Erwinia chrysanthemi HarpinEch: An Elicitorof the Hypersensitive Response that Contributesto Soft-Rot Pathogenesis. David W. Bauer . Department of Plant Pathology, Cornell University, Ithaca, NY 14853-4203 U.S.A. Zhong-Min Wei, Steven V. Beer, and Alan Collmer. Department of Plant Pathology, Cornell University, Ithaca, NY 14853-4203 U.S.A. MPMI 8:484-491. Accepted 8 March 1995 . 1995 The American Phytopathological Society.

Mutants of the soft-rot pathogen Erwinia chrysanlhemi EC16 that are deficient in the production of the pectate lyase isozymes PelABCE can elicit the hypersensitive response (HR) in tobacco leaves. The hrpNEch gene was identified in a collection of cosmids carrying E. chrysanlhemi hrp genes by its hybridization with the Erwinia amylovora hrpNEa gene. hrpNEch, appears to be in a monocistronic operon, and it encodes a predicted protein of 340 amino acids that is glycine-rich, lacking in cysteine, and highly similar to HrpNEa in its C-terminal half. Escherichia coli DH5( cells expressing hrpNEch from the lac promoter of pBIuescript II accumulated HrpNEch in inclusion bodies. The protein was readily purified from cell lysates carrying these inclusion bodies by solubili-zation in 4.5 M guanidine-HCl and reprecipitation upon dialysis against dilute buffer. HrpNEch suspensions elicited a typical HR in tobacco leaves, and elicitor activity was heat-stable. Tn5-gusA1 mutations were introduced into the cloned hrpNEch and then marker-exchanged into the genomes of E. chrysanthemi strains AC4150 (wild type), CUCPB5006 (? pelABCE), and CUCPB5030 (? pelABCE outD::TnphoA). hrpNEch::Tn5-gusAl mutations in CUCPB5006 abolished the ability of the bacterium to elicit the HR in tobacco leaves unless complemented with an hrpNEch subclone. An hrpNEch::Tn5-gusAl mutation also reduced the ability of AC4150 to incite infections in witloof chicory leaves, but it did not reduce the size of lesions that did develop. Purified HrpNEch and E. chrysanthemi strains CUCPB5006 and CUCPB5030 elicited HR-Iike necrosis in leaves of tomato, pepper, African violet, petunia, and pelargonium, whereas hrpNEch mutants did not. HrpNEch thus appears to be the only HR elicitor produced by E. chrysanlhemi EC 16, and it contributes to the pathogenicity of the bacterium in witloof chicory.