Phytotoxic and Eustress Effects of Metal Oxide Nanoparticles (CuO, MnxOx, and ZnO NPs) on Plants

Nanoparticles (NPs) have great potential for stimulating plant growth and development, reducing the negative impact of various types of stress on plants, and increasing the yield of agriculturally important crops. Metal oxide NPs (MONPs) have been shown to have a significant effect on the physiological and biochemical processes in plants, enhancing plant resilience. Among them, CuO, MnxOx, and ZnO NPs are of particular interest because they contain elements essential for plant function. However, widespread use in agrochemistry and plant protection requires a preliminary risk assessment due to their potential phytotoxic effects. Phytotoxicity manifests through the development of oxidative stress, genotoxicity, and transcriptional disruption. A decrease in plant growth and photosynthesis, increased lipid peroxidation (LPO), and the accumulation of toxic NPs in plant tissues were also observed. Among the studied MONPs, CuO and ZnO NPs exhibit the greatest phytotoxic effects. However, the effects of MONPs are dose-dependent. Numerous studies have shown that MONPs can stimulate plant biometric parameters and productivity, as well as influence biochemical processes. MONPs have been shown to influence the functioning of the plant antioxidant system, manifested by modulating the content of reactive oxygen species (ROS), the activity of antioxidant enzymes (AOEs), and the regulation of signaling pathways mediated by ROS and reactive nitrogen species. Furthermore, MONPs influence the accumulation of proline and phenols in plant tissues. MONPs have a pronounced effect on the functioning of the plant photosynthetic apparatus, manifested by changes in pigment content, the activity of photosynthetic enzymes, and the functioning of photosystems. MONPs can improve nutrient absorption, regulate osmotic balance, and activate plant defense mechanisms. ZnO NPs are effective in mitigating salt stress. CuO and MnxOx NPs have shown promise in mitigating biotic stress. Furthermore, these NPs were found to reduce the toxicity of heavy metals to plants. Overall, when used wisely, MONPs hold promise for enhancing the physiological, biochemical, and agronomic performance of crop plants under conditions of global climate change, effectively addressing food security issues.