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 Kþ 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 Kþ 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 peptidebinding
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.