Analysis for Nodulation and Nitrogen Metabolism Genes in the Genome of Bradyrhizobium japonicum Strains Isolated in the Lower Volga Region

Seven indigenous strains of Bradyrhizobium japonicum subsp. saratovii isolated from soybean nodules (Glycine max (L.) Merr.) grown in the arid Lower Volga region of Russia were investigated. A complete set of the major symbiotic genes was detected in all strains. Single synonymous nucleotide substitutions were identified in nifH and nifD, whereas a missense mutation, Ser149Phe, was found in the nifK gene of strain II-2. Homology modelling showed that this substitution did not cause any noticeable rearrangement of the overall structure of the nitrogenase β-subunit, although it was located near the conserved Cys152 residue. The nodB and nodC genes were completely conserved; the nodC sequence corresponded to symbiovar glycinearum. Structural disruptions in the nap operon were detected in strains II-2 and III-2, which was consistent with the absence of nitrate-reducing activity. Comparison of the genomic data with the inoculation results showed that the most effective strain, II-2, combined strong growth-promoting and yield-enhancing effects with the presence of a complete set of symbiotic genes and disruption of napA. These findings indicate that the integration of genomic analysis with phenotypic assessment improves the accuracy of selecting promising indigenous Bradyrhizobium strains for soybean inoculation.