Spécificité de la coopération phytostimulatrice Azospirillum lipoferum / riz – AzoRiz

Plants are members of complex communities and interact both with deleterious and beneficial microorganisms. Beneficial relationships are frequent in nature where microorganisms improve plant nutrition or help the plant to overcome abiotic and biotic stresses. These interactions involve fungi, such as mycorrhizal symbionts or bacteria, such as rhizobial symbionts or plant growth-promoting rhizobacteria (PGPR). The establishment of mutualistic or pathogenic associations usually involves partner recognition and requires a specific molecular crosstalk between the plant and the invading microorganism. In the associative symbiosis (cooperation) between crop plants and Azospirillum, a PGPR bacterium that is directly beneficial to the plant mainly by the synthesis of phytohormones (notably indole-3-acetic acid, IAA), several lines of evidence point towards a certain degree of specificity, which has been overlooked in previous studies. Indeed, a plant genotype effect has been reported with several crops, with responses to inoculation varying among cultivars of the same crop. Our hypothesis is that the specificity of the Azospirillum-plant cooperation is based on the exchange of molecular signals, allowing both actors to perceive each other and to respond to its partner by activating or repressing specific sets of genes. Thus far, this associative symbiosis has been mainly studied through agronomic aspects such as root morphology and crop yield increase. The proposed study aims at characterizing the molecular basis of the specificity of the associative symbiosis on both partners. The model association Azospirillum lipoferum-rice will be chosen as genomes of both partners are now available. The specificity of the association will be analyzed by combining transcriptomics and metabolomics to decipher the interactions between two different A. lipoferum strains and either their corresponding host rice cultivar or the one of the other strain. First, the metabolomic response of the host will be assessed by rice metabolome profiling (focusing on secondary metabolites). Second, the molecular response of both partners will be analyzed by a global transcriptomic approach. Third, the putative genome dynamics triggered by the association will be investigated. In the case of rice, the transcriptomic approach will also take into account the expression of the transposable elements (taken here as putative mediators of gene expression). For the bacterial partner, the occurrence of phenotypic switch linked with genome rearrangements (a phenomenon generating intra-population diversity and affecting several phenotypes) will be assessed in the presence of rice. It is expected that this first omic approach on both Azospirillum and rice will enable a molecular understanding of the specificity of this phytostimulatory cooperation.