How genetic pressure of Medicago trunculata acts on the molecular selection and population pattern of its favourite symbiotic bacteria, Sinorhizobium sp.?

Nitrogen-fixing Sinorhizobium sp. bacteria interact with Medicago truncatula plants, forming root nodules in this nitrogen fixation symbiosis. When rhizobia colonize rhizosphere’s legumes, symbiotic specificity is expressed at two levels, among-species for nodulation ability and within-species for nodulation competitivity. We investigated the genetic diversity of 223 Sinorhizobium sp. strains isolated from nodules sampled from four Medicago truncatula fixed lines, each issued from distinct geographic areas, and cultivated on the same French soil. The DNA molecular polymorphism of isolates was analyzed by CE-SSCP and RFLP on 5 loci chosen either for species characterization or for their involvement in symbiosis and effectors translocation pathways. Although more than 90% of the sample bacteria belonged to Sinorhizobium medicae (others belonging to S. meliloti), statistical tests showed a significant differentiation among M. truncatula lines for bacterial species proportion. Conversely, on the basis of the four other loci analysed, we could not detect any significant among-lines population differentiation, for each bacterial species. Nevertheless, several rare strains isolated were specific from a single plant line. Thus, we compared the nodulation ability and the fixation efficiency of these isolates on the different M. truncatula lines used for trapping. Moreover, to enlighten the role played by bacterial genetic diversity on nodulation competitivity, intraspecific competition assays were performed with genetically different strains on 80 plants. Hence, SNP molecular genotyping of 200 nodules allowed the choice of the most competitive strain for each of the two Sinorhizobium species. Therefore, those two strains were tested for their ability to compete in the formation of nodules on the four Medicago truncatula lines, with inoculation of strains at different ratios (90/10, 50/50 and 10/90 proportions) on 30 plants each. Results led us to formulate hypotheses explaining the S. medicae versus S. meliloti proportions found initially in this bacterial trapping. This work outlines the influence of the genetic diversity of the host plant on the distribution of symbiotic genotypes in rhizospheric bacterial populations, and allows a better understanding of bacterial selection in natural and cultivated ecosystems.