MdCIbHLH1 modulates sugar metabolism and accumulation in apple fruits by coordinating carbohydrate synthesis and allocation

The content of soluble sugars is a vital parameter that indicates the quality of fleshy fruits such as apple (Malus domestica Borkh.). Studying the patterns of accumulation of soluble sugars and regulatory mechanisms associated with fruit development is crucial for breeding improved fruit varieties. Here, we report that MdCIbHLH1, a low temperature-induced bHLH transcription factor, inhibits the accumulation of soluble sugars by regulating sugar-metabolizing enzyme activities, photosynthetic performance, and the expression of sugar-related genes in developing apple fruits. MdCIbHLH1 inhibits MdFBP and MdPEPCK expression, thus blocking the conversion of acids to sugars in apple fruits. We also discovered that MdCIbHLH1 decreases the photosynthetic rate and carbohydrate accumulation in apple leaves. Our results suggest that soluble sugar accumulation in apple fruits is influenced by multiple factors, including metabolic status, photosynthesis, and carbohydrate allocation. MdCIbHLH1 is critically involved in controlling the accumulation of soluble sugars by coordinating carbohydrate synthesis and allocation, thus influencing sugar transport and its metabolism during the development of apple fruits.