Mesorhizobium Robiniae Sp. Nov., A Novel Species Isolated From Root Nodule Of Robinia Pseudoacacia

Thirty three isolates were obtained from individual R. pseudoacacia nodules collected in Yangling, in the Shaanxi province of China, were characterized using RFLP and sequencing of 16S rDNA, nodA, nodC and nifH genes. The result showed that five type strains of group I were different from known species in Mesorhizobium, were probable new species in Mesorhizobium. We used some methods, including the phylogeny housekeeping genes and symbiotic genes with rep-PCR fingerprinting, DNA base composition, DNA-DNA hybridization and phenotype characteristic, in order to identify the taxonomic status of five new rhizobia.In the phylogeny tree based on 16S rDNA gene sequences, the five strains were classified in the genus Mesorhizobium, sequence similarities were 99.5% and 99.6%, respectively, in relation to those of the most related species M. mediterraneum and M. temperatum. And five strains had some hereditary distance with known species in Mesorhizobium. So it was possible that five strains were new species in the genus Mesorhizobium, which were different from whole type strains in this genus.In the phylogenetic analyses of the housekeeping genes, recA and atpD, showed five rhizobia were also within Mesorhizobium, supporting the conclusion from 16S rDNA gene analysis. But five strains were different in the status of three evolutionary trees. These obvious differences in genes between five strains and some type strains could make sure the remarkable status of these new founded rhizobia.In the ERIC fingerprint profiles, the new group was distinct from related Mesorhizobium species. Furthermore, the new rhizobia and reference strains were together at the similarity level of 50%, showing much lower confidence.The DNA G+C mol% were from 61.41%～63.35% among the new rhizobial species. Besides, the DNA-DNA homology analysis showed that DNA-DNA hybridization value was 90.1% and 96.9% among CCNWYC 115T and the other four strains in the novel group, but was 47.5% between CCNWYC 115T and M. tarimense and this value was lower than that of standard (70%) for new species report.119 physiological and chemical characteristic were tested for five tested strains and four reference strains. The results indicated five tested strains had extensive use of carbon and nitrogen source, strong resistence. In addition, there were thirty one differences in phenotype characteristic between five tested strains and four reference strains. And some differences existed among five tested strains.All of these could illustrate that the Mesorhizobium group could be reported as a unique genomic species and a novel species Mesorhizobium robiniae sp. nov. is proposed. The type strain is CCNWYC115T.