Evaluation of genetic diversity of bradyrhizobia strains nodulating soybean [Glycine max (L.) Merrill] isolated from South Brazilian fields

The massive inoculation of Brazilian soils with few bradyrhizobia strains recommended for soybean has resulted in an established population in most soils cropped with this legume. Besides, several environmental conditions are limiting factors to the growth and activity of rhizobia in soil. These features can shape soil and plant-associated habitats, modifying the composition and activities of their microbial communities. In this work, five bacterial populations from distinct regions of Rio Grande do Sul State were analyzed by rep-PCR and AFLP methodologies. A high level of genetic diversity within populations was observed. The Shannon index was estimated considering a level of 70% of similarity in the profiles and, varied from 3.95 to 6.17 in the different areas. Using the principal coordinate analysis as statistical approach to correlate the bacterial diversity to the soil parameters, it was found that pH, clay and organic matter contents were the major soil factors affecting diversity. Soil pH was the main characteristic that affected bradyrhizobial diversity, whereas clay and organic matter contents had less influence in bacterial diversity. The present study emphasizes that there is a high level of genetic diversity in bradyrhizobia populations that nodulate soybean in Southern Brazilians fields. This information could be useful in the formulation of new inoculants containing strains better adapted to the local environmental conditions, resulting in the improvement of the cropping systems into which these inoculants can be most profitably applied, increasing significantly the productivity of soybean in Brazilian fields.