Copper oxide nanoparticles alleviate cadmium toxicity in cereal crops

Cadmium (Cd) pollution in soil severely affects crop production and food safety. Currently, cupric oxide nanoparticles (CuO NPs) are considered as potential regulators alleviating the toxicity of heavy metals in plants, but their physiological functions and molecular mechanisms are not completely clarified. In this study, we found that CuO NPs had the capacity to adsorb ionic Cd2+ and their suspension (<= 100 mg L-1) remained stable within 24 h. High concentrations of CuO NPs (>= 10 mg L-1) inhibited plant growth, while 5 mg L-1 CuO NPs could be appropriate for Cd remediation in rice and barley. We also investigated the influence of 5 mg L-1 CuO NP pre-treatment on growth and Cd toxicity of two rice genotypes (Nipponbare and 9311) and two barley genotypes (Golden Promise and XZ26) exposed to Cd stress. The results showed that pre-treatment with low concentration of CuO NPs (5 mg L-1) alleviated Cd toxicity, which is attributed to inhibition of Cd uptake in roots related to CuO NPs adhering on the rhizosphere surface to inhibit Cd2+ influx, down-regulation of some key Cd-transporter genes (Nramp5 and HMA2), and enhancement of antioxidative capacity (SOD, POD and CAT) and up-regulation of ROS scavenging related genes. Moreover, the regulation of Cd-transporter and ROS scavenging related genes by CuO NP pre-treatment showed a little difference between paddy rice and upland barley, which may account for the difference in the response to Cd stress and CuO NP pre-treatment between them. It may be concluded that low concentration of CuO NPs alleviates Cd toxicity in plants acting as a nano-scale protector, and provides an alternative approach for fighting against Cd pollution.