The role of the phytotoxin coronatine in the virulence of Pseudomonas syringae pv. tomato in Arabidopsis thaliana was evaluated by comparing symptom development, in planta bacterial multiplication, and the induction of defense-related genes in Arabidopsis plants inoculated with the coronatine-producing (Cor+) P. s. pv. tomato strain DC3000 and the coronatine-defective (Cor-) strain DC3661 by either infiltration or dipping methods. The Cor+ strain, P. s. pv. tomato DC3000, caused severe disease symptoms and multiplied by 4-6 logs after inoculation by either infiltration or dipping. P. s. pv. tomato DC3661 failed to produce any disease symptoms and multiplied by only 1-1.5 logs in dipped plants, whereas it caused mild symptoms and multiplied 6 logs over the 4-day experimental period in plants inoculated by infiltration. Parallel experiments using a natural host, tomato, yielded similar results. Analysis of the accumulation of mRNAs encoded by several distinct defense-related genes in Arabidopsis leaves infiltrated with either DC3000 or DC3661 demonstrated that the Cor- strain consistently induced higher levels of these transcripts. These results demonstrate that coronatine production is required under more natural inoculation conditions for the successful infection of Arabidopsis by DC3000, and that coronatine may play a critical role during the early stages of infection by suppressing the activation of defense-related genes.