Cultured parsley (Petroselinum crispum) cells respond to treatment with elicitors derived from different species of the genus Phytophthora with transcript accumulation of defense-associated genes and the production of furanocoumarin phytoalexins. Pep-25, an oligopeptide fragment of a Phytophthora sojae 42-kDa cell wall protein, and a cell wall elicitor preparation derived from Phytophthora parasitica (Pp-elicitor) stimulate accumulation of the same gene transcripts and formation of the same pattern of furanocoumarins. Treatment of cultured cells and protoplasts with proteinase-digested Pp-elicitor identified proteinaceous constituents as active eliciting compounds in parsley. Similar to Pep- 25, Pp-elicitor induced effluxes of K+ and Cl- and influxes of protons and Ca2+. Concomitantly, as monitored in aequorin-transgenic parsley cell lines both elicitors induced an immediate increase in the cytoplasmic Ca2+ concentration up to sustained levels of 175 nM (Pp-elicitor) or 300 nM (Pep-25), respectively. The signature of the Ca2+ response differed greatly between the two elicitors tested. Extracellular Ca2+ proved essential for activation of an oxidative burst, MAP kinase activity and phytoalexin production by either elicitor. While Pp-elicitor induced a qualitatively similar spectrum of defense responses as did Pep-25, elicitor-specific quantitative differences in response intensity and kinetics suggest activation of a conserved signaling cascade through separate ligand binding sites.