High Light Promotes Anthocyanin Biosynthesis via MsCOP1‐MsMYB17‐ MsDFR/MsANS Module in Alfalfa

Anthocyanin is an antioxidant that enhances plant resilience against diverse biotic and abiotic stresses. Although high light intensity is known to enhance anthocyanin production, the underlying regulatory mechanisms are poorly understood. In this study, we identified an R2R3 MYB transcription factor MsMYB17 in alfalfa that promotes anthocyanin biosynthesis under both normal and high light conditions. Overexpression of MsMYB17 in alfalfa reduces plant height, internode length, and stem diameter without affecting plant internode number. Genetic and biochemical experiments demonstrate that MsMYB17 directly binds to and activates the promoters of MsDFR and MsANS, thereby promoting anthocyanin accumulation, which in turn reduces lignin content and inhibits plant growth under normal light conditions. Furthermore, high light enhances MsMYB17 expression and stabilises MsMYB17 protein by attenuating MsCOP1-mediated MsMYB17 degradation via the 26S proteasome pathway, thereby promoting anthocyanin production through the MsMYB17-MsDFR/MsANS module. Collectively, our findings reveal a light adaptation mechanism in alfalfa that integrates transcriptional and post-translational regulation of MsMYB17 to modulate anthocyanin biosynthesis. This study provides novel insights into the molecular mechanisms by which plants fine-tune specialised metabolism under fluctuating light conditions, and positions MsMYB17 as a potential target for enhancing stress resilience and forage quality in alfalfa.