MdMYB305-MdbHLH33-MdMYB10 regulates sugar and anthocyanin balance in red-fleshed apple fruits.

Sugar and anthocyanin are important indicators of fruit quality, and understanding the mechanism underlying their accumulation is essential for breeding high-quality fruit. We identified an R2R3-MYB transcription factor (TF) MdMYB305 in the red-fleshed apple progeny, which was positively correlated with fruit sugar content but negatively correlated with anthocyanin content. Transient injection, stable expression (overexpressing and CRISPR/Cas9 knockdown) and heterologous transformation tomato confirmed that MdMYB305 promotes the accumulation of sugar and inhibits the synthesis of anthocyanin. Through a series of molecular experiments (such as EMSA, LUC) confirmed that MdMYB305 combines with sugar-related genes (MdCWI1/MdVGT3/MdTMT2) and anthocyanin-related genes (MdF3H/MdDFR/MdUFGT), promote and inhibit their activities, and finally regulate the sugar and anthocyanin content of fruits. In addition, this study also found that MdMYB305 competes with MdMYB10 for the MdbHLH33 binding site to balance sugar and anthocyanin accumulation in the fruits, which provides a reference value for exploring more functions of MYB-bHLH-MYB complex and the balance relationship between sugar and anthocyanin in the future.