Early signalling events in the Avr9/Cf‐9‐dependent plant defence response

Resistance of tomato to the leaf mould fungus Cladosporium fulvum is controlled by the interaction between a plant‐encoded resistance gene ( Cf‐9 ) and pathogen‐encoded avirulence ( Avr9 ) gene. Our objective is to understand the underlying molecular mechanisms that transmit the Cf‐9/Avr9‐dependent pathogen perception event and activate the plant defence response. Our approach toward the understanding of Cf ‐function is based on the analysis of early Cf‐9/Avr9‐mediated responses and signalling events. Because Cf‐9 transgenically expressed in tobacco retains its specificity and activity to the Avr9 elicitor, signalling experiments were conducted in the heterologous system using these transgenic lines or derived Cf9 tobacco cell cultures. Among the earliest responses to the Avr9/Cf‐9 elicitation event were rapid changes in ion‐fluxes, the synthesis of active oxygen species (AOS), probably catalysed by a plant NADPH‐oxidase, and the transient activation of two MAP kinases. These kinases were identified as WIPK (wounding‐induced protein kinase) and SIPK (salicylic‐acid induced kinase) from tobacco. Studies with pharmacological inhibitors suggested that the MAP kinases are located in an independent signalling pathway from the Avr9/Cf‐9‐dependent synthesis of AOS. SIPK and WIPK were involved in pathogen‐related elicitation processes as well as in abiotic stress responses. This indicates that the plant defence is triggered via a signalling network that shares components with pathways originating from abiotic environmental stress stimuli.