Studies on the hrp gene cluster of Erwinia amylovora have revealed the existence of a "pathogenicity island", which includes a Hrp (type III) protein secretion system similar in function and protein sequence to systems in Pseudomonas, Raistonia, and Xanthomonas spp. The interaction between E. amylovora and its host and nonhost plants depends on secreted proteins. Genes of the cluster encode secreted proteins and those that function in regulation or secretion of them. HrpN harpin, an acidic, heat-stable, glycine-rich secreted protein, elicits the hypersensitive response (HR) following infiltration of leaf apoplast with the protein solution. When applied to plants at low concentrations by either infiltration or spraying, HrpN induces systemic acquired resistance (SAR), enhances plant growth and promotes insect repellency. Mutagenesis of hrpN essentially knocks out HR induction and pathogenicity of the weakly virulent strain Ea32 1. HrpW, a second harpin, elicits the HR, but it is dispensable for the Hrp phenotype in E. amylovora. HrpW also elicits SAR and its activity in the other beneficial effects is under investigation. The hrp gene cluster also encodes Avr-like proteins. DspE and DspF are required for pathogenicity but not for HR elicitation. DspEF is homologous to AvrEF of Pseudomonas syringae, and cross-functionality between the homologs has been demonstrated. DspF has characteristics of chaperones. A second avr-like gene homologous to avrRxv of Xanthomonas campestris and its putative chaperone are present between hrpN and hrpW. The hrp gene cluster of E. amylovora secretes Avr proteins of P. syringae, and recombinant E. amytovora cells express and secrete AvrPto and AvrB into media and induce the HR on cultivars of tomato and soybean carrying the corresponding R genes, respectively. Other Hrp-secreted proteins of E. amylovora are being discovered by expressing its pathogenicity island in Escherichia coli.