Selenium nanoparticles alleviate chromium toxicity in maize via melatonin-mediated regulation of cell wall polysaccharide metabolism and redox homeostasis.

Selenium nanoparticles (Se NPs) and melatonin have been shown to alleviate chromium (Cr) toxicity in plants. However, the relationship between Se NPs and melatonin in Cr detoxification remains unclear. This study employed hydroponic experiments to examine the role of melatonin in the Cr stress tolerance induced by Se NPs in maize (Zea mays L.). Cr stress increased Cr accumulation in maize leaves and roots, reduced chlorophyll content and photosynthesis, and decreased dry weight. In contrast, foliar application of 50 μM Se NPs significantly alleviated growth inhibition. Notably, Se NPs significantly upregulated the expression of melatonin biosynthetic genes (T5H, SNAT, and ASMT) and increased melatonin accumulation, thereby promoting the synthesis of cell wall polysaccharides (pectin, hemicellulose) and activating the antioxidant enzyme system (SOD, CAT, POD), enhancing the cell wall's capacity to bind Cr and reducing oxidative damage. However, treatment with the melatonin synthesis inhibitor p‑CPA, as well as the utilization of melatonin‑deficient snat mutants in Arabidopsis and maize, significantly reduced the protective efficacy of Se NPs. In summary, this study elucidates that melatonin functions as a downstream signaling molecule in the Cr detoxification activated by Se NPs, mitigating Cr toxicity by facilitating cell wall polysaccharide synthesis and reestablishing redox homeostasis.