Genome rearrangements of indigenous soybean bradyrhizobia and their ecological significance

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In order to understand the generation and ecological roles of Bradyrhizobium japonicum HRS strains possessing high copy numbers of insertion sequences (IS), we have investigated (1) geographical and phylogenetic distributions of HRS strains, (2) their excellent features in field ecosystem, (3) their genome rearrangements, and (4) horizontal transfer of nodulation (nod) gene from HRS strains. We found a correlation among the IS copy number, phylogenetic position, serogroup and growth rate of HRS strains, which are derived from USA, China and Japan. Nodule occupancy of HRS strains increased in a water-logged soil condition. Moreover, HRS strains showed stress resistance. The nif gene cluster of HRS strain were subjected to DNA rearrangement probably by IS elements. We have observed horizontal transfer of nod genes from B. japonicum HRS strain to B. elkanii USDA94 mutant lacking common nod genes as a recipient. In this project, we could show the ecological and genetic features of B. japonicum HRS strains.