Phylogenetic analyses of Bradyrhizobium strains nodulating soybean (Glycine max) in Thailand with reference to the USDA strains of Bradyrhizobium

To elucidate the phylogenetic relationships between Thai soybean bradyrhizobia and USDA strains of Bradyrhizobium, restriction fragment length polymorphism (RFLP) analysis using the nifDK gene probe and sequencing of the partial 16S rRNA gene were performed. In our previous work, Thai isolates of Bradyrhizobium sp. (Glycine max) were separated clearly from Bradyrhizobium japonicum and Bradyrhizobium elkanii based on the RFLP analysis using the nodDYABC gene probe. RFLP analysis using the nifDK gene probe divided 14 Thai isolates and eight USDA strains of B. japonicum into different groups, respectively, but categorized into the same cluster. All of seven strains within these Thai isolates had the same sequence of the partial 16S rRNA gene, and it was an intermediate sequence between those of B. japonicum USDA 110 and B. elkanii USDA 76T. Furthermore, three USDA strains of B. japonicum, USDA 6 (B. japonicum ATCC 10324T), USDA 115 and USDA 129, had the same partial 16S rRNA gene sequence that seven Thai isolates had. These results suggest that Thai isolates of Bradyrhizobium sp. (Glycine max) are genetically distinct from USDA strains of B. japonicum and B. elkanii, but also indicate a close relationship between Thai isolates and USDA strains of B. japonicum.Key words: Bradyrhizobium, 16S rRNA gene, nif gene, soybean, Thailand.