Harpin, Elicitor of the Hypersensitive Response Produced by the Plant Pathogen Erwinia amylovora

A proteinaceous elicitor of the plant defense reaction known as the hypersensitive response was isolated from Erwinia amylovora , the bacterium that causes fire blight of pear, apple, and other rosaceous plants. The elicitor, named harpin, is an acidic, heat-stable, cell-envelope-associated protein with an apparent molecular weight of 44 kilodaltons. Harpin caused tobacco leaf lamina to collapse and caused an increase in the pH of bathing solutions of suspension-cultured tobacco cells. The gene encoding harpin ( hrpN ) was located in the 40-kilobase hrp gene cluster of E. amylovora , sequenced, and mutated with Tn 5 tac1. The hrpN mutants were not pathogenic to pear, did not elicit the hypersensitive response, and did not produce harpin.