Genomic and Functional Diversity of the Sinorhizobial Model Group

Sinorhizobium meliloti (Syn. Ensifer meliloti) is a model species for the study of nitrogen fixation and genomic diversity. This species is distributed worldwide in many different soil types, where it occurs as free living form or as symbiont of leguminous plants (Fabaceae), in which it induces the formation of nodules, where bacteria fix nitrogen. Population surveys of this species have shown an extremely high genetic polymorphism among strains, in particular in genomic regions involved in the symbiosis. In recent years the genomics of natural isolates has enabled us to uncover the nature of the high polymorphism, shedding light on the connection between the genetic diversity and functions related to symbiotic interaction, nutrients utilization and tolerance to challenging conditions in soils. Here we review the most important features of the genomic and phenotypic diversity of S. meliloti in comparison to the closely related species S. medicae, trying to depict the biological significance of the genomic variability of sinorhizobia in connection with soil- and host-associated lifestyles.