Abscisic acid biosynthesis in vascular plants is a constitutive process

The biosynthetic pathway leading to the formation of the plant hormone abscisic acid (ABA) in plants exposed to stress is believed to involve dioxygenase-mediated cleavage of a 9-Z-xanthophyll followed by oxidation of the product xanthoxal (XAN) via abscisic aldehyde (AB-ald). However, there are inconsistencies in the current model. Use of ABA-deficient mutants has not adequately explained the origin of basal endogenous ABA. The ubiquitous occurrence of AB-ald as a natural plant product has not been unequivocally established, Its conversion to ABA In vivo and in vitro occurs at rates below 30%, and the relationship between AB-ald and 2-trans-abscisic alcohol (trans-AB-ol) has not been wholly elucidated. This review explores the constitutive nature of ABA biosynthesis. Schemes accounting for the metabolism of XAN and 2-trans-AB-ol are proposed. It is concluded that there exist two precursor pools viz., beta,beta-carotenoids, and E,E- and E,Z-farnesol. These precursors are converted to ABA via 4'-keto-xanthoxic acid and 2-trans-AB-ol respectively presumably after incorporation into, and 'stress-induced' release from, ABA-adduct.