Microsomal fractions isolated from parsley cell suspension cultures, which had been challenged with an elicitor from either Alternaria carthami or PI1 ~~tlrophthora megasperma f. sp. glycinea, catalyzed the formation of psoralen from synthetic [3-'4C](+)marmesin. Whereas psoralen was the only product formed in incubations with Alternaria-induced microsomes, another unidentified product was isolated from incubations with Phythophthorainduced microsomes. The latter product is neither a precursor nor a product of psoralen. In contrast, microsomes isolated from non-induced parsley cells lacked both of these catalytic activities. The formation of psoralen depends on NADPH as a cofactor and molecular oxygen. Blue-light-reversible CO inhibition and inhibition by various synthetic chemicals known to bind to cytochromes P450 indicated that the reaction is catalyzed by an elicitorinducible cytochrome P,,,-dependent psoralen synthase. Fractionation of microsomal preparations by centrifugation revealed that psoralen synthase is associated with the endoplasmic reticulum. Our results suggest that the endoplasmic reticulum of cultured parsley cells is the primary target in the previously reported differential induction by elicitors from these two non-pathogenic strains of fungi.Continuously dark-cultured parsley cells accumulate and excrete various linear furanocoumarin and a pyronocoumarin phytoalexins in response to elicitor challenge, as was shown previously using crude cell-wall fractions from two nonpathogenic fungi (Alternaria carthami and Phythophthora megasperma f. sp. glycinea) [I, 21. The relative composition of the coumarins induced varied significantly in response to the two elicitors. Furthermore, the increase in fresh weight of A. carthami (Ac) elicitor-induced parsley cells was comparable to that of untreated controls [3], suggesting that the primary metabolism of the cells was not severely altered, whereas comparable amounts of P. megasperma (Pmg) elicitor inhibited the growth of the cells. Since massive synthesis of the coumarins was observed within a few hours of addition of either one of the elicitors, parsley cells appeared to represent Correspondence to U. Matern, Lehrstuhl fiir Biochemie der

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