Transcription factor MdMYB6L represses anthocyanin biosynthesis of apple skin at low-altitude regions

Transcriptional repressor MdMYB6L is induced by heat stress and plays a negative regulatory role in apple skin colouration in low-altitude regions. Pericarp colour is a critical economic trait in apple, with the red colouration of apple skin in high-altitude regions being significantly more pronounced than that in low-altitude regions. However, the regulatory mechanisms governing anthocyanin accumulation in the apple pericarp in these regions have not been thoroughly explored. In this study, multiple MYB transcription factors were significantly downregulated in 'Xiangfu' apple fruit under high-altitude climate conditions compared to that in low-altitude regions. MdMYB6L exhibited an inverse expression pattern compared to MdANS, MdCHS, MdDFR, and MdUFGT. MdMYB6L functions as a nuclear-localised transcriptional repressor induced by heat stress. UV treatment and cold stress decreased its expression, with this effect becoming more pronounced when these stressors were alternated. Stable transformation in 'Orin' callus and apple fruit injections of MdMYB6L showed that MdMYB6L was a negative regulator of anthocyanin biosynthesis and inhibited the expression of the above four structural genes. Yeast one-hybrid (Y1H) and dual-luciferase reporter (DLR) assays demonstrated that MdMYB6L directly bound to the promoters of these genes, regulating their expression levels. Our findings indicate that MdMYB6L is the key regulator controlling apple skin colouration under the climate conditions of high- and low-altitude regions, providing a highly valuable candidate for application in breeding apple varieties with a higher nutritional value.