AvrBsT is a type III effector from Xanthomonas campestris pv vesicatoria that is translocated into plant cells during infection. AvrBsT is predicted to encode a Cys protease that targets intracellular host proteins. To dissect AvrBsT function and recognition in Arabidopsis thaliana, 71 ecotypes were screened to identify lines that elicit an AvrBsT-dependent hypersensitive response (HR) after Xanthomonas campestris pv campestris (Xcc) infection. The HR was observed only in the Pi-0 ecotype infected with Xcc strain 8004 expressing AvrBsT. To create a robust pathosystem to study AvrBsT immunity in Arabidopsis, the foliar pathogen Pseudomonas syringae pv tomato (Pst) strain DC3000 was engineered to translocate AvrBsT into Arabidopsis by the Pseudomonas type III secretion (T3S) system. Pi-0 leaves infected with Pst DC3000 expressing a Pst T3S signal fused to AvrBsT-HA (AvrBsT HYB -HA) elicited HR and limited pathogen growth, confirming that the HR leads to defense. Resistance in Pi-0 is caused by a recessive mutation predicted to inactivate a carboxylesterase known to hydrolyze lysophospholipids and acylated proteins in eukaryotes. Transgenic Pi-0 plants expressing the wild-type Columbia allele are susceptible to Pst DC3000 AvrBsT HYB -HA infection. Furthermore, wild-type recombinant protein cleaves synthetic p-nitrophenyl ester substrates in vitro. These data indicate that the carboxylesterase inhibits AvrBsT-triggered phenotypes in Arabidopsis. Here, we present the cloning and characterization of the SUPPRESSOR OF AVRBST-ELICITED RESISTANCE1.