Root nodule bacteria: perspectives of monitoring symbiotic properties by applying genetic markers

Alfalfa and soybeans are widely cultivated economically valuable fodder and leguminous crops the yield of which directly depends on bacterial microsymbionts. Processing seeds of legumes with strains of nodule bacteria (rhizobia) helps to increase the productivity of the plant-microbial system both in typical and in adverse growing conditions, for example, on degraded soils, including those subjected to salinization, waterlogging, aridity, etc. That is why obtaining new strains capable of forming highly productive and stress-resistant symbiotic systems with leguminous plants is extremely popular in agriculture. Modern technologies for the production of highly productive and environmentally differentiated varieties of legumes suggest the use of a biogeocenotic approach based on the consideration of symbiotrophic indicators (Z.S. Shamsutdinov, 2014). The formation of highly productive plant-microbial systems is based on the principle of complementarity of macro- and microsymbiont genomes (I.A. Tikhonovich et al., 2015), the complementarity of the interaction of which determines their successful introduction into agrocenoses, which differ in agroclimatic and soil conditions. Virulence, competitiveness, specificity and effectiveness of nitrogen-fixing activity, which rhizobia exhibit in relation to a certain species, and sometimes to the variety of the leguminous host plant, are among the symbiotically significant and genetically determined properties of bacteria. All of the above symbiologically significant characteristics are determined by numerous groups of rhizobia genes. The review presents an analysis of data on the genes of soybean and alfalfa microsymbionts, for which participation in the control of symbiotic activity and stress resistance has been experimentally proven. Nodule bacteria of the species Sinorhizobium meliloti , S . fredii , and Bradyrhizobium japonicum are the most studied, but contrastingly differing in the genetic and morphophysiological characteristics. Analysis of recent data on the main groups of symbiologically significant genes (i.e. nod genes involved in the synthesis and decoration of the Nod factor signaling molecule which initiates the nodulation process during plant-microbial interaction, the nif , fix , and eff groups of genes responsible for the nitrogen fixation and symbiotic effectiveness) indicates a continuing high degree of incompleteness and fragmentation for both fast- and slow-growing rhizobia species. At the same time, according to published data, allelic polymorphism for these genes is a factor that plays an important role in varying signaling, host specificity, and symbiotic efficacy in both fast and slow-growing species of nodule bacteria. It is concluded that a coupled analysis of sequences of interest genes from functionally different groups of genes involved in the formation of highly effective stress-resistant symbioses, the sym genes (symbiosis), srg (stress related genes; genes of resistance to stress factors) and QS (quorum sensing genes), or sym - srg -QS genes, are promising for the search and creation of molecular markers associated with the symbiotic and adaptive properties of nodule bacteria necessary for monitoring them under laboratory conditions and in microbiome of agrocenosis.