Changes in electrophoretic profiles of lipopolysaccharides from competitive strains of Bradyrhizobium spp. induced by soybean roots

The soybean Bradyrhizobium strain Semia 566 was introduced into soils of the Cerrados (Brazilian edaphic savannas) in the late 1960s. Then, nodule occupancy by this strain was not greater than 2%. Recently, this serogroup has been found in approximately 60% of nodules formed on soybeans cultivated in the Cerrados, replacing the strains 29W and Semia 587, the Brazilian commercial inoculant for soybean. Although some re-isolates of Semia 566, adapted to Cerrado soils, were more competitive than 29W under both field and aseptic conditions, they did not differ from the parental strain, based on their lipopolysaccharide (LPS) electrophoretic profile. The only exceptions were the isolates 4A-5 and CPAC-15 which presented an additional polysaccharidic band of low molecular weight or higher mobility. On the other hand, this same band may be induced and intensified in LPS extracted from competitive strains (29W, 220, 204, 370, 372, 516, 122 and CPAC-15) after bacterial contact with soybean roots for 6 or 12 h. In addition, a 29W Tn5 mutant with a phenotype of delayed nodulation showed a delayed induction of this polysaccharidic band. Conversely, the LPS of less competitive strains was not modified or showed a weak intensification of this band. As this band alteration was correlated with the concurrent elevation of dominance in nodules, it may be suggested that LPS plays a role in the competitive ability of rhizobia strains for nodulation.