Plant-Derived Melatonin Inhibits Bacterial Virulence via CpxA/R Two-Component System

In defending against pathogens, plants deploy diverse secondary metabolites and signaling molecules. Among these, melatonin orchestrates plant growth and development, modulates stress responses, and regulates intracellular redox homeostasis and signaling. However, the mechanisms of melatonin in plant-pathogen interaction are rarely reported. Using Pseudomonas syringae pv. tomato DC3000 (Pst DC3000) as model bacteria, we designed a two-step high-throughput screening strategy to screen the plant natural product library and the bacterial mutant library. This study reveals that melatonin is perceived by a bacterial receptor histidine kinase CpxA, which subsequently modulates bacterial virulence. In detail, bacterial CpxA senses melatonin through Glu48 and Thr51 sites located in the periplasmic sensor region. Thus, melatonin inhibits autophosphorylation of CpxA and decreases transphosphorylation of the response regulator CpxR. The DNA-binding capacity of CpxR to promoters of type III secretion system (T3SS) genes is weakened by reduced phosphorylation cascade of CpxA/R, inhibiting bacterial T3SS genes expression and virulence. We also showed that increasing melatonin synthesis in plants can enhance disease resistance and sustain crop productivity. This study illustrates a previously unknown mechanism by which plants disarm the pathogenicity of bacteria, as well as provide effective molecular targets for crop genetic improvement and biopesticides development.