RESUMO
The importance of horizontal gene transfer (HGT) in the evolution and speciation of bacteria has been emphasized; however, most studies have focused on genes clustered in pathogenesis and very few on symbiosis islands. Both soybean (Glycine max [L.] Merrill) and compatible Bradyrhizobium japonicum and Bradyrhizobium elkanii strains are exotic to Brazil and have been massively introduced in the country since the early 1960s, occupying today about 45% of the cropped land. For the past 10 years, our group has obtained several isolates showing high diversity in morphological, physiological, genetic, and symbiotic properties in relation to the putative parental inoculant strains. In this study, parental strains and putative natural variants isolated from field-grown soybean nodules were genetically characterized in relation to conserved genes (by repetitive extragenic palindromic PCR using REP and BOX A1R primers, PCR-restriction fragment length polymorphism, and sequencing of the 16SrRNA genes), nodulation, and N(2)-fixation genes (PCR-RFLP and sequencing of nodY-nodA, nodC, and nifH genes). Both genetic variability due to adaptation to the stressful environmental conditions of the Brazilian Cerrados and HGT events were confirmed. One strain (S 127) was identified as an indigenous B. elkanii strain that acquired a nodC gene from the inoculant B. japonicum. Another one (CPAC 402) was identified as an indigenous Sinorhizobium (Ensifer) fredii strain that received the whole symbiotic island from the B. japonicum inoculant strain and maintained an extra copy of the original nifH gene. The results highlight the strategies that bacteria may commonly use to obtain ecological advantages, such as the acquisition of genes to establish effective symbioses with an exotic host legume.
Assuntos
Bradyrhizobium/genética , Transferência Genética Horizontal , Genes Bacterianos , Sinorhizobium fredii/genética , Microbiologia do Solo , Simbiose/genética , Bradyrhizobium/isolamento & purificação , Brasil , Impressões Digitais de DNA , DNA Bacteriano/química , DNA Bacteriano/genética , DNA Ribossômico/química , DNA Ribossômico/genética , Genes de RNAr/genética , Ilhas Genômicas , Dados de Sequência Molecular , Fixação de Nitrogênio/genética , Filogenia , Raízes de Plantas/microbiologia , Reação em Cadeia da Polimerase , Polimorfismo de Fragmento de Restrição , RNA Bacteriano/genética , RNA Ribossômico 16S/genética , Homologia de Sequência do Ácido Nucleico , Sinorhizobium fredii/isolamento & purificação , Glycine max/microbiologiaRESUMO
Two DNA fragments, a 730-bp and a 900-bp fragment, one homologous to host cultivar specificity genes nolBT of Sinorhizobium fredii and the other one homologous to RSalpha, an insertion-like sequence present in Bradyrhizobium japonicum, were generated by polymerase chain reaction (PCR) with two pairs of primers. The amount of each fragment generated by the multiplex PCR was proportional to the amount of template DNA present. The amplification of the 900-bp RSalpha fragment was more sensitive, since it was amplified from a smaller amount of template DNA than the 730-bp nolBT fragment. By running the multiplex reaction in the presence of template DNA isolated from different sources, we confirmed that the reaction can discriminate between S. fredii, Bradyrhizobium japonicum and Sinorhizobium xinjiangensis.