The structure of ABA strongly
suggests a derivation from
isoprenoids. Two distinct
pathways have been proposed
for the synthesis of ABA (Fig.
3). In the “direct pathway”,
which occurs in some
phytopathogenic fungi, ABA
is derived from the C15
isoprenoid, farnesyl
diphosphate (FPP1
). When [14C]-mevalonic acid (MVA) was fed to ABAproducing
cultures of Cercospora rosicola, there was a significant incorporation
of label into ABA. Several potential intermediates in the pathway were also
labeled and when fed back to cultures these compounds were also converted to
ABA (24). Based upon these studies, several likely intermediates for the ABA synthetic pathway in fungi were identified (Fig. 4). More recently, the pathway of ABA biosynthesis has been studied by feeding [1-13C]-D-glucose to cultures
(15). The 13C-labeling pattern, determined by NMR, was also consistent with a direct pathway from farnesyl diphosphate in fungi.Because of structural similarities, an "indirect pathway" in which ABA is produced from the cleavage of carotenoids also had been proposed. A number of studies indicated that ABA in plants is derived from carotenoids (Fig. 5).