Combination of Genetic Variants of nifH, nifK, and nodC Genes in the Formation of a Symbiotic Cluster in the Genome of Rhizobium leguminosarum

Abstract The ability of nodule bacteria to form a nitrogen-fixing legume–rhizobium symbiosis is ensured by a cluster of symbiotic genes that determine both the processes of interaction with the legume, leading to the formation of specialized structures called nodules on the roots, and the fixation of atmospheric nitrogen. The organization and localization of these genomic clusters facilitate their active participation in horizontal gene transfer. The frequency of transfer correlates with the degree of relationship between the bacteria. Recombination events lead to formation of numerous rhizobial strains with varying symbiotic efficiency. In this study, we investigated the possible mechanisms underlying the formation of genetic diversity in nodule bacteria and the involvement of the legume in this diversity. It was demonstrated that the pangenome of a rhizobial population in the rhizosphere of wild legumes may contain several allelic variants of symbiotic genes, which, upon recombination, can form various combinations. This suggests that the evolution of the symbiotic system of nodule bacteria may occur not through the formation of an optimal cluster of symbiotic genes, but through individual evolution of each gene component, which, when assembled into different mosaics, produce diverse symbiotic phenotypes of rhizobia. This allows the host plant to select the most effective microorganisms based on the changing environmental conditions.