Successful pathogens like Pseudomonas syringae translocate type III effector proteins (T3SE) into host cells. Plant hosts react by specifically recognizing these effectors via R proteins that trigger defense responses. The T3SE family HopZ1 has evolved into three allelic forms as a result of diversifying selection. In this thesis, I investigated how virulence and immune-eliciting functions are determined in HopZ1a and HopZ1b in Arabidopsis. Mutational analysis of HopZ1a identified ten residues important for immune elicitation and at least three are involved in virulence functions. These results suggest that distinct key amino acid residues in HopZ1a mediate the two activities. The closely related HopZ1b T3SE elicits an inconsistent immune response in Arabidopsis. We found that HopZ1b-triggered immune response involves a TIR-type R protein and plastid-derived SA. Together, these results highlight an uncharacterized ETI response to the HopZ1 family of T3SEs.