Legume growth-promoting rhizobia: An overview on the Mesorhizobium genus

The need for sustainable agricultural practices is revitalizing the interest in biological nitrogen fixationand rhizobia-legumes symbioses, particularly those involving economically important legume crops interms of food and forage. The genus Mesorhizobium includes species with high geographical dispersionand able to nodulate a wide variety of legumes, including important crop species, like chickpea or bis-errula. Some cases of legume-mesorhizobia inoculant introduction represent exceptional opportunitiesto study the rhizobia genomes evolution and the evolutionary relationships among species. Completegenome sequences revealed that mesorhizobia typically harbour chromosomal symbiosis islands. Thephylogenies of symbiosis genes, such as nodC, are not congruent with the phylogenies based on coregenes, reflecting rhizobial host range, rather than species affiliation. This agrees with studies showingthat Mesorhizobium species are able to exchange symbiosis genes through lateral transfer of chromo-somal symbiosis islands, thus acquiring the ability to nodulate new hosts. Phylogenetic analyses of theMesorhizobium genus based on core and accessory genes reveal complex evolutionary relationships anda high genomic plasticity, rendering the Mesorhizobium genus as a good model to investigate rhizobiagenome evolution and adaptation to different host plants. Further investigation of symbiosis genes aswell as stress response genes will certainly contribute to understand mesorhizobia-legume symbiosisand to develop more effective mesorhizobia inoculants