Low pH attenuates anthocyanin accumulation via the STOP1-MYB75 module in Arabidopsis

Anthocyanins are important secondary plant metabolites involved in diverse physiological processes. In acidic soils, elevated proton (H⁺) concentrations (i.e., Low pH) constitute a major constraint on plant growth and crop productivity. Low pH has been reported to reduce anthocyanin biosynthesis in plants, but the underlying molecular mechanisms remain poorly understood. Here, we verified that low pH negatively regulates anthocyanin accumulation in Arabidopsis thaliana. Consistently, low pH stabilizes the SENSITIVE TO PROTON RHIZOTOXICITY1 (STOP1) transcription factor, which in turn represses anthocyanin accumulation. Notably, numerous anthocyanin biosynthetic genes, particularly those encoding components of the WD-repeat/bHLH/MYB (MBW) transcriptional complex, were significantly down-regulated under low pH conditions. Further analyses revealed that the MBW complex is essential for mediating low pH-induced anthocyanin repression. Mechanistically, STOP1 binds to the promoter of MYB75 to repress its transcription. Genetic analyses confirmed that STOP1 functions upstream of MYB75 in this regulatory pathway. Collectively, our findings reveal that low pH attenuates anthocyanin accumulation through STOP1-mediated transcriptional repression of MYB75, expanding our understanding of the multidimensional regulation of STOP1 to secondary metabolism. It provides novel mechanistic insights into how the STOP1-MYB75 module orchestrates the endogenous anthocyanin biosynthesis pathway under acidic conditions.