Knot formation caused by Pseudomonas syringae subsp. savastanoi on olive plants is hrp-dependent

The virulence of Pseudomonas syringae subsp. savastanoi (Pss), which causes hyperplastic symptoms (knots) on olive plants, is associated with secreted phytohormones. We identified a Tn5-induced mutant of Pss that did not cause disease symptoms on olive plants although it was still able to produce phytohormones; in addition the mutant also failed to elicit a hypersensitive response in a nonhost plant. Molecular characterization of the mutant revealed that a single Tn5 insertion occurred within an open reading frame encoding a protein 92% identical to the HrcC protein of P. syringae pv. syringae. Moreover, sequence analysis revealed that the gene encoding the HrcC protein in Pss was part of an operon that included five genes arranged as in other phytopathogenic bacteria. These results imply that hrp/hrc genes are functional in Pss and that they play a key role in the pathogenicity of this plant pathogen.