Abstract: In this study, alfalfa was genetically engineered to improve its salt tolerance. The SOS1-SOS2-SOS3 genes from Arabidopsis thaliana were incorporated into Algonquin using tumefaciens-mediate transformation technology. Transgenic plants were identified by PCR, herbicide resistance screening and RT-PCR. The salt resistance of the transgenic plants was evaluated by exposing them to different concentration (100, 200 and 300 mmol·L^-1) of NaCl for 6 days and comparing them to unmodified plants. Physiological and biochemical indexes, plant height, Na⁺ and K⁺ content, cell membrane permeability and chlorophyll content were measured. The results showed that under different salt treatments, the height of all plants increased but transgenic plant height was significantly higher than that of unmodified plants at 100 and 200 mmol·L^-1 NaCl. After salt exposure the chlorophyll content of all plants initially increased before declining but the chlorophyll content of unmodified plants was lower than that of transgenic plants. Under 100 and 200 mmol·L^-1 NaCl treatments, membrane permeability, superoxide dismutase activity and proline content of transgenic plants were lower than that of unmodified plants. However, peroxidase activity, catalase activity and soluble sugar were higher than that of unmodified plants. Malondialdehyde content declined in all plants but the decline was greater in unmodified plants. The accumulation of Na⁺ in roots of transgenic plants was less than in unmodified plants, while K⁺ content was more than that of unmodified plants. Transgenic plants displayed increased Na⁺ efflux, reducing cellular ion toxicity, alleviating the damage from salt stress.

Key words: Medicago sativa, AtSOS genes, genetic transformation, salt-resistance