Abstract

Anthocyanin biosynthesis is tightly controlled by the MYB-bHLH-WD40 (MBW) complex in plants. Here, we investigated the regulatory mechanism underlying anthocyanin accumulation in passion fruit (Passiflora edulis Sims) peels. We identified PebHLH42, a nucleus-localized IIIf bHLH transcription factor that interacts with activator PeMYB114. The PeMYB114-PebHLH42 complex strongly activates PeCHS and PeANS transcription, promoting anthocyanin accumulation in tobacco leaves, whereas PebHLH42 alone shows no inductive effect. We also characterized PeMYB6, a nuclear R2R3-MYB repressor that harbors EAR and TLLLFR motifs. PeMYB6 overexpression reduces the anthocyanin and proanthocyanidin levels and represses flavonoid biosynthetic genes. Mechanistically, PeMYB6 competes with PeMYB114 for binding to PebHLH42, thus suppressing the activation of PeCHS and PeANS. Moreover, PeMYB114 activates PeMYB6 transcription, forming an activator-repressor feedback loop. These results reveal a refined regulatory module for passion fruit pigmentation and benefits molecular breeding for color modification.