Activation of antioxidant and detoxification gene expression in cucumber plants exposed to a Cu(OH)2 nanopesticide

Abstract

Cu-containing nanopesticides are increasingly being used as fungicides in modern agriculture. However, their fate, transport and toxicity in crop plants have been less studied. Here, we exposed 3 week-old cucumber plants cultivated in artificial media to different concentrations of a Cu(OH)₂ nanopesticide (0, 2.5 and 25 mg) for 7 d. The physiological and molecular responses were investigated. In order to elucidate the contribution of copper ions to the response, we also exposed the plants to CuSO₄. Results showed that the Cu(OH)₂ nanopesticide did not reduce the photosynthetic pigment production. In contrast, 10 mg Cu ions induced a significant decrease in photosynthetic pigment levels (around 25%) and leaf chlorosis symptoms. Foliar exposure to 25 mg Cu(OH)₂ nanopesticide induced significant changes in mRNA levels of antioxidant and detoxification-related genes; 6 genes (SOD, GPX4, GPX, MDAR, POD, WRKY6) were up-regulated up to 9-fold, and one (cAPX) was down-regulated by 32%. The Cu(OH)₂ nanopesticide at both dose levels (2.5 and 25 mg per plant) decreased the transcript production of a stress-related gene (DNAJ) by 40% and 80%, respectively. The up-regulation of the transcript levels of SOD, GPX4, GPX, MDAR, POD, and WRKY6 and down-regulation of DNAJ was also observed in CuSO₄ treated plants (with increases of up to 7-fold), indicating that most of the responses are due to released copper ions. We postulate that the increased mRNA levels of antioxidants and detoxification enzymes reflect plant adaptation to over-generated reactive oxygen species (ROS) triggered by copper ions. The activated genes could serve as potential biomarkers of nanopesticide exposure and may be applicable to other plant/Cu nanopesticide interactions.