Genetic diversity of elite rhizobial strains of subtropical and tropical legumes based on the 16S rRNA and glnII genes

Abstract

Biodiversity of diazotrophic symbiotic bacteria in the tropics is a valuable but still poorly studied resource. The objective of this study was to determine if a second housekeeping gene, glnII, in addition to the 16S rRNA, can be employed to improve the knowledge about taxonomy and phylogeny of rhizobia. Twenty-three elite rhizobial strains, very effective in fixing nitrogen with twenty-one herbal and woody legumes (including species from fourteen tribes in the three subfamilies of the family Leguminosae) were selected for this study; all strains are used as commercial inoculants in Brazil. Complete sequences of the 16S rRNA and partial sequences (480bp) of the glnII gene were obtained. The same primers and amplification conditions were successful for sequencing the glnII genes of bacteria belonging to five different rhizobial genera-Bradyrhizobium, Mesorhizobium, Methylobacterium, Rhizobium, Sinorhizobium)-positioned in distantly related branches. The analysis of the concatenated genes (16S rRNA+glnII) considerably improved information about phylogeny and taxonomy of rhizobia in comparison to the single analysis of the 16S rRNA. Nine strains might belong to new species. The complementary analysis of the glnII gene was successful with all strains and improved the phylogenetic clustering and clarified the taxonomic position of several strains. The strategy of including the analysis of glnII, in addition to the 16S rRNA, is cost- and time- effective for the characterization of large rhizobial culture collections or in surveys of many isolates.