The apple bHLH transcription factor MdbHLH3 functions in determining the fruit carbohydrates and malate

Changes in carbohydrates and organic acids largely determine the palatability of edible tissues of horticulture crops. Elucidating the potential molecular mechanisms involved in the change in carbohydrates and organic acids, and their temporal and spatial crosstalk are key steps in understanding fruit developmental processes. Here, we used apple (Malus domesticaBorkh.) as research materials and found that MdbHLH3, a basic helix-loop-helix transcription factor (bHLH TF), modulates the accumulation of malate and carbohydrates. Biochemical analyses demonstrated that MdbHLH3 directly binds to the promoter ofMdcyMDHthat encodes an apple cytosolic NAD-dependent malate dehydrogenase, activating its transcriptional expression, thereby promoting malate accumulation in apple fruits. Additionally,MdbHLH3overexpression increased the photosynthetic capacity and carbohydrate levels in apple leaves and also enhanced the carbohydrate accumulation in fruits by adjusting carbohydrate allocation from sources to sinks. Overall, our findings provide new insights into the mechanism of how the bHLH TF MdbHLH3 modulates the fruit quality. It directly regulates the expression of cytosolic malate dehydrogenaseMdcyMDHto coordinate carbohydrate allocation and malate accumulation in apple.

Automated summary

The study reveals that the bHLH TF MdbHLH3 modulates the accumulation of malate and carbohydrates in apple fruits, promoting carbohydrate accumulation by adjusting carbohydrate allocation from sources to sinks. By directly regulating the expression of cytosolic malate dehydrogenase MdcyMDH, MdbHLH3 coordinates carbohydrate allocation and malate accumulation in apple.