Pseudomonas viridiflava and P. syringae--natural pathogens of Arabidopsis thaliana.

We report the isolation and identification of two natural pathogens of Arabidopsis thaliana, Pseudomonas viridiflava and Pseudomonas syringae, in the midwestern United States. P. viridiflava was found in six of seven surveyed Arabidopsis thaliana populations. We confirmed the presence in the isolates of the critical pathogenicity genes hrpS and hrpL. The pathogenicity of these isolates was verified by estimating in planta bacterial growth rates and by testing for disease symptoms and hypersensitive responses to A. thaliana. Infection of 21 A. thaliana ecotypes with six locally collected P. viridiflava isolates and with one P. syringae isolate showed both compatible (disease) and incompatible (resistance) responses. Significant variation in response to infection was evident among Arabidopsis ecotypes, both in terms of symptom development and in planta bacterial growth. The ability to grow and cause disease symptoms on particular ecotypes also varied for some P. viridiflava isolates. We believe that these pathogens will provide a powerful system for exploring coevolution in natural plant-pathogen interactions.