Effects of 1-aminocyclopropane-1-carboxylate (ACC) deaminase-overproducing Sinorhizobium meliloti on plant growth and copper tolerance of Medicago lupulina

Background and aims Rhizobia typically produce a lower level of ACC deaminase compared with free-living plant growth-promoting bacteria. While the endogenous rhizobial ACC deaminase is important in legume nodulation, it is not sufficient to protect host plants against environmental stresses. The main goal of this study was to assess the effects of a genetically engineered Sinorhizobium meliloti strain overproducing ACC deaminase, and its symbiotic performance in Medicago lupulina under copper stress conditions. Methods The engineered strain was transformed with an exogenous acdS gene by triparental conjugation. A plant growth assay was conducted to assess its plant growth promotion ability under copper stress conditions. The expressions of antioxidant genes in these plants were analyzed using quantitative real-time PCR. Results Plants nodulated with the engineered strain showed a greater dry weight, a decreased ethylene level in roots, a higher total copper uptake but a lower level of copper translocation to aerial parts, as compared with the plants nodulated with the wild-type strain under copper stress conditions. These results were positively correlated with higher expression of antioxidant genes in the roots of these plants exposed to severe copper stress. Conclusions The engineered strain could improve plant growth as well as copper tolerance of M. lupulina, and enhance the antioxidant defense system.