Bacterial canker of kiwifruit, caused by Pseudomonas syringae pv. actinidiae (Psa), poses a serious threat to the global kiwifruit industry. Although flavonoids are widely recognized as natural antibacterial compounds, the transcriptional regulatory networks controlling their synthesis in kiwifruit and their relationship with production of downstream antibacterial metabolites remain poorly understood. In this study, we identified the transcription factor AcMYB5 as a key mediator of salicylic acid (SA) signaling that activates flavonoid biosynthesis and enhances resistance to Psa. Comparative analysis between the resistant cultivar ‘Jinkui’ and the susceptible cultivar ‘Hongyang’ revealed that Psa infection induced a rapid accumulation of endogenous SA, accompanied by a decrease in jasmonic acid (JA) levels in ‘Jinkui’. From a pool of SA-induced candidate genes, we identified AcMYB5, which is rapidly up-regulated by SA and encodes a nuclear localization protein. Overexpression of AcMYB5 in susceptible kiwifruit significantly enhanced resistance to Psa. Mechanistically, AcMYB5 directly binds to and activates the promoter of the chalcone isomerase (AcCHI), a key structural gene in the flavonoid pathway, leading to a marked increase in total flavonoid content. Notably, AcMYB5 did not activate any other genes in the flavonoid synthesis pathway in our assays, underscoring its target specificity. Our findings reveals a novel AcMYB5-AcCHI module that finely tunes flavonoid-mediated defense responses, offering valuable genetic targets and strategic insights for kiwifruit-resistant breeding.

Plants coordinate phenylpropanoid metabolism across different tissues to defend against pathogens, but the underlying transcriptional mechanisms remain unclear. Here, we identify the kiwifruit transcription factor AcMYB15 as a regulator that lignin deposition and salicylic acid (SA) accumulation, conferring bacterial canker resistance in kiwifruit through a tissue-specific regulatory module during Pseudomonas syringae pv. actinidiae (Psa) infection. In leaves, AcMYB15 directly binds to the CAACCC motif in the AcPAL1 promoter to activate its transcription, promoting localized lignin accumulation. In stems, which are the primary conduits for systemic Psa colonization, AcMYB15 forms a protein complex with the stem-enriched transcription factor AcWRKY24. Formation of the AcMYB15-AcWRKY24 protein complex enhanced transcriptional activation of the stem-prevalent AcPAL2 by strengthening its DNA-binding preferences for the CAACCA motif. Functional characterization indicates that AcPAL1 and AcPAL2 exert distinct defensive roles: AcPAL1 overexpression primarily leads to increased lignin deposition, whereas AcPAL2 overexpression predominantly enhances SA accumulation. Therefore, the AcMYB15-AcWRKY24 synergistic module provides a coordinated defense mechanism in the stem by simultaneously promoting lignin deposition and SA accumulation, thereby conferring enhanced resistance to bacterial canker. Our study uncovers a two-tiered transcriptional mechanism that coordinately regulates the tissue-specific accumulation of lignin and SA during Psa infection. These findings provide a solid theoretical basis and potential genetic resources for the genetic improvement and molecular breeding of disease-resistant fruit crops.