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If plant hormones are the ideal metabolic markers (see: Plant hormones: Ideal metabolic markers. www.plantevolution.info), then the regulation of plant growth is directly linked to the metabolic status of the plant. The theory presented here explains the function of the different plant hormones by linking them with the metabolic status of a plant cell during the life cycle of the plant. All is looked upon being a product of evolution. Evolution linked the metabolic status with the function of the plant hormones. The metabolic status depends on environmental factors as there are light, water availability and mineral nutrition. Evolution is the competition for these elements. Trying to explain the origin of the function of plant hormones requires one to go back in time and asses the benefit of a signal that is translated in a growth response. The proceeding theory “Plant hormones: Ideal metabolic markers” allowed one to explain co-ordinated callus growth and only explains the basic function of the plant hormones as there are: auxin promote cell growth; cytokinin promote cell division; gibberellines enhance overall growth. The starting point of the theory explaining the advanced functions of plant hormones is the end point of the proceeding theory that identified plant hormones as ideal metabolic markers. Now the search for the benefits of signalling hormone molecules begins.

And let there be light... Light starts up photosynthesis in plants. Photosynthesis equals CO2 fixation and the formation of carbon compounds starts. If the basic respiratory needs are fulfilled and the production of carbon compounds exceeds the requirements for the maintenance of the cells, than a plant can grow. Evolution requires the plant to use its energy efficiently. That is why more expensive building blocks will only be present at a higher concentration when enough energy is present. In these circumstances also auxin will be produced when the tryptophane concentration rises. Auxin tells neighbouring cells that growth takes place. A plant that responds to this signal by coordinating the growth of neighbouring cells will have an evolutionairy advantage.

A second interpretation of elevated auxin can be: “We growing cells have extra growth potential. Give us more mineral nutrition and we will grow even better.” After coordinated cell growth is addressed, a second, even more larger benefit, can be achieved by a plant which fulfils the need for extra nutrition. This is how and why vascular tissue arose. Auxin is known to induce xylem formation in plants. Xylem vessels transport water and nutrition to the upper part of the plant in order to sustain and promote the growth of the photosynthetic active part of the plant. The development of a transport system for nutrient is a big leap forward. At this moment the first real differentiation between photosynthetic active tissue and supporting tissue becomes apparent. The supporting tissue will provide nutrients to the photosynthetic tissue in return for sugars. As plants start to redirect nutrients in the plant, also the nutrient demand rises. The competition for nutrients becomes very important. Plants with the best nutrient uptake will win. At this time point a new plant structure arises: the root. In the beginning, roots were nothing more than supporting plant tissue with increased nutrient uptake capabilities. The plant however must keep a balance between the amount of supporting tissue and photosynthetic tissue. Competition will favour those plant that have an optimal nutrient supply system for the cheapest price. A plant that can regulate the growth of the root in relation to nutrient demand is the next step in the evolution. The nutrient demand is linked to activity of the photosynthetic tissues. There is growth potential if the photosynthetic tissue can produce more building blocks than needed to sustain the existing cells. Auxin is the best candidate to reflect this growth potential (see : Plant hormones: Ideal metabolic markers. www.plantevolution.info). Auxin is therefore also the best candidate to control root growth. More auxin equals growth potential, which equals nutrient demand, and leads to root and xylem induction.

The creation of extra nutrient uptake tissue requires energy and building blocks which are provided by the photosynthetic active tissues of the plant. At this moment, sugar is transported by diffusion from the photosynthetic active tissues to the supporting tissues. A plant that can speed up and direct the transport of the sugars to the newly developed roots will have an evolutionairy advantage. The sugars must be directed to those uptake cells that deliver the most nutrients. The cells with the highest or best nutrient uptake are also the most nutritionally balanced cells of the plant. The better the nutritional demands are fulfilled, the more the cells will grow and divide. A plant that can direct the sugar transport of the synthetic active parts of the plants towards the best nutritionally balanced uptake cells of the plant will obtain the extra and optimal nutrient uptake for enhancing the growth. Cytokinins, that already play a role in coordinated cell growth (see Plant hormones: Ideal metabolic markers. www.plantevolution.info), signals the presence of dividing cells, in this case the presence/location of the best nutritionally balanced cells of the plant. A plant that reacts upon the presence of an elevated cytokinin concentration by differentiating sugar transporting tissue is the next big step in evolution. Cytokinin is known to be responsible for the induction of phloem. Phloem however can not be induced without the presence of auxin. The presence of auxin is as important as that of cytokinin. The requirement for both auxin an cytokinin can be translated as: Only phloem differentiation when there is nutritional demand and potential increased mineral uptake.

...And there was light. Where there is light, there is growth and growth potential. Where there is light, there is auxin. Auxin induces xylem and roots to increase mineral nutrition and enhance plant growth as such. Evolution however requires a plant to be efficient and competitive. The solution is directed root growth, driven by the presence of nutrients. When nutrients are present, cells can growth and divide. Where there is cell division, there are auxins and cytokinins. The combination of auxins and cytokinins induce phloem. Phloem transports the sugars to the root tip, enhancing nutrient uptake and root growth. Evolution of plants is driven by competition of light and nutrients, translated and stirred by auxin and cytokinin.

Growth regulation only based on auxin and cytokinin is far from ideal. As auxin production is linked with photosynthetic activity, the production of auxin fluctuates together with light intensity. Growth regulation based on a fluctuating signal can lead to the production of extra plant tissue that can not be sustained on the long term. There are two possible solutions to overcome this problem. The first solution is a moderated response that does not lead to unsustainable growth. The second solution is reducing the fluctuations. The modern plant has both solutions at work. The occurrence of gibberellins proves that the plants reaction upon auxin and cytokinin alone is moderated. Reducing the fluctuations is achieved by producing plant hormone conjugates.

During the course of evolution it is likely to assume that conjugation only became apparent after the moderated response. Without a response there is no need and no selection pressure for reducing fluctuation by conjugation. The original response must have had benefits for the plants and no disadvantages, for example the production of tissue which could not be sustained on the long term. This prerequisite also means that sometimes there could be extra growth potential that will not be used because of the risk on the long term. Estimating the sustainability of the extra growth potential required an other signal molecule that does not only reflect the overall metabolic activity and availability of nutrients. This is the time point when gibberellins became plant hormones. As advanced status marker molecules (see Plant hormones: Ideal metabolic markers. www.plantevolution.info), they give a better estimation of the possibility of extra growth.

Besides a moderate signal response, a second solution is reducing the fluctuations of the plant hormone concentration. This solution probably became quickly apparent during evolution because reducing the fluctuations increases the quality of the signal. When the production of plant hormones is high, the plant will convert part of the hormone molecules into conjugates. These conjugates are reconverted to hormone molecules when the hormone concentration goes down. By doing so, the fluctuations in the hormone concentration are reduced. According to my opinion, this is the only reason why there are hormone conjugates. There are however some exceptions. In some cases conjugation in plant is also used to lower the hormone concentration as not all conjugates can be reconverted into hormones. Also in this case the fluctuations are reduced, however only during the time point when the synthesis of the hormone occurs and not on moment with low hormone production rates. The end result is shorter signalling in time. In some cases, this can be beneficial to the plant. This type of deactivation is especially useful for example when plant hormone production peaks and conjugation alone is not enough to reduce these large fluctuations.

Coordinated growth stirred by auxin and cytokinin allows a plant to differentiate. Roots appear and photosynthetic tissue can be optimised to capture more light. Leaves and stems are formed. Leaves are directed towards the light and stems allow plants to grow towards the light. Stems also give the plants the opportunity to position the different leaves at different heights, optimising light capturing of the plant without hindering the other leaves of the plant. The next step is branching of the plants. Branching will increase light capturing even more. The question that has to answered now is when to branch. Not to early, otherwise the leaves of the different branches will hinder each other and not to late, otherwise other neighbouring plants will take the light instead. Branching must be coordinated and the coordination must be based on energy efficiency and the competition for light with other plants. Evolution will benefit a plant that develops a signal for efficient coordinated branching. This signal must enable the plant to estimate the distance on the stem from the growing tip. Ideally it also has to give information on the light distribution in the plant. This assumption means that the candidate precursors for the branching signal are limited. A branching signal must be derived from a molecule that plays a role in capturing light. Only a few molecules play a role in capturing light, namely chlorophylls and carotenes. Furthermore, the signal molecule must be produced in different quantities in young and old tissue, in tissue near the growth apex of the stem and tissue near the bottom of the stem. Only carotenes fulfils this requirement. Carotene molecules are part of the light harvesting complex of the photosynthetic apparatus. The size and the composition of the light harvesting complex, and therefore also the production of carotene, is dependent on the light that the plant receives. The LHC has two functions: (1) capturing and transferring light to the chlorophyll molecules and (2) protecting the photosynthetic apparatus from oxidative damage. There are two groups of carotenes produced by two different pathways, namely carotenes produced by the β-pathway and those produced by the α-pathway Studies have shown that especially the pigments of the β-pathway, including the xanthophyll cycle pigments (zeaxanthin, antheraxanthin, violaxanthin), that detoxify oxygen radicals, increase under high light conditions. On the contrary the α-pathway, represented by the major higher plant xanthophyll lutein, does not respond to changes in the light environment. This means that only carotenes produced by the β-pathway are real candidate precursors for the branching signal. Concerning the xanthophylls of the β-pathway, a distinction can be made between the different molecules based on the regulation of their production. Whereas the production of the xanthophyll cycle pigments fluctuates greatly during the day, there are other pigments of the pathway that do not show the same diurnal fluctuations in production, although they are also found more abundantly under high light conditions. The absence of diurnal variations make the latter pigments the best candidates for precursor of a branching signal. It is β-carotene that became the precursor of the branching signal molecules now known under the name strigolactone. In comparison with the other signal molecules, strigolactone does not induce a response but instead inhibits branching. This makes sense because the signal is based on measuring the light intensity and not the measurement of darkness. The selection process benefited those plants which could enhance their light capturing capabilities in the most economical way.

Light drives branching and it does so by inducing higher β-carotene levels in leaves that receive more light and/or not full grown. Derivatives of β-carotene, namely the strigolactone, are aspecific products of enzymes that normally play a role in other metabolic pathways. During the course of evolution, plants reacted upon this signal and translated the concentration of strigolactone in a signal that measures the distance from the apex that also takes into account the light intensity under which the plant is growing. Branching induced upon this light distance measuring system resulted in the most optimal branching patterns which can be observed nowadays in plants.

Now that a plant has different branches, a new question arises: Which branch may grow the most? Although this looks like a new problem, it is not and the plant did not address this issue as a new problem during the course of evolution. In fact, noting changed, although the auxin signal got a new dimension. When a new branch is formed, this branch will form leaves that produce auxin as a result of active growth and photosynthesis. Auxin will induce xylem to govern more nutrient transport. The direction of xylem vessel formation is directed by the auxin gradient. This key feature of the auxin signal explains what will happen at the branching point. Xylem vessel orientation will be based upon the auxin gradient that is present at the branching point. At the branching point two auxin gradients meet and xylem will be formed in the direction of both gradients. However, as the biggest branch will have a higher auxin concentration, most of the xylem vessel will develop in the direction of the biggest branch. The end result is that the biggest branch will also receive more nutrition and will have therefore more growth potential. Apical dominance is born, although nothing changed in the plant. What changes is our interpretation of the auxin signal. From now on the auxin signal can also be seen as the cry for food and those that cry the most get the most.

Evolution puts organisms for dilemmas. One of the biggest dilemma is: Do I grow or do I multiply? Phrased otherwise: When does a plant have sex? Again evolution links all with the competition for energy. Which is the most suitable/economical strategy for the plant at a given moment? If a plant starts multiplying too fast, the competitors that grow bigger will win the competition for light. If a plant starts multiplying too late, it could get into trouble when for example drought stress occurs and it is not able to produce enough energy for the production of the seeds. Finding the best time point for multiplying is finding the best balance between the competition for light with other plants and the production of enough seeds to ensure survival. As long as a plant does not experience stress that limits growth, it can compete with other plants. However, as soon stress occurs, the plant has to make a choice: keep on competing or multiplying. This choice can only be made if the plant is able to estimate the condition of the plant. The plant already has several signals that are linked to the well-being of the plant, namely gibberellins, ethylene, salicylic acid, absisic acid and jasmonic acid.

As mentioned earlier, gibberellins reflect the extra growth potential of a plant. However, gibberellins do not give an estimate of the level of stress the plant experiences. The absence of gibberellins does not mean that the plant is not able to grow and compete with other plants. A plant can grow without gibberellins, even if there is no stress. Stress is a better measuring parameter to estimate the need to replicate. Ethylene, salicylic acid, absisic acid and jasmonic acid all signal a stress condition. These signal molecules however have the disadvantage that they can show large fluctuations over time. A plant that will react upon a temporary stress event will lose the competition with other plants that ignore this smaller event. Furthermore, a plant needs a signal molecule that is not linked to one type of stress, but a signal molecule that gives the overall status of the plant. The combination of different levels of different types of stress must also be evaluated by the plant. A new signal molecule is needed that is linked to a cellular process that takes place during all types of stress. Plant stress can be defined as a condition that is a threat for the survival of the cells. In “plant hormones: ideal metabolic markers”, different types of stress were linked with different stress signals. Mechanical stress linked to salicylic acid, drought stress to ethylene, oxidative stress to absisic acid and cell leakage/chemical stress with jasmonic acid. Independently of the factor that inflicted the stress, in all cases the cellular content of the cell is changed. Drought stress increases the ion concentration in the cells. Cell leakage can lead to an increase or decrease of the ion concentration. The same is true for mechanical damage. In the case of oxidative stress, the ion balance will be influenced indirectly because cellular components that play a role in maintaining the osmotic equilibrium and components that posses an ionic charge become damaged. In order to function normally, cells need to correct the shifted ionic balance in the cells. This is achieved by the production of polyamine, cationic molecules. Monitoring the content of the polyamine equals monitoring the status of the ionic balance which reflects the overall well-being of the plant. During the course of evolution, plants started the use the balance of polyamine as an estimate of the condition of the plant and linked decision making with polyamines. When to replicate is one of the questions answered by the polyamine content of the plant cells. The use of polyamine as a signal molecule is obtained early in the evolution process as spermidine is know to initiate reproduction not only in plants but also in fungi. Furthermore not only reproduction is linked with polyamines. The story of polyamine only begins. More will be explained in the article: “Polyamines: Do plants have feelings?”.

Starting from coordinated callus growth in the theory “Plant hormones: ideal metabolic markers” and evolving to an economical efficient and durable plant that knows when to form branches and when to multiply. This evolution needed coordination which was achieved by adding secondary function to the plant hormones and developing new signal molecules. No extra enzymes needed to be created for the production of the new signal molecules and for the signalling of the secondary function. The creation of extra enzymes is against the logic of evolution as there first had to be a signal present representing a status upon which a plant reacts. This argument was already discussed in the proceeding theory. If no new enzymes arose, other things changed. In the case of the extra secondary functions, it is a new receptor that had to emerge that reacts upon the elevated level of the signal molecule and activates a response in the plant cells. For example, plants started to react upon auxin by differentiating cells into xylem vessels. Optimal light conditions are translated into auxin and the reaction upon auxin is xylem: the demand for more nutrients. A new secondairy function for a plant hormone was born. A second example is the role cytokinins got during evolution in redirecting the nutrient flow. Plant started to react upon the combination of auxin and cytokinin by differentiating cells into phloem vessels. If auxin signals growth potential and the nutritional requirements are fulfilled, cell division increases. Increased cytokinin levels coincide cell division and that cytokinin will induces phloem. By inducing phloem , a new secondairy function for cytokinin arose, namely: induction of phloem that can deliver sugars to the most nutritional balanced parts of the roots.

In literature, there are many other function appointed to auxin and cytokinins. When an extra function was linked to these signal molecules during evolution, one may not forget that auxin and cytokinin always invoke the basic reaction addressed above and the function mentioned in the previous theory on which this article was build. The extra function must therefore always coincide the original reaction invoked by these hormones. Concerning the extra functions, one must also always be careful when appointing extra function based upon feeding experiments and mutant analysis. A good example is the increase of the ethylene production upon high auxin concentrations. When invoking extra cell enlargement that can not be sustained by the plant, these enlarged plant cells will experience drought stress that will be corrected by producing betaine. The burst of methylation that coincides the production of betaine also induces the production of ethylene. It is however questionable that in nature a plant will induce growth that can not be sustained. (There are also some misunderstandings concerning hormone balances and the interaction of the different hormones. This will be the topic of next article/hypothesis that can be found in the near future at www.plantevolution.info.) As mentioned in the theory in this article, the plant has two safety mechanisms in place that hinder the uncontrolled and unsustainable production of plant tissue, namely plant hormone conjugates and growth potential that can only be unleashed by gibberellins. Both control the hormone signal by levelling the fluctuating signal and only allowing growth when other requirements besides the basic requirements for light and nutrition are fulfilled. Another fault made in literature is appointing new functions that are in fact not new. An example of this was given in this theory, namely apical dominance. Apical dominance was not a new function, but a new interpretation of a function that was already linked with auxin.

The different interpretations of the same function and wrong explanations given upon feeding and mutant analysis made hormone signalling difficult to interpret. The theory presented above not only tries to understand the evolution of the plant hormones but also simplifies the interpretation of functions of hormones. Two good examples of this are the explanation given for the branching signal strigolactone and the flowering signal polyamine. The theory presented above outlines the basic rules for branching and flowering to which a plant obeys. Both branching and flowering however are complex processes in which many parameters play a role. During the course of evolution, plants competed and tried to find and fill new niches. New criteria were linked to branching and especially flowering. Flowering of modern plants is not only regulated by polyamines and polyamines do not only play a role in flowering. The role of polyamines will be discussed in more detail in the theory “Polyamines: do plants have feelings?”.

Where we started with light... we ended up with environmental factors that are translated by the plant hormones into a simple plant design. This theory is my view on how evolution designed the plants. The theory does not stop here. First of all, the theory itself evolves as new things are discovered that support the theory or refute the theory. This theory is published on the web, so that readers can react and the theory can be revised if necessary. Secondly, only basic plant design was addressed. In the following theories, different aspect of advanced plant design will be discussed.

The theory was build by reading many articles. Not only articles that sustain this theory were important, but also articles that refuted alternative hypotheses. Making a choice/selection which articles to refer to is therefore difficult. All researchers on plants must be thanked, even those that were wrong in linking new functions and for example performed feeding experiments that lead to the wrong conclusion. Furthermore the list of articles is still growing as this is not a static theory. References are also normally published to sustain a theory. In this case the sustainability is put to the test by the readers. If the theory proofs to be wrong, adjustments will be made and published on www.plantevolution.be. This theory waits upon your reaction and your view.

The theory stated above can be found at www.plantevolution.info. Also the proceeding theory “Plant hormones: ideal metabolic markers” can be found on this website. Other theories on plant evolution and plant design will follow. Next theory foreseen: “Polyamines: do plants have feelings?”.

Some of the ideas in this theory are new, others are not. Good ideas were taken over, other ideas were adjusted and new ideas were added. The new ideas in this theory are first of all the explanation of the origin of the signal molecules strigolactones and polyamines. The simplification of the idea on apical dominance is an adjusted version of an existing theory. The same accounts for the ideas of the function of plant hormone conjugates and gibberellines. In the case of auxin and cytokinin nothing new is told, however the simplification of linking auxin to light and cytokinin to nutrition is a new view point.

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ORIGINALITY PROTECTED BY

BOIP i-DEPOT