ABI5 regulates ABA-induced anthocyanin biosynthesis by modulating the MYB1-bHLH3 complex in apple.

Abscisic acid (ABA) induces anthocyanin biosynthesis in many plant species. However, the molecular mechanism of ABA-regulated anthocyanin biosynthesis remains unclear. As a crucial regulator of ABA signaling, ABSCISIC ACID-INSENSITIVE5 (ABI5) is involved in many aspects of plant growth and development. Nevertheless, whether ABI5 regulates anthocyanin biosynthesis has not been reported. In this study, we found that MdABI5, the homolog of Arabidopsis ABI5, positively regulated ABA-induced anthocyanin biosynthesis in apple. A series of biochemical tests showed that MdABI5 specifically interacts with MdbHLH3, a positive regulator of anthocyanin biosynthesis. MdABI5 enhanced the binding of MdbHLH3 to its target genes MdDFR and MdUF3GT. In addition, MdABI5 directly bound to the promoter of MdbHLH3 to activate its expression. Moreover, MdABI5 acted as an enhancer in the ABA-promoted interaction between MdMYB1 and MdbHLH3. Finally, antisense suppression of MdbHLH3 significantly reduced anthocyanin biosynthesis promoted by MdABI5, indicating that MdABI5-promoted anthocyanin biosynthesis was dependent on MdbHLH3. Taken together, our data suggest that MdABI5 plays a positive role in ABA-induced anthocyanin biosynthesis by modulating the MdbHLH3-MdMYB1 complex. Our work enriches the functional studies of ABI5 and further broadens the regulatory network of ABA-mediated anthocyanin biosynthesis, providing new insights for further study of the transcriptional regulatory mechanisms of anthocyanin biosynthesis.