Pathways and Related Enzymes of ABA Biosynthesis in Higher Plants

Two pathways have been proposed for the biosynthesis of ABA: the ‘direct pathway’ and ‘indirect pathway’. In the direct pathway, which operates in some fungi, ABA is derived from farnesyl diphosphate; in indirect pathway, ABA is produced from the oxidative cleavage of carotenoids. The first committed step for ABA biosynthesis in plants is the oxidative cleavage of a 9-cis-epoxycarotenoid (C₄₀) to produce xanthoxin (C₁₅) and a C₂₅ by-product (Fig.2). Xanthoxin is then oxidized to a ketone by an NAD-requiring enzyme. Consequently, there is a nonenzymatic desaturation of the 2'-3' bond and opening of the epoxide ring to form abscisic aldehyde. In the final step of the pathway, abscisic aldehyde is oxidized to ABA. These endogenous enzymes such as zeaxanthin epoxidase (ZEP), 9-cis-epoxycarotenoid dioxygenase (NCED) and aldehyde oxidase (AO) may play key roles in the regulation of ABA biosynthesis in higher plants.