title: A harpin binding site in tobacco plasma membranes mediates activation of the pathogenesis-related gene HIN1 independent of extracellular calcium but dependent on mitogen-activated protein kinase activity.
abstract: Harpin from the bean halo-blight pathogen Pseudomonas syringae pv phaseolicola (harpin(Psph)) elicits the hypersensitive response and the accumulation of pathogenesis-related gene transcripts in the nonhost plant tobacco. Here, we report the characterization of a nonproteinaceous binding site for harpin(Psph) in tobacco plasma membranes, which is assumed to mediate the activation of plant defense responses in a receptor-like manner. Binding of 125I-harpin(Psph) to tobacco microsomal membranes (dissociation constant = 425 nM) and protoplasts (dissociation constant = 380 nM) was specific, reversible, and saturable. A close correlation was found between the abilities of harpin(Psph) fragments to elicit the transcript accumulation of the pathogenesis-related tobacco gene HIN1 and to compete for binding of 125I-harpin(Psph) to its binding site. Another elicitor of the hypersensitive response and HIN1 induction in tobacco, the Phytophthora megasperma-derived beta-elicitin beta-megaspermin, failed to bind to the putative harpin(Psph) receptor. In contrast to activation by beta-megaspermin, harpin(Psph)-induced activation of the 48-kD salicylic acid-responsive mitogen-activated protein kinase (MAPK) and HIN1 transcript accumulation were independent of extracellular calcium. Moreover, use of the MAPK kinase inhibitor U0126 revealed that MAPK activity was essential for pathogenesis-related gene expression in harpin(Psph)-treated tobacco cells. Thus, a receptor-mediated MAPK-dependent signaling pathway may mediate the activation of plant defense responses induced by harpin(Psph).
authors.affiliation: Department of Stress and Developmental Biology, Leibniz Institute of Plant Biochemistry, Weinberg 3, D-06120 Halle/Saale, Germany.
authors.forename: J
authors.initials: J
authors.lastname: Lee
authors.forename: D F
authors.initials: DF
authors.lastname: Klessig
authors.forename: T
authors.initials: T
authors.lastname: Nürnberger
mesh_terms: D001425:Bacterial Outer Membrane Proteins / Q000378:metabolism*; D001665:Binding Sites; D001667:Binding, Competitive; D002462:Cell Membrane / Q000378:metabolism*; D005656:Fungal Proteins / Q000494:pharmacology; D005786:Gene Expression Regulation; D018506:Gene Expression Regulation, Plant; D020928:Mitogen-Activated Protein Kinases / Q000378:metabolism*; D010446:Peptide Fragments / Q000378:metabolism; D010935:Plant Diseases / Q000235:genetics; D010937:Plant Growth Regulators / Q000494:pharmacology; D010940:Plant Proteins / Q000096:biosynthesis / Q000235:genetics*; D010947:Plants, Toxic; D020156:Salicylic Acid / Q000494:pharmacology; D014026:Nicotiana / Q000235:genetics* / Q000378:metabolism
chemical_list: D001425:Bacterial Outer Membrane Proteins; D005656:Fungal Proteins; C102693:HIN1 protein, Nicotiana tabacum; D010446:Peptide Fragments; D010937:Plant Growth Regulators; D010940:Plant Proteins; C075462:harpin protein, Erwinia amylovora; C087295:HrpZ protein, Pseudomonas syringae; D020928:Mitogen-Activated Protein Kinases; D020156:Salicylic Acid