Apple bZIP transcription factor MdbZIP44 regulates abscisic acid‐promoted anthocyanin accumulation

Abstract

Phytohormone abscisic acid (ABA) induces anthocyanin biosynthesis; however, the underlying molecular mechanism is less known. In this study, we found that the apple MYB transcription factor MdMYB1 activated anthocyanin biosynthesis in response to ABA. Using a yeast screening technique, we isolated MdbZIP44, an ABA-induced bZIP transcription factor in apple, as a co-partner with MdMYB1. MdbZIP44 promoted anthocyanin accumulation in response to ABA by enhancing the binding of MdMYB1 to the promoters of downstream target genes. Furthermore, we identified MdBT2, a BTB protein, as an MdbZIP44-interacting protein. A series of molecular, biochemical, and genetic analysis suggested that MdBT2 degraded MdbZIP44 protein through the Ubiquitin-26S proteasome system, thus inhibiting MdbZIP44-modulated anthocyanin biosynthesis. Taken together, we reveal a novel working mechanism of MdbZIP44-mediated anthocyanin biosynthesis in response to ABA.

MdbHLH93, an apple activator regulating leaf senescence, is regulated by ABA and MdBT2 in antagonistic ways

“…The blue line represents posttranslational regulation; the green line represents transcriptional regulation; (1) An et al . ().…”

“…In Arabidopsis , the transcription of AtBT2 is responsive to various hormone and stress treatments (Mandadi et al ., ). In apple, MdBT2 mediates Fe homeostasis and anthocyanin accumulation by regulating the stability of MdbHLH104 and MdbZIP44 proteins, respectively (Zhao Q. et al ., ; An et al ., ).…”

“…EMSA and Y1H assay were performed as described previously (An et al ., , ). For EMSA, the fusion protein of MdbHLH93‐HIS and biotin‐labeled probes were used.…”

“…Y2H, pull‐down and BiFC assays were performed to determine the interaction between MdBT2 and MdbHLH93 proteins. These assays were carried out as described previously (An et al ., ). For the Y2H assays, the indicated fragments of the MdBT2 gene were cloned into the pGBD vector (pGBD‐MdBT2, pGBD‐MdBT2‐N: 1‐195 aa and pGBD‐MdBT2‐C: 196‐297 aa), and the full‐length bHLH93 gene was inserted into the pGAD vector.…”

“…Apple calli were used for in vitro protein degradation and in vivo ubiquitination assays, as described previously (An et al ., ). For the in vitro degradation assays of the MdbHLH93 protein, WT and MdBT2 transgenic apple calli (MdBT2‐OX, MdBT2 ‐overexpressing apple calli; MdBT2‐Anti, MdBT2 antisense suppressing apple calli), as well as the MdbHLH93‐HIS fusion protein, were prepared.…”