Grandpa's biology - 08










PLANT EVOLUTION ACCORDING TO
TREATMENTS WITH AUXIN
AND GIBBERELLIN

IAA treatments slowed the natural development of leaves and
accelerated that of roots..., GA treatments had
the opposite effect in every case




Encouraged by these results, Grandpa now decided to push the envelope a little further; but in which direction? Studying what happens at basic levels (DNA, RNA, enzymes) was not really in his field. He therefore decided to work with the intermediate metabolism, more specifically, by studying nitrogen and sugar concentrations in the plants… which, with a bit of luck, would not be too difficult. We know that sugar and nitrogen levels in plant tissues evolve according to the plant's age and physiological condition. It is easy to imagine that this evolution of the environment in which the parasite is developing is the cause of the phenomena observed. In 1953, R.W. LEWIS summarised the situation in these words: "The possible combinations of Foods present plus the capacity of the parasite to respond differently to these combinations account for the varying degrees of resistance and susceptibility not explicable in other terms. One can imagine combinations of metabolites that would permit any intermediate condition between complete resistance and complete susceptibility". So this was the direction chosen for research to continue.


I am not going to describe this laborious work in details. If you are interested, you can contact the director of the INRA who will, I'm sure, be delighted to go through the archives to give you the information you want. Let's just say that this work, seen from the point of view of Papy's biology, from what was known about the evolution of sugar/nitrogen ratios according to the age of the plants and data provided at the time in the literature, on the hormone content of various plant organs, led to the following conclusions:
- IAA treatment slowed the natural development of leaves and accelerated that of roots (it makes leaves younger and roots older);
- GA treatment had the opposite effect in every case, which was more marked the higher the dose of hormone given. e.g.




These observations are not surprising. They confirm the data given in the literature, according to which senescence is reflected:
- in the leaves, by hypoauxinia (in terms of balance, a decrease in auxin concentration linked to an increase in gibberellin concentration);
- in the roots, by hyperauxinia (in terms of balance, an increase in auxin concentration linked to a decrease in gibberellin concentration).



All this may appear complicated, so here is a diagram giving all this data to help understand why IAA treatment can both "rejuvenate" the leaves and "age" the roots.





This diagram is based on the following principle: every plant starts as a single cell in which there can only be one auxin/gibberellin balance and one sugar/nitrogen ratio. In view of the previous results, it may now be thought that this auxin/gibberellin balance experiences a double evolution from the first cell divisions:
- evolution in favour of gibberellins in cells which will give rise to the aerial parts of the plant;
- evolution in favour of auxins in cells which will give rise to roots.

The gradual growth of the plant's aerial parts from egg to embryo, embryo to youth, youth to adulthood, old age and death would then correspond to a gradual reading of half the genome, linked to the evolution of hormone balance in favour of gibberellins and leading to the hypoauxinia revealed in old leaves.

The gradual growth of the roots from egg to embryo, etc. would correspond to a gradual reading of the other half of the genome, linked to the evolution of hormone balance in favour of auxins and leading to the hyperauxinia revealed in old roots.



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 - 08