EXPRESSION OF THE TYPE III SECRETION CHAPERONE PROTEIN DSPF OF ERWINIA AMYLOVORA IN APPLE INCREASES RESISTANCE TO FIRE BLIGHT

Erwinia amylovora causes fire blight of apple, pear and other rosaceous plants and elicits plant defense responses in non-host plants. Required for these interactions are the clustered bacterial hrp genes, which encode a large set of proteins broadly conserved among plant and animal pathogens and constitute a type III secretion pathway. The “disease-specific” (dsp) region next to the hrp gene cluster of Erwinia amylovora is required for pathogenicity but not for elicitation of the hypersensitive reaction. A 6.6-kb apparent operon, dspEF/AB, was found responsible for this phenotype. DspE/A is a pathogenesis effector protein for E. amylovora. DspF/B is a small (16 kDa), acidic protein with predicted amphipathic alpha helices in its C terminus and is a type III secretion chaperone protein for the translocation of DspE/A into the host cell. Because the function of type III secretion chaperone proteins is mediated by physical interaction with the cognate effector proteins, we hypothesized that expression of DspF/B in apple might interfere with the virulence function of DspE/A in the host cell, therefore reducing fire blight disease. Gene expression constructs designed to express the DspF gene were used to transform apple. Transgenic lines harboring the DspF gene have been recovered. RT-PCR and western blot assays showed expression of DspF in the transgenic lines. Some transgenic lines were evaluated for resistance to fire blight by inoculation of shoots of own-rooted potted plants with the virulent strain Ea273 of E. amylovora. Preliminary results indicate that the susceptibility of several lines to E. amylovora was decreased by 50% and one line by 80%.