Antagonistic action of harpin proteins: HrpW sub(ea) from Erwinia amylovora suppresses HrpN sub(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 sub(ea) and HrpW sub(ea). In the present study, we showed that an Erwinia amylovora hrpW sub(ea) mutant induces stronger electrolyte leakages in Arabidopsis thaliana foliar disks than the wild-type strain, thus suggesting that HrpW sub(ea) could function as a HR negative modulator. We confirmed this result by using purified HrpW sub(ea) and HrpN sub(ea). HrpW sub(ea) has dual effects depending on its concentration. At 200 nM, HrpW sub(ea), like HrpN sub(ea), provoked the classical defense response - active oxygen species (AOS) production and cell death. However, at 0.2 nM, HrpW sub(ea) inhibited cell death and AOS production provoked by HrpN sub(ea). HrpW sub(ea) probably inhibits HrpN sub(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 sub(ea) harpin can act antagonistically to the classical HrpN sub(ea) harpin by suppressing plant defense mechanisms.