Melatonin alleviates cadmium toxicity in Solanum nigrum by enhancing pectin-mediated cell wall sequestration and antioxidant.

Cadmium (Cd) contamination in agricultural soils threatens crop production and food safety. Although melatonin is known to enhance antioxidant defense, its role in cell-wall remodeling and Cd compartmentalization remains unclear. We investigated the effects of 100 μmol L-1 melatonin on Solanum nigrum exposed to 10 mg kg-1 Cd using physiological, biochemical, subcellular, transcriptional, and random forest analyses. Cd reduced biomass, photosynthetic rate, and Fv/Fm, whereas melatonin partially restored these traits. Melatonin also decreased Cd concentrations in roots, stems, and leaves and reduced root-to-shoot Cd translocation. Meanwhile, Cd partitioning to the cell-wall fraction increased from 56.7% to 67.5% in roots and from 55.1% to 66.5% in leaves, while organellar Cd partitioning was approximately halved. These changes were accompanied by increased pectin content and pectin-bound Cd, upregulation of pectin metabolism and Cd detoxification genes, enhanced antioxidant activity, and improved redox status. Random forest modeling identified ascorbate, photosynthetic rate, and pectin-bound Cd as key predictors of Cd tolerance. These findings indicate that melatonin enhances Cd tolerance through coordinated cell-wall sequestration and antioxidant defense while altering Cd allocation, with implications for phytoremediation efficiency.