Genetic Characterization of Rhizobia Isolated From Soybean Nodules and Inoculants.

Field trials evaluating the effectiveness of new commercial formulations of rhizobia inoculants were conducted over a three-year period at three sites with or without a recent history of soybean production. Soybean seed inoculation did not result in statistically significant increase in nodule count or soybean yield at any location. To determine whether the inoculated rhizobial strains were responsible for nodulation of soybean plants, we isolated rhizobia from six inoculants and soybean root nodules in inoculated treatments and uninoculated controls. A total of 107 rhizobial isolates from soybean nodules and 12 isolates from the inoculants were obtained. Rep-PCR DNA fingerprinting technique was used to determine the similarity of rhizobial isolates. The results show that rhizobia from the commercial inoculants were different from those inside soybean nodules. Rhizobia isolated from three different locations formed three sub-clusters by location within the large cluster formed by all root nodule isolates. The data suggest that rhizobia in the soybean root nodules were not the same as those in the inoculants and that rhizobia in the nodules of soybean without inoculation were similar to those in the nodules of inoculated soybean. There were geographical differences among rhizobial isolates from three experimental sites.