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Microsomal fractions isolated from parsley cell suspension cultures, which had been challenged with an elicitor from either Alternaria carthami or Phytophthora megasperma f. sp. glycinea, catalyzed the formation of psoralen from synthetic [3-14C](+)marmesin. Whereas psoralen was the only product formed in incubations with Alternaria-induced microsomes, another unidentified product was isolated from incubations with Phythophthora-induced microsomes/ The latter product is neither precursor nor a product of psoralen. n 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 elicitor-inducible cytochrome P450-dependent psoralen synthase. Fractionation of microsomal preparations by centrifugation revealed that psoralen synthase is associated with 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. [ABSTRACT FROM AUTHOR]