Caracterização da diversidade de estirpes de Bradyrhibium japonicum e B. elkanii estabelecidas por inoculação em solos do Cerrados, isoladas de nódulos de soja

Não disponibilizado pelo autorThe plasticity of rhizobial genomes is far greater than previously thought, with complex genomic recombination events that may be accelerated by the often-stressful environmental conditions of the tropics. This study aimed at evaluating changes in soybean rhizobia due to adaptation to inhospitable environmental conditions (high temperatures, drought and acid soils) in the Brazilian Cerrados. Both the host plant and combinations of four strains of soybean Bradyrhizobium were introduced in an uncropped soil devoid of rhizobia capable of nodulating soybean. After the third year, seeds were not reinoculated. Two hundred and sixty-three isolates were obtained from nodules of field-grown soybean after the seventh year, and their morphological, physiological, serological and symbiotic properties determined, followed by genetic analysis of conserved and symbiotic genes. B. japonicum strain CPAC 15 (same serogroup as USDA 123) was characterized as having high saprophytic capacity and competitiveness, representing up to 70% of the population by the seventh year, in contrast to B. japonicum strain CPAC 7 (same serogroup as CB 1809). In general, adapted strains had increased mucoidy, and up to 43% of the isolates showed no serological reaction. High variability, presumably resulting from the adaptation to the harsh environmental conditions, was verified in rep-PCR profiles, being lower in strain CPAC 15, intermediate in B. elkanii and higher in CPAC 7. RFLP-PCR types of the 16S rDNA corresponded to: one type for B. elkanii species, two for B. japonicum, associated to CPAC 15 and CPAC 7, and unknown combinations of profiles. However, when nodC sequences and RFLP-PCR of the nifH region data were considered, only two clusters were observed with full congruence with B. japonicum and B. elkanii species. Combining the results, variability was such that even within a genetically more stable group, such as that of CPAC 15, only 6.4% of the isolates showed high similarity to the inoculant strain, whereas none was similar to CPAC 7. The genetic variability in our study seems to result from a variety and combination of events including strain dispersion, genomic recombination and horizontal gene transfer. Furthermore, the genetic variability appears to be mainly associated with adaptation, saprophytic capacity and competitiveness, and not with symbiotic effectiveness, as the similarity of symbiotic genes was higher than that of conserved regions of the DNA