Impacts of inoculation with Sinorhizobium meliloti strains differing in salt tolerance on the productivity and habitus of alfalfa (Medicago sativa L.)

Symbiotic efficiency (SE) in nodule bacteria (rhizobia) characterizers their adaptive potential and practical significance. This trait is closely related to fixation of molecular nitrogen (N2) which is implemented by rhizobia in root nodules and is controlled by genotypes of both partners. The highest intensity of N2 fixation is typical for fast-growing rhizobia ( Rhizobium, Sinorhizobium ). However, SE (impact of the rhizobia inoculation on the plant biomass) is determined by many gene systems of rhizobia including those not related to operation of nitrogenase. Therefore, N2-fixing activity and SE should be considered as two different symbiotic features. Specifically, rhizobia can synthesize biologically active substances (phytohormones, vitamins, lumichrome) which influence the development of host plants. The aim of our research was to explore the impacts of Sinorhizobium meliloti strains which are fixing actively N2 but are contrasting in salt resistance on productivity and on the indices of habitus (ratios of different plant morphometric traits) in perennial tetraploid alfalfa ( Medicago sativa ). These impacts were studied under influences of the salinity stress and of the microsymbiont mutations affecting SE. We used S. meliloti strains: 1021 (standard laboratory strain), СХМ1-188 (UV-induced mutant of strain СХМ1 possessing an increased SE) and СА67 (strain isolated from the Tajikistan soils). Besides, we used Tn5-induced mutants with a decreased SE: F (obtained from strain СА67 in this research) and GL1 (obtained from strain 1021 at Dalhouse University, Canada). Seeds of alfalfa cultivar Agnia resistant to edaphic stresses were obtained from All-Russia Research Institute of Fodders named after V.R. Williams (Lobnya, Moscow Province). The legume-rhizobia symbiosis was studied in the microvegetative trials by analyzing different morphometric traits (mass and length of shoots and roots) and their ratios as dependent on the salinity stress (0.6 % NaCl). In the absence of stress, all S. meliloti strains possessing the high N2-fixing activities but contrasting in salt tolerance resulted in the increased mass and length of alfalfa shoots combined with the decreased length of roots. These effects lead to a drastic change in the plant habitus (shoot:root ratios for length increased 2-3 fold) which are perhaps responsible for the high intensity of symbiotrophic alfalfa development. Under the salinity stress, no significant changes of the shoot:root length ratio were revealed, while the mass:length ratio in shoots was increased from 0.16-0.34 mg/mm to 0.52-0.68 mg/mm. In plants inoculated with the salt-sensitive S. meliloti strain CA67 (unable to grow in the presence of 3 % NaCl), loss of SE induced by the salinity stress was accompanied by the increase of shoot and root length (by 36 % and 79 %, respectively) although their masses were not changed with respect to non-inoculated control. In plants inoculated with the salt-tolerant strains CXM1-188 and 1021, SE under the salinity stress was retained while the stimulatory impacts on the shoot and root growth were not revealed. The Tn5 mutants F and GL1 characterized by the decreased SE did not differ from parental strains in their influences on plant habitus. In this work we demonstrated for the first time that the SE depends on two factors - nutritional (determined by N2 fixation) and regulatory (possibly mediated by modulations of hormonal status in plants which influence the activities of shoot and root meristems). The genetic modifications of regulatory (growth-stimulatory) rhizobia activities may represent an important approach for optimizing the symbiotrophic nitrogen nutrition in the leguminous crops and for their adaptations to the edaphic (osmotic) stresses.