Biodiversity of bradyrhizobia nodulating Lupinus spp.

The genetic structure of Bradyrhizobium isolates recovered from three Lupinus species (Lupinus campestris, Lupinus montanus, and Lupinus exaltatus) grown in Mexico was examined. Among 41 Bradyrhizobium isolates, 18 electrophoretic types (ETs) were distinguished by multilocus enzyme electrophoresis of five metabolic enzymes. The mean genetic diversity, 0.64, indicated that there was great genetic diversity in the population sampled. Most isolates (63%) fell into two closely related clusters (clusters I and II) and were the types most frequently isolated from the root nodules of L. montanus and L. campestris. ET cluster III isolates were frequent nodule occupants of L. exaltatus. The isolates also were assigned to three main groups by using Curie point pyrolysis mass spectrometry. In general, the multilocus enzyme electrophoretic data and pyrolysis mass spectrometric data agreed. We determined the 16S rRNA sequences of representative Lupinus isolates and of Bradyrhizobium japonicum USDA 6T and found that the lupine isolates were highly related to the B. japonicum type strain, although not all B. japonicum type strains (subcultures maintained in different bacterial collections) had identical small-subunit rRNA.