A critical review on the toxicity regulation and ecological risks of zinc oxide nanoparticles to plants

The advancement and application of nanotechnology inevitably cause the release of nanoparticles (NPs) into the environment, in particular, zinc oxide (ZnO) NPs may seriously threaten the ecological safety of plants. This review focuses on the translocation and physiological responses of ZnO NPs in plants to systematically summarize the toxicological effects and molecular mechanisms of NPs. The results indicated that ZnO NPs migrated to tissues via uptake and translocation, gradually accumulated in intracellular and intercellular spaces, and led to physiological inhibition, nutrient imbalance, and photosynthesis perturbation in plants. This abiotic stress triggered the overproduction of reactive oxygen species (ROS) by perturbing cellular redox homeostasis, while the activation of antioxidant genes and alteration of KEGG pathways enhanced the ability of plants to resist phytotoxicity. Furthermore, ZnO NPs significantly altered metabolites associated with oxidative stress, antioxidant defense, membrane disorder and energy expenditure, affected carbon/nitrogen metabolism via the TCA cycle and glycolysis pathway, and augmented cytotoxicity and genotoxicity by inducing DNA damage and inhibiting mitosis. More notably, the composite exposure of ZnO NPs with other substances is bifacial and may create potential mitigation or synergistic effects on plants in ecosystems, thus posing uncertain ecological risks. This review systematically provides clarification on the environmental fate of ZnO NPs in plants at the physiological and molecular levels, theoretical references on the toxicity mechanisms and potential risks of NPs, and directions and insights for future research to achieve strategies that minimize risks and maximize benefits. This review clarifies the translocation of zinc oxide nanoparticles (ZnO NPs) in plants, reveals the molecular mechanisms of toxicity regulation, and emphasizes the potential risks of composite systems of NPs.