Bradyrhizobium tropiciagri sp nov and Bradyrhizobium embrapense sp nov., nitrogen-fixing symbionts of tropical forage legumes

Biological nitrogen fixation is a key process for agricultural production and environmental sustainability, but there are comparatively few studies of symbionts of tropical pasture legumes, as well as few described species of the genus Bradyrhizobium, although it is the predominant rhizobial genus in the tropics. A detailed polyphasic study was conducted with two strains of the genus Bradyrhizobium used in commercial inoculants for tropical pastures in Brazil, CNPSo 1112(T), isolated from perennial soybean (Neonotonia wightiD, and CNPSo 2833(T), from desmodium (Desmodium heterocarpon). Based on 16S-rRNA gene phylogeny, both strains were grouped in the Bradyrhizobium elkanii superclade, but were not clearly clustered with any known species. Multilocus sequence analysis of three (gin/l, gyrB and recA) and five (plus atpD and dnaK) housekeeping genes confirmed that the strains are positioned in two distinct clades. Comparison with intergenic transcribed spacer sequences of type strains of described species of the genus Bradyrhizobium showed similarity lower than 93.1 %, and differences were confirmed by BOX-PCR analysis. Nucleotide identity of three housekeeping genes with type strains of described species ranged from 88.1 to 96.2 %. Average nucleotide identity of genome sequences showed values below the threshold for distinct species of the genus Bradyrhizobium (<90.6 %), and the value between the two strains was also below this threshold (91.2 %). Analysis of nifH and nodC gene sequences positioned the two strains in a Glade distinct from other species of the genus Bradyrhizobium. Morphophysiological, genotypic and genomic data supported the description of two novel species in the genus Bradyrhizobium, Bradyrhizobium tropiciagri sp. nov. (type strain CNPSo 1112(T)=SMS 303(T)=BR 1009(T)=SEMIA 6148(T)=LMG 28867(T)) and Bradyrhizobium embrapense sp. nov. (type strain CNPSo 2833(T)=CIAT 2372(T)=BR 2212(T)=SEMIA 6208(T)=U674(T)=LMG 2987).