Overexpression of blackcurrant RnAPX6 gene delays fruit ripening and enhances abiotic stress tolerance by reshaping ascorbic acid redox balance.

The ascorbate-glutathione (AsA-GSH) antioxidant system is essential for plant growth, fruit ripening, and abiotic stress adaptation, while the functional mechanisms of genes participating in the AsA recycling pathway in blackcurrant remain largely unclear. A candidate differentially expressed gene, RnAPX6, which potentially regulates AsA biosynthesis and recycling, was screened via transcriptome profiling of blackcurrant cultivars with high- and low-total AsA contents at four distinct fruit developmental stages. Transient overexpression and VIGS silencing in blackcurrant fruit, as well as stable heterologous transformation in tomato, were performed to explore the biological roles of RnAPX6. RnAPX6 reshaped AsA redox balance by coordinating AsA recycling and biosynthetic pathways. This gene acted as a negative regulator of fruit ripening; its overexpression delayed peel pigment accumulation, maintained fruit firmness, suppressed ethylene production and MDA generation, and reduced total soluble solids. In addition, heterologous RnAPX6 overexpression in tomato alleviated cold- and salt-induced injury by boosting antioxidant enzyme activity, eliminating excess ROS, and stabilizing photosystems. This work uncovers the dual roles of RnAPX6 in fruit quality modulation and stress defense, offering a valuable gene resource for breeding superior horticultural crops.