RESUMO

Symbiotic nitrogen-fixing bacteria, commonly called rhizobia, are agronomically important because they can provide significant amounts of nitrogen to plants and help in recovery of impoverished soils and improvement of degraded environments. In recent years, with advances in molecular techniques, several studies have shown that these bacteria have high levels of genetic diversity, resulting in taxonomic reclassifications and descriptions of new species. However, despite the advances achieved, highly conserved 16S ribosomal genes (16S rRNA) do not elucidate differences between species of several genera, including the genus Bradyrhizobium. Other methodologies, such as multilocus sequence analysis (MLSA), have been used in such cases, with good results. In this study, three strains (SEMIAs 690T, 6387 and 6428) of the genus Bradyrhizobium, isolated from nitrogen-fixing nodules of Centrosema and Acacia species, without clear taxonomic positions, were studied. These strains differed from genetically closely related species according to the results of MLSA of four housekeeping genes (dnaK, glnII, gyrB and recA) and nucleotide identities of the concatenated genes with those of related species ranged from 87.8 % to 95.7 %, being highest with Bradyrhizobium elkanii. DNA-DNA hybridization (less than 32 % DNA relatedness) and average nucleotide identity values of the whole genomes (less than 90.5 %) indicated that these strains represented a novel species, and phenotypic traits were determined. Our data supported the description of the SEMIA strains as Bradyrhizobium viridifuturi sp. nov., and SEMIA 690T ( = CNPSo 991T = C 100aT = BR 1804T = LMG 28866T), isolated from Centrosema pubescens, was chosen as type strain.

Assuntos

Bradyrhizobium/classificação , Fabaceae/microbiologia , Fixação de Nitrogênio , Filogenia , Técnicas de Tipagem Bacteriana , Composição de Bases , Bradyrhizobium/genética , Bradyrhizobium/isolamento & purificação , DNA Bacteriano/genética , Ácidos Graxos/química , Genes Bacterianos , Esterco , Dados de Sequência Molecular , Tipagem de Sequências Multilocus , Hibridização de Ácido Nucleico , RNA Ribossômico 16S/genética , Análise de Sequência de DNA

RESUMO

Symbiotic association of several genera of bacteria collectively called as rhizobia and plants belonging to the family Leguminosae (=Fabaceae) results in the process of biological nitrogen fixation, playing a key role in global N cycling, and also bringing relevant contributions to the agriculture. Bradyrhizobium is considered as the ancestral of all nitrogen-fixing rhizobial species, probably originated in the tropics. The genus encompasses a variety of diverse bacteria, but the diversity captured in the analysis of the 16S rRNA is often low. In this study, we analyzed twelve Bradyrhizobium strains selected from previous studies performed by our group for showing high genetic diversity in relation to the described species. In addition to the 16S rRNA, five housekeeping genes (recA, atpD, glnII, gyrB and rpoB) were analyzed in the MLSA (multilocus sequence analysis) approach. Analysis of each gene and of the concatenated housekeeping genes captured a considerably higher level of genetic diversity, with indication of putative new species. The results highlight the high genetic variability associated with Bradyrhizobium microsymbionts of a variety of legumes. In addition, the MLSA approach has proved to represent a rapid and reliable method to be employed in phylogenetic and taxonomic studies, speeding the identification of the still poorly known diversity of nitrogen-fixing rhizobia in the tropics.