The bHLH transcription factor MdbHLH3 promotes anthocyanin accumulation and fruit colouration in response to low temperature in apples

ABSTRACT Low environmental temperatures promote anthocyanin accumulation and fruit colouration by up‐regulating the expression of genes involved in anthocyanin biosynthesis and regulation in many fruit trees. However, the molecular mechanism by which fruit trees regulate this process in response to low temperature (LT) remains largely unknown. In this study, the cold‐induced bHLH transcription factor gene MdbHLH3 was isolated from an apple tree and was found to interact physically and specifically through two regions (amino acids 1–23 and 186–228) at the N terminus with the MYB partner MdMYB1 (allelic to MdMYB10). Subsequently, MdbHLH3 bound to the promoters of the anthocyanin biosynthesis genes MdDFR and MdUFGT and the regulatory gene MdMYB1 to activate their expression. Furthermore, the MdbHLH3 protein was post‐translationally modified, possibly involving phosphorylation following exposure to LTs, which enhanced its promoter‐binding capacity and transcription activity. Our results demonstrate the molecular mechanism by which MdbHLH3 regulates LT‐induced anthocyanin accumulation and fruit colouration in apple. The molecular mechanism by which fruit trees regulate fruit coloration in response to low temperature remains largely unknown. In this study, it was found that the apple transcription factor MdbHLH3 is regulated at transcriptional, translational and posttranlational levels by low temperature, and therefore involved in the modulation of temperature‐controlled anthocyanin biosynthesis and fruit coloration in apple.