Mesorhizobium sangaii sp. nov., isolated from the root nodules of Astragalus luteolus and Astragalus ernestii

Our previous published data indicated that the two rhizobial strains SCAU7T and SCAU27, which were isolated from the root nodules of Astragalus luteolus and Astragalus ernestii respectively, in Sichuan Province, China, might be novel species of the genus Mesorhizobium. Their exact taxonomic position was determined in the present study by using polyphasic approaches. Comparative analysis of nearly full-length 16S rRNA gene sequences showed that these strains belonged to the genus Mesorhizobium, with Mesorhizobium ciceri USDA 3383T, Mesorhizobium loti NZP 2213T, Mesorhizobium shangrilense CCBAU 65327T and Mesorhizobium australicum WSM2073T as the closest neighbours (>99 % 16S rRNA gene sequence similarity). Phylogenies of the housekeeping genes atpD and recA confirmed their distinct position, showing low similarity with respect to those of M. loti LMG 6125T (96.5 % and 92.3 % similarity respectively), M. ciceri USDA 3383T (96.8 % and 93.3 % similarity, respectively), M. shangrilense CCBAU 65327T (96.5 % and 92.7 % similarity, respectively) and M. australicum WSM2073T (95.4 % and 90.6 % similarity, respectively). The DNA–DNA relatedness values between strain SCAU7T and strain SCAU27 were 83.0 %, showing that they belong to the same species. The DNA–DNA relatedness values of SCAU7T with M. loti NZP 2213T, M. ciceri USDA 3383T and M. shangrilense CCBAU 65327T were 41.1 %, 48.8 % and 23.4 %, respectively, clearly indicating that strain SCAU7T represents a novel species. A series of phenotypic and genotypic tests and comparison of cellular fatty acids indicated that the novel group of isolates was distinct from previously described species. Therefore, we propose that strains SCAU7T and SCAU27 represent a novel species of the genus Mesorhizobium, Mesorhizobium sangaii sp. nov., with strain SCAU7T ( = HAMBI 3318T = ACCC 13218T) as the type strain.