Activation of the oxidative burst in aequorin-transformed Nicotiana tabacum cells is mediated by protein kinase- and anion channel-dependent release of Ca2+ from internal stores

The source of Ca2+ involved in transducing an oxidative-burst defense signal was examined in aequorin-transformed tobacco (Nicotiana tabacum L.) cells using modulators of Ca2+ entry. Treatments that either increased or decreased the influx of Ca2+ from external stores were found to have little effect on the magnitude or kinetics of an osmotically stimulated oxidative burst, In contrast. treatments that reduced the discharge of Ca2+ from internal stores inhibited dilution-activated H2O, production. Curiously, most of the modulators commonly employed in animal studies as internal Ca2+ release inhibitors were neither effective in blocking discharge of intracellular Ca2+ nor in preventing the oxidative burst. When three different biochemical elicitors of the oxidative burst were similarly examined. both the H2O2 production and Ca2+ fluxes stimulated were found to be sensitive to modulators of internal Ca2+ release, but neither was impacted by alterations in externally derived Ca2+ influx. We hypothesize. therefore. that the oxidative burst does not depend on the influx of external Ca2+. but instead may generally be mediated by the release of internal Ca2_ in a manner that depends on the proper function of kinases and anion channels. These Ca2+ pulses trigger downstream signaling events that include the activation of Ca2+-regulated protein kinases. which are required for stimulation of the oxidative burst.