Grandpa's biology - 07


If AG treatments attenuated the visible effects of fusariosis
until they disappeared completely, IAA treatments
showed an opposite effect in promoting
the fusarium attack (photo 4).

One month later, the plants were ready. Grandpa went to work, but without any great success at first. It was summer and successive experiments only gave mixed results. One month before the following photos were taken, all the plants were absolutely identical. They had reached the "4-6 leaves" stage and were ready for the experiment. They were depotted and their roots were carefully washed. The plants in the first photo were then simply repotted: these were the control plants, free of any disease. The plants in the second photo were inoculated by soaking the roots in a suspension of fusarium spores before repotting. Fusarium is a fungus which normally lives in the soil. It penetrates plants via the roots and gradually colonises all the other organs via the sap carrying vessels. This is a particularly suitable means of access: progress is easy, food is choice and abundant… so the method of inoculation described above was most appropriate. The next day, diseased and healthy plants were split into nine separate batches and each batch was treated by spraying the leaves as follows:

- the first batch with water, in other words, no treatment at all, to avoid modifying the plant's natural hormone balance. These are the plants shown in the centre of each photo, which are called "control plants".
- the others with increasing doses of IAA, the best-known auxin (towards the right starting from the control plant) and GA, the most representative gibberellin (towards the left starting from the same control plant) so as to alter the plants' natural auxin / gibberellin balance more and more strongly, in favour of either auxins (right-hand plants) or gibberellins (left-hand plants).

Result: Although the influence of the various treatments on the phenotypic aspect of healthy plants was not surprising (it matched the descriptions found in the literature in every way, photo n° 1), although it may also confirm the hypothesis in the case of diseased plants treated with gibberellin (left-hand plants in photo n° 2, the visible effects of disease were attenuated until they had completely disappeared), on the other hand it raised serious questions in the case of plants inoculated with auxin, the various treatments remaining apparently without effect on the evolution of the disease (right-hand plants in the same photo).

However, at the end of October, things changed. The last experiment finally gave the expected results. It was autumn. Environmental conditions were less favourable to plant growth as well as development of the parasite. Even though the GA treatments (left-hand plants in photo n° 4) still showed better tolerance to the disease, IAA treatments (right-hand plants) now also showed that they were effective in promoting the fusarium attack.

One incident occurred: during these last experiments, one plant inoculated with fusarium was accidentally contaminated with tobacco mosaic virus. Whereas the other plants in the same batch displayed classic symptoms of fusariosis (small size, leaf epinasty), the plant which had been secondarily inoculated with tobacco mosaic virus only differed from the healthy controls in the mosaic leaf symptoms (yellow and green patches juxtaposed, reminiscent of a mosaic). One month later, the leaves suddenly turned yellow and fell off. The sap carrying vessels in the stems and roots had turned brown, which is characteristic of plants suffering from fusariosis.

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 - 07