Melatonin Regulates N Metabolism and Wheat Growth via Facilitating Li Fixation in the Root Cell Wall and Redox Homeostasis

Soil lithium (Li) pollution threatens agricultural systems, yet sustainable mitigation strategies remain limited. This study investigated the protective effects and underlying mechanisms of melatonin (Mel) in wheat under Li stress. Li stress severely reduced wheat growth, while Mel application significantly alleviated the detrimental effects by reducing shoot Li accumulation and enhancing root sequestration by reprogramming the cell wall, increasing pectin synthesis, and enhancing the Li adsorption capacity of the root cell wall. Furthermore, isotherm models and FTIR analyses indicated a higher Li adsorption capacity in Mel-treated isolated root cell walls. Concurrently, Mel restores nitrogen assimilation and the antioxidant systems. Mechanistically, these responses are coordinated by the Mel-mediated specific activation of key transcription factors, including TaCIGR2 and the MYB repressor TaMYB4, to enhance pectin/cellulose synthesis, while upregulation of TaWRKY46/24 and TaMYB1 supported nitrogen metabolism and antioxidant response. These findings establish Mel as a potent agrochemical that enhances crop resilience in Li-contaminated soils.