Elucidation of the Indirect Pathway of Abscisic Acid Biosynthesis by Mutants, Genes, and Enzymes

Abstract

Abscisic acid (ABA) was discovered independently by several groups in the early 1960s. Originally believed to be involved in the abscission of fruit and dormancy of woody plants, the role of ABA in these processes is still not clear. ABA is, however, necessary for seed development, adaptation to several abiotic stresses, and sugar sensing. The regulation of these processes is in large part mediated by changes in de novo synthesis of ABA. Two pathways have been proposed for the synthesis of ABA. In the “direct pathway,” which operates in some fungi, ABA is derived from farnesyl diphosphate (Hirai et al., 2000). Because of structural similarities, an “indirect pathway” in which ABA is produced from the cleavage of carotenoids also had been proposed (Taylor and Smith, 1967). The first committed step for ABA synthesis in plants is the oxidative cleavage of a 9-cis-epoxycarotenoid (C40) to produce xanthoxin (C15) and a C25 by-product (Fig. 1). The 4′-hydroxyl of xanthoxin is oxidized to a ketone by an NAD-requiring enzyme. As a consequence, 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.