Highly effective root nodule inoculants of alfalfa (Medicago varia L.): molecular-genetic analysis and practical usage in cultivar creation

In grass stands of perennial fodder legumes, alfalfa is a preferred leguminous plant for the creation of cultivated pastures and restoration of degraded soils. Medicago sativa L. nothosubsp. Varia (Martyn) Arcang is a high-yielding crop and resistant to adverse cultivation conditions, which is important for risky farming zones in Russia. Alfalfa productivity largely depends on the success of the formation of a plant-microbial symbiotic system with root nodule bacteria (rhizobia), which becomes able to fix atmospheric nitrogen. Modern symbiogenetics has shown that the effectiveness of symbiotic systems depends on the complementary interaction of the plant and microsymbiont genomes. Proceeding from this, the biological products that are used to treat legume seeds should also contain selected rhizobia strains with corresponding genotypic characteristics. In the present work, a comparative analysis of the yield of 73 variety-strain combinations, including Sinorhizobium meliloti represented by isolates from salt affected areas and by two strains (425a and 415b) of commercial importance, with alfalfa varieties obtained by classical and symbiotically depended plant breeding approaches was performed. The prospects of selecting highly effective strains those complementary to economically valuable varieties of alfalfa in model experiments have been shown. The strains A1 and A2 were found to be symbiotically more active with tested alfalfa varieties than strains of commercial importance. It has been evaluated that cultivar-strain combinations based on cultivars obtained by a symbiotically depended plant breeding approach are characterized by increased adaptability, and their potential to increase yield significantly exceeds 50 %. It was revealed that yields of variety-strain combinations formed by strain 425a are under the influence of uncontrolled factors according to two-factor analysis of variance. A high complementarity of strains A1 and A2 to the Agnia variety, and the functional significance of the genetic characteristics of strain 425a, led us to study their genomic characteristics. A comparative analysis of genomes done by using DNA biochips approach revealed significant differences between symbiotically highly effective strains. It was established that genes related to symbiotic activity and stress tolerance of rhizobia had a divergent structure mainly in strains recovered from salinized soils. The data of the first stages of the molecular genetic analysis of highly effective strains strongly indicate the need to continue research that will allow targeted selection of microsymbiont strains for modern varieties of alfalfa. Data produced through the research of a number of variety-strain combinations grown in various climatic conditions of the Russian Federation clearly demonstrate the need for the widespread introduction of a symbiotically depended plant breeding approach in order to create new economically valuable varieties of legumes needed to form a sustainable forage base for farmers.