Antagonistic action of harpin proteins: HrpW_ea from Erwinia amylovora suppresses HrpN_ea-induced cell death in Arabidopsis thaliana

Harpins are proteins secreted by the type-three secretion system of phytopathogenic bacteria. They are known to induce a hypersensitive response (HR) in non-host plant leaf tissue. Erwinia amylovora, the fire blight pathogen of pear and apple trees, secretes two different harpins, HrpN_ea and HrpW_ea. In the present study, we showed that an Erwinia amylovora hrpW_ea mutant induces stronger electrolyte leakages in Arabidopsis thaliana foliar disks than the wild-type strain, thus suggesting that HrpW_ea could function as a HR negative modulator. We confirmed this result by using purified HrpW_ea and HrpN_ea. HrpW_ea has dual effects depending on its concentration. At 200 nM, HrpW_ea, like HrpN_ea, provoked the classical defense response - active oxygen species (AOS) production and cell death. However, at 0.2 nM, HrpW_ea inhibited cell death and AOS production provoked by HrpN_ea. HrpW_ea probably inhibits HrpN_ea-induced cell death by preventing anion channel inhibition, confirming that anion channel regulation is a determinant feature of the plant response to harpins. Collectively our data show that the HrpW_ea harpin can act antagonistically to the classical HrpN_ea harpin by suppressing plant defense mechanisms.