Phage biocontrol reduces the disease burden and modulates plant immunity through suppression of bacterial virulence

Bacteriophages are emerging as modulators of plant-microbe interactions and promising biocontrol agents against bacterial plant pathogens. In this study, we investigate the tripartite interaction between Arabidopsis thaliana, Xanthomonas campestris pv. campestris (Xcc), and the virulent phage Seregon. Parallel transcriptomic profiling shows that application of a single-phage treatment does not eradicate Xcc but strongly mitigates disease symptoms, restoring plant growth within 14 days post-inoculation. Phage-mediated protection is further associated with reduced plant immune activation, including lower jasmonate-related responses and reactive oxygen species levels, confirmed using nuclear reporter lines and histochemical staining. In parallel, Xcc shows reduced expression of key virulence-associated genes, including type III secretion system components, effectors, and flagella biosynthesis genes. Notably, phage treatment does not promote substantial resistance emergence in planta, contrasting with in vitro conditions. Together, our findings show that phage biocontrol can enhance plant resilience by modulating both host immunity and pathogen behavior.