We have studied the response of Arabidopsis thaliana to the bacterial pathogen Pseudomonas syringae pv. maculicola (Psm) strain ES4326. Several previously unknown Arabidopsis defense-related genes were identified including ones encoding a glutathione-Stransferase, a superoxide dismutase, a lipoxygenase, and two calmodulin-like proteins. Interestingly, mRNA corresponding to each of these genes displayed markedly different patterns of accumulation during the defense response to Psm ES4326. We have isolated three categories of Arabidopsis mutants that show an aberrant defense response to Psm ES4326. Three mutants were isolated that do not mount a hypersensitive response (HR) when infiltrated with Psm ES4326/avrRpt2 but are still able to display an HR in response to other avr genes. At least two of these mutants are allelic and map to chromosome IV. To facilitate the identification of additional Arabidopsis mutants that do not mount an HR in response to an avr gene, we developed a new method that involves vacuum infiltration of seedlings growing in petri plates. We also isolated three mutants that synthesize decreased levels of camalexin, an indole-based Arabidopsis phytoalexin. Two of the three camalexin mutants are significantly more permissive for the growth of Psm ES4326 than wild-type plants. Finally, five Arabidopsis mutants were isolated that display accelerated disease symptoms in response to Psm ES4326. These latter mutants, which were given the name acd for accelerated cell death, were assigned to two complementation groups.
