Characterization of rhizobia that nodulate legume species of the genus Lespedeza and description of Bradyrhizobium yuanmingense sp. nov.

Legume species belonging to the genus Lespedeza are annual or perennial herb or shrub plants that grow in the northern hemisphere. They are known for the formation of root nodules, but little information is available about their microsymbionts. In this study, 58 root-nodule isolates from Lespedeza spp., obtained from China and the USA, were characterized using numerical taxonomic analysis of phenotypic features, SDS-PAGE analysis of whole-cell proteins, DNA-DNA hybridization, 16S rRNA gene sequence analysis and cross-nodulation with selected legume species. From the results generated using these approaches, it was concluded that Lespedeza spp. were promiscuous hosts for rhizobia. Four main clusters of bacteria, which included 35 of the strains isolated from Lespedeza spp., were defined upon numerical taxonomic analysis; these groups corresponded to those determined from analyses of protein electrophoretic and DNA-DNA hybridization data. The four clusters were found to define strains belonging to one of four species, Sinorhizobium saheli, Bradyrhizobium japonicum, Bradyrhizobium elkanii or a novel species of the genus Bradyrhizobium. The strains of B. japonicum and B. elkanii were all from the USA soil samples, and their representative strains could not nodulate soybean. The seven strains found to represent the novel Bradyrhizobium sp. were from China. These were differentiated from recognized species of the genus Bradyrhizobium by all of the taxonomic methods used here; hence, it is proposed that the novel strains isolated from Lespedeza spp. represent a novel species of the genus Bradyrhizobium, Bradyrhizobium yuanmingense. The type strain of the novel species, CCBAU 10071(T) (= CFNEB 101(T)), formed ineffective nodules on Medicago sativa and Melilotus albus but did not nodulate soybean. The other 23 bacterial strains isolated from Lespedeza spp. were found to form single branches or small groups (two to three strains) that were related to Bradyrhizobium, Mesorhizobium, Rhizobium and Sinorhizobium spp. on the basis of numerical taxonomic analysis, indicating the possibility that other rhizobial species are also associated with Lespedeza spp.