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1US Department of Agriculture, ARS, Soybean and Alfalfa Research Laboratory, HH-19, Bldg 011, BARC-West, Beltsville, MD 20705, USA
2Grasslands Research Institute, Hohehott, People's Republic of China
3Penn State University, Berks Campus, Tulpohocken Road, Reading, PA, USA
Author for correspondence: Peter van Berkum. Tel: +1 301 504 7280. Fax: +1 301 504 5728. e-mail: pberkum{at}asrr.arsusda.gov
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.
Key Words: Rhizobium mongolense sp. nov. • Medicago ruthenica • root-nodule bacteria
The GenBank accession numbers for the SSU rRNA gene sequences are U89816--U89832.
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