AcCOMT9 positive regulates Cd tolerance in kiwifruit by modulating heavy metal transport and antioxidant capacity.

Soil cadmium (Cd) contamination poses a severe threat to crop productivity and food safety. Melatonin, a potent endogenous antioxidant and signaling molecule, plays a pivotal role in mitigating plant abiotic stresses, including Cd stress. However, the molecular mechanisms underlying melatonin-mediated Cd tolerance in kiwifruit remain poorly understood. Here, we systematically dissected the regulatory role of melatonin and its biosynthesis gene AcCOMT9 in modulating kiwifruit Cd tolerance. Exogenous melatonin application (150 μM) significantly alleviated Cd-induced oxidative damage and upregulated core melatonin biosynthesis genes, including AcCOMT9. Overexpression of AcCOMT9 sbusential enhanced Cd tolerance in transgenic kiwifruit seedlings, reducing hydrogen peroxide by 20%, superoxide anion by 50%, and shoot Cd accumulation by 37.5%. Transcriptome profiling and RT-qPCR analysis revealed that AcCOMT9 overexpression activated multiple stress-responsive pathways, upregulating heavy metal transporter genes (ABC, HMA2, HMA4, VIT3, MTP11, MTP4), flavonoid biosynthesis genes, photosynthesis-related genes, and antioxidant-related genes. WGCNA analysis further identified core Cd-responsive transcription factors WRKY2, BIM2, WRKY4, bHLH144, bHLH121, and MYBS3 as key regulatory candidates. Collectively, our findings establish a mechanistic framework wherein AcCOMT9-driven melatonin biosynthesis orchestrates antioxidant defense, metal homeostasis, and photosynthetic resilience to enhance kiwifruit Cd stress tolerance. These findings provide novel insights into melatonin-governed heavy metal tolerance in perennial fruit crops and offer valuable candidate gene resources for future Cd-tolerant molecular breeding.