Salicylic acid (SA) plays a central role in plant acquired resistance and induces gene expression of a group of pathogenesis-related (PR) proteins. We have previously reported that tobacco cells display Ca2+-dependent and Ca2+-independent excretion modes of SA in suspension culture depending on the applied SA concentrations (Chen et al. 2001). In this study, same concentrations of SA (200 μmol/L and 20 μmol/L) were used to compare their effects on PR-N, an acidic β-1,3-glucanase, gene induction and mRNA accumulation in tobacco cell suspension culture. SA at both concentrations induced PR-N gene expression. The induction by 200 μmol/L SA was significantly inhibited by EGTA (a Ca2+ ion chelator) or reduced glutathione (an active oxygen species scavenger). However, these two compounds have little or no effect on PR-N gene induction by 20 μmol/L SA. Either calcium ionophore (A23187) or catalase inhibitor (3-amino-1,2,4-triazole; 3AT) activated PR-N gene expression. As observed for 200 μmol/L SA, the induction by A23187 and 3AT was also blocked by EGTA and reduced glutathione, respectively. A SA functional analog, 2,6-dichloroisonicotinic acid (INA), induced PR-N gene expression. However, EGTA and reduced glutathione had little or no effect on the induction by 20 μmol/L INA, same as observed for the induction by 20 μmol/L SA. Based on these data, we conclude that SA could mediate alternative signal transduction pathways leading to PR-N gene induction in tobacco cell suspension culture. SA concentration seems to play a key role in the activation of different components of signal transduction pathways. Active oxygen species elevation and external Ca2+ influx are components likely associated with 200 μmol/L SA activation mechanism. Another signal transduction pathway not associated with these two components, however, may be responsible for the effect of SA at 20 μmol/L.
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