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We examined the biosynthetic pathway of abscisic acid (ABA) after isopentenyl diphosphate in a fungus, Cercospora cruenta. All oxygen atoms at C-1, -1, -1 0 , and -4 0 of ABA produced by this fungus were labeled with 18 O from 18 O 2 . The fungus did not produce the 9Z-carotenoid possessing -ring that is likely a precursor for the carotenoid pathway, but produced new sesquiterpenoids, 2E,4E--ionylideneethane and 2Z,4E--ionylideneethane, along with 2E,4E,6E-allofarnesene. The fungus converted these sesquiterpenoids labeled with 13 C to ABA, and the incorporation ratio of 2Z,4E--ionylideneethane was higher than that of 2E,4E--ionylideneethane. From these results, we concluded that C. cruenta biosynthesized ABA by the direct pathway via oxidation of ionylideneethane with molecular oxygen following cyclization of allofarnesene. This direct pathway via ionylideneethane in the fungus is consistent with that in Botrytis cinerea, except for the positions of double bonds in the rings of biosynthetic intermediates, suggesting that the pathway is common among ABAproducing fungi.

A new biosynthetic intermediate of ABA, (2Z,4E)-gamma-ionylideneacetaldehyde, was isolated from young mycelia of Cercospora cruenta. Under an (18)O2 atmosphere, an oxygen atom of this endogenous aldehyde was exclusively labeled. Similarly, three (18)O atoms were incorporated into the ABA molecule recovered after prolonged incubation; selectively labeled were one of the carboxyl oxygen atoms and the two on the ring portion of ABA. A feeding experiment with [1-(13)C]glucose proved the exclusive operation of the mevalonate pathway for the formation of both ABA and beta-carotene. These results suggest that (2Z,4E)-gamma-ionylideneacetaldehyde can be a key ABA biosynthetic intermediate formed by the oxidative cleavage of a carotenoid precursor.

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