Abstract
Plants have the capacity to adapt to environmental factors using the phytohormone, auxin, as a major regulator of development and tropisms. Among these responses, gravitropism is essential for plants roots to grow downward in the search for nutrients and water. Importance of auxin was demonstrated by the root agravitropic phenotype of the polarized transport mutant of this phytohormone : pin2. A new mutant allele of PIN2 revealed that this agravitropic phenotype is caused by the nutrients available in the culture media. My thesis project aims to understand what is the impact of nutrients on the already existing link between auxin gradient and gravitropism, to modulate root architecture. To do this, different culture media were used to follow auxin gradient, using DR5 and R2D2 probes, during the response to gravistimulation in different mutants. It is clear that pin2 and aux1 mutants, transporters of auxin efflux and influx respectively, respond to gravity only on media lacking phosphorus and nitrogen, independently of the osmolarity and the primary roots growth rate. In pin2 mutant, we were able to establish that this reversion of agravitropic phenotype occurs through the asymmetric and ectopic expression of PIN1 in the epidermis. This is associated with a restoration of the asymmetric accumulation of auxin necessary for the reorientation of the root according to gravity. These observations along with the use of auxin inhibitors have highlighted the importance of auxin homeostasis in this process. Auxin transport represents the main lever for explaining the importance of nutrients during gravitropism. We also demonstrated that cytokinins and brassinosteroïds, through their action in synergy with auxin, have an effect that could modulate the auxin threshold to respond to gravity.Thus, the work carried out during this thesis has provided new knowledge about the agravitropic phenotype of pin2 and aux1 and about the effects of nutrients on auxin transport during gravitropism. The regulation of auxin distribution by nutrient availability directly impacts the ability of roots to drive their gravitropic response. This suggests that setting the root growth angle by modulating the gravitropic response can be both genetically and environmentally controlled. These observations open the way to the improvement of soil exploration by roots with a better use of resources.