Changes in protein profiles (SDS-PAGE) of competitive Bradyrhizobium sp. strains during soybean pre-infection stage

Abstract

When the Bradyrhizobium japonicum SEMIA 566 (parental strain) was introduced into Cerrados soils (Savanna soils) in 1976, this strain was found to be able to occupy only 2% of soybean nodules. In contrast, strains 29W and SEMIA 587 both presented 90% of nodule occupancy. Nowadays, in the same type of soil, the serogroup 566 dominates (60%) in the soybean nodules, dislocating the 29W and SEMIA 587 strains. For comparative analysis of parental and adapted SEMIA 566, some strains SEMIA 566 were re-isolated from Cerrados and used as inoculant to soybean cultivated in two Cerrado areas. The first area showed a resident soil population of serogroup 29W (area 1), and in the second area the established soil population belongs to serogroup SEMIA 587 (area 2). The re-isolates that were more competitive in 29W area were different from those that have shown to be more competitive than SEMIA 587. After six hours of contact with soybean roots, the re-isolates SEMIA 566 that were more competitive than 29W presented alterations of electrophoretic (SDS-PAGE) polypeptidic bands of cellular envelope with molecular weight (MW) of 44-46kD and 37kD. On the other hand, the re-isolates that showed higher nodule occupancy levels in relation to SEMIA 587 registred alterations in proteins of 44kD, 42Kd, 40kD and 37kD. The low competitive strains presented delayed alterations (12 hours) or did not modify these polypeptidic bands. The reisolates SEMIA 566 as well as the parental strain SEMIA 566, are serologically identical, differing from each other in the protein of 44kD which was absent in the parental strain. The strains SEMIA 566, isolated from Cerrados soils differed from the parental SEMIA 566 through the presence of a 44kD protein. Therefore, those isolates, besides being competitive, constitute a new SEMIA 566 subpopulation.