Regulation of MdANR in Anti-Burning Process of Apple Peel.

Sunburn in apple peel significantly affects fruit appearance and reduces its commercial value. Previous research has shown that apple peel reduces sunburn by increasing the accumulation of proanthocyanidins (PAs) and other protective compounds. However, the precise molecular regulatory mechanism remains unclear. In this study, we systematically investigated MdANR, a key gene involved in PAs biosynthesis. We found that MdANR expression in apple peel is responsive to temperature and light fluctuations, with higher expression levels observed under increased temperature and light exposure. Functional analysis revealed that MdANR overexpression in apple peel and callus enhanced resistance to high-temperature and -light-intensity stress, accompanied by a corresponding increase in PAs and chlorogenic acid contents. In addition, we demonstrated that MdMYBR9 can activate MdANR promoter activity and promote its expression through yeast one-hybrid, dual-luciferase, and electrophoretic mobility transfer experiments. The results indicated that MdMYBR9 was an upstream regulator of MdANR. Based on these findings, this study proposes the MdMYBR9-MdANR-PAs regulatory model for apple sunburn resistance, providing a molecular framework for enhancing sunburn tolerance in apple breeding programs.