Rhizobia-inoculation enhances the soybean's tolerance to salt stress

Our recent research revealed that several key enzymes involved in flavonoid biosynthesis played vital roles in soybean's tolerance to salinity. Since the flavonoids also act as important signals mediating the establishment of symbiosis between rhizobium and leguminous plants, the Sinorhizobium meliloti 1021 was tested to find out whether it had any impact on soybean's tolerance to salt stress. The roots of soybean seedling (Glycine max cultivar Union85140) were inoculated by soak in diluted S. meliloti 1021 suspension and then the seedlings were transferred in sphagnum peat and pearlite soil. Five days after inoculation, the seedlings were treated with salt solutions every four days until harvest. Soybean seedlings inoculated with S. meliloti 1021 demonstrated significantly stronger lodging-resistance and the fresh weight of the whole plants increased by 10.95-30.95 % compared with that of the non-inoculated plants. These changes in soybean seedlings might be resulted from: 1. S. meliloti 1021 playing a role in helping soybean plant to exclude Na+ while absorb K+, reducing ionic stress caused by salts; 2. S. meliloti 1021 enhancing the antioxidant enzymes' activities on reducing oxidative stress caused by salts; 3. S. meliloti 1021 increasing the content of osmotic compounds, thus reducing osmotic stress caused by salts. In addition, S. meliloti 1021 played significant roles in regulating the transcription of several key enzymes related to flavonoids metabolism (cytochrome P450 monooxygenase, chalcone synthase and chalcone isomerase), ROS scavenging (catalases, ascorbate peroxidase, glutathione S-transferase and superoxide dismutase), and other salt-responsive genes (Stress-induced protein SAM22, PR10-like protein and Phosphatidylinositol-specific phospholipase C) in soybean seedlings. These results showed that S. meliloti 1021 significantly enhanced soybean's ability to adapt to saline soil, mostly due to its effect on flavonoids metabolism in plants.