Zinc oxide nanoparticles alleviate cadmium stress by modulating plant metabolism and decreasing cadmium accumulation in Perilla frutescents

Cadmium (Cd), one of the most toxic heavy metals, severely represses plant growth and development. Previous studies have demonstrated that engineered nanomaterials can modulate heavy metal toxicity tolerance in plants. However, the mechanisms of zinc oxide nanoparticle (Zn NP)-mediated Cd tolerance in plants remain largely unclear. In this study, Zn NPs were sprayed on the leaves of perilla (Perilla frutescents) seedlings under Cd stress, and related morphological and physiological indicators were recorded and analyzed. The results showed that Zn NPs could increase the root and leaf dry weight of perilla seedlings by 58% and 12%, respectively, under Cd stress and could increase the photosynthetic capacity index qP under high Cd stress by 23%. Physiological analysis showed that Zn NPs could increase the superoxide dismutase (SOD) and peroxidase (POD) activities under Cd stress by 114% and 10%, and reduce catalase (CAT) activity and malondialdehyde (MDA) content by 81% and 13%, respectively. Meanwhile, Zn NPs increased the content of nutrient elements and promoted the growth of perilla seedlings under Cd stress. Moreover, compared with Cd treatment alone, Zn NPs decreased Cd accumulation by 82% and 32% in the leaves and roots of Cd-treated perilla leaves, respectively. Further metabolomics analyses revealed that treatment with Zn NPs increased the contents of organic acids (citric acid, malic acid and maleic acid) and amino acids (arginine, glutamate and phenylalanine) in the Cd-treated plants. These stress-related metabolites alleviated Cd toxicity in plants. These results collectively indicated that Zn NPs might be used as a potential regulator to improve plant growth and decrease Cd accumulation in vegetables, and provided insights into a better understanding of the detoxification mechanism of Zn NPs for sustainable agriculture.