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, HrpNea and HrpWea. In the present study, we showed that an Erwinia amylovora hrpWea mutant induces stronger electrolyte leakages in Arabidopsis thaliana foliar disks than the wild-type strain, thus suggesting that HrpWea could function as a HR negative modulator. We confirmed this result by using purified HrpWea and HrpNea. HrpWea 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, HrpWea inhibited cell death and AOS production provoked by HrpNea. HrpWea probably inhibits HrpNea-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 HrpWea harpin can act antagonistically to the classical HrpNea harpin by suppressing plant defense mechanisms.

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.

A CFTR chloride channel activator prevents HrpN(ea)-induced cell death in Arabidopsis thaliana suspension cells.

Erwinia amylovora is a necrogenic bacterium that causes fire blight of the Maloideae subfamily of Roseacae, such as apple and pear. It provokes necrosis in aerial parts of susceptible host plants and the typical hypersensitive reaction in non-host plants. The secreted harpin, HrpN ea, is able by itself to induce an active cell death in non-host plants. Ion flux modulations were shown to be involved early in such processes but very few data are available on the plasma membrane ion channel activities responsible for the pathogen-induced ion fluxes. We show here that HrpN ea induces cell death in non-host Arabidopsis thaliana suspension cells. We further show that two cystic fibrosis transmembrane conductance regulator modulators, glibenclamide and bromotetramisole, can regulate anion channel activities and HrpN ea-induced cell death.

Comparison of different physiological parameter responses in Lemna minor and Scenedesmus obliquus exposed to herbicide flumioxazin.

The sensitivity of different physiological parameters in Scenedesmus obliquus and Lemna minor exposed to herbicide (flumioxazin) was investigated to indicate the most convenient and sensitive parameter. To assess toxicity of flumioxazin, we used a panel of biomarkers: pigment contents, chlorophyll fluorescence parameters and antioxidative enzyme activities. Algae and duckweed were exposed to 48-h IC50 for growth rate. In L. minor, the sensitivity of the parameters was as follows: QN > Oxygen emmision > phiS(PSII) > QP > phi(PSII) > CAT, GR > Pigment> APO > Growth. For S. obliquus, this ranking was as follows: CAT > Oxygen emission > QP > APO > GR > Pigment > phiS(PSII) > Growth > phi(PSII) > QN (from the greatest to the least sensitive). The results demonstrated that the observed toxicity is related not only to interspecific variations but also to the selected parameter.