Rhizobial symbiosis effect on the growth, metal uptake, and antioxidant responses of Medicago lupulina under copper stress

The effects of rhizobial symbiosis on the growth, metal uptake, and antioxidant responses of Medicago lupulina in the presence of 200 mg kg−1 Cu2+ throughout different stages of symbiosis development were studied. The symbiosis with Sinorhizobium meliloti CCNWSX0020 induced an increase in plant growth and nitrogen content irrespective of the presence of Cu2+. The total amount of Cu uptake of inoculated plants significantly increased by 34.0 and 120.4 % in shoots and roots, respectively, compared with non-inoculated plants. However, although the rhizobial symbiosis promoted Cu accumulation both in shoots and roots, the increase in roots was much higher than in shoots, thus decreasing the translocation factor and helping Cu phytostabilization. The rate of lipid peroxidation was significantly decreased in both shoots and roots of inoculated vs. non-inoculated plants when measured either 8, 13, or 18 days post-inoculation. In comparison with non-inoculated plants, the activities of superoxide dismutase and ascorbate peroxidase of shoots of inoculated plants exposed to excess Cu were significantly elevated at different stages of symbiosis development; similar increases occurred in the activities of superoxide dismutase, catalase, and glutathione reductase of inoculated roots. The symbiosis with S. meliloti CCNWSX0020 also upregulated the corresponding genes involved in antioxidant responses in the plants treated with excess Cu. The results indicated that the rhizobial symbiosis with S. meliloti CCNWSX0020 not only enhanced plant growth and metal uptake but also improved the responses of plant antioxidant defense to excess Cu stress.