MdSWEET4, a glucose transporter from apple (Malus × domestica Borkh.), interacts with MdSWEET23 and contributes to fruit sugar accumulation, as validated in tomato

The apoplasmic unloading pathway is crucial for regulating sugar accumulation in fruits, thereby determining both yield and quality. SWEETs are key facilitators of sugar transport, yet their specific functions in apple fruit sugar accumulation remain unclear. Here, we identified and characterized MdSWEET4 in apple (Malus × domestica Borkh). Spatiotemporal expression, subcellular localization, and yeast complementation assays confirmed that MdSWEET4 encodes a plasma-membrane glucose transporter predominantly expressed in the phloem tissue of fruit vascular bundles. Silencing of MdSWEET4 significantly altered sugar accumulation in apple fruits and upregulated the expression of sucrose transporters. Heterologous overexpression of MdSWEET4 in tomato enhanced soluble sugar accumulation in fruits. MdSWEET4 formed heterooligomers with MdSWEET23, and their co-expression in tomato unexpectedly reduced sugar accumulation. We conclude that MdSWEET4 is a key regulator of sugar unloading and metabolism in apple, and that its interaction with MdSWEET23 may modulate sugar-transport efficiency.