Light-modulated MdbHLH14L–MdGMP1 module regulates ascorbate accumulation and oxidative stress tolerance in apple

Ascorbate (Asc) is a central antioxidant that helps maintain cellular redox homeostasis and is involved in fruit nutritional quality and stress resilience. Although light stimulates Asc accumulation in plants, the mechanisms by which environmental light cues are transcriptionally integrated into the Asc biosynthetic program, particularly in fruit crops, remain poorly understood. Here, we identify a light-responsive regulatory module that directly links environmental light signals to Asc biosynthesis in apple. We show that light enhances Asc production by inducing MdGMP1, a key gene in the D-mannose/L-galactose pathway. In field-grown apple fruits under natural light conditions, Asc accumulation was higher in sun-exposed peel than in shaded peel, coinciding with elevated MdGMP1 expression. Using the MdGMP1 promoter as bait, we identified a transcription factor, MdbHLH14L. Biochemical and transcriptional assays demonstrated that MdbHLH14L directly binds to the MdGMP1 promoter and represses its transcription. Overexpression of MdbHLH14L markedly reduced Asc accumulation in apple fruits, with similar effects observed in leaves and calli, whereas silencing MdbHLH14L significantly elevated Asc levels. Continuous artificial light exposure suppressed MdbHLH14L expression, whereas continuous darkness induced its expression, and its abundance was negatively correlated with MdGMP1 expression and Asc accumulation. Functionally, MdbHLH14L overexpression increased sensitivity to oxidative stress, whereas silencing conferred enhanced tolerance. Together, these results reveal a light-modulated MdbHLH14L–MdGMP1 transcriptional module that integrates environmental light cues with Asc biosynthesis and redox homeostasis. This study provides mechanistic insight into how external light signals are transduced into metabolic regulation and highlights potential molecular targets for improving fruit nutritional quality and stress resilience in horticultural crops.