Melatonin Attenuates Cadmium-Induced Oxidative Stress in Potato by Remodeling Redox Homeostasis and Activating Multi-Pathway Antioxidant Networks

Cadmium pollution severely suppresses potato growth, impairs photosynthesis, and disrupts cellular redox balance, threatening crop yield and food safety. Exogenous small-molecule regulators represent low-cost strategies to alleviate heavy-metal phytotoxicity, yet systematic comparisons of N-acetyl-L-cysteine (NAC), L-tryptophan (Trp), and melatonin (MT) on Cd-stressed potatoes remain insufficient. In this study, two rounds of concentration-gradient screening using morphological and physiological assays identified 5 μM of melatonin as the optimal treatment to alleviate cadmium-induced damage in potato seedlings. This concentration significantly restored shoot height, root length, fresh weight, and chlorophyll content. Relative to the Cd-only control, 5 μM of MT reduced MDA and cadmium accumulation by 51.88% and 49.26%, respectively, while increasing proline content by 40.99%. Transcriptomic analysis revealed that MT reversed Cd-induced suppression of photosynthesis and carbon fixation pathways and activated carbohydrate metabolism. Weighted gene co-expression network analysis identified four key modules significantly correlated with Cd tolerance and MT-mediated recovery. Eight hub genes, including StANNAT1 (Ca2+-ROS homeostasis), StNRT1.1 (nitrogen transport), StSWEET10 (carbon allocation), and StHMG1 (MVA pathway), were validated by qPCR, confirming their involvement in Ca2+-ROS signaling, membrane integrity, and photosynthetic recovery. Mechanistically, MT rebalanced cellular redox status, promoted Cd sequestration, optimized carbon–nitrogen distribution, and enhanced antioxidant defense. These findings demonstrate that exogenous MT alleviates Cd toxicity through coordinated regulation of multiple pathways, providing a practical strategy for safe potato cultivation in Cd-contaminated soils and establishing a molecular framework for understanding MT-mediated heavy-metal tolerance in crops.