UC San Diego

Bacterial phytopathogens employ a type III secretion system to deliver effector proteins into the plant cell to suppress defense pathways, however the underlying molecular mechanisms of these effectors largely remain a mystery. Here, we demonstrate that the Pseudomonas syringae pv. tomato (Pst) type III effector, HopE1, specifically binds Arabidopsis With No Lysine (K) protein kinase 8 (WNK8) using a yeast two-hybrid screen, purified recombinant proteins, wheat germ in vitro transcribed and translated proteins, as well as Arabidopsis cell extracts. We also clearly show that WNK8 phosphorylates HopE1 and that HopE1 inhibits the autophosphorylation of WNK8 as well as the WNK8 phosphorylation of subunit C of the Arabidopsis thaliana vacuolar H⁺-ATPase (AtVHA-C). Furthermore, we demonstrate that WNK8 expression is induced upon Pst infection suggesting a possible role in early defense response. HopE1 and other effectors have been shown to suppress the plant hypersensitive response in order to promote pathogen survival. Although the precise molecular mechanism or in planta substrates of HopE1 remain obscure, our work uncovers a potential substrate and creates the tools necessary to further examine the role HopE1 plays in plant pathogenesis