HrpZ harpins from different Pseudomonas syringae pathovars differ in molecular interactions and in induction of anion channel responses in Arabidopsis thaliana suspension cells

HrpZ, a type three secretion system helper protein from the plant-pathogen Pseudomonas syringae, can be recognized by many plants as a defence elicitor. Responses of Arabidopsis thaliana suspension cells to different HrpZ variants were studied by electrophysiological methods and cell death assay. Purified HrpZ originating from a compatible pathogen P. syringae pv. tomato DC3000 (HrpZPto) and incompatible P. syringae pv. phaseolicola (HrpZPph) both promoted Arabidopsis cell death. As an early response, both HrpZ variants induced an increase in time dependent K+ outward rectifying current. In contrast, the effects of HrpZ proteins on anion currents were different: HrpZPph had no effect, and HrpZPto induced an anion current increase. This suggests that the observed responses of the K+ channels and anion channels resulted from different and separable interactions and that the interaction implied in anion current modulation is host-specific. HrpZPto and HrpZPph also had a different sequence preference in phage display screen for peptide-binding. These peptides presumably represent a part of a putative target protein in the host, and HrpZ proteins of different P. syringae pathovars might have different binding specificities to match the allelic variation between plant species. Supporting the idea that the peptide-binding region of HrpZ is important for interactions with host cell components, we found that a mutation in that region changed the anion channel response of Arabidopsis cells.