Characterization of soybean Bradyrhizobium strains adapted to the Brazilian savannas

Brazilian soils are originally free from soybean bradyrhizobia and the firs t inoculants were brought to the country in this century, but a search for adapted strains started immediately and still continues. A strain selection program was established at Embrapa based on the reisolation of strains aft er a long period of adaptation to the soils followed by a search for varian t genotypes with higher N-2 fixation capacity and competitiveness. A second approach of this program consists of searching for variant colonies of a s ingle strain with higher N-2 fixation rates and competitiveness, following a short period of adaptation to the soil. In this study, using both approac hes, strains belonging to three serogroups, CB 1809, 532C and SEMIA 5020, w ere obtained. In general, the variant strains showed differences in colony morphology (mucoidy) but produced similar protein and lipopolysaccharide pr ofiles. Within serogroup CB 1809, containing variants obtained via the seco nd approach, a low level of DNA polymorphism was detected relative to the p arental genotype by ERIC and REP-PCR. However, within the two other serogro ups, containing variant strains obtained via the first approach, a high lev el of polymorphism in ERIC and REP-PCR fingerprints was observed relative t o the putative serologically related parental genotypes. These results show that a great variability can be detected following adaptation of Bradyrhiz obium strains to the soil, although other potential explanations for the DN A polymorphisms observed are discussed. Some of the variant strains obtaine d by both methodologies were found to have higher rates of N-2 fixation and almost all were more competitive than the parental genotypes, suggesting t hat it is possible to select variant strains which can contribute to an imp roved plant N nutrition status. (C) 1999 Federation of European Microbiolog ical Societies. Published by Elsevier Science B.V. All rights reserved.