Transcription Factor AcMYB5 Activates Flavonoid Biosynthesis and Enhances Resistance of Kiwifruit to Bacterial Canker

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.