Evaluation of exogenous melatonin in conferring tolerance to cadmium toxicity of Lycium barbarum: Oxidative stress, physiological and gene expression analysis

• Cadmium stress inhibits Lycium barbarum growth, reduces chlorophyll, and increases ROS. • Melatonin alleviates cadmium stress, enhances tolerance, and regulates hormone signaling, MAPK pathways, and key transcription factors. • WGCNA identifies co-expressed modules and hub genes linked to cadmium content, elucidating melatonin's role in tolerance. Cadmium impairs plant growth and fruit quality; the stress-tolerant medicinal woody plant Lycium barbarum could remediate Cadmium-contaminated soils, but its Cadmium-response mechanisms remain unknown. This study applied different concentrations of cadmium ions to L. barbarum spikes under hydroponic conditions, with exogenous melatonin added, to investigate the phenotypic, physiological and molecular regulatory mechanisms. Results showed cadmium stress inhibited L. barbarum growth, reduced chlorophyll content and increased MDA, POD and SOD content. Melatonin treatment alleviated these effects and activated antioxidant enzyme activity. Transcriptome analysis revealed melatonin significantly affected genes related to plant hormone signal transduction and MAPK signaling pathways, especially auxin pathway and key cadmium tolerance genes like PP2C-2, AUX1–4, MPK8–5, MYC2–7 and WRKY33–2 , inhibiting cadmium ion transport and accumulation in L. barbarum . Three core transcription factors AP2–56, AP2–136 and bHLH-125 regulated cadmium accumulation by regulating gene expression. Weighted gene co-expression network analysis identified a MEyellow module highly correlated with cadmium content change, with genes like Lba03g01205 involved in the response. Thanks for the valuable comments. For the first time in a woody plant, Lycium barbarum , this study demonstrates that melatonin alleviates cadmium stress via the MAPK–hormone signalling network, elucidating the physiological and molecular mechanisms underlying enhanced cadmium tolerance and offering a novel strategy for cadmium-resistant breeding of L. barbarum and bioremediation of cadmium-contaminated soils.