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The symbiotic system of Legumes and root-nodule bacteria is a significant process for enriching barren and dry soils and supporting plant growth. The aim of this study was to isolate root-nodule bacteria from Medicago falcate and to select high activity strains with resistance to environmental stress. As a result, only MSZ-25 strain grew in the medium containing 0.8M NaCl, with pH4.8-9.8 and at temperatures of 4-45°С. All rhizobial isolates were tolerant to the concentrations of 0.01-0.1% Pb(NO3)2, 0.01% ZnSO4, CdCI2, CuCI2 and NiCI2. The strains were also resisted to penicillin 10μ/ml, erythromycin 10μ/ml, and chlorampenicol 10μ/ml but the growths of all isolates were inhibited by kanamycin 30μ/ml, novobiocin 30μ/ml and tetracycline 10μ/ml.

The importance of root nodule bacteria in biotechnology is determined by their distinctive feature: symbiotic nitrogen fixation resulting in the production of organic nitrogen-containing compounds. While interacting with host legume plants, the cells of these bacteria undergo global changes at all levels of expression of genetic information leading to the formation in root nodules of so-called bacteroids functioning as nitrogen fixation factories. The molecular mechanisms underlying plant-microbial symbiosis are actively investigated, and one of the most interesting and poorly studied aspects of this problem is the species-specificity of interaction between root nodule bacteria and host plants. In this work we have performed the proteomic analysis of the Sinorhizobium meliloti bacteroids isolated from two legume species: alfalfa (Medicago sativa L.) and yellow sweet clover (Melilotus officinalis L.). It has been shown that the S. meliloti bacteroids produce a lot of proteins (many of them associated with symbiosis) in a host-specific manner, i.e., only in certain host plant species. It has been demonstrated for the first time that the levels of expression in bacteroids of the genes encoding the ExoZ and MscL proteins responsible for the synthesis of surface lipopolysaccha-rides and formation of a large conductance mechanosensitive channel, respectively, depend on a host plant species that confirms the results of proteomic analysis. Overall, our data show that the regulation of bacteroid development by the host plant has species-specific features.