Abstract
Bacterial speck of tomato, caused by Pseudomonas syringae pv. tomato (Pst), is a globally significant disease leading to substantial yield losses. This study demonstrated that methyl caffeate (MC), a plant-derived phenolic acid, exhibited dual functionality against Pst strain DC3000 through direct antibacterial action and indirect host resistance priming. In vitro assays revealed MC's potent inhibitory activity against bacterial growth, characterised by disruption of membrane integrity, leading to intracellular content leakage and reactive oxygen species (ROS) accumulation. Transcriptomic analysis identified 434 upregulated and 326 downregulated genes, with differentially expressed genes primarily enriched in pathways related to flagellar assembly, motility, quorum sensing, chemotaxis and the type III secretion system (T3SS), thereby impairing bacterial motility and virulence. Concurrently, MC enhances plant defence responses by inducing the transcriptional expression of defence hormone signalling pathway-related genes and increasing the activities of defence enzymes such as phenylalanine ammonia-lyase (PAL) and polyphenol oxidase (PPO). This modulation helps maintain ROS homeostasis and alleviates oxidative damage caused by Pst infection, thereby attenuating disease progression. These findings established MC as a multifunctional agent combining direct bactericidal effects with plant immunity induction, providing a sustainable strategy for bacterial speck management.