The response of Arabidopsis thaliana land race Columbia to the bacterial pathogen Pseudomonas syringae pv. maculicola 4326 harboring cloned avirulence genes avrB and avrC from P. syringae pv. glycinea and avrA and avrD from P. syringae pv. tomato was examined. Only avrB was recognized by Columbia, as evidenced by attenuation of disease symptoms, slower bacterial multiplication in planta, and differentially greater induction of mRNA for several defense-related genes. This contrasts with two A. thaliana land races where P. s. pv. maculicola strains containing avrB were not recognized. These plants showed typical disease symptoms, and bacterial multiplication in planta was not reduced in response to P. s. pv. maculicola containing avrB. In addition, there was no differential induction of defense-related mRNAs in these susceptible land races after infiltration with bacteria containing or lacking avrB. These results extend previous observations that avirulence genes from pathogens of one host plant can be recognized by "nonhost" plants and provide the genetic framework for analysis of the plant-specified response to the bacterial avrB gene product in A. thaliana.