HRP SECRETED PROTEINS: THEIR ROLE IN PATHOGENICITY AND HR ELICITATION

Erwinia amylovora, like many Gram negative bacteria required a functional Hrp-secretion pathway to be pathogenic.
To date 3 proteins secreted through this pathway have been identified: Harpin (HrpN), DspA/E and HrpW. Harpin is a glycine rich heat stable protein able to elicit a HR on tobacco when purified.
DspA/E is homologous and functionaly equivalent to AvrE, an avirulence protein of Pseudomonas syringae pv. tomato. HrpW shows structural similarity with Harpin and its C-terminal region is homologous to class III pectate lyases.
The contribution of each protein to E. amylovora pathogenicity was determined. hrpN mutants are still able to produce weak fire blight symptoms showing that HrpN is rather a virulence factor than a pathogenicity factor.
In contrast dspA mutants are non pathogenic.
Thus DspA appears to be an essential pathogenicity factor. hrpW mutants are as pathogenic as the wild type strain CFBP1430 indicating that HrpW is not required for pathogenicity.
These mutants were then tested for their ability to induce HR and electrolyte leakage on tobacco leaf.
It could be concluded that although Harpin is the main HR elicitor, DspA also participates to HR elicitation.
Surprisingly, HrpW acts as a negative HR effector since hrpW mutants induce stronger HR than the wild type strain CFBP1430.

VIII International Workshop on Fire Blight

M.-A. Barny, S. Gaudriault, M.-N. Brisset, J.-P. Paulin

fire blight, type III secretion, hrp, chaperone

https://doi.org/10.17660/ActaHortic.1999.489.62