|Van Berkum, Peter
Submitted to: International Journal of Systematic and Evolutionary Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/5/1997
Publication Date: N/A
Interpretive Summary: Sesbania species are short-lived leguminous plants, which are potentially useful to low input sustainable agriculture as green-manure because of a nitrogen-fixing symbiosis with soil bacteria. The Sesbania symbionts are important tools to investigate evolutionary relationships among bacteria because they are highly diverse. We showed that the closest relatives of the symbionts of S. herbacae were Rhizobium galgae, which are the symbionts of Galega orientalis and G. officionalis. Estimates of species limits provided evidence that the Sesbania isolates were distinct from R. galeae. Therefore, we proposed a new species and suggested the name Rhizobium huautlense based upon the region where the symbionts originated. The significance of our findings is the genetic relationships between our new species, R. galegae, Rhizobium leguminosarum and Agrobacterium vitis, which is a plant pathogen were elucidated. Our work provides evidence that R. galegae and R. leguminosarum share a common ancestry. This contrasts with accepted theory that R. galegae and A.vitis share a common ancestry. Previous to our contribution, suggestions were made to change the genus affiliation of R.galegae from Rhizobium to Agrobacterium. Our report does not support such a change. This impacts evolutionary biology of the legume symbionts and is useful to microbiologists.
Technical Abstract: The nitrogen-fixing rhizobial symbionts of Sesbania herbacea growing in the Nature Reserve at the Sierra de Huautla, Mexico, were isolated and characterized. All one hundred and four isolates together with the type strain for Rhizobium galegae, HAMBI 540T, had similar 16S rRNA genes as revealed by PCR-RFLP analysis. Similarity in the sequences of the 16S rRNA genes placed the isolates on a phylogenetic branch shared with R. galegae. Among 66 randomly selected isolates, three closely related electrophoretic types were identified, which were distinct from ten ETs distinguished among 23 strains of R. galegae. We propose a new species Rhizobium huautlense, represented by the Sesbania isolate SO2, based upon low estimates of DNA relatedness between our chosen type strain and the type strains for the other species, the dissimilarity of the nucleotide sequence of the 16S rRNA genes, and their distinct ETs compared with R. galegae. Our description of R. huautlense is significant because in our reconstruction of the phylogeny of R. huautlense there was a shift in the node of the branch of A. vitis relative to that of R. galegae. The revised phylogenetic tree would tend to indicate common ancestry between R. galegae and the type strain for the genus, R. leguminosarum.