Phenotypic and genotypic diversity of similar serotypes of soybean bradyrhizobia from two soil populations

The physiological and genetic diversity within two major serotypic groups of bradyrhizobial isolates obtained from soybean (Glycine max L. Merr.) plants grown on a Dothan and a Cape Fear soil was examined. All isolates serotyped as 3194 had large colonies with smooth borders and high resistances to erythromycin, streptomycin and spectinomycin with minimal inhibitory concentration values (MIC) ranging from 200 to 400 μg ml−1. Pulsed-field gel-electrophoresis (PFGE) separation of DNA fragments generated with the rarely cutting restriction endonuclease, Xba I, revealed six genotypes among 28 different 3194 isolates. Four of these genotypes were common to both soils and only 21% of the isolates were classified as having high N2-fixation capacity. Leaf chlorosis was induced by 46% of the 3194 isolates. Among the 122124 isolates, MIC values were lower than for 3194 isolates ranging from < 13 to 200 μg ml−1. These 122124 isolates produced small colonies (50%) and large colonies with rough borders (50%) when plated on YEM. The genetic diversity of serotype 122124 isolates differed with soil type as PFGE patterns revealed nine genotypes among the 16 isolates from the mineral organic (Cape Fear) soil and only three genotypes among the 14 isolates from the sandy mineral (Dothan) soil. Only two of the 12 genotypes were common to both soils. Sixty percent of the 122124 isolates were classified as having high N2-fixation capacity and none induced foliar chlorosis. Pulsed-field gel-electrophoresis pattern was the only trait that generated groups of isolates that were similar with respect to other measured traits.