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Biological Activity of Harpin Produced by Pantoea stewartii subsp. stewartii

October 2001 , Volume 14 , Number  10
Pages  1,223 - 1,234

Musharaf Ahmad , 1 Doris R. Majerczak , 1 Sharon Pike , 2 Mary Elizabeth Hoyos , 2 Anton Novacky , 2 and David L. Coplin 1

1Department of Plant Pathology, The Ohio State University, Columbus 43210-1087, U.S.A.; 2Department of Plant Pathology, University of Missouri, Columbia 65211, U.S.A.

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Accepted 25 June 2001.

Pantoea stewartii subsp. stewartii causes Stewart's wilt of sweet corn. A hypersensitive response and pathogenicity (Hrp) secretion system is needed to produce water-soaking and wilting symptoms in corn and to cause a hypersensitive response (HR) in tobacco. Sequencing of the hrp cluster revealed a putative harpin gene, hrpN. The product of this gene was overexpressed in Escherichia coli and shown to elicit the HR in tobacco and systemic resistance in radishes. The protein was designated HrpNPnss. Like other harpins, it was heat stable and protease sensitive, although it was three- to fourfold less active biologically than Erwinia amylovora harpin. We used antibodies to purified HrpNPnss to verify that hrpN mutants could not produce harpin. This protein was secreted into the culture supernatant and was produced by strains of P. stewartii subsp. indologenes. In order to determine the importance of HrpNPnss in pathogenesis on sweet corn, three hrpN::Tn5 mutants were compared with the wild-type strain with 50% effective dose, disease severity, response time, and growth rate in planta as parameters. In all tests, HrpNPnss was not required for infection, growth, or virulence in corn or endophytic growth in related grasses.

Additional keywords: Erwinia stewartii , type III secretion.

© 2001 The American Phytopathological Society