Physiological, transcriptomic, and metabolomic analyses reveal zinc oxide nanoparticles modulate plant growth in tomato

With the increasing use of zinc oxide nanoparticles (ZnO NPs) in industry, there is an increased release of these NPs into ecosystem, with potential impact on the ecological environment. Herein, we investigated the physiological and molecular mechanisms underlying ZnO NP-mediated plant growth in tomato plants. Foliar spraying with ZnO NPs (20 and 100 mg L-1) improved tomato growth by increasing the chlorophyll content and photosystem II activity. Comparative transcriptomic analysis revealed that ZnO NPs upregulated the expression of a set of genes involved in nutrient element transport, carbon/nitrogen metabolism, and the secondary metabolism in tomato, with the metabolome analysis further supporting this result. Foliar spraying with ZnO NPs increased iron (Fe) accumulation by 12.2% in tomato leaves; we thus examined the effects of ZnO NPs in tomato plants in response to Fe deficiency. Interestingly, foliar spraying with ZnO NPs markedly improved Fe deficiency tolerance in tomato. Physiological analysis indicated that ZnO NPs reduced Fe deficiency-induced oxidative damage and improved the metal nutrient element contents in tomato. Further, transcriptomic and metabolomic analyses indicated that foliar spraying with ZnO NPs increased the expression of genes encoding antioxidative enzymes, transporters, and the enzymes or regulators involved in carbon/nitrogen metabolism and secondary metabolism, thereby improving the levels of antioxidation, sugars, and amino acids in Fe-deficient tomato plants. Taken together, these results contribute to our understanding of the ecological effects of ZnO NPs.