<jats:title>Significance</jats:title> <jats:p> Pathogenic bacteria inject effector proteins into the host to suppress its defenses. However, bacteria produce the effector proteins and injection machinery only upon recognition of a potential host. Here we identified an <jats:italic>Arabidopsis</jats:italic> mutant, <jats:italic>mapk phosphatase 1</jats:italic> ( <jats:italic>mkp1</jats:italic> ), with decreased levels of chemical signals recognized by the bacterium, thus making the plant more resistant by suppressing the ability of the pathogen, <jats:italic>Pseudomonas syringae</jats:italic> , to express and inject effector proteins. Reapplying these chemical signals not only eliminated resistance in the <jats:italic>mkp1</jats:italic> mutant but also suppressed resistance in wild-type plants with a preinduced immune response. These results demonstrate an important layer in determining the biological outcome during host–pathogen interactions and may provide new targets for enhancing resistance against bacterial pathogens.