Copper-Regulated Modulation of the MdbZIP11-MdHAT5 Transcriptional Module Enhances ABA-Mediated Immunity in Apple

Transcription factor-mediated integration of hormonal signalling is central to plant immunity, yet how such regulatory modules are activated by external signals in perennial fruit crops remains largely unknown. Here, we show that the effector protein SyCD1 from Saccharothrix yanglingensis Hhs.015 enters apple (Malus domestica) leaf cells and reduces Cu²⁺ levels, thereby relieving Cu²⁺-associated repression of the basic leucine zipper transcription factor MdbZIP11. This Cu²⁺ reduction promotes the accumulation and transcriptional activation of MdbZIP11, enabling downstream activation of abscisic acid (ABA) biosynthesis-mediated immune responses. Mechanistically, MdbZIP11 physically interacts with the ABA-related protein kinase MdSnRK2A, and this interaction is associated with phosphorylation of MdbZIP11 and increased transcriptional activity. MdbZIP11 directly binds to the promoter of the HD-ZIP transcription factor gene MdHAT5, transcriptionally activating its expression; MdHAT5 subsequently functions as a positive regulator of ABA accumulation and disease resistance. Importantly, genetic epistasis analysis confirms that MdHAT5 is indispensable for MdbZIP11-mediated ABA accumulation and disease resistance. Together, our findings uncover a bZIP-HD-ZIP transcription factor relay that amplifies ABA signalling to orchestrate immune responses in apple, providing a conceptual framework for hormone-guided disease resistance in perennial fruit crops.