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

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.