Melatonin enhances metallic oxide nanoparticle stress tolerance in rice via inducing tetrapyrrole biosynthesis and amino acid metabolism

The overuse of metallic oxide nanoparticles (NPs) has caused a burst of their enrichment in paddy fields. Several findings have reported that the NPs of ZnO and CuO have an adverse impact on plant growth, while the potential mechanism of rice response to the NPs of metallic oxides remains largely unknown. In our research, we found that pretreatment with different concentrations of ZnO and CuO NPs greatly increased the biosynthesis and accumulation of melatonin and its precursors (serotonin, tryptamine, and tryptophan), and induced the transcription of melatonin biosynthetic genes in rice seedlings. Foliage sprayed melatonin greatly enhanced the tolerance to metallic oxide NPs toxicity in rice plants as evidenced by reduced inhibition of plant growth and enhanced biosynthesis of tetrapyrrole in rice seedlings. Exogenous melatonin significantly alleviated metallic oxide NPs-induced oxidative stress with decreased reactive oxygen species (ROS), and increased the activity and gene expression of major antioxidant enzymes. Moreover, the melatonin supplement greatly increased the total amino acid content under metallic oxide NPs treatment in rice leaves. In summary, our findings reveal that melatonin is related to metallic oxide nanoparticle-participated molecular, biochemical, and physiological alterations by accelerating tetrapyrrole biosynthesis and amino acid metabolism in rice.