Study Of Rhizobium Helanshanense Sp. Nov.

This article gave a brief introduction about the current taxonomy of rhizobia and the polyphasic taxonomy methods applied in rhizobia. Meanwhile, we used some methods, including the phylogeny research of 16S rDNA genes, housekeeping genes and symbiotic genes with rep-PCR fingerprinting, DNA base composition, DNA-DNA hybridization and numerical taxonomy, in order to identify the taxonomic status of five new rhizobia.During the study of rhizobia that existed in the root nodules of leguminous Sphaerophysa salsula (Pall.) DC in northwest of China, we isolated five rhizobial strains which have representative 16S rDNA gene sequences. In the phylogeny tree based on 16S rDNA gene sequences, five strains were classified in the genus Rhizobium. CCNWQTX14 sequence similarities with their close species R. sphearophysar were 98.0%. So it was possible that five strains were the new species in the genus Rhizobium, 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 Rhizobium. From the comparison, CCNWQTX14 sequence similarities with their close type strain R. sphearophysar were 94.5% and 96.2%, respectively in the phylogeny trees of recA and atpD gene sequences.The phylogeny trees on basis of symbiotic gene sequences weren't totally similar with that of 16S rDNA gene sequences. Through symbiotic gene sequences comparison, we found that the sequences of CCNWQTX14 nifH and nodD genes were phylogenetically close to that of some reference strains in genus Sinorhizobium. For example, the nifH gene sequence similarity between CCNWQTX14 and S. kummerowiae was 100%. So, that was possible the symbiotic genes of new group may get from genus Sinorhizobium through horizontal transfer.In the ERIC fingerprint profiles, the new group was distinct from related Rhizobium species. Furthermore, the new rhizobia and reference strains were together at the similarity level of 42%, showing much lower similarity.The DNA G+Cmol% were from 59.20%-62.33% among the new rhizobial species. Besides, the DNA-DNA homology analysis showed that DNA-DNA hybridization value was 41.80% between CCNWQTX14 and R. daejeonense and this value was lower than that of standard (70%) for new species report.The results of physiological and chemical characteristics indicated there was lot of differences between the new rhizobia and reference strains. The phylogeny tree of numerical taxonomy based on differences of physiological and chemical characteristics showed that the new group and reference strains were together at the similarity level of 42% and the new rhizobia were also distinct with each other. This evidence illustrated that the five strains could be divided from type strains in the genus of Rhizobium, belonging to the novel species.All of these could illustrate that the Rhizobium group could be reported as a unique genomic species and a novel species Rhizobium helanshanense sp. nov. is proposed. The type strain is CCNWQTX14~T.