DIFFUSIBLE SIGNAL(S) INVOLVED IN CELL-TO-CELL COMMUNICATION AND LOCALIZED DEFENSE RESPONSES IN TOBACCO
J CHAPPELL, M LUSSO, A MANDUJANO-CHAVEZ, M SCHOENBECK and L RAISTON
Plant Physiology/Biochemistry/Molecular Biology Program, University of Kentucky, Lexington, KY 40546-0091, USA
We previously demonstrated that tobacco cell cultures sealed within 1000 MW cut-off dialysis tubing with a 10 kDa elicitin protein secrete a diffusible signal(s) capable of activating defense responses in subsequent bioassays . In the current work, we have attempted to better document the breadth of responses induced by this diffusible factor, as well as to dissect the transduction cascade leading up to the production of the diffusible signal. Like elicitin induction, treatment of tobacco cell cultures with the diffusible signal induces the production of reactive oxygen species and triggers cell death. Likewise, the diffusible factor induces a suppression of squatene synthase and an induction of sesquiterpene cyclase enzyme activities, indicating a diversion of carbon flow from sterols to sesquiterpenes. Somewhat surprising, however, is that the cells treated with the diffusible factor do not accumulate capsidiol, the typical sesquiterpene phytoalexin found in tobacco. Induction of capsidiol accumulation in elicitin-treated cells is correlated with the selective induction of both a sesquiterpene cyclase enzyme activity and a P450-type hydroxylase enzyme activity. In contrast, the diffusible signal does not induce either the enzyme activity or the steady-state MRNA level for aristolochene hydroxylase. In another series of experiments, a pharmacological approach was used to initially dissect the signal transduction pathway(s) induced by elicitins and the diffusible factor. The induction of sesquiterpene cyclase activity by elicitin is sensitive to staurosporine and EGTA, while induction by the diffusible signal is not. Elicitin-induced production of the diffusible factor is itself sensitive to the protein kinase inhibitor. Taken together, these results are consistent with the notion that incipient cells perceiving a pathogen activate a protein kinase-dependent transduction cascade resulting in the release of a diffusible signal to their neighboring cells (localized response). The diffusible factor then serves to potentiate the neighbors' ability to respond defensively to subsequent pathogen ingress.