Effects of potential signal transduction antagonists on phytoalexin accumulation in tobacco

Abstract

Tobacco cell suspensions produce sesquiterpene phytoalexins when treated with cellulase. This response is enhanced by CaCl 2, MgCl 2 or CdCl 2. For CaCl 2 and MgCl 2, enhancement extends for 8 hr after elicitor treatment. Several classes of signal transduction antagonists were tested on this plant system. EGTA inhibits sesquiterpene accumulation with an effective concentration range suggesting that Ca 2 + is the divalent cation being affected. Mg 2 + is as effective as Ca 2 + in relieving the inhibition by EGTA. This type of shared role by Ca 2 + and Mg 2 + has not been described previously and is yet to be characterized. The timing of the EGTA effect suggests a role of Ca 2 + in signal transduction. Calcium ion influx across the plasma membrane is one possibility since inhibitors of Ca 2 + influx inhibit sesquiterpene accumulation induced by cellulase. Verapamil is an inhibitor and its action is relieved by the Ca 2 + ionophore A23187. Among the cations which inhibit Ca 2 + influx, La 3 + is the most effective inhibitor of sesquiterpene accumulation. Calmodulin, or a Ca 2 + -dependent protein kinase, is also implicated in the signal transduction sequence, based on the results with W7, W5 and staurosporine. Neomycin and LiCl, inhibitors of phosphoinositide cycle steps, enhance cellulase-elicited sesquiterpene accumulation, suggesting a complex interaction of this cycle in the phytoalexin response. The results indicate that Ca 2 + and Mg 2 + play several roles in sesquiterpene biosynthesis. The involvement of a multicomponent elicitor signal transduction pathway, with calcium ion functioning at more than one step, is suggested.