A Rhizobium strain that nodulates and fixes nitrogen in association with alfalfa and soybean plants

A Rhizobium meliloti strain that forms effective nodules on soybean (Glycine max) and alfalfa (Medicago sativa) is described. The strain, 042B, was isolated from root nodules of alfalfa in Xinjiang Autonomous Region of China. Experiments showed that strain 042B was able to nodulate soybean as effectively as Bradyrhizobium japonicum USDA110, a widely used inoculant strain. Under hydroponic conditions, both strains performed similarly in many respects such as time required for the appearance of nodules, total nitrogenase activity, plant top dry weight and total plant nitrogen. In soil pot experiments, total nitrogen, seed weight and seed nitrogen of soybean plants inoculated with strains 042B or USDA110 were much higher than those of the uninoculated control, but there were no differences between plants with strain 042B relative to strain USDA110. However, when strain 042B was the inoculant, the number of nodules was greater, but the nodules were smaller than those of strain USDA110. Results from ELISA showed that nodule occupancy of strain 042B in soybean ranged from 82-90% and that of strain USDA110 ranged from 78-86%. Strain 042B can grow in the presence of 5% (w/v) NaCl, at 42 °C, and at pH 10·7. Strain 042B was shown to have two large plasmids (molecular sizes 200 kb and > 1000 kb, respectively). The DNA G + C mol% of strain 042B was 63·6. The DNA level of homology between strain 042B and type-strain R. meliloti USDA1002 was 76%, while DNA homologies of strain 042B were 10% and 12% with R. fredii USDA205 and B. japonicum USDA6, respectively.