Biosynthesis and Signalling of Abiscisic Acid in Plants

Abscisic acid (ABA) (Appendix for the figure), a phytohormone, regulates various processes in plants such as physiological responses to environmental stresses (e.g. drought and salinity) and induction of primary dormancy during seed maturation (Milbrow, 2001; Liotenberg, North & Marion-Poll, 1999).  Its synthesis in vascular plants proceeds via a multi-stage pathway, with its immediate precursor being an aldehyde xanthoxin (referred to as xanthoxal), an intermediate that results from the breakdown of carotenoids (Milbrow, 2001; Liotenberg, North & Marion-Poll, 1999).  Such link to carotenoids as precursors of ABA was first evident from the structural similarity between ABA and end groups of some carotenoids, with biochemical data confirming such observations (Liotenberg, North & Marion-Poll, 1999). The identification of the exact carotenoid precursors was proposed following observations that increases in ABA following water stress corresponded with decreases in violaxanthin and neoxanthin (Liotenberg, North & Marion-Poll, 1999). Subsequently, observations that such changes have a stoichiometry of 1:1 confirmed that violaxanthin and neoxanthin are precursors of ABA (Liotenberg, North & Marion-Poll, 1999).

The complete pathway of ABA biosynthesis (appendix for reactions) may be divided into three stages.  In the early stages, small-phosphorylated intermediates (e.g. isopentenyl pyrophosphate – IPP, and geranylgeranyl pyrophosphate – GGPP) are synthesized from the compounds generated in the glycolytic pathway – pyruvate or glyceraldehyde 3- phosphate (Milbrow, 2001; Liotenberg, North & Marion-Poll, 1999).  In the intermediate stage, such small-phosphorylated intermediates are converted to phytoene, an uncyclized C40 carotenoid in a reaction catalysed by phytoene synthase (Milbrow, 2001; Liotenberg, North & Marion-Poll, 1999). This intermediary stage then follows a series of reactions that culminate with the cleavage of 9’-cis-neoxanthin to form xanthoxal in a reaction catalysed by 9’-cis-epoxy carotenoid dioxygenase (NCED) (Milbrow, 2001; Liotenberg, North & Marion-Poll, 1999).  In the final stage, xanthoxal is converted to ABA in a process thought to proceed via the oxidation of xanthoxal to xanthoxic acid (Milbrow, 2001), rather than via an AB-aldehyde intermediate as was previously thought (Liotenberg, North & Marion-Poll, 1999). Go to part 2 here.

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