Genetic Diversity and Nodulation Potential of Bradyrhizobium Strains in Cowpea and Soybean.

The genus Bradyrhizobium comprises phylogenetically diverse bacteria that establish symbioses with numerous legumes, including economically important crops such as cowpea (Vigna unguiculata) and soybean (Glycine max). This study characterized 34 Bradyrhizobium strains isolated from nodules of cowpea and soybean cultivated in adjacent tropical soils in Brazil to elucidate the relationships between the symbionts and their hosts. Phylogenetic analyses of 16S rRNA, gyrB, recA, and nodC genes, combined with genome sequencing and comparative analyses (ANI and dDDH), revealed high genetic diversity and distinct taxonomic affiliations. Most strains clustered within the B. elkanii superclade, although several occupied divergent lineages, some potentially representing new taxons. Genome-based analyses confirmed these patterns, showing ANI values above 95–96% within groups and below 94% across distant strains. One group of cowpea-derived strains exhibited high symbiotic efficiency and low similarity to all known type strains, suggesting the presence of a novel species with potential for the crop. Conversely, some soybean-derived strains were genetically identical to commercial inoculants, indicating persistence or re-isolation from inoculated soils. Remarkably, strain BR 13971, isolated from soybean, nodulated both hosts efficiently, suggesting a distinct symbiovar with broad host compatibility. Cross-inoculation assays demonstrated that soybean-derived strains nodulated cowpea, but the reverse was not observed. These findings expand the understanding of Bradyrhizobium diversity in tropical soils and highlight the potential of native strains for inoculant development.