The AcMADS45-AcMADS64 Module Modulates Carotenoid Accumulation by Directly Regulating Carotenogenic Genes in Kiwifruit

Carotenoids are essential nutrient components and flesh pigments in kiwifruit. However, the regulatory mechanisms underlying carotenoid biosynthesis in kiwifruit remain elusive. In this study, we identified a transcription factor, AcMADS45, that is specifically expressed in fruit and exhibits a strong positive correlation with carotenoid accumulation. Overexpression of AcMADS45 in kiwifruit resulted in increased carotenoid content and upregulation of carotenoid biosynthesis genes. In contrast, virus-induced gene silencing (VIGS) of AcMADS45 reduced carotenoid levels and downregulated the expression of carotenoid biosynthesis genes, particularly AcBCH1/2. Further experiments using Y1H, EMSA, and luciferase assays confirmed that AcMADS45 directly binds to the AcBCH1/2 promoters to activate their transcription. In addition, we demonstrated that AcMADS64 acts as a positive transcriptional activator and interacts with AcMADS45 to synergistically enhance AcBCH1 expression, thereby increasing the carotenoid content in kiwifruit calli. These findings provide new insights into the molecular regulation of carotenoid accumulation in nonmodel plants.