory II)

· A climactic or sustained level of SA may occur if a cell has reached its peak metabolic levels and may signal that a plant’s resources can be turned to growth (Not true in Theory II)

· This climactic or sustained level of SA may be a prerequisite for the synthesis of Auxin and/or Cytokinin, because only then does a plant know that it has enough resources to turn them to growing bigger (Not true in Theory II)

Information provided initially by: http://www.sidwell.edu/sidwell.resources/bio/virtuallb/plant/hormone.html, http://www.psc.ttu.edu/ps3323/PPT%20Files/HORMONES.ppt,

http://www.biologie.uni-hamburg.de/b-online/e31/31f.htm, http://styx.nsci.plu.edu/~dhansen/hormones2.ppt, http://www.pasionflow.co.uk/horm.htm,

http://www.umanitoba.ca/faculties/afs/plant_science/courses/39_768/l18/l18.1.html and http://www.nslc.wustl.edu/courses/Bio4021/2003/L18.htm.

Theory

1. The goal of a plant is to germinate, survive, grow, and reproduce (and either exploit or contribute to life in general – see my summary of a future paper in progress here).

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2. The role of the shoot is to create sugars from sunlight, water, and Carbon Dioxide harvested from the air. It also harvests most of the Oxygen needed by the plant for respiration. The shoot may serve as a reserve store for water and minerals. This may be far fetched as a general principle but the storage of water occurs in at least the cactus. The best place for storing all nutrients may be out of harms way in the soil, in the root. The shoot also provides the structure that supports the leaves, flowers, and fruit, but this will not be important here.

3. The role of the root is to harvest water and minerals from the soil. In order to function, the root also needs to harvest some Oxygen from spaces between the soil particles. The root also provides a place for storing reserves of sugar in the form of starch and may even store Oxygen. It also anchors the plant in a propitious place for it to grow and prevents it from being physically uprooted by the elements or fauna. The anchoring role of the root will not be important here.

4. If they face conditions where they have to make a choice, plants will invest in promising new meristematic cells (i.e. those that are functioning well in their role, like young leaves making good amounts of sugar and successfully harvesting Oxygen) and withdraw nutrients from mature cells to feed these “babies,” even if that means withdrawing nutrients from mature cells that are functioning “adequately”. There is a cost to the transfer of nutrients from mature to juvenile cells. This cost is measured in the loss of some nutrients during the process. It is true that minerals cannot be destroyed and are usually not excreted. However, they are probably not fully recoverable from a mature cell, leaf, or root, in the same way that they would be if they were merely stored in some kind internal reserve, such as a vacuole within a cell.

5. There are three general groups of plant hormones. The Growth Hormones are released under long term good growth conditions and are separated into one predominantly synthesized in the shoot and one in the root. The Stress Hormones are released under various kinds of long term stress and are separated into one synthesized predominantly in the shoot and one in the root. Lastly, there are the Shock/Synchronizer Hormones. The idea with the Shock/Synchronizer Hormones is that they are released under rapidly developing stress of any kind or return from stress good conditions, confronting the individual cells, parts of the plant, or the plant as a whole. To elaborate, these are the first hormones released when the physical survival of the cells is under threat or when the cells return to secure environmental and nutrient conditions. They quickly shut a plant or plant part down or restore it to normal functioning. That is, they are released when nutrient or environmental conditions call into question the very survival of the cells or conversely guarantee the security of an individual cell, regardless of the role it is expected to play in the plant. They may also play a secondary role as modulators of the rate of cell metabolism slowing it down to survivable levels according to local conditions, or speeding it up so that full use may be made of current nutrient levels and environmental conditions. In fact, a final climactic high or sustained level of these hormones may be needed to kick off the synthesis of the stress hormones (GA and Ethylene) on one end, or the growth hormones (Auxin and Cytokinin) on the other.

6. All plant cells are totipotent not just under the right conditions, such as in tissue culture, but also in the way that they behave in response to environmental and nutrient conditions. That is, a shoot cell will always act somewhat like a root cell, and a root cell will always act somewhat like a shoot cell. In addition, a mature cell will act somewhat like an immature cell, and vice versa. For example, below, it is suggested that just like a shoot meristem cell, when any cell is met with good environmental conditions and more than enough sugar and Oxygen to support growth, it will make Auxin or at least a tiny amount of it.

7. The Growth Hormones include Auxin and CKs. These are made by all cells when conditions are good for growth. Auxin is made when any plant cell is facing the conditions that would be propitious for the growth of a shoot meristematic cell. These include freedom from environmental stresses and the production or existence of more than enough sugar and Oxygen to support it and any cells depending on it. (Root cells, except for the few that are meristematic, have no cells depending on them for their sugar and Oxygen needs.) CKs are made by any cell under the conditions that would be propitious for a root meristematic cell to grow. These include freedom from environmental stresses and the uptake or existence of more than enough minerals and water to support it and any dependent cells. To go into more detail, a root cell, for example, would make a CK if it was taking in more than enough minerals and water to support both it and any dependent shoot cell of similar size in the shoot (i.e. a cell in the shoot that depends on that cell in the root for its water and minerals). A similar relationship exists for a shoot cell vis-à-vis sugar and Oxygen, and the sister cell in the root that depends on it for the nutrient.

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8. The Stress Hormones include the GAs, ET and the BAs. These are made by all plant cells under some kind of stress where the plant must remove resources from the site of stress and redistribute the resources to another part of the plant so that the stress is ameliorated. They also initiate the freeing of stored resources to address the particular shortfall. GAs, for instance, are made by all cells under which conditions that would be disadvantageous to a mature root cell (e.g. in any cell when there is less than enough sugar and Oxygen to support both it and any dependent cells). GA removes the resources from this cell and redistributes them into some part of the shoot so that the prospect of increasing the supply of sugar and Oxygen to the entire plant is increased. (The root would keep the resulting sugar and Oxygen for itself and route the minerals and water to where they were needed in the shoot.) Also GA causes the release of enzymes in the root, which turn starch stored in vacuoles into sugar. This sugar also helps, at least temporarily, to resolve deficiencies. I hypothesize that Oxygen is also stored in the roots and GA initiates its freeing from storage and availability to the root. Gas is a much less compact stored item than starch, but nevertheless this phenomenon is possible.

Similarly, ET is made by all cells under similar conditions and would be disadvantageous for the existence of a mature shoot cell. For instance, in any cell when there are not enough minerals and water to support both it and any dependent, similar-sized cell, ET causes the withdrawal of all nutrients and redirects the sugar and Oxygen to the root, keeping the minerals and water in the shoot. This is an attempt to produce productive new root growth and an eventual surpassing of the previous levels of minerals and water. This is an attempt by the plant at a wise reinvestment of resources. It is a gamble to jump start the growth in mineral and water levels to facilitate the growth of the shoot. These jump starts always have costs. These include some use of energy and thus an overall net loss in the weight of the plant. I hazard that minerals and water are stored in either the shoot (for theory symmetry) or the root (because of the practicality of its inaccessibility to the hostile outside world). I suggest that ET initiates the freeing of stored minerals and water in addition to its resource stress role.

Like many involved in the study of plant hormones, I believe that BAs may actually be part of a hormone cascade that involves GA or a parallel path with many similarities. That is, BAs may just be a step along the way in a scheme from a stress to the root cells generally and the plant’s reaction to the stress. In fact, BAs may be the primary hormone igniter of the chemical domino path that leads to the reaction of a plant to stress and GAs may be just a step along the path. Future experimentation will elucidate this. Because of the similarities, I will refer, as some scientists do, to GA and BA together as the GA/BA class of hormones.

9. The Metabolism Synchronizer Hormones are the ABAs, and SAs. They fall into two categories: general metabolic inhibitors/senescence stimulators, and general metabolic stimulators/senescence blockers. They act rapidly. I am defining ABAs as general metabolic inhibitors/senescence activators. They would be made when there is any kind of nutrient or environmental stress to a cell, such as a low or high temperature, wounding, mechanical stress from wind or other plants or animals, pests, disease, or the dropping below sustenance level of minerals, water, sugar, or Oxygen. In terms of nutrient stress, the survival ABA is made when any of the resources of the cell fall below sustenance level. This hormone would not be activated if the levels of these nutrients fell below what the particular cell nutrient level was supposed to be for that type of cell if it was supposed to harvest or synthesize that nutrient for others. For instance in a root cell, ABA would not be synthesized until either minerals or water fell below what was necessary for the survival of the root cell itself. The hormone would not be made when the cell had a level of minerals and water below what was necessary to support both it and a sister cell in the shoot. Falling below that level would initiate ET emanation. The level of sugar and Oxygen necessary for ABA synthesis in the root would of course be below the sustenance level, which might be a frequent occurrence because the cell has to get these nutrients from the opposite end of the plant. In an attempt to re-establish good supplies GA/BA would also be released when sugar and Oxygen fell below sustenance level. Low levels of ABA would be to slow the metabolism so that less sugar and Oxygen were needed. At higher ABA levels, ABAs would work with GA/BAs to senesce the cell. At low levels, the slowing down of the metabolism itself might retard the GA/BAs’ proposed initiation of that root cell’s senescence. ABA, I hypothesize, always work first at a low level to slow metabolism, so that the nutrient sustenance level is actually lowered to a “bearable level” or in the case of environmental stress of any kind, less damage ensues than would occur at a higher rate of metabolism. When and if the plant or cell “calculates” that survival is not possible or “not warranted,” it releases higher levels of ABA, initiating the senescence of the stressed cell or damaged part of the plant.

The SAs on the other hand are categorized as metabolic stimulators/senescence inhibitors that hasten all metabolic activities and oppose the action of ABA and the Stress when they are trying to inappropriately move nutrients out of an efficiently working cell. SA would be released when a cell is in no physical danger of survival and might increase with better nutrients or any kind of internal and external environmental conditions above a base line. SA might be a “foundation” indicator, indicating that a cell is in good health without regard to how it is supposed to be functioning (i.e. as a root or as a shoot cell). For example, it might be released after a good rain after a drought, when ABA and the drought action of overall metabolism inhibition, stomate closing, and progressive senescence with plant size shrinkage is no longer warranted.

These Metabolism Synchronizer Hormones may also act in low quantities on the metabolism as day-to-day status quo regulators. These actions would involve neither growth nor redirection and its cost of plant shrinkage. ABA and SA levels may rise and fall many times a day. When the levels are low, they may be the equivalent of “moods” in animals or humans or a Circadian Rhythm, alternating between levels of depression and inaction and excitement and action, as the conditions and growth opportunities warrant.

ABA and SA often seem to have counteracting effects (here and here).

10. The Growth Hormones are made primarily in meristematic tissue and the Stress Hormones are made in mature tissue. The Metabolism Synchronizer Hormones are made in all tissues equally. Although the amount of most hormones made by cells may differ according to their maturity, small amounts of each of these hormone groups are made in all cells under the right conditions. The exception is that perhaps the Metabolism Synchronizer Hormones are made in all cells in equal amounts under the right conditions. Alternatively, perhaps under the same conditions, the survival hormone ABA are made in larger amounts in mature cells than in juvenile cells. The suggestion is that juvenile cells can recover from stress more easily than mature cells. Therefore, they have less need of the stress-protecting effects of low levels of ABA and do not need to be sent to the “glue factory” of senescence when they are experiencing high levels of stress (i.e. the plant has “confidence” that they will recover). Perhaps we can even say that plant cells, like all living things, are most susceptible to stress at the beginning and end of their lives. Thus, under the same stress, higher amounts of ABA would be made at the beginning and end of the lives of cells, because the plant would “know” that this was when the cells were most susceptible and least likely to survive stress. Conversely, SA might be more easily made in young cells and mature cells. That is, the highest amounts of SA would be made in cells after they have moved out of the fragile juvenile stage and before they move into senility or close to it.

Getting back to the Growth and Stress Hormones, let’s take Auxin as an example. It is a Growth hormone and primarily a shoot hormone. The largest amounts are made in shoot meristematic cells. Smaller amounts are made in root meristematic cells and also in mature shoot meristematic cells, but there is still a small or very small amount in mature root cells under the right conditions (i.e. under conditions that would induce a shoot meristematic cell to produce it, involving good external conditions and a good level of sugar and Oxygen in the older root cells). Looking at ET, it is a Stress hormone and primarily a “shoot hormone” too. It is made when mature shoot cells are experiencing deficiencies in water and minerals, but would also be made when mature root cells are not taking in appropriate amounts of water and minerals. It would also be made in the shoot meristems when they were experiencing deficiencies of minerals and water. Perhaps the ET levels would only rise to levels that would cause hibernation of these meristematic regions, like in the secondary buds. Finally a small or very small amount would be made in meristematic tissue of roots experiencing water and mineral shortage.

11. Growth Hormones and SA gather all nutrients and perhaps also attract proximate supplies of Growth Hormones and SA while repelling Stress Hormones and ABA. This makes meristematic tissue get involved in positive feedback loops that are responsible for apical dominance, where the better the conditions for dividing cells, the more Growth Hormones and SA are made at the site. This happens in an exponential way. Eventually one meristematic tissue wins out over all the rest, and this becomes the apically dominant meristem. The others go into hibernation. Stress Hormones and ABA repel all nutrients from the site. They may also repel Growth Hormones and SA and attract Stress and ABA.

12. The Growth Hormones and SA may both need to be at high levels for cell division to take place. The Stress Hormones and ABA may all also need to be “in attendance” before cell death is “signed off on.” If this is true of cell division, the addition of SA may greatly increase the ease of raising calluses from single cells in tissue culture. Where success has been had in the past, the cell lines may have natively synthesized unusually high levels of SA.

13. The ratio of endogenously synthesized Growth Hormones to exogenously available ones will be an important determining factor in morphology. For instance, in a shoot meristem, a cell will store up enough sugars, gases (Oxygen and Carbon Dioxide), minerals (solute concentrations) and water (water pressure) until it “knows” it can reach a mature size. It will then poll its exterior environment and measure the levels of Auxin and CK. If there are high enough levels of these outside the cell, then the cell “knows” that it is a good bet that the supply of sugar, gases, minerals and water will increase. Instead of using the nutrients stored and the resulting hormones synthesized within to make one mature cell, it can risk dividing into two, and trying to create two mature cells.

14. Increasing levels of Growth Hormones directly inhibit the levels and/or transport of Stress Hormones until a threshold is reached when they directly induce it. This threshold is not tied directly to the Growth Hormone level but is a moving target based on the ratio of the level of the Growth Hormone to the level of the nutrient it is intending to increase. One of the most important reasons for Auxin’s existence and its movement down to the roots is to increase the supply of water and minerals. One of the most important reasons for CK's existence and movement up to the shoots is to increase the supply of sugar and Oxygen to the successful new cells in the roots. For example, Auxin will kick off ET production not after Auxin reaches a certain threshold amount, but after it reaches a certain ratio in comparison with the amount of water and or minerals that exist where the Auxin is, in the root or shoot. The point of Auxin is generally to increase the amount of water and minerals with new growth in the roots. If the level of Auxin gets too high in relation to the amount of water and minerals (and the gap is increasing), the plant “knows” that the growth angle is not presently working, and so by promoting the synthesis of Auxin, it tries to “kick start” the mineral and water growth by temporarily inhibiting the synthesis and transport or activating the degradation of itself. ET also has the effect of causing senescence in less efficient mature leaves, thus diminishing the need for water and minerals. The resulting sugar and Oxygen are funneled downward to induce a temporary bloom in root growth. The extra minerals and water from this leaf senescence may be sent up to where water is limited, mainly in the shoot meristematic regions that are producing the high levels of Auxin to begin with. This again is a gamble because with the stressing of any nutrients within the plant there are opportunity energy costs.

There are three behaviors that Auxin can induce in the root to increase water and mineral supplies. These are:

a. If CK, minerals, and water are high, it can induce cell division in the root meristems and thus increase the supply through new growth without any changes. b. If CK is low, but minerals and water are high, it can induce new root growth to replace the ineffective root apical meristems and restart mineral and water supply growth. c. If CK is high but minerals and water are low than this would indicate there is a problem with the functioning of the mature roots. This could be due to inefficient roots but as we will see below it could also be due to healthy roots malfunctioning because of a lack of sugar and Oxygen. Either way, the root would want to release at least some ET in order to lower the root nutrient requirements of the shoot and free some sugar and water from cannibalized, less efficient mature leaves. In the case of inefficient roots, the sugar and water would be used to support root growth through cell division. For that, it would also need Auxin. So under this condition, Auxin levels would not be totally suppressed by the new synthesis of ET. On the other hand if the low minerals and water conditions were due to sugar and Oxygen starvation of perfectly good roots, then the root would ?? ?? d. If both CK and minerals and water supplies are relatively low in the roots, this means that neither the old roots nor the meristematic roots are working. Auxin will then induce ET. This is done, as mentioned, to lower the mineral and water load, to free both sugar and Oxygen for new root growth and to free minerals and water for the shoot. This is “emergency jump start” growth.

In actuality, the first two conditions can also lead to ET emanation if sugar and Oxygen levels are low in the roots. When this is true, even if levels of CK and/or minerals and water are high, the roots will not risk cell division or new root initiation because the lack of extra sugar and Oxygen is limiting their growth. Auxin’s inducing of ET in the root is tied to its ratio to minerals and water and sugar and Oxygen.

To elaborate further, the threshold over which Auxin will induce ET is also tied to the level of GA/BA. Clearly, high levels of GA/BA are an indicator that mature root cells are starving for sugar and Oxygen. It is hard to imagine that sugar and Oxygen starved roots would be good harvesters of water and minerals from the surrounding soil. The root could monitor just GA/BA to get an idea of the sugar and Oxygen needs of the root. Perhaps the root monitors both sugar and Oxygen levels and GA/BA levels, with one being the confirmation of the other. The bottom line, however, is the sugar and Oxygen level and the root may be able to safely ignore GA/BA levels. [I can’t make sense of the following sentence.] Knowing biological systems, however, and their complexity of control, I would not be surprised if Auxin threshold level that would induce ET synthesis is tied to though they would confirm In the end, low levels of sugar and Oxygen may lower the threshold for Auxin’s induction of ET. ET inhibits the root senescence promoted by GA/BA. It’s obvious that if this is true it is because it wants to preserve its existing supply of water and minerals

Conversely, increasing levels of Stress Hormones directly inhibit the synthesis and/or transport of the Growth Hormones until again a threshold is reached. Then they encourage the synthesis and/or transport of the Growth Hormones. Again, this threshold is not absolute but is dependent on the ratio of the Stress to the nutrient the Stress are trying to increase. For example, if ET levels have been increasing for a while but minerals and water levels have also been increasing for a while and the ratio between ET and these nutrients has been closing, the plant “knows” the attempt at “jump starting” water and mineral supply has succeeded and ET is no longer needed.

16. The hormones can be seen as complementary pairs. Auxin’s complement is ET, CK's complement is GA/BA, and SA’s complement is ABA. This is important because Auxin transportation to the root can be seen in part as an attempt to increase water and minerals (even if it is natively synthesized in the root, it leads to cell division or new root growth). If the levels of Auxin rise too high, the plant abandons the attempt temporarily and switches to ET, trying to jump start production increases. Since Auxin causes cell lengthening and ET causes cell broadening, we can surmise that this kind of thing happens often and provides for balanced growth of the root. ET is perhaps a radical at least temporary change in the root’s strategy for increasing mineral and water supply. If you look back at the chart for ET, there is an entry for a well-known finding that ET induces root hair growth. Root hairs greatly increase the surface area of the root, aiding mineral and water absorption. However, this may make the plant more vulnerable to loss of water during drought. It may also make the plant more susceptible to root predation and disease, because, I hypothesize, the root hair cell is more vulnerable to all these things than the normal wall of the root. So ET represents a major change in strategy for the root. Whereas Auxin causes it to grow down and to make no special arrangements for absorption, ET may cause it to grow out laterally and to make a special of arrangement of growing root hairs. This is all to increase water and mineral levels.

Root hair growth may be a normal part of the life of a plant, or it may be a growth gamble not always taken. At any rate, I hypothesize that normally ET, GA/BA, and ABA “rule” the night, in that they are normally released during the night when the plant cannot synthesize sugar or take in as much water and minerals. It is the normal course of events. The lowest levels of Auxin, CK and SA would occur at night and the highest during the day. GA/BA and ET do cause both growth and senescence. That is, at night, when ET is high in the roots, it will be causing lateral growth of the roots, while GA/BA will be causing some senescence of older, less efficient roots. The growth in the roots at night will be balanced, because GA/BA will be lengthening the roots that they are not killing off. In the shoot, ET is doing the converse, pruning older, less efficient leaves while GA is lengthening the good young ones. The good young ones are also broadened by ET at night. I suggest that the plant does the bulk of its self-pruning at night. Also at night, the plant lives off the nutrients it has necessarily stored from the day. In fact, pruning may not be necessary if all parts remain efficient and enough nutrients have been stored from the day to allow for sustenance and even growth.

CK and GA/BA have the same relationship as Auxin and ET. An important reason for transporting CK to the shoot is to increase its sugar and Oxygen supply, either by cell division in the meristem in concert with Auxin or by the outgrowth of the secondary buds out of concert with it. If this attempt is unsuccessful CK will induce GA/BA which is a completely different strategy for the shoot. With GA/BA the stem lengthens, in an attempt to move the leaves out of a possible shade and more into the sunlight where more sugar can be synthesized. One would also guess that GA/BA would induce some kind of increase in the efficiency of the leaves, just like root hairs. Of course GA/BA does not cause induction of Chlorophyll in seedlings grown in the dark, but in perhaps in low light they might I don’t know and haven’t seen the findings. Perhaps GA/BA causes an increase in either or both photosynthesis efficiency itself or by increasing the amount of Chlorophyll in plants that are actually in the light and maybe especially in mature plants as opposed to seedlings.

17. When the plant needs to trim or prune parts of itself for reasons other than nutrient deficiencies, such as disease or pestilence, it can and does use the appropriate Stress Hormone as well as ABA. JAs are volatile and may induce a spreading area of senescence as a defense mechanism. For instance, if a leaf gets infected with a disease, the plant will want to limit the spread of the disease, so it will sacrifice cells surrounding the place of infection in order to quarantine the spread. This is done with both ET and ABA. Indeed, this is described in postulates 11 and 12. Even GA/BA will eventually be made as ET and ABA push out nutrients, including sugar and Oxygen, from the cells that are being sacrificed. This run-away effect feeds on itself until the cell dies. The spread of self-catalysis after injury or infection is halted perhaps directly by SA and then perhaps indirectly by new Auxin and CK synthesized in response to the influx of nutrients from cannibalized cells from the quarantine area. Indeed wounding, infection, or parasitism may only initially activate ABA directly, and the ET and GA/BA synthesis may be in response to repelling of nutrients that ABA activate. ABA may directly induce SA so that the cannibalization does not go too far. There have been many references in the literature to the induction of ABA and SA after wounding or disease. I hypothesize that wounding or infection may only directly activate ABA but this leads to the synthesis of SA. ET and BA/GA would be synthesized indirectly in response to the nutrient repelling action of ABA and Auxin and CK would be indirectly synthesized in response to nutrients being both attracted by SA and pushed out of the cannibalized cells by ABA, ET and GA/BA. Like most biological systems, I hypothesize, there are multiple controls and the notion here of indirect synthesis is incorrect and the plant has more control over the process. Thus, wounding or disease may start just with the release of ABA, but this induces ET and GA/BA at the site and SA, Auxin and CK at some distance from the problem area.

18. Another “raison d’être” for the growth, and possibly the Stress Hormones too, is to facilitate nutrient transport. Auxin is transported downward, and certainly attracts sugar and Oxygen to itself as it travels the phloem subway down to the roots. Incidentally Auxin also attracts minerals and water, so there may be a circulation system in the plant of minerals and water. That goes up the Xylem, but some comes back down in the phloem with Auxin. Similarly, CK is transported up the Xylem, and may take water and minerals coming up with it in the root (although the Xylem is dead wood, so I am not sure about this). Again, similarly to the above, CK might attract sugar and Oxygen, as it goes up the hollow tube to the stomata.

Predictions

1. The reasons for the existence of the “shoot” Growth Hormone Auxin include:

a. To attract all nutrients and Growth Hormones to new shoot cells that look like good investments for the plant in that they are producing high amounts of sugar and Oxygen or most probably will do so in the future. b. To induce orderly upward growth through apical dominance, so that the young, most efficient cells reach the most sunlight and the plant is “pointed” at the top and can pierce through the forest canopy if necessary. The Christmas tree growth pattern of apical dominance is also bottom heavy, adding to the balance and stability of the plant. c. To cause the movement of sugar and Oxygen down the phloem by attracting these nutrients from the leaves as Auxin is transported down the phloem. d. To induce an increase in the minerals and water supply by:

i) Causing more root cell division in concert with CK; ii) Inducing new roots in the absence of CK; or iii) Inducing a “jump start” to the water and mineral supply growth promoting ET synthesis, which:

(1) Induces root hairs (2) Cuts back on some of the less efficient leaves that are unnecessary water and mineral sinks themselves; (3) Frees sugar and Oxygen (for the starving mature roots and new root growth); and (4) Releases water and minerals back to the shoot.

2. The reasons for the existence of the “root” Growth Hormone CK similarly include:

a. To attract all nutrients and Growth Hormones to new root cells that look like good future investments to the plant, in that they are harvesting high amounts of water and minerals or most probably will do so in the future. b. To induce orderly downward growth of the root through apical dominance, so that the young, most efficient root cells reach the deepest, most humid, soil, which is expected to be rich in minerals. The deeper the root goes, the more even the temperature the less energy is wasted on heating and cooling the root cells. c. To cause water and minerals to move up the xylem, by attracting these nutrients from the roots into the transpiration stream. d. To induce an increase in the sugar and Oxygen supply by:

i) Causing more shoot cell division in concert with CK; ii) Inducing secondary bud outgrowth in the absence of Auxin; or iii) Inducing a “jump start” in the sugar and Oxygen supply growth by promoting GA/BA synthesis, which:

(1) Induces improvements in the efficiency of photosynthesis; (2) Cuts back on some of the less efficient roots that are unnecessary sugar and Oxygen sinks; (3) Frees sugar and Oxygen for the roots; and (4) Releases some water and minerals back to the shoot for:

(a) Shoot meristem cell division; (b) Secondary bud outgrowth; and (c) Increased activity of mature shoot cells.

3. The reasons for the existence of the Stress" shoot” Hormone ET include:

a. The induction of hibernation of secondary buds under water and mineral shortage produced by Auxin and apical dominance. Because they are juvenile cells, ET may not be synthesized in large amounts there normally. Perhaps only enough ET is made to induce hibernation. As above, ABA is probably involved in slowing down the metabolism to a hibernating state. Low levels of ABA may be able to effect the hibernation alone. In fact, they may prevent senescence-inducing effects of ET! b. To induce an increase in water and minerals by:

i) Promoting the use of stored minerals and water; ii) Inducing root hairs; iii) Cutting back on some of the less efficient leaves that are unnecessary water and mineral sinks; iv) Freeing sugar and Oxygen (for improved mature root performance and new root growth); v) Releasing some water and minerals back to the shoot; vi) Inducing Auxin synthesis after a successful jump start of water and mineral level growth.

c. To prune wounded and diseased tissue from the shoot, possibly in concert with ABA.

4. The reasons for the existence of the Stress "root” Hormone GA/BA include:

a. The induction of hibernation of secondary root buds under sugar and Oxygen shortage produced by CK and root apical dominance. Because they are juvenile cells, perhaps GA/BA is not usually synthesized in large amounts there. Perhaps only enough GA/BA is made to induce hibernation. As above, ABA are probably involved in slowing down the metabolism to a hibernating state. Low levels of ABA may be able to effect the hibernation alone, and in fact may prevent the senescence-inducing effects of GA/BA. b. To induce an increase in sugar and Oxygen by:

i) Promoting the use of stored sugar and Oxygen; ii) Inducing an increase in the efficiency of photosynthesis; iii) Cutting back on some of the less efficient roots that are unnecessary sugar and Oxygen sinks; iv) Freeing sugar and Oxygen (for improved mature root performance and new root growth); v) Releasing some water and minerals back to the shoot for improved mature shoot performance and new shoot growth; and vi) Inducing CK synthesis after a successful jump start of sugar and mineral level growth.

c. To prune wounded and diseased tissue from the root, possibly in concert with ABA

5. The reasons for the existence of the “survival” hormones ABA include:

a. To slow down of metabolism of nutrient stressed cells so that their nutrient needs fall below those provided by the environment (they lower the “sustenance level” for the cell); b. To induce senescence of nutrient stressed cells whose nutrient supply falls below the lowest possible sustenance level; c. To slow down the metabolism of injured or diseased cells or tissue in order to limit damage; d. To induce senescence of injured or diseased cells or tissue in order to limit the spread of the disease or of autocatalysis of tissue induced by ET.

6. The reasons for the existence of the “survival” hormone SA include:

a. To speed up the metabolism of healthy cells, so that all available nutrients are used up to the point of one limiting nutrient, to provide the most efficient growth or cell functioning; b. To induce the division (in concert with Auxin and CK) of cells that have more than enough nutrients to support normal functioning and reach mature size; c. To speed up the metabolism of healthy efficient cells near the site of injury or disease, to preserve them from autocatalysis of tissue induced by ET; d. Possibly to re-induce cell division in tissue near cells that have senesced and abscised, in order to replace these cells.

7. In general, the following hormones inhibit senescence of the shoot and leaves: Auxin, CK, GA/BA and SA. ABA and ET are usually the only hormones that will inhibit shoot growth or induce shoot tissue senescence. In general, the following hormones inhibit the senescence of the root core and peripheral roots: Auxin, CK. ET and SA. ABA and GA/BA are normally the only hormones that inhibit root growth or initiate root senescence.

8. Growth Hormones and SA are at their highest levels during the day when there are the most opportunities for nutrient synthesis and harvesting. Stress and ABA reach their highest levels at night, when the plant must rely on nutrient reserves built up at night or prune inefficient tissue and use the resources to sustain itself through the period when little nutrient procurement is possible. The plant can also slow down its metabolism at night and thus need fewer nutrients at night. This last strategy may be induced by ABA alone but is more likely to be induced by ET and GA/BA as well. The slower metabolism is also aided by the colder night temperatures.

9. Stress and ABA hormones are at their highest level at the beginning and end of the life of the plant or growing season, when a plant must rely on stored nutrients from senesced tissue and must be content with a slower rate of metabolism because of lower temperatures. Growth and SA hormones are generally at their highest level during the middle of the growth life of the plant or season.

10. Growth Hormones tend to move resources toward the edges of the plant to facilitate the procurement of more nutrients from the environment. This means they move resources to the top and bottom of the plant in the apical meristems but also perhaps move them laterally, to where the shoot and roots branch out like river deltas or capillary beds. Also, we can expect to find the highest concentration of the Growth Hormones at these edges of the plant, where the youngest cells reside. On the other hand, Stress Hormones tend to move resources inward, toward the stem and root core and closer to the soil line on both the root and shoot sides of the plant. The Stress Hormones engage the plant in a “hunkering down,” conservative, “smaller but stronger” posture. Also, we should expect to find the highest amounts of Stress Hormones near the soil line and in the shoot and root core.

11. It might seem that apical dominance could run away in growth like a cancer, but it doesn’t for seven reasons. These will be explained for Auxin and shoot apical meristems but a similar analysis could be done for CK and root apical dominance.

a. Auxin is transported down the stem through the phloem away from the shoot apex. b. Auxin does induce xylem, so the shoot apical meristem hard wires a supply of water and minerals to itself. However, it also induces phloem, which takes the sugar and Oxygen. away from the cells as they change to the mature cell morphology, so the multiplicative positive feedback effect of Auxin synthesis trails off as the meristem cells mature in the shoot. c. Once a leaf has made xylem to itself as above, it only has to produce a small amount of Auxin to protect itself from senescence being initiated. The xylem guarantees that the leaf’s water and mineral supply cannot be hijacked by nearby meristematic tissue. d. At any rate, as leaves increase in maturity and the plant grows, the older leaves move further and further away from the nutrient attractive forces of the Meristem (the leaves stay at the same level above the ground, and the meristem moves higher and higher off of it). e. Relatively speaking also, the apical dominance may get weaker over time. As the older mature cells are closer to the source of the minerals and water, they are closer to the root and have “first dibs” on the supply of these nutrients. Meanwhile the apical meristem is moving further and further away from the source, the root. f. If the apical meristem makes too much Auxin, this will induce ET both directly and indirectly. ET in turn inhibits the transport of Auxin and may inhibit its synthesis, thereby dampening the process. Auxin induces ET indirectly (and perhaps GA/BA and ABA) because the meristem’s draining of nutrients from surrounding tissue promotes the synthesis of these hormones. In addition to ET, we can expect at least ABA to inhibit Auxin synthesis. g. There is an equally potent attractor of nutrients at the other end of the plant – the root apical meristem. These two “superpowers” probably work out a balance – an agreement to split the resources down the middle so to speak.

12. A phenomenon called Epinasty (leaf stems – called petioles – lengthen and the leaf droops down and curls in on itself) is induced by ET. When ET is released it causes the senescence and abscission of some of the leaves. Since the leaves are the primary organs for harvesting Oxygen, with the roots harvesting only some of the Oxygen needed, the plant may be left with a rapidly developing partial anoxia. I hypothesize that the supply of Oxygen becomes a growth and metabolism limiting factor more quickly than the supply of sugar. Thus, Epinasty is the plant’s choice as the lesser of two evils. The plant loses some photosynthesis efficiency because the leaves are no longer parallel to the ground with their face to the sun, but they may be able to trap more Oxygen within their curled undersides. Alternatively or additionally, the Epinastic state may act like a sail, increasing the plant’s ability to trap Oxygen and blowing up like a spinnaker. This may be bad physics, in that more gas diffusion occurs when the leaf surface is parallel to the direction of the wind. The leaves may also be trapping Carbon Dioxide, but this is unlikely since they only need to harvest enough Carbon Dioxide for their own photosynthesis needs. They do not support other cells with their supply of the gas. Thus, the leaves left after a bout of ET senescence need not configure themselves in any special way in order to get the Carbon Dioxide they need.

13. In addition to the three behaviors high levels of Auxin can engender in the roots, there may be a fourth behavior. That is, the plant may try for quite a while to expand its minerals and water level, but if cell division, new root growth, and “jump starting” repeatedly prove unsuccessful, the plant may “decide” to store the extra sugar and Oxygen until a more opportune time comes for the root. How the plant would measure repeated lack of success here, I don’t know.

14. Finally, there is one other mysterious hormone effect that I would like to explain. It is again induced by ET. It is the phenomenon of the induction of adventitious roots (roots growing out of the shoot above the soil line and back into the ground) by the hormone under flooding conditions. I argue that this is because roots need to procure some Oxygen from the soil, and flooding prevents this. With the roots starting out above the flooded soil in the atmosphere, they can absorb the Oxygen needed and then dip back down into the soil to get the minerals and maybe even the water.

Theory II

1. Auxin and Cytokinin are released any time nutrients go above survivable levels. Auxin is stimulated by above survivable levels of sugar and essential gases. Cytokinin is stimulated by above survivable levels of minerals and water. Auxin and Cytokinin being signals of nutrients levels, stimulates their use by increasing the speed of metabolism or inducing growth and cell division.

2. GA and Ethylene are released when nutrients fall below that which is needed for peak metabolism. GA is released when sugar and essential gases fall below the level needed for peak metabolism. Ethylene is released when minerals and water fall below this peak level. GA and Ethylene then do their best to attempt to address the shortfall by decreasing the speed of metabolism, inducing cell dormancy and inducing senescence.

3. As is well accepted by most plant physiologists, in this Second Theory ABA takes on the role of a “Shock” Hormone. That is it would be induced by rapidly developing threats to plant survival and would put them in a defensive and dormant mode making them more impervious to the threat. As is beginning to be accepted Salicylic Acid here would be kind of the “All Clear” Hormone returning the cell to normal functioning after a serious threat of some kind.

4. Because Auxin is an indication that the levels of sugar and gases are above survivable levels, it increases that part of metabolism that uses sugar and gases. This partial increase in metabolism will cause an increase in the need for the complimenting water and mineral nutrients in order to round out an increase in metabolism. If water and minerals are not in adequate supply they will quickly decrease and this will cause an increase in the amount of Ethylene. Ethylene puts brakes on that part of a cell metabolism involved with water and minerals. Thus an artificial stimulation of that part of metabolism dependent on sugar and gases by adding Auxin to a plant, will lead to an emanation of Ethylene. This is especially so in the root where that part of metabolism dependent on sugar and gases is less likely to be operating at peak levels. Of course this argument is somewhat specious because that part of metabolism connected with water and minerals should be already operating at full throttle in the root. The attempt of course here is to describe why scientists have found that Auxin leads to Ethylene emanation more quickly in the roots than the shoots. Perhaps I have the details wrong here, but maybe someone using the concepts introduced here, could tease out a satisfactory explanation.

5. Because Cytokinin is an indication of at least survivable levels of water and minerals, it increases the speed of metabolism of that part of metabolism that depends on water and metabolism. This naturally leads to an increase need for the complimentary nutrients, sugar and gases. Thus the emanation of Gibberellin will occur if the supplies of sugar and gases are inadequate. I believe GA puts brakes on that part of metabolism that uses sugar and gases. The artificial raising of the levels of CK will induce GA especially in the shoot. There is a problem here too though because as explained before, that part of metabolism depending on sugar and gases should be at full throttle in the shoot.

6. Whenever nutrient levels fall between peak metabolism and survivable nutrient levels, we can expect both Stress and Growth Hormones to be synthesized. Since shoots are the organs that make sugar and harvest essential gases, we can expect that levels rarely fall below peak metabolism nutrient levels. Thus we can expect that Gibberellin is rarely synthesized in the shoot. I know there is supposedly experimental evidence that this is untrue, but perhaps it should be looked at again more closely. This is not to say that GA is not found in the shoot. From the definition of a hormone we can expect that the effect of a hormone occurs at a distant location from its synthesis. I would expect GA to be mostly made in the root where sugar and essential gases may often fall below peak metabolism levels. Synthesis may occur in the roots, but the effects GA has are in the shoot as well as the root. As in the first version of theory, I believe all of GA’s effects are meant to address shortfalls in sugar and essential gases. This would include root growth inhibition and senescence, shoot lengthening, shoot preservation from senescence, and the changing of photosynthesis from C3 to C4 (which I believe is more efficient but builds up toxins in the leaves).

Thus on the converse, we can expect Ethylene to be rarely synthesized in the root, where levels of minerals and water seldom fall below those needed for peak metabolism. This is not to say that similar to GA and the shoot, that Ethylene does not have a big effect on roots. Again just like in the first theory, I believe everything Ethylene does is to address nonideal levels of minerals and water. Thus Ethylene inhibits shoot and leaf growth, induces leaf senescence, initiates the somewhat risky growth of root hairs, preserves roots from senescence and causes roots to branch out and to make new lateral growth in the soil.

This is at least what would be true during the day. Some kind of converse or complimentary situation would exist at night when few nutrients are synthesized or harvested. So for instance GA and Ethylene would be at high levels at night in both the root and shoot with Auxin at some level in the shoot and rarely found in the root at night. Conversely Cytokinin would be synthesized in the root at night but rarely in the shoot or only in the beginning of the period trailing off as the night progresses.

7. In this scheme, it is fairly clear what would be a signal for growth. It would be the existence of fairly high levels of Growth Hormones (Auxin and Cytokinin), and the lack of existence of the Stress Hormones (GA and Ethylene). Conversely the signal for the presence of senescence conditions would be the presence of Stress Hormones and the absence of Growth Hormones. This would signal the lack of survivability of cells in that local tissue.

8. If there is a linear relationship between the levels of sugar and gases and Auxin we should expect to be roughly twice the level of the hormone in the shoot as the root. This is because the shoot is making sugar and harvesting gases not just for itself but also for the root. It makes or harvests two parts, keeps one for itself and sends the other to the root. The same would be true if there is a linear relationship between minerals and water and Cytokinin. Experimental evidence suggests that a linear relationship does not exist but more likely a logarithmic scale of some sort. The idea would be that the amount of Auxin in the shoot would be some power of say 2 of the amount that is found in the root. Let’s say it is the square. Thus there would be 4 times the amount of Auxin found in the shoot as in the root. Thus Auxin can be seen as primarily a shoot hormone. As mentioned in Theory I it has been found that Auxin is found to be synthesized in much greater amounts in young leaves than mature leaves. As in the previous theory I believe the relative levels of synthesis of all hormones holds for this one too. Thus Auxin is made the most in young leaves and the least in mature roots. Cytokinin would be made the most in young roots and the least in mature leaves. GA would be the synthesized the most in mature roots and the least in young leaves. Ethylene would be synthesized the most in mature leaves and the least in young roots. I also believe the relationship holds of Growth Hormones attracting all nutrients and hormones to itself excepting Ethylene and GA and the Stress Hormones pushing out all nutrients and hormones excepting Ethylene and GA.

9. In this scheme we can also postulate an explanation for cell dormancy like the dormancy that exists in secondary buds. That would be perhaps at low levels GA and Ethylene in addition to the direct causation of dormancy, would attract all nutrients instead of pushing them out. On the other side, perhaps at low levels Auxin and Cytokinin would push nutrients out of a cell. Thus a dynamic yet stable equilibrium would exist at very low levels of nutrients. The movement into real metabolism or back toward senescence would need amounts or deficiencies in nutrients beyond which caused the synthesis of GA and Ethylene acting in their nutrient attraction mode, and Auxin and Cytokinin acting in their nutrient pushing mode. Perhaps another similar equilibrium exists at the peak metabolism point.

Qualifications, Contact Information and Guestbook

My name is Paul Pruitt. I received a BA from Swarthmore College in 1984, where I studied under Dr. 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 s.socrtwo@verizon.com or sign my guestbook. If you would like to see what others have written, click here to view my guestbook.

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