Mediator subunit MED18 interacts with HY5 to regulate anthocyanin accumulation under high light in Arabidopsis

One of the hallmark responses of plants to high light (HL) stress is the rapid transcriptional activation of anthocyanin biosynthesis. The Mediator complex, a transcriptional coactivator, is involved in multiple developmental and stress response pathways in plants, but its role in the HL response remains unclear. In this study, we demonstrated that the MED18 subunit of the Arabidopsis (Arabidopsis thaliana) Mediator complex is specifically required for HL-induced anthocyanin biosynthesis-related gene expression and anthocyanin accumulation. RNA-seq analyses showed that MED18 shares an overlapping set of HL-regulated genes, particularly anthocyanin biosynthesis-related genes, with LONG HYPOCOTYL 5 (HY5), a key transcription factor in the HL response. Biochemical analyses revealed that this functional link between MED18 and HY5 is underpinned by a direct protein-protein interaction. Furthermore, the direct binding of HY5 to the promoter of anthocyanin biosynthesis-related genes partially depends on MED18. Moreover, MED18 also associates specifically with anthocyanin biosynthesis-related genes in a HY5-dependent manner. In summary, our findings uncover a role for MED18 in HL-induced anthocyanin accumulation and the function of the MED18-HY5 interaction in the transcriptional regulation of anthocyanin biosynthesis under HL stress in plants.