Antagonistic action of harpin proteins:: HrpWea from Erwinia amylovora suppresses HrpNea-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, HrpWea, like HrpNea, 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 HrpNea. HrpWea 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.