Plantevolution.info

Plant hormones or plant growth regulators are the signal molecules responsible for directing the growth of plants. The function of the different hormones are diverse and have been described in detail. The unravelling of the signalling pathways is ongoing. The same accounts for the biosynthetic pathways. Every year the amount of knowledge on plant hormones is growing. There is however one question which was not addressed until now. Why did these specific molecules became plant hormones? In this article an evolution theory will be presented that gives a possible answer to this question. According to the theory, the origin of the plant hormones is linked to the metabolic changes that occur just before or during the process for which the hormone signals. As many metabolic changes occur simultaneously, it is clear that a selection process has taken place selecting the most suitable molecule. In the theory presented, this selection process is linked to the energy loss or energy use of different metabolic processes. In the case of growth promoting signal molecules (auxin, cytokinin, gibberellins and brassenosteriods), a derivative of or a side product of the compound or building stone which acquires the most energy to build is the most suitable signal molecule. To minimize energy loss and optimize the energy efficiency of cells, these building stones will only be produced abundantly when the energy status of the cell is high. The same rule of energy efficiency as a selecting procedure applies also for the stress signal molecules (ethylene, absisic acid, salicylic acid and jasmonic acid). However in this case, it is not only the energy cost for the production of a building stone that can serve as a selecting procedure. The importance of the role a molecule plays in the stress response holds the key. For both the growth promoting hormones and the stress hormones it can be assumed that these molecules became a hormone because they were the most suitable metabolic marker molecules.

The evolution of a one cellular organism to a multiple cellular organism assumes coordinated growth. Coordinated growth is only possible when signal molecules are available that can be exchanged between the cells. Not only signal molecules are a prerequisite, but also the enzymes that produce the signal molecules and the receptors that respond to these molecules have to be present before a signal can turn into a response. It is however unlikely that all this appeared at the same time during the course of evolution. In order for the receptor to function, the signal molecule had to be present before the receptors. And in order for a signal molecule to occur, the enzymes that help to synthesis these molecules have to be there. In the beginning of the evolution of the multicellular organism, the cells however had no reason to produce these molecules and certainly had no reason to produce specific enzymes for the production of a molecule that still had to receive a function. Therefore it can be assumed that in the beginning the “to be signal” molecules were the products of a-specific conversion by enzymes of other metabolic pathways. The by-products of these metabolic pathways were later recognized by new receptors as being the perfect molecules to represent the status of the multicellular organism. The candidate signal molecule do not only had to represent the status of the cell, but also had to fulfill other requirements. The molecule had to be able to pass the membrane and cell wall without specific transporters as non were available at the beginning of the evolution of signal molecules. Furthermore the stability of the candidate signal molecules was very important. Highly unstable molecules will react before transported to another cell. If a molecule is too stable, the signal will last longer than the status it represents. Beside all this, the molecule also had to had unique structural properties that enables a receptor to recognize this specific molecule. The above criteria all had to be fulfilled before a molecule could become a signal molecule reflecting the growth and stress status of cells. The evolution and selection of the different types of growth and stress signals are looked upon in the following paragraphs.

Growth in a unicellular organism is determined by the ability to produce enough energy and building stones to allow cell enlargement. Proteins play the central role in this process. Besides being essential components of the cell structure, they are also necessary for the production of all the other building stones of the cell and the energy production. The ability to produce enough proteins perfectly indicates the growth status of the cell. Proteins itself will only be produced in sufficient amounts if the building stones of the proteins, namely the amino acid, can be produced in sufficient amounts. The synthesis of these amino acids is highly regulated, especially the synthesis of these amino acids which require a large energy input during the production. An overproduction of amino acids equals energy loss for the cell. It can be assumed that during the course of evolution, natural selection has selected these cells which minimize energy loss. Natural selection not only favoured a highly regulated amino acid production, even the amino acid composition of proteins is affected. Amino acids with high energy building costs are found more sparsely and seem to be present only when their special characteristics are necessary for the function of the protein. Natural selection selects the solutions which require the smallest amount of energy and/or minimize energy loss. The selection of a molecule that indicates the growth status of the cell and which can function as a growth signal for other cells will be based on the same principal. However in this case the molecule cannot be the molecule that requires the least energy input because the energy input of the molecule must also reflect the high energy status of the cell. The candidate molecule therefore may only be produced abundantly when the energy status of the cell is high and when the cell is growing. A lot of proteins fulfill this requirement, however due to their size, proteins or derivatives of proteins are no candidates for becoming a growth signal molecule. Other candidates are the derivatives of the building blocks of proteins, namely the amino acids. Because the synthesis of these amino acids is highly regulated, they will only be produced abundantly when the cell has enough energy to grow. Based on the presumptions made above, the correlation between the growth status and the production of an amino acid will be the greatest with the amino acid that requires the most energy to be build, namely tryptophan. Tryptophan, being a building block of proteins, cannot be a signal molecule itself. Derivatives of tryptophan do not have this restriction. Indol acetic acid is derived from tryptophan and functions as a plant hormone regulating the growth and enlargement of cells in plants.

In multicellular organisms cell growth occurs coordinated. Otherwise irregular structures will be formed. In order to coordinate growth, cells have to be able to signal their growth status to other neighboring cells and likewise have to be able to sense the growth status of the other cells. In growth, a distinction can be made between cell enlargement and cell division. As mentioned above, cell enlargement is reflected by the production of the tryptophane derivative indole acetic acid, which is the ideal metabolic marker reflecting the growth status and/or the capability for a cell to grow. Cell division can only occur when abundant energy is available. As the energy status is reflected by the amount of indole acetic acid, it is logical to assume that cell division can only occur when the level of indole acetic acid is high enough. Besides energy, also the nutritional demands have to be fulfilled. Only then the cell will start duplicating its DNA. Cell division requires the cells to produce large quantities of nucleic acids. A rise in the concentration of the nucleic acids reflects that the cell is going to divide. Neighbouring cells have to be able to sense this. A rice in the concentration of free nucleic acids excreted by the dividing cells would be the perfect signal for signalling division if they were not a common building stone present in every cell. Derivatives of nucleic acids do not have this restriction and could therefore better fulfil this role. The question remains, which derivative of which nucleic acid is the most suited for presenting the dividing status of the cell. Because the production of nucleic acids requires a lot of energy and the cell uses its energy in an economical way avoiding energy loss and energy overconsumption, it can be presumed that the dividing status is presented the best by a derivative of the nucleic acid that requires the most energy to be build. This conclusion is reached by following the same logical selection procedure for signal molecules explained above for the signal representing cell enlargement. The nucleic acid with the highest building costs is adenine. Zeatine, a derivative of adenine, is the signal molecule in plants signalling and promoting cell division.

Basic growth can be signalled by the metabolic markers indole acetic acid and zeatine. However, a growth solely based on these two signals is insufficient because they only give a rough estimate of the status of the cell. Auxins only represent a status of high metabolic building activity and cytokinins represent a status of cell divisions. A higher status, for example signalling the presence of a seed to the surrounding tissue to induce the growth of fruit, requires a more advanced status marker. Furthermore, growth cannot only be based on metabolic building activity and status of division because both processes fluctuate. Weather conditions can change radically and photosynthesis, which is the main driving force for metabolic activity, fluctuates during the day and between days. Fluctuating signals are not the best signals for regulating growth. They can result in promoting growth of extra cell tissue which cannot be supported later on when photosynthetic activity is low. Auxins and cytokinins, two signal molecules derived from fluctuating processes, can only account and signal for a limited growth. This does not endanger the plant at later time points when unfavourable weather conditions occur. In some cases however extra growth is possible and an extra signal molecule is necessary, reflecting the extra growth potential of the cells. When applying the same logic which was used for auxin and cytokinin, the signal molecule in this case must be a derivative of the most expensive molecule/building block of the cell. The two most expensive basic building blocks of the cell are cholesterol and chlorophyll. Brassenosteriods are derivatives of cholesterol and therefore ideal candidates for signaling extra growth potential. The history of gibberellins is less clear, although there is a relation with the synthesis of chlorophyll, the other most expensive building block of plant cells. Chlorophyll itself is far from an ideal candidate to become the precursor of a signal molecule. Chlorophyll derivatives cannot fulfil the requirements of a signal molecule stated in the beginning of the article. It is highly unlikely that chlorophyll derivatives can pass the membrane without a specific transporter. Also, the presence of enzymes able to catalyse drastic changes in the structural properties of chlorophyll to make it a more diffusible molecule is not evident. Chlorophyll itself is build from several other basic building molecules/blocks. The most expensive building block of chlorophyll is geranylgeranyl pyrophosphate. Gibberellins are derived from geranylgeranyl pyrophosphate. Geranylgeranyl pyrophosphate is the precursor of many diterpenoid compounds, including quinines, the phytol side chain of chlorophyll, and tetraterpenoids, including carotenoids. The synthesis of geranylgeranyl pyrophosphate is the last chemical reaction that these different groups of compounds share. Many of the molecules derived from geranylgeranyl pyrophosphate play critical roles in photosynthesis. All this ma kes geranylgeranyl pyrophosphate an ideal candidate for a precursor of a signal molecule. Most of the time the concentration of geranylgeranyl pyrophosphate will be relatively low because different biosynthetic enzymes compete for geranylgeranyl pyrophosphate. Only when the requirements for extra photosynthetic apparatus rises and the cell possesses enough energy to fulfill these needs, the concentration of geranylgeranyl pyrophosphate will rise. The need for extra photosynthetic apparatus will occur when the light intensity is high. Building the extra photosynthetic apparatus will require the presence of nutrients and an already present optimal functioning photosynthetic apparatus that can deliver the energy needed. If this is true, geranylgeranyl pyrophosphate will only rise under optimal conditions for the plant. The same accounts for gibberellins which are derived from geranylgeranyl pyrophosphate. A rise in geranylgeranyl pyrophosphate reflects a status of energy and nutrient abundance, and an optimal functioning cell apparatus. An advanced status is however more than that. A rice in geranylgeranyl pyrophosphate does not give detailed information on the presence and/or activity of a particular process in the cell. A molecule signalling an advanced status has to be a “check list” molecule. A “check list” molecule is a molecule that can only be produced if the advanced status has been reached. The “check list” molecule must be the result of a process that checked if all the requirements of the advanced status were fulfilled. The basic rules phrased at the beginning however stated that no specific hormone synthese enzymes can be present in a cell before the signal molecule. This leads to the logical conclusion that for the production of the check list molecule, one or more of the enzymes only present in the case of the advanced status are used. Only in this way the check list molecule can represent and signal the presence of the advanced status. The molecules signalling an advanced status in a plant, allowing/inducing extra growth, are the gibberellins. The production of gibberellins requires the presence of numerous enzymes. Logical deduction leads to the conclusion that the enzymes needed for the production of gibberellins are enzymes that are also used for metabolic processes linked to the advanced status for which the gibberellins signal. Gibberellins are the “check list” molecules of the plant. Different gibberellins represent different check lists. This does not mean that one specific gibberellin only represents one specific advanced status. It is possible that different advanced statuses lead to the production of the same gibberellin in different plant tissues or during different physiological ages of the plant. The production of gibberellins can even be plant species specific. During the course of evolution, new advanced statuses had to be signalled in the plant. Every time a selection process took place during which a gibberellin was selected. This gibberellin had to represented the status in the best way and may not be in conflict with an already coupled function to that specific gibberellin.

Until now only the positive markers for plant growth were discussed. The positive markers are linked with light and nutrition. A plant however also needs negative markers signalling unfavourable growth conditions. As plant growth can experience different types of stresses that constrain the growth, it will need more than one negative growth marker. The most stringent growth restrain is experienced during drought. Plant growth is highly dependent on the availability of water. Water is needed for the transport of nutrients. A limited water supply will therefore also limit the availability of nutrients and concordantly also the growth. Furthermore the growth will be inhibited because the plant has to adept itself to the lack of water. The plant cells posses several mechanisms to overcome the negative effects of the lack of water. Plants tend to cope with water deficit stress by a process known as osmotic adjustment. In this process, plants decrease their cellular osmotic potential by the accumulation of solutes. The first response in case of water loss will be an increase in the free sugar content of the cells. By doing so, the plant increases the osmotic potential of the plant cell and this without interfering the activity of the enzymes present in the cells. The sugars do not only preserve the osmotic equilibrium, but also serve as osmoprotectants of proteins. The negative effects upon the growth are limited at this stage. There is no need for the plant cells to signal to other plant cell to alter their growing behaviour. This changes when drought stress increases. At that time point a second defence mechanism will become more active. The production of specific osmolytes is enhanced and an increase in polyols, methylated proteins, methylated amino acids and betaine is observed. The question now is: which of these compounds would be the best precursor for the signal molecule for osmotic stress? Polyols, derivatives of sugars, are no candidates for signalling drought stress as they are easily metabolised by the neighbouring cells. Proteins are no candidates due to their size. The remaining candidates for osmotic stress signalling are betaine and the methylated amino acids. Other candidates that can not be ruled out are the precursors of these molecule because also the concentration of these molecules will rise in the case of drought stress. When looking at the synthesis of both betaine and methylated amino acids and proteins, one candidate stands out: S-adenosyl methionine. Methylation of amino acid and proteins requires the methyl donor S-adenosyl methionine. Also the synthesis of betaine requires S-adenosyl methionine. Choline, the precursor of betaine, is the most methylated compound that can be found in a plant cell. Three molecules of S-adenosyl methionine are required to produce choline from serine. Osmotic stress leads to a major burst of methylation of a magnitude that only occurs then. This makes S-adenosyl a good candidate for a precursor of the osmotic stress signal molecule. Ethylene, a plant hormone signalling stress in plants, is produced from S-adenosyl methionine. Ethylene is linked with situations of extreme stress in which plant cells experience problems with sustaining their activity. As soon as the vascular tissue does not function optimal and/or the vascular tissue can not fulfil the needs of a plant organ, an increase in osmotic stress will occur. Ethylene signals the plant that drastic measures must be undertaken, such as the abscission of leaves. The action upon the hormone ethylene results on the long term in the restoration of the osmotic equilibrium in the plant. The fact that a volatile molecule became a plant hormone for osmotic stress probably is linked with the properties of the molecule and the status of the plant. A more volatile molecule will move quicker and this is especially important in a plant where sap flow ceased due to drought stress.

The opening and closing of stomata were not discussed in the paragraphs above. Stomata are only found on higher plants and the movement of stomata do not act upon drought stress, but close before the plant experience drought stress. Therefore the opening of stomata is regulated in a different way. Also the role of ethylene in fruit ripening was not discussed. The role of ethylene in ripening is only partly linked to stress. The production of ethylene for ripening fruits probably evolved from the ethylene production during stress. During the course of evolution, the ethylene production capacity and the response of tissue upon exposure of ethylene was selected to govern the ripening of the fruit so that animals could be attracted that feed the fruits and disperse the seeds in the fruits.

Plants need sunlight to grow. To much sunlight however can damage the plant. High UV irradiation can cause oxidative stress in the plant. Oxidative stress can also be induced by toxic substances, for example heavy metals. During the course of evolution plants obtained several mechanism to concur oxidative stress. These mechanism capture and/or convert the oxygen radicals. Because oxidative stress can be live threatening, it is important that plants posses a plant hormone that signals an increase of oxidative stress. Elevated oxidative stress levels enhance the production of antioxidants in a cell. The rice in antioxidants would be the ideal signal for oxidative stress. However, antioxidants themselves can not serve as a signal molecule because they would not be recognized as such by neighbouring cells. Similar to auxin and cytokinin, the better potential candidates are the derivatives of the antioxidants. Because we are dealing with oxidative radicals reacting with the antioxidants, the most likely candidate derivatives are oxidative breakdown products of the antioxidants. The next question to answer is: the derivative of which antioxidant is the best representative of an increase in oxidative stress? There are numerous antioxidants, however the most important are ascorbic acid and glutathione. Also the carotenoids can not be ruled out as they protect the photosynthetic apparatus of the plant cell. Ascorbic acids and derivatives of ascorbic acid are easily metabolised in the neighbouring cells and are therefore not suited as a signal molecule. This does not account for glutathione and carotenoids. When comparing the potential of glutathione and carotenoids, the carotenoids take the upper hand. The level of oxidative stress a plant experiences varies during the day and depends largely on the level of sunlight radiation. Oxidative stress specifically rises in the neighbourhood of the photosynthetic apparatus. It is there that the carotenoids are present. Carotenoids in the chloroplast help capturing the sunlight and also protect the photosynthetic apparatus from oxidative stress. The protection is achieved by the xanthophyll cycle. Excess light energy is dissipated through the xanthophyll cycle, with the formation of zeaxantin from violaxanthin. A rice in oxidative stress only occurs when the level of irradiation exceeds the xanthophyll cycle protection potential. Oxidative stress due to irradiation is the highest in chloroplast which makes the oxidative breakdown products of carotenoids the best candidate for signalling oxidative stress. The building cost of carotenoids is the second reason why carotenoids are better candidates then glutathione. The loss of an expensive component which is not recycled that easily as in the case of glutathione, gives a better idea of the severity of the oxidative stress a plant cell experiences. Evolution as a driving force of the building plan of the cell favours energy efficient architecture and operation. The loss of high energy cost building blocks must be avoided. The fittest plant will be the plant that is able to react to the loss of expensive building blocks and avoid further losses of those building blocks. The fittest plant is also the plant that only switches its extra defence mechanisms when necessary. Although derivatives of glutathione would give a good estimate of the overall oxidative stress in the cell, they would not estimate the effect of the oxidative stress on photosynthetic apparatus as good as the derivatives of carotenoids. A regulation of oxidative defence based on a signal coming from the breakdown of carotenoids is therefore likely to be more effective and efficient. In plants, a signal molecule is found which is derived from the oxidative breakdown of carotenoids, namely absisic acid. Absisic acid is produced from the oxidative breakdown of violaxanthin and neoxanthin. Being a carotenoid and playing a role in the xanthophyll cycle made violaxanthin the ideal candidate precursor for the oxidative stress signal. Absisic acid however is almost never linked to oxidative stress by scientists. Absisic acids best known function is to signal drought stress in plants. Water shortage induces absisic acid synthesis in roots, which is transported to the upper part of the plant where it induces stomatal closure. Water shortage also induces root elongation. Root elongation requires cell wall loosening to enable cell expansion. Cell wall loosening in roots is achieved by enzymes and by the production of hydrogen peroxide and superoxide. These extremely reactive molecules attack cell wall polysaccharides, breaking the bearing structure of the cell wall. The oxidative burst starts at the onset of elongation. The reactive oxygen species do not only react with the polysaccharides but also other cell components. It is only logical to assume that the level of the signal molecule for oxidative stress will also rise in the roots, starting at the onset of elongation. This links the production of absisic acid in the roots with water shortage. For a plant to react to an increase of absisic acid in the roots has evolutionary advantages. A plant which is able to react to water shortage before an osmotic imbalance occurs is fitter then the plant that only reacts when the osmotic imbalance occurred. The reactions of a plant to the osmotic stress signalling molecule ethylene illustrate this assumption.

A plant subjected to extreme weather conditions, for example hail and high wind velocity, or to grazing by animals, for example cows or caterpillars, will experience mechanical stress. The same accounts for pathogenic micro-organisms that breakdown the cell wall. Wounding a plant causes mechanical stress in the form of structural instability. The strength of the plant is determined by the thickness and composition of the cell walls and the architecture of the plant tissue. Wounds inflict structural instability in the surrounding tissue. The stability is reinstated by reinforcing the cell walls in the surrounding tissue. Sometimes wound tissue is formed, often cork tissue. The cell wall consist out of cellulose, hemicellulose, pectin and lignine. Cellulose and hemicellulose form a network which is embedded in the pectin matrix. Reinforcement of the cell wall comprises a thicker cell wall and more cross links in the network. Furthermore extra lignin is produced that penetrates the spaces in the network, driving out water and strengthening the cell wall. During the course of evolution, natural selection favoured those plants that could signal mechanical stress and react upon that signal avoiding further damage. The molecule that became the signal molecule for mechanical stress had to be a molecule that is produced more abundantly after a mechanical injury. The injury itself does not lead to an increase of a certain molecule. However, during the reaction after the injury of the plant cells, the concentration of different molecules rises, especially the molecules that are produced to fix the injury, namely cellulose, hemicellose, pectin and lignine. Non of these molecule poses the characteristics for becoming a signal molecule as they are all to large, do not diffuse true the cell membrane and cell wall, and will not be recognized as a signal molecules by other cells. Derivatives of these molecule could have unique characteristics so that they potentially could be recognized by other cells as a signal. However they are still to large and it is highly unlikely that they can diffuse true the membrane without a specific transporter. Other candidates for becoming the precursor of the signal molecule are the precursors of cellulose, hemicellulose, pectin and lignin. The precursors of cellullose, hemicellulose and pectin are sugars. Sugars and their derivatives will never be signal molecules. Lignin is build by alanine. The lignin production of a plant is highly regulated because building lignin costs a lot of energy. This is reflected in the pectine composition of plant cell walls of different plants. Cell walls wil only contain lignin or beter stated just enough lignin to ensure rigidity of the cell wall. The large increase of lignine production after cell wall injury is there fore exceptional. The increase of lignin will inflict also a large increase of alanine. A derivative of alanine would be an ideal signal molecule for signalling mechanical stress to the neighbouring cells. In the modern plant salicilyc acid is linked with signalling mechanical stress. Salicilyc acid is produced from alanine. From this it can be concluded that salicilyc acid is the result of a rice of alanine that is produced for the production of lignine to repair the mechanical injury.

The last type of stress discussed in this theory is membrane leakage. Membrane leakage can be inflicted by chemicals that react or interact with the membrane. The same accounts for pathogens that produce toxins that disturb the membrane permeability. Also oxidative stress can lead to membrane leakage when oxygen radicals react with the membrane. A cell reacts upon membrane leakage by adjusting the permeability of the membrane. The cells will increase the fluidity of the membrane. By doing so, the cell membrane becomes less permeable. The reason why membranes are not always more fluid has to do with the energy cost making the membrane more fluid. As plant cells have to compete, they will normally produce a membrane that fulfills the functions for which it is designed and they build it in the most economical way. All this changes when membrane leakage occurs. The fluidity of the membrane will be increased by the production of cholesterol, membrane associated proteins and the conversion of the lipids of the membranes from saturated to unsaturated types. Especially the modification of the lipids increases the fluidity of the membrane. Different types of unsaturated fatty acid are formed and these can have up to 3 double bindings. The influence of the unsaturated fatty acids upon the liquidity of the membrane is linked to the amount of double bindings. Fatty acids with 3 double bindings have the greatest effect upon the liquidity. They are also the most costly to build. The most common unsaturated fatty acid with 3 double bindings found in plants is linolenic acid. When membrane leakage occurs, the production of unsaturated fatty acids will increase. The production of the most costly saturated acid, namely linolenic acid will only be increased if it is really necessary. A boost in the production of linolenic acid is therefore probably exceptional. This special characteristic of linolenic acid production was the reason why in plants jasmonic acid became the signal molecule for membrane leakage. Jasmonic acid is a derivative of linolenic acid. Jasmonic acid signals neighbouring cells membrane leakage stress, upon which the plant will react and defend itself.

From one cell to multicellular asked for collaboration. Evolution selected signal molecules based upon existing metabolic processes in the plant that needed signalling for efficient coordinated growth. The precursors of the signal molecules had to be key molecules in these metabolic processes in order to give a good reflection of the activity of these processes. Furthermore, these signal molecules produced from these precursors had to be products of existing enzymes. In my opinion it is illogical to assume that later on specific biosynthetic enzymes arose for the production of the signal molecules. Because the biosynthesis of plant hormones uses enzymes of other metabolic processes, it is linked to these processes and signals also the activity of these processes. It is unlikely that this extra value was lost later on during the evolution. Furthermore, if specific biosynthetic enzymes would arise, new signals would be needed to activate these enzymes. Signals for signalling the production of biosynthetic enzymes to produce a signal molecule for signalling is like contacting a person by telephoning another person and asking him if he would phone to the person you would like to contact.

There could however be one exception to this rule, namely in the case where during evolution plants benefited from drastic morphological changes that were only possible by redirecting normal growth. For example, the development and growth of fruit asked for adjustment of the nutrient flow to fulfil the nutritional needs for growing fruit. Fruit produce cytokinin to induce extra phloem to redirect the sugars coming from photosynthetic active organs. This could be a case where the plant gained the capacity to produce cytokinins during evolution in a plant organ that normally does not produce large amounts of cytokinin. Normally cytokinin production is linked to nutrient availability according to the theory presented above. However, in this case the plant needs extra nutrient for the growth of the fruits. The expression of a specific cytokinin biosynthetic enzyme in the tissue that is destined to become fruit would solve this problem. Extra cytokinin produced by such an enzyme would change the nutrient flow in the plant by directing more phloem vessels towards the fruit tissue. There is however also a second possibility, namely the production of cytokinin by enzymes from another metabolic process that is specifically active in fertilised seeds. This possibility could be an explanation why for many plant hormones there does not exist one biosynthetic pathway, but many different biosynthetic pathways. Normally one would expect that evolution would only benefit and select the most energy efficient biosynthetic pathway. This is the case for example in plant associated bacteria that produce auxin. Auxin biosynthesis in bacteria that specifically produce IAA to influence plant growth is done through the indole pyruvate and the indole acetamide biosynthetic pathway. The latter is only the most economical in an environment with high nitrogen availablility. The acetamide biosynthetic pathway is therefore more found in plant pathogenic bacteria that operate in and on the surface of the plant where there is enough nitrogen present for the bacteria. The indole pyruvate pathways is more common in plant beneficial bacteria that operate in a less rich environment with more competition for nutrients. The situation in plants is totally different. Almost all possible biosynthetic pathways seem to occur in plants. Some of the pathways occur in all plants, whereas others are limited to certain plant species. Researchers have also noticed differential biosynthetic activity in different parts of the plant and during different time point in the live time of the plant. According to me, this can only be explained if the enzymes responsible for the production of the signal molecules play a role in the process that needed signalling. The enzymes that govern the production of signal molecules can be looked at as sensors for certain metabolic processes that the signal molecules signal. According to me this is the only possible explanation why evolution did not opted only for the most energy efficient pathways, but many different pathways occur. This assumption stated above also means that different metabolic states can lead to an increase of the same signal molecule. The response upon the signal molecule however can not differ every time in the plant. This problem will be addressed by the next hypothesis: “Building a plant: plant hormone functions revised”. In this hypothesis also the relation between plant growth regulators and basic plant architecture will be addressed. The hypothesis presented in this article is the foundation on which the plant will be build in the next article.

From one cell to a multicellular plant with coordinated growth governed by derivatives of metabolic processes. The theory presented above is my view on the origin of plant hormones. The question now is: Is this view a correct view of what happened? Non of the above can be proven. However, the theory can be put to the test by looking at the modern plants as sources of archaeological evidence. This theory is published on the web, so that readers can react when finding evidence that sustains or refutes the hypotheses. As more evidence comes available, the theory itself can evolve. In a way, one can state that the evolutionary selection process of the theory starts now.

Although only plant hormones were discussed in this article, a similar theory building can be done for animal hormones. The best example are the steroids. These hormones are produced from cholesterol. The arguments given when explaining the origin of gibberellins and brasinosteroides also account for steroids of animals. Steroids are advanced status marker molecules. Furthermore, the explanation given for auxin and cytokinin also explain why these plant hormones do not fulfil the same function in animals. Animals do not produce there own tryptophane. Signalling the concentration of tryptophane is not linked to a metabolic process that reflects the growth state potential of an animal cell. In a way, tryptophane itself performs the role of growth hormone in animals as the availability of amino acids determines if a cell can grow or not. The explanation for cytokinin is somewhat different as animals do produce adenine the precursor of cytokinin. Cytokinins reflects nutrient availability in plants according to the theory presented above. Animals cells however do not need such a signal. In plants, the role of cytokinins in the growth has to be looked at in relation to auxin that reflects the activity of photosynthesis. It is the crosstalk between those two hormones that determines the growth. The crosstalk between nutrient availability and photosynthetic capacity is something that does not take place in animals. It is the nutrient availability itself that determines the growth. No specific growth signal is needed. There are however some similarities between growth regulated by nutrients in animals and growth regulation by auxin and cytokinin in plants. Also in the case of animals cells, growth based on nutrients alone is limited to ensure that growth can be sustained on the long term. In the case of plants, two other growth signals appeared during evolution that signalled the possibility for extra sustainable growth, namely gibberellines and brassenosteroids. A similar thing happened in animal cells. Steroids have a similar function as gibberellines and brassenosteroids in plant cells, namely signalling extra growth potential. Furthermore, both signals have been linked with advanced growth regulation.

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