Nitrate Efflux Is an Essential Component of the Cryptogein Signaling Pathway Leading to Defense Responses and Hypersensitive Cell Death in Tobacco

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Abstract
There is much interest in the transduction pathways by which avirulent pathogens or derived elicitors activate plant defense responses. However, little is known about anion channel functions in this process. The aim of this study was to reveal the contribution of anion channels in the defense response triggered in tobacco by the elicitor cryptogein. Cryptogein induced a fast nitrate (NO(3)(-)) efflux that was sensitive to anion channel blockers and regulated by phosphorylation events and Ca(2+) influx. Using a pharmacological approach, we provide evidence that NO(3)(-) efflux acts upstream of the cryptogein-induced oxidative burst and a 40-kD protein kinase whose activation seems to be controlled by the duration and intensity of anion efflux. Moreover, NO(3)(-) efflux inhibitors reduced and delayed the hypersensitive cell death triggered by cryptogein in tobacco plants. This was accompanied by a delay or a complete suppression of the induction of several defense-related genes, including hsr203J, a gene whose expression is correlated strongly with programmed cell death in plants. Our results indicate that anion channels are involved intimately in mediating defense responses and hypersensitive cell death.