Recovery of soybean inoculant strains from uncropped soils in Brazil

Soybean (Glycine max (L.) Merrill) and the corresponding bradyrhizobia were introduced in Brazil several decades ago and have been intensively used since then. However, in the past decade the possibility has been raised of native bradyrhizobia strains able to nodulate soybean. To clarify this, a modem cultivar and six unimproved promiscuous genotypes were inoculated with soil dilutions from 14 diverse uncropped soils beating native vegetation. Isolates of Bradyrhizobium were obtained from seven of the soils, and most proved intolerant of acidity, salinity and high temperature. Thirty-nine of the 40 isolates showed similarity to seven strains that have been, or are, used in commercial inoculants. Characteristics evaluated included: synthesis of indole acetic acid, profiles of protein, lipopolysaccharide and DNA after amplification by PCR with ERIC primer, partial sequence of 16S rRNA and symbiotic properties. From the similarity of these strains to inoculant bradyrhizobia dispersal from soybean-cropped areas seems likely. However, in some strains (as those belonging to serogroup SEMIA 5039), PCR clustering was different from groupings based on serology, profiles of protein and lipopolysaccharide, indicating the presence of indigenous strains with similar properties, or genetic transfer from inoculant strains to indigenous bradyrhizobia, or variability due to the adaptation to different soil conditions. Only one isolate did not fit into the characteristics of known strains and could represent an indigenous soybean Bradyrhizobium, but this isolate showed poor symbiotic performance. (C) 2002 Elsevier Science B.V. All rights reserved.