The Interaction of Harpin_Pss, with Plant Cell Walls. M. E. Hoyos. Plant Pathology Department, University of Missouri, Columbia, MO 65211 U.S.A. C. M. Stanley,(2) S. Y. He,(3) S. Pike,(1) X-A. Pu,(1) and A. Novacky(1). (1)Plant Pathology Department, (2)Division of Biological Sciences, University of Missouri, Columbia, MO 65211 U.S.A. (3)Plant Pathology Department, University of Kentucky, North Lexington, KY 40546 U.S.A. MPMI 9:608-616. Accepted 25 June 1996. Copy right 1996 The American Phytopathological Society.

Erwinia amylovora and Pseudomonas syringae pv. syringae produce elicitors of the hypersensitive reaction (HR), harpin_Ea and harpin_Pss, respectively. Harpin_Ea causes K⁺ efflux and extracellular alkalinization in suspension-cultured cells of tobacco. These responses are associated with disease resistance. We treated living, fixed, and permeabi-lized cells and protoplasts from tobacco suspension cultures with harpin_Pss, antiharpin_Pss antibody, and fluoro-chrome-tagged antibody and examined them with confocal laser microscopy. The fluorescent signal was localized in the outer part of the cell and was not observed in protoplasts. EGTA, a chelating agent that extracts Ca⁺⁺ and pectins from cell walls, blocked harpin_Pss binding, as evi-denced by the absence of fluorescent signal. The pH of the external medium of suspension cultures alkalinized in response to harpin_Pss and P. s. syringae. Bacteria and harpin_Pss-induced alkalinization of the extracellular medium were also completely blocked upon EGTA treatment. Protoplasts alkalinized the medium at much reduced levels in response to P. syringae pv. syringae and did not alka-linize the medium in response to harpin_Pss. These results suggest that the cell wall is crucial for HR induction in the suspension culture cell.