The biosynthetic pathway to abscisic acid via ionylideneethane in the fungus Botrytis cinerea.

The biosynthetic pathway to abscisic acid (ABA) from isopentenyl diphosphate in the fungus, Botrytis cinerea, was investigated. Labeling experiments with (18)O2 and H2(18)O indicated that all oxygen atoms at C-1, -1, -1' and -4' of ABA were derived from molecular oxygen, and not from water. This finding was inconsistent not only with the known carotenoid pathway via oxidative cleavage of carotenoids, but also with the classical direct pathway via cyclization of farnesyl diphosphate. The fungus produced new C15-compounds, 2E,4E-alpha-ionylideneethane and 2Z,4E-alpha-ionylideneethane, along with 2E,4E,6E-allofarnesene and 2Z,4E,6E-allofarnesene, but did not apparently produce carotenoids except for a trace of phytoene. The C15-compounds labeled with 13C were converted to ABA by the fungus, and the incorporation ratio of 2Z,4E-alpha-ionylideneethane was higher than that of 2E,4E-alpha-ionylideneethane. From these results, it was concluded that farnesyl diphosphate was reduced at C-1, desaturated at C-4, and isomerized at C-2 to form 2Z,4E,6E-allofarnesene before being cyclized to 2Z,4E-alpha-ionylideneethane; the ionylideneethane was then oxidized to ABA with molecular oxygen. This direct pathway via ionylideneethane means that the biosynthetic pathway to fungal ABA, not only before but also after isopentenyl diphosphate, differs from that to ABA in plants, since plant ABA is biosynthesized using the non-mevalonate and carotenoid pathways.