The AcWRKY40‐ <i>AcICS1</i> Module Regulates Kiwifruit Resistance to <i>Pseudomonas syringae</i> pv. <i>actinidiae</i> via Salicylic Acid Synthesis

Kiwifruit bacterial canker, caused by Pseudomonas syringae pv. actinidiae (Psa), is one of the most destructive diseases affecting kiwifruit production worldwide. Salicylic acid (SA) plays a pivotal role in plant immunity, with its levels and signaling tightly regulated to maintain a balanced immune response. In plants, SA is biosynthesized via two distinct pathways: the isochorismate synthase (ICS) pathway and the phenylalanine ammonia-lyase (PAL) pathway. However, their roles in kiwifruit remain unclear. Here, we demonstrate that AcICS1 positively regulates kiwifruit resistance to Psa by promoting SA biosynthesis. Overexpression of AcICS1 significantly enhanced disease resistance, whereas silencing AcICS1 compromised immunity. Through dual-luciferase assays, AcWRKY40 was identified as a candidate regulator of AcICS1. Its direct binding to the W-box motif in the AcICS1 promoter was further confirmed by electrophoretic mobility shift assay (EMSA) and yeast one-hybrid (Y1H) assays. Functional analyses revealed that AcWRKY40 acts as a negative regulator of kiwifruit resistance to Psa: overexpression of AcWRKY40 repressed AcICS1 expression, inhibited SA accumulation, and increased disease susceptibility, whereas silencing AcWRKY40 alleviated these effects. Collectively, these findings identify AcWRKY40 as a transcriptional repressor of AcICS1, establish the AcWRKY40-AcICS1 regulatory module in kiwifruit, and provide new insights into SA-mediated resistance to bacterial canker.