Polyphenol oxidases (PPOs; EC 1.14.18.1 or EC 1.10.3.2) catalyze the oxidation of phenolics to quinones, highly reactive intermediates whose secondary reactions are responsible for much of the oxidative browning that accompanies plant senescence, wounding, and responses to pathogens. To assess the impact of PPO expression on resistance to Pseudomonas syringae pv. tomato we introduced a chimeric antisense potato PPO cDNA into tomato (Lycopersicon esculentum L.). Oxidation of caffeic acid, the dominant o-diphenolic aglycone of tomato foliage, was decreased ca. 40-fold by antisense expression of PPO. All members of the PPO gene family were downregulated: neither immunoreactive PPO nor PPO-specific mRNA were detectable in the transgenic plants. In addition, the antisense PPO construct suppressed inducible increases in PPO activity. Downregulation of PPO in antisense plants did not affect growth, development, or reproduction of greenhouse-grown plants. However, antisense PPO expression dramatically increased susceptibility to P. syringae expressing the avirulence gene avrPto in both Pto and pto backgrounds. In a compatible (pto) interaction, plants constitutively expressing an antisense PPO construct exhibited a 55-fold increase in bacterial growth, three times larger lesion area, and ten times more lesions cm(-2) than nontransformed plants. In an incompatible (Pto) interaction, antisense PPO plants exhibited 100-fold increases in bacterial growth and ten times more lesions cm(-2) than nontransformed plants. Although it is not clear whether hypersusceptibility of antisense plants is due to low constitutive PPO levels or failure to induce PPO upon infection, these findings suggest a critical role for PPO-catalyzed phenolic oxidation in limiting disease development. As a preliminary effort to understand the role of induced PPO in limiting disease development, we also examined the response of PPO promoter::beta-glucuronidase constructs when plants are challenged with P. syringae in both Pto and pto backgrounds. While PPO B inducibility was the same in both compatible and incompatible interactions, PPO D, E and F were induced to higher levels and with different expression patterns in incompatible interactions.