Receptor-Mediated Increase in Cytoplasmic Free Calcium Required for Activation of Pathogen Defense in Parsley

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Abstract
Transient influx of Ca2+ constitutes an early element of signaling cascades triggering pathogen defense responses in plant cells. Treatment with the Phytophthora sojae–derived oligopeptide elicitor, Pep-13, of parsley cells stably expressing apoaequorin revealed a rapid increase in cytoplasmic free calcium ([Ca2+]cyt), which peaked at ∼1 μM and subsequently declined to sustained values of 300 nM. Activation of this biphasic [Ca2+]cyt signature was achieved by elicitor concentrations sufficient to stimulate Ca2+ influx across the plasma membrane, oxidative burst, and phytoalexin production. Sustained concentrations of [Ca2+]cyt but not the rapidly induced [Ca2+]cyt transient peak are required for activation of defense-associated responses. Modulation by pharmacological effectors of Ca2+ influx across the plasma membrane or of Ca2+ release from internal stores suggests that the elicitor-induced sustained increase of [Ca2+]cyt predominantly results from the influx of extracellular Ca2+. Identical structural features of Pep-13 were found to be essential for receptor binding, increases in [Ca2+]cyt, and activation of defense-associated responses. Thus, a receptor-mediated increase in [Ca2+]cyt is causally involved in signaling the activation of pathogen defense in parsley.