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The HrpN_ea Harpin from Erwinia amylovora Triggers Differential Responses on the Nonhost Arabidopsis thaliana Cells and on the Host Apple Cells

Erwinia amylovora is a gram-negative necrogenic bacterium causing fire blight of the Maloideae subfamily of Rosaceae such as apple and pear. It provokes progressive necrosis in aerial parts of susceptible host plants (compatible interaction) and a hypersensitive reaction (HR) when infiltrated in nonhost plants (incompatible interaction). The HrpN_ea harpin is a type three secretion system effector secreted by E. amylovora. This protein is involved in pathogenicity and HR-eliciting capacity of E. amylovora. In the present study, we showed that, in nonhost Arabidopsis thaliana cells, purified HrpN_ea induces cell death and H₂O₂ production, two nonhost resistance responses, but failed to induce such responses in host MM106 apple cells. Moreover, HrpN_ea induced an increase in anion current in host MM106 apple cells, at the opposite of the decrease of anion current previously shown to be necessary to induce cell death in nonhost A. thaliana cells. These results suggest that HrpN_ea induced different signaling pathways, which could account for early induced compatible or incompatible interaction development.