Suspension cultures of Ammi majus L. cells produce various linear furanocoumarins in response to treatment with elicitor preparations from either Alternaria carthami Chowdhury or Phytophthora megasperma f.sp. glycinea. Microsomes which were isolated from these cells 14 h after addition of the elicitor eficiently catalyzed the conversion of demethyl[3 -''C]suberosin into labelled (+)marmesin in the presence of NADPH and oxygen. In contrast to the chemical cyclization of demethylsuberosin by rn-chloroperoxybenzoic acid, the reaction catalyzed by the marmesin synthase proceeded rapidly and no intermediate demethylsuberosin epoxide could be recovered. Significant blue-light-reversible inhibition by carbon monoxide and inhibition by various chemicals known to inhibit reactions dependent on cytochrome P450 suggested that the marmesin synthase is a cytochrome-P450-dependent monooxygenase.Upon prolonged incubation, a subsequent major labelled product originated from (+)marmesin, which was identified as psoralen. The psoralen synthase was also characterized as a cytochrome-P4,0-dependent monooxygenase. Both the marmesin synthase and the psoralen synthase, as well as enzymes catalyzing the formation of demethylsuberosin and 0-prenylumbelliferone from umbelliferone and dimethylallyl diphosphate, were associated with the endoplasmic reticulum in Ammi majus cells and their activities were concomitantly induced by elicitor treatment of the cells. We propose that in vivo these enzymes are active in the lumen of the endoplasmic reticulum from where the furanocoumarin phytoalexins are excreted into the cell culture fluid.

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Microsomes prepared from cultured Ammi majus cells that had been challenged for 14 h with an elicitor derived from the cell walls of Phytophthora megasperma f.sp. glycinea (Pmg) converted psoralen to bergaptol (5‐hydroxypsoralen) in the presence of NADPH and oxygen. The enzymatic activity was characterized as an inducible cytochrome‐P‐450‐dependent monooxygenase associated with the endoplasmic reticulum. All of the steps involved in bergapten (5‐methoxypsoralen) biosynthesis in Ammi majus have now been demonstrated in vitro. The results suggest that bergaptol and not hydroxymarmesin in the precursor of bergapten.

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