REGULAR ARTICLE Effects of 1-aminocyclopropane-1-carboxylate (ACC) deaminase-overproducing Sinorhizobium meliloti on plant growth and copper tolerance of Medicago lupulina Zhaoyu Kong & Bernard R. Glick & Jin Duan & Shulan Ding & Jie Tian & Brendan J. McConkey & Gehong Wei Received: 9 December 2014 /Accepted: 1 March 2015 /Published online: 15 March 2015 # Springer International Publishing Switzerland 2015 Abstract Background and aims Rhizobia typically produce a lower level of ACC deaminase compared with free- living plant growth-promoting bacteria. While the en- dogenous 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 condi- tions. 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. Keywords ACC deaminase . Symbiosis . Excess copper . Antioxidant responses . Phytoremediation Introduction Copper (Cu) is an essential micronutrient for normal growth and development of plants, as it is directly involved in a variety of metabolic pathways, including respiration, photosynthesis, protein synthesis, cell wall metabolism and lignification, ethylene sensing and ox- idative stress protection. These properties make the cu- pric ion indispensable for the life of plants; however, they are also the reason why the copper ion could be strongly toxic for plants when it is present in excess. Over the centuries, as a result of industrial production, Plant Soil (2015) 391:383–398 DOI 10.1007/s11104-015-2434-4 Responsible Editor: Katharina Pawlowski. Electronic supplementary material The online version of this article (doi:10.1007/s11104-015-2434-4) contains supplementary material, which is available to authorized users. Z. Kong : S. Ding : G. Wei (*) State Key Laboratory of Soil Erosion and Dryland Farming on the Loess Plateau, College of Life Sciences, Northwest A&F University, Yangling, Shaanxi 712100, China e-mail: weigehong@nwsuaf.edu.cn Z. Kong : B. R. Glick : J. Duan : B. J. McConkey (*) Department of Biology, University of Waterloo, 200 University Avenue West, Waterloo, Ontario N2L 3G1, Canada e-mail: mcconkey@uwaterloo.ca J. Tian College of Horticulture, Nanjing Agricultural University, Nanjing 210095, China