Summary auto-generated
This study describes Rhizobium mongolense sp. nov., a new species of nitrogen-fixing root-nodule bacteria that forms symbioses with Medicago ruthenica, a wild legume native to Inner Mongolia. Researchers collected rhizosphere samples from six locations in Inner Mongolia and isolated 106 rhizobial cultures from plant nodules. Using multilocus enzyme electrophoresis on 94 isolates, they identified 56 distinct electrophoretic types. One isolate (USDA 1920) was highly divergent, while the remaining isolates formed two weakly divergent subgroups. Analysis of small subunit rRNA genes confirmed that isolates belonged to genus Rhizobium rather than Sinorhizobium, which typically associates with Medicago species. DNA-DNA hybridization analysis showed genomic similarity between USDA 1920 and USDA 1844 (representing the other isolates) was below 20%, indicating they represent distinct genomic species. The study identified at least three genomic species forming nitrogen-fixing symbioses with M. ruthenica: R. tropici, USDA 1920 (single isolate), and R. mongolense (represented by USDA 1844). Phenotypic characterization distinguished these isolates based on antibiotic resistance patterns and carbon substrate utilization.
Key findings
- At least three genomic species of rhizobia form nitrogen-fixing symbioses with Medicago ruthenica, including the newly described Rhizobium mongolense
- Multilocus enzyme electrophoresis identified 56 distinctive electrophoretic types among 94 isolates, with two major subgroups and one highly divergent isolate (USDA 1920)
- DNA-DNA hybridization showed less than 20% genomic similarity between USDA 1920 and USDA 1844, confirming they represent distinct species
- Small subunit rRNA gene sequences confirmed isolates belonged to genus Rhizobium, not Sinorhizobium as typically found in other Medicago species
- R. mongolense is phylogenetically distinct from other Rhizobium species and distinguishable by antibiotic resistance patterns and ability to utilize sodium acetate as sole carbon source
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Abstract
Medicago ruthenica [(L.) Ledebour] is native to Inner Mongolia where rhizosphere samples were collected for the isolation of 106 rhizobial cultures. Besides nodulating the original trap host, the isolates formed nitrogen-fixing symbioses with Phaseolus vulgaris. Only half of the isolates nodulated alfalfa (Medicago sativa). but these did not form nitrogen-fixing symbioses. Rhizobium tropici also formed nitrogen-fixing symbioses with Medicago ruthenica. A total of 56 distinctive muitilocus electrophoretic types (ETs) were identified among 94 of the 106 isolates which were analysed 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. It was concluded from small subunit rRNA gene sequences of eight isolates of Medicago ruthenica that they belonged to the genus Rhizobium and not to the genus Sinorhizobium which is more commonly associated with Medicago. 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 it was concluded that at least three genomic species of rhizobia form nitrogen-fixing symbioses with Medicago ruthenica. One of these genomic species is R. tropici, another is represented by the single isolate USDA 1920 and the name Rhizobium mongolense is proposed for the third genomic species represented by USDA 1844.