Population polymorphism of the alfalfa nodule bacteria ( Sinorhizobium meliloti) for the genes encoding for symbiotic efficiency and competitiveness

The most important traits of nodule bacteria (rhizobia), which are tested for selection of commercially valuable strains are the symbiotic efficiency (ability to increase the mass of the inoculated host plants due to intensive fixation of molecular nitrogen) and competitiveness (ability to inoculate the plants in the presence of other rhizobia strains of the same species). By PCR analysis of alfalfa rhizobia ( Sinorhizobium meliloti ) of various origins we for the first time showed polymorphism of natural populations for genes Smc03879 or phbA (negative regulator of the effective symbiosis, encodes for synthesis of poly-β-hydroxybutyrate) and Sma0907 or cmp-107 (involved in the competition for nodultion). The population of the Uralsk region of Kazakhstan living in the impoverished saline soil, phbA gene was detected in 100 % of the strains, while population of the rich Chernozem soil of the Ternopil region of Ukraine in only 82.2 % of the strains. Even lower occurrence of phbA (30.8 %) is characteristic of the strains isolated from alfalfa nodules collected in the Ternopil region. This polymorphism may reflect a low adaptive value for conversion of C-compounds into poly-b-hydroxybutyrate for rhizobia strains from the Ternopil region in comparison with the strains of the Uralsk region. Participation of gene phbA in the control symbiotic efficiency is confirmed by the fact that the gene nifH, which encodes for the structure of the small subunit of nitrogenase, but does not affect the intensity of its synthesis, differences in polymorphism between strains of two populations were not identified. The absence of polymorphism for gene Smb21375 or eff-798 in all studied populations may be due to the fact that it encodes an ABC transporter required for the uptake by bacteria the broad range of essential nutrients obtained from both the host plant or from the soil. An increased incidence of effective symbionts among strains lacking gene phbA, indicates the prospects to use the molecular probes containing this gene for the selection of practically important rhizobia genotypes that can provide the plant host with the significant amounts of fixed nitrogen.