Abstract: Apple Alternaria blotch, caused by Alternaria alternata f. sp. mali (A. alternata), is a devastating fungal disease threatening global apple production. However, the early signaling pathway underlying apple resistance to A. alternata remains unclear. In this study, through transcriptomic analysis, we identified that MdWRKY9 is a key positive regulator of apple resistance to A. alternata, whose expression is induced in the resistant “Hanfu” while being reduced in the susceptible “Gala” following A. alternata infection. Stable overexpression of MdWRKY9 enhanced apple resistance to A. alternata, whereas RNA interference (RNAi)-mediated knockdown of MdWRKY9 increased susceptibility. We further demonstrated that MdWRKY9 directly represses the expression of MdJAZ8, a key repressor of jasmonate (JA) signaling. Accordingly, stable overexpression of MdJAZ8 in apple calli reduced resistance to A. alternata and suppressed JA signaling. Consistently, transient overexpression of MdJAZ8 in apple leaves decreased resistance to A. alternata, whereas transient silencing of MdJAZ8 via RNAi enhanced disease resistance. Furthermore, we identified that the receptor-like cytoplasmic kinase MdBIK1 (BOTRYTIS-INDUCED KINASE1, a core component of the PTI pathway) interacts with and phosphorylates MdWRKY9 at Thr-2. The phosphorylation of MdWRKY9 enhances resistance to A. alternata by further suppressing MdJAZ8 and activating JA signaling. Together, our findings define a complete PTI-associated signaling module in which the MdBIK1-MdWRKY9 complex enhances JA-mediated immunity against A. alternata by directly repressing MdJAZ8. This study refines the regulatory mechanism by which the BIK1 family kinases modulate JA signaling, providing new insights into the plant immune signaling network.

Key words: A. alternata, apple, BIK1, JA signaling, phosphorylation, transcriptional regulation, WRKY transcription factor