Grandpa's biology - 05









GRANDPA'S HYPOTHESIS

Auxins and gibberellins are thought to act on DNA AT and GC groups,
their role being to selectively open the double DNA chains
(gene activation) by breaking the H+ bonds
which provide stability.




In May 1965, Grandpa, a foreign researcher, was recruited by the INRA and assigned to the plant physiopathology laboratory in Dijon. His job was to study the mode of action of plant growth hormones in host/parasite relations between a variety of tomato and a pathogenic fungus. At least this was what was written in his contract. However, the laboratory director knew that Grandpa was not really interested in either the mode of action of plant hormones or the host/parasite relations noted above. He simply wanted to verify, in a specific case chosen at random, the conclusions he had reached two years earlier when writing his final year dissertation.

These conclusions concerned the mode of action of auxins and gibberellins, these mysterious substances known as "growth hormones" which act always, everywhere and at all levels, of which we have already spoken. They are summarised as follows:

"Auxins and gibberellins, plant hormones of which only the various mobile forms are known to us, once they reach their destination in the plant, are thought to act on DNA AT and GC groups, their role being to selectively open the double DNA chains (gene activation) by breaking the H+ bonds which provide stability. This can be expressed as follows: at a given moment in a given cell, the only active genes are those with a structure (AT/GC ratio) which matches the auxin/gibberellin equilibrium reigning at that moment in that cell".



THREE PRELIMINARY REMARKS


This hypothesis in no way prejudices the mode of action of other plant hormones, substances which are not chemically related to the two hormones envisaged here, and which therefore necessarily have a different activity.

Neither does it prejudice groups on which auxins and gibberellins are thought to act, or the fact that this action concerns the entire length of the DNA double chains rather than their initial part, known as an "operator" in molecular biology. It is strictly incapable of this and in any case, it is not important at present.

Finally, for convenience, from now on the term "hormones" will only apply to active forms of auxins and gibberellins, which are the substances concerned here. Their mobile forms will be called "free hormones".



WHY HAVE THIS HYPOTHESIS?


We have seen that the fact that a plant is treated with an auxin or a gibberellin causes deep modifications to its growth, differentiation, reproduction, senescence, sensitivity or resistance to disease of various kinds… , all phenomena result from a multitude of enzyme reactions. Like many others at the time, Grandpa thought that the action of these substances could not be explained without involving either all the enzymes or at least a large number of them.

However, there was something about this type of evidence that bothered the dear man: the contradiction between the non-specific nature of auxins and gibberellins and the specificity of enzymes. In the infinite diversity of enzymatic structures, how could the sites of action be located, these chemical groups which are always similar, in such a way as to explain how non-specific hormone actions are performed at these locations?

He therefore came to the following conclusion: since direct action of auxins and gibberellins on enzymes is difficult to envisage, this action, since it must take place, must be indirect. Auxin and gibberellin should therefore act on substances with a non-specific structure, substances which must also reveal the infinite diversity of enzyme structures.

He thought that the choice was limited, in this case, and that it was necessary to go through DNA, its double helix structure and more precisely the AT and GC groups directly involved in enzyme synthesis.

This led to the previous hypothesis and the following diagrams to which we shall refer constantly from now on, because they form the basis of what we must refer to as "Grandpa's biology".



IF YOU HAVE THE CAT'S TAIL, YOU
HAVE THE ENTIRE CAT...


For Grandpa, this hypothesis could only be right or wrong: it was right if the auxins and gibberellins really act on DNA AT and GC groups, their role being to control gene activity. It was wrong in any other case.

If it was wrong, it would simply be another hypothesis to be discarded. If it was right, on the other hand, then it would be something really important, because it would lead to a better understanding, not only of the mode of action of the two main plant hormones, but also of cell regulation phenomena, cell function, plant function, concepts relating to health and sickness, different types of disease and doubtless many other things as well.

If the hypothesis proved correct, it could be said without much risk of being mistaken:



CONCERNING UNICELLULAR ORGANISMS




The black arrows indicate reactions of synthesis and degradation requiring intervention of specific enzymes.

White arrows indicate that these are simple reactions of exchange, balance, regulation.


1. At any given time in a given cell, only genes with a structure (AT/GC group ratio) matching auxin/gibberellin balance reigning at that time in that cell would be active. If this balance changes, these genes lose their active status to other genes with structures matching the new balance now found in the cell. This gives rise to the following:
- everything that happens in a given cell at a given moment is directly linked to the auxin/gibberellin balance at that moment in that cell;
- everything that happens in a given cell over a given time is directly linked to the evolution of the auxin/gibberellin balance in that cell during that same time.

2. Regulation of cell activity provided by these two hormones cannot be absolute. Indeed, it concerns only enzyme synthesis, the activity of the enzymes in question being subject to the influence of the environment (the cell environment, itself influenced by the external environment).

3. This role is important, however. Like any self-respecting metabolic operation, hormone balance results from complex enzyme activity (gibberellins are produced by carbon metabolism, auxins by nitrogen metabolism). It is therefore a sort of reflection of what is happening in the organism, considering its age, physiological status and environmental conditions…

4. This little nucleus computer does not regulate enzyme synthesis simply by following a predefined programme of events (the genetic programme). It takes into account information provided by the AUX/GIB balance in the evolution stage of the cell, its state of health and the environmental conditions.



CONCERNING MULTICELLULAR ORGANISMS



The black arrows indicate reactions of synthesis and degradation requiring intervention of specific enzymes.

White arrows indicate that these are simple reactions of exchange, balance, regulation.



5. - everything that happens at a given moment in a given plant is directly linked to the auxin/gibberellin balance at that moment in each of this plant's cells.
- everything that happens in a given plant over a given time is directly linked to the evolution of the auxin/gibberellin balance in each of this plant's cells during that same time.

6. Unlike what happens in unicellular organisms, the hormone balance in the cell does not only reflect cell metabolism. It largely depends on free hormones, also, in its way, reflecting what happens in all the plant's other cells. These billions of tiny computers which are the cell nuclei and are strictly identical within the same plant, are connected together in a form of network, just like the computers used by bank or business employees.

7. The roots therefore take into account the variations in temperature or light to which the aerial parts of the plant are subject. The stems or leaves react to changes in soil humidity, fertiliser supply or the damage done by a pathogenic fungus attacking the roots.

8. We can finally add this: every organism (uni- or multicellular) not meeting these conditions will be eliminated by natural selection, unless there are exceptional circumstances enabling it to bypass the problem (as in certain cases of symbiosis, parasitism or human intervention).


It remains to discover how closely this theory matches reality. This leads to a new question: how can it be verified?



presentation/contentsa work of popularizationstory of modern biologythe point of view of French citizenssome basic concepts to recallgrandpa's hypothesishow to verify this hypothesisfirst testsevolution of plants according to auxin and gibberellin treatmentshost-parasite relationsaction of the fungus on the plantaction in return of the plant on the fungusaction of the virus on the plantaction in return of the plant on the virusa plant subjected to double attack by both fungus and virusthe scientific debatethe Peter principleconclusion - answer to some questionsimages




Grandpa's biology - 05