Harpin, An Elicitor of the Hypersensitive Response in Tobacco Caused by Erwinia amylovora, Elicits Active Oxygen Production in Suspension Cells

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Abstract
Active oxygen (AO) production and a K+/H+ exchange response (XR) are two concurrent early events associated with incompatible plant-bacteria interactions that result in a hypersensitive response (HR). Recently, a protein, termed harpin, produced by Erwinia amylovora has been reported to be the elicitor responsible for the HR caused by this pathogen. Although both the bacterium and harpin are reported to induce XR in tobacco (Nicotiana tabacum) cell suspensions, there have been no reports regarding the concurrent production of AO in this system. Here we report that E. amylovora stimulates the AO response, whereas an E. amylovora mutant that does not produce harpin does not elicit the AO response. In addition, a cell-free preparation of harpin induces AO production. This study indicates that harpin may be the bacterial elicitor of the XR and AO responses during the development of E. amylovora-induced HR.