MdWRKY125 bridges cold condition and fruit flavor by activating the sucrose synthase gene MdSUS3 in apple

The flavor quality of apple (Malus domestica) fruit is largely determined by sugar content. While moderate cold condition is known to enhance sugar accumulation and improve fruit flavor, the transcriptional regulatory networks that integrate cold signals into the sucrose metabolic pathway remain unclear. Here, we identified the transcription factor MdWRKY125 as a crucial integrator of cold signaling and sugar metabolism. Functional studies in apple fruit and calli demonstrated that MdWRKY125 increases soluble sugar (fructose, glucose, sucrose) contents while decreasing malate contents, thereby increasing the sugar-acid ratio. MdWRKY125 directly binds to and activates the promoter of the sucrose synthase gene MdSUS3, which promotes sugar accumulation and suppresses malate accumulation. Genetic analysis indicated that MdSUS3 acts downstream of MdWRKY125. Furthermore, 4 °C cold condition induces MdWRKY125 expression and enhances MdWRKY125-mediated activation of MdSUS3, forming a cold-amplified transcriptional cascade that reprograms soluble sugar metabolism under cold condition. These findings establish the MdWRKY125-MdSUS3 transcriptional module as a central molecular bridge that directly improves the flavor quality of apple fruit under cold condition. This module provides valuable genetic targets for the molecular design breeding of apple cultivars with excellent flavor and enhanced resistance to cold condition.