Nitrogen dioxide (NO₂) is a common outdoor air pollutant, which has adverse effects on the environment and human health. Herein, NO₂ inhibited photosynthesis and antioxidant capacity in plants. Melatonin (Mel) is a neurohormone found in the pineal gland. Exogenous Mel alleviated chlorophyll degradation and increased the expression of key proteins and genes in the process of chlorophyll synthesis in tobacco leaves exposed to NO₂. Additionally, the activities of photosystem II (PSII) and photosystem I (PSI) were enhanced. PSII and PSI reaction center proteins and genes were upregulated. Mel pre-treatment enhanced enzyme activities and expression of proteins related to the ascorbic acid–glutathione cycle and thioredoxin-peroxiredoxin pathway in leaves exposed to NO₂, thus regulating their redox balance. Furthermore, exogenous Mel mediated the polyamine synthesis pathway and increased the expression of the key enzyme proteins SAMS1, SAMS2, and SAMS3 in the polyamine synthesis pathway in leaves under NO₂ stress. Mel regulated ABA signal transduction and calmodulin binding transcription factors CAMTA12 and NtCaM calmodulin NtCaM2 in Ca²⁺ signal transduction. Collectively, these results elucidate that Mel can alleviate high-concentration NO₂, thus suitable for agricultural application.

二氧化氮（NO ₂ ）是一种常见的室外空气污染物，对环境和人体健康产生不利影响。其中，NO ₂抑制植物的光合作用和抗氧化能力。褪黑激素 (Mel) 是松果体中发现的一种神经激素。外源Mel减轻了暴露于NO ₂的烟叶叶绿素的降解，增加了叶绿素合成过程中关键蛋白和基因的表达。此外，光系统 II (PSII) 和光系统 I (PSI) 的活性也得到增强。 PSII和PSI反应中心蛋白和基因上调。 Mel预处理增强了暴露于NO ₂的叶片中与抗坏血酸-谷胱甘肽循环和硫氧还蛋白-过氧化还原蛋白途径相关的酶活性和蛋白质表达，从而调节其氧化还原平衡。此外，外源Mel介导了多胺合成途径，并增加了NO ₂胁迫下叶片多胺合成途径中关键酶蛋白SAMS1、SAMS2和SAMS3的表达。 Mel 在 Ca ²⁺信号转导中调节 ABA 信号转导和钙调蛋白结合转录因子 CAMTA12 和 NtCaM 钙调蛋白 NtCaM2。总的来说，这些结果表明Mel可以缓解高浓度NO ₂ ，​​因此适合农业应用。