Rapid Avr9- and Cf-9-dependent activation of MAP kinases in tobacco cell cultures and leaves: Convergence of resistance gene, elicitor, wound, and salicylate responses

The Cf-9 resistance (R) gene from tomato confers resistance to the fungal p athogen Cladosporium fulvum expressing the corresponding, pathogen-derived avirulence gene product Avr9. To understand how an initial R/Avr recognitio n event is transmitted and triggers the induction of plant defenses, we inv estigated early Avr9/Cf-9-dependent activation of protein kinases in transg enic tobacco expressing the Cf-9 gene. We identified two protein kinases of 46 and 48 kD, using myelin basic protein as substrate, that became rapidly activated in a strictly gene-for-gene manner within 2 to 5 min after Avr9 elicitation in both Cf9 tobacco plants and derived cell cultures, Studies w ith pharmacological inhibitors and effecters revealed that Ca2+ influx and a phosphorylation event(s) are required for kinase activation, but neither enzyme is involved in the Avr9-dependent synthesis of active oxygen species , The activation of both kinases is achieved via post-translational mechani sms, and the activation but not inactivation step includes tyrosine phospho rylation, Using specific antibodies, we found that the 46- and 48-kD kinase s were similiar to WIPK (for wound-induced protein kinase) and SIPK (for sa licylic acid-induced protein kinase), two previously characterized mitogen- activated protein (MAP) kinases from tobacco, In addition, Cf9 tobacco plan ts and cell cultures showed an Avr9-dependent accumulation of the WIPK tran script, Cf9 tobacco suspension cultures are thus a unique system in which t o analyze the earliest events in R gene function. These data indicate that (1) the R/Avr-mediated induction of plant defense is accomplished via sever al parallel signaling mechanisms, and (2) R/Avr-dependent signal transducti on pathways are interlinked at MAP kinases with responses of plants not onl y to non-race-specific elicitors but also to abiotic stimuli, such as wound ing and mechanical stress.