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

Harpin from the bean halo-blight pathogen Pseudomonas syringae pv phaseolic ola (harpin(Psph)) elicits the hypersensitive response and the accumulation of pathogenesis-related gene transcripts in the nonhost plant tobacco. Her e, we report the characterization of a nonproteinaceous binding site for ha rpin(Psph) in tobacco plasma membranes, which is assumed to mediate the act ivation of plant defense responses in a receptor-like manner. Binding of I- 125-harpin(Psph) to tobacco microsomal membranes (dissociation constant = 4 25 nM) and protoplasts (dissociation constant = 380 nM) was specific, rever sible, and saturable. A close correlation was found between the abilities o f harpin(Psph) fragments to elicit the transcript accumulation of the patho genesis-related tobacco gene HIN1 and to compete for binding of I-125-harpi n(Psph) to its binding site. Another elicitor of the hypersensitive respons e and HIN1 induction in tobacco, the Phytophthora megasperma-derived beta - elicitin beta -mesaspermin, failed to bind to the putative harpin(Psph) rec eptor. In contrast to activation by beta -megaspermin, harpin(Psph)-induced activation of the 48-kD salicylic acid-responsive mitogen-activated protei n kinase (MAPK) and HIN1 transcript accumulation were independent of extrac ellular calcium. Moreover, use of the MAPK kinase inhibitor U0126 revealed that MAPK activity was essential for pathogenesis-related gene expression i n harpin(Psph)-treated tobacco cells. Thus, a receptor-mediated MARK-depend ent signaling pathway may mediate the activation of plant defense responses induced by harpin(Psph).