The feasibility of using the plant growth promoting bacteria isolated from the nodules to increase the alfalfa (Medicago sativa L.) plant resistance to salinity stress

Introduction Salinity stress negatively impacts agricultural yield throughout the world affecting production whether it is for subsistence or economic gain. The plant response to salinity consists of numerous processes that must function in coordination to alleviate both cellular hyperosmolarity and ion disequilibrium. In addition, crop plants must be capable of satisfactory biomass production in a saline environment (yield stability).Soil salinity is a major constraint to food production because it limits crop yield and restricts use of land previously uncultivated. The United Nation Environment Program estimates that approximately 20% of agricultural land and 50% of cropland in the world is salt-stressed Natural boundaries imposed by soil salinity also limit the caloric and the nutritional potential of agricultural production. These constraints are most acute in areas of the world where food distribution is problematic because of insufficient infrastructure or political instability. Water and soil management practices have facilitated agricultural production on soils marginalized by salinity, but additional gain by these approaches seems problematic. Salinity is one of the most brutal environmental factors limiting the productivity of crop plants because most of the crop plants are sensitive to salinity caused by high concentrations of salts in the soil, and the area of land affected by it is increasing day by day. For all important crops, average yields are only a fraction – somewhere between 20% and 50% of record yields; these losses are mostly due to drought and high soil salinity, environmental conditions which will worsen in many regions because of global climate change. A wide range of adaptations and mitigation strategies are required to cope with such impacts. Efficient resource management and crop/livestock improvement for evolving better breeds can help to overcome salinity stress. However, such strategies being long drawn and cost intensive, there is a need to develop simple and low cost biological methods for salinity stress management, which can be used on short term basis. Microorganisms could play a significant role in this respect, if we exploit their unique properties such as tolerance to saline conditions, genetic diversity, synthesis of compatible solutes, production of plant growth promoting hormones, bio-control potential, and their interaction with crop plants.   Materials and methods Salinity is a major abiotic stress limiting growth and productivity of plants. Hence, the aim of this study was to investigate the effect of the plant growth promoting bacteria (isolated from alfalfa plant nodules) on reducing the effects of salinity stress in the alfalfa plant. For this purpose, 63 bacterial isolates were isolated from the nodules of 13 samples of alfalfa plants grown in the agricultural fields of Qom province. These bacterial isolates were characterized in terms of the resistance to salinity and some the plant growth promoting traits. Finally, three superior isolates including two non-rhizobial isolates A36 and A37 and one rhizobial isolate ARh29 were selected for greenhouse testing. Based on the 16S rRNA gene sequences, the isolates A36, A37, and ARh29 were closely related to Klebsiella sp., Kosakonia cowanii and Sinorhizobium meliloti, respectively. Greenhouse experiment was conducted in a completely randomized design with factorial arrangement in three replications. Salinity levels and bacterial levels included 0, 50, 100, 150, and 200 mM sodium chloride (NaCl), the isolates A36 + A37, ARh29, A36 + A37 + ARh29, negative controls (plants non-inoculated with bacterial isolates and fertilized with N-free Hoagland's solution), and positive control (plants non-inoculated with bacterial isolates and fertilized with N- containing Hoagland's solution plants), respectively.   Results and discussion The results showed that bacterial strains could increase plant dry weight and proline content in all salinity levels compared to non-inoculated plants. In addition, bacterial strains increased the uptake of potassium ions and decreased the absorption of sodium ions in alfalfa plants under salinity stress. At salinity level of 200 mM NaCl, plant dry weight and proline content in the plants inoculated with every three bacterial isolates were respectively 29 and 35% more than those in the positive control plants and the ratio of potassium to sodium was 36% higher than this control. In general, the results of this study showed that alfalfa plant root nodules harbor salinity tolerant-and plant growth promoting non-rhizobial bacteria and that their use along with rhizobial bacteria can improve alfalfa plant growth under salinity stress.