The MpbZIP46‐MpERF105 Module Responds to ABA and Ethylene Signalling to Promote Anthocyanin Synthesis in <i>Malus</i> ‘Profusion’ Under Rust Stress

Malus 'Profusion' synthesizes anthocyanins at the spots as a defence mechanism against rust fungi. While ethylene and abscisic acid (ABA) are known to synergistically regulate anthocyanin biosynthesis via transcription factors (TFs) in plants, their regulatory roles in M. 'Profusion' under rust stress remain elusive. In this study, we found that the release of ABA and ethylene significantly increased during rust infection. Crucially, we identified MpbZIP46, a novel ABA-responsive bZIP TF, as playing a key role in anthocyanin biosynthesis. Overexpression of MpbZIP46 significantly promoted anthocyanin synthesis, while CRISPR/Cas9-mediated knockdown of MpbZIP46 significantly reduced anthocyanin content. Further studies showed that under rust conditions, the release of ABA and ethylene synergistically promoted the accumulation of anthocyanins in M. 'Profusion' rust spots. Mechanistically, MpbZIP46 physically interacts with the ethylene-responsive TF MpERF105, forming a functional complex that synergistically transactivates the promoter of MpMYB10b-the core regulator of anthocyanin synthesis, thereby driving anthocyanin production in rust-stressed leaves. In conclusion, this study established the molecular mechanism by which ABA and ethylene regulate anthocyanin synthesis in M. 'Profusion' leaves under rust stress through the MpbZIP46-MpERF105-MpMYB10b module.