Aiming at investigating the overall diversity, biogeography, and symbiosis gene evolutionary history of the Sesbania cannabina-nodulating rhizobia in China, a total of 874 rhizobial isolates originating from the root nodules of this plant grown at different sites were characterized and compared with those of some reference strains. All of the S. cannabina-nodulating rhizobia were classified into 16 (geno) species, including seven novel genospecies in the genera Ensifer, Rhizobium, Neorhizobium, and Agrobacterium, with Ensifer sesbaniae and Neorhizobium huautlense as the dominant and universal species. Ten of these species were found to nodulate other leguminous hosts or to lack nodulating abilities and were defined as symbiovar sesbania. Biogeographic patterns were observed, for which pH, TN, AK, and AP were the main determinants. The effects of pH were opposite to those of TN and AK, while AP presented effects independently of TN, AK, and pH. Symbiotic genes of these rhizobia showed a common origin, but nodA evolved faster than nifH. Point mutation is the main driving force in the evolution of both nodA and nifH, and lateral transfer of symbiotic genes might play an important role in the formation of diverse S. cannabina-nodulating rhizobial species. S. cannabina only nodulates with Sesbania rhizobia, demonstrating its severe selection on rhizobial symbiosis genes. Soil pH and physiochemical characteristics could affect rhizobial survival and competitive nodulation. This study provides insight into the community shifts and evolution of rhizobia in relation to their host and soil environments.
Keywords: Adaptation; Horizontal transfer; Rhizobia; Sesbania cannabina; Soil properties; Symbiotic genes.