Submitted to: International Journal of Systematic and Evolutionary Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: February 14, 1997
Publication Date: N/A
Interpretive Summary: Medicago ruthenica potentially is a new agricultural forage crop and may be a source of genes for the genetic improvement of alfalfa. Since M. ruthenica is a legume it benefits from a nitrogen-fixing symbiosis. In agriculture, the benefit is in the form of enhanced efficiency of crop production. Management of biological nitrogen fixation in legume crop production involves the inoculation of the appropriate bacterial cultures at the time of sowing. However, the bacteria forming symbioses with M. ruthenica have not been cultured or characterized. Therefore, these bacteria are not available to industry for the production of inoculants, and hence they are not available to the farmers who wish to grow M. ruthenica. This publication describes the isolation and characterization of the rhizobia of M. ruthenica. These bacteria do not associate with commercial alfalfa, nor does this crop form symbioses with the rhizobia of M. ruthenica. The isolates obtained are characterized as a new species of Rhizobium, R. mongolense. This information will be useful to scientists and to industry because these rhizobia are available for further study and for the production of inoculants.
Medicago ruthenica [(L.)Ledebour] is native to Inner Mongolia edwhere rhizosphere samples were collect for the isolation of 105 rhizobial cultures. Besides nodulating the original trap host, the isolates formed nitrogen-fixing symbioses with Phaseolus vulgaris. Only half of the isolates nodulated alfalfa (M. sativa), but these did not form nitrogen-fixing symbioses. Rhizobium tropici also formed nitrogen-fixing symbioses with M. ruthenica. A total of 56 distinctive multilocus electrophoretic types (ETs) were identified among 94 of the 105 isolates which were analyzed for variation in electrophoretic mobility of 12 enzyme loci. One isolate (USDA 1920) possessed a unique ET, while the ETs of the other isolates formed two weakly divergent subgroups approximately equal in size. We conclude from SSU rRNA gene sequences of eight isolates of M. ruthenica that they belonged to the genus Rhizobium and not to Sinorhizobium, which is the genus more commonly associated with Medicago species. Genomic similarity, determined from DNA hybridization analysis, between USDA 1920 and the strain representing the remaining isolates (USDA 1844) was lower than 20%. Based upon these observations we concluded that at least three species of rhizobia form nitrogen-fixing symbioses with M. ruthenica. One of these species is R. tropici, a second unnamed species is represented by the single isolate USDA 1920 and we propose the name Rhizobium mongolense for the third species represented by USDA 1844.