Elucidating the Mechanistic Role of Exogenous Melatonin in Salt Stress Tolerance of Maize (Zea mays L.) Seedlings: An Integrated Physiological, Metabolomic, and Proteomic Profiling Analysis

Maize (Zea mays L.), as a globally significant cereal crop, exhibits high sensitivity to salt stress during early seedling stages. Although melatonin (MT) has demonstrated potential in mitigating abiotic stresses, the specific mechanisms underlying MT-mediated alleviation of salt stress in maize seedlings remain unclear. In this study, we established four treatment groups: control (CK), melatonin treatment (MT), salt stress (NaCl), and combined treatment (NaCl_MT). Metabolomic and proteomic analyses were performed, supplemented by photosynthesis-related experiments as well as antioxidant-related experiments. Metabolomic analysis identified key metabolites in MT-mediated salt stress mitigation. Both metabolomic and proteomic analyses underscored the critical roles of photosynthetic and antioxidant pathways. Salt stress significantly decreased the net photosynthetic rate (Pn) by 67.7%, disrupted chloroplast ultrastructure, and reduced chlorophyll content by 41.6%. Conversely, MT treatment notably mitigated these detrimental effects. Moreover, MT enhanced the activities of antioxidant enzymes by approximately 10–20% and reduced the accumulation of oxidative stress markers by around 10–25% in maize seedlings under salt stress. In conclusion, this study conducted a systematic and multidimensional investigation into the mitigation of salt stress in maize seedlings by MT. Our results revealed that MT enhances antioxidant systems, increases chlorophyll content, and alleviates damage to chloroplast ultrastructure, thereby improving photosystem II performance and strengthening photosynthesis. This ultimately manifests as improved seedling phenotypes under salt stress. These findings provide a meaningful entry point for breeding salt-tolerant maize varieties and mitigating the adverse effects of salinized soil on maize growth and yield.