Trends in Plant Science
ReviewFormation and breakdown of ABA
Section snippets
Early steps in the plastid
Initial research focused on two possible routes to ABA: a ‘direct’ pathway from farnesyl pyrophosphate (similar to the fungal route discussed later) or an ‘indirect’ pathway via cleavage of a carotenoid precursor3. In each case the ultimate precursor was thought to be mevalonic acid (MVA).
Several persuasive lines of evidence favored the ‘indirect’ pathway to ABA via a C40 carotenoid intermediate. Some reports show that corn viviparous mutants (Vp), defective in carotenoid biosynthesis, are also
ABA catabolism in higher plants
There are several metabolic pathways by which ABA can be removed or degraded in plant tissues as a means of further regulating ABA concentrations. In the simplest case, ABA is exported by passive or carrier-mediated efflux from the cells. In the majority of plant tissues, catabolic inactivation of (+)-ABA proceeds via hydroxylation at the 8′ position to form the unstable intermediate, 8′-hydroxyABA, which subsequently cyclizes spontaneously (and/or enzymatically) to form (−)-PA (Ref. 3). The
Metabolism and physiology
There is substantial biochemical and physiological evidence that ABA concentrations in plant cells are maintained dynamically by continual synthesis and degradation. Therefore, measurements of extracted ABA reflect only a ‘snapshot’ of the net effect of these processes and provide little information about flux, cellular or subcellular distribution and/or the potential for rapid change. By perturbing either ABA synthesis or degradation, it has been poss-ible to gain some sense of the relative
ABA metabolism in lower plants and photosynthetic procaryotes
ABA is ubiquitous among green plants and pre-dates the evolution of seed-bearing landforms. It has been identified in ferns, bryophytes (mosses and liverworts) and all algal classes, including oxygenic photosynthetic procaryotes35 (cyanobacteria). In many lower plants, it has been unclear until recently whether ABA is a hormone or a secondary metabolite. However, there is mounting evidence that ABA has a hormonal function in mosses, liverworts and algae35.
Little is known about the metabolic
Future prospects
The description of metabolic pathways and the identification of intermediates is mostly complete with respect to ABA (Fig. 1). However, several questions remain. Firstly, there is a significant deficiency in our knowledge of the xanthophyll trans-to-cis-isomerization reaction. This is a potentially important step in ABA biosynthesis, but to date, no mutants with defects in the isomerization of carotenoids have been isolated in plants13, and no biochemical information has been obtained.
Acknowledgements
We thank David Taylor, Pat Covello and Sue Abrams for helpful comments and suggestions, and June McClintick for preparing Fig. 1, Fig. 2. This paper is NRCC number 42624.
References (46)
- et al.
Molecular biology and regulation of abscisic acid biosynthesis in plants
Plant Physiol. Biochem.
(1999) Abscisic acid metabolism and its regulation
- et al.
Synthesis, metabolism and compartmentation of abscisic acid in Riccia fluitans L.
J. Plant Physiol.
(1997) - et al.
Some caveats for bioengineering terpenoid metabolism in plants
Trends Biotechnol.
(1998) 8′-Acetylene ABA: an irreversible inhibitor of ABA 8' hydroxylase
Bioorg. Med. Chem. Lett.
(1997)- et al.
Conversion of xanthoxin to abscisic acid by cell-free preparations from bean leaves
Plant Physiol.
(1987) - et al.
Endogenous biosynthetic precursors of (+)-abscisic acid. VI. Carotenoids and ABA are formed by the ‘non-mevalonate’ triose-phosphate pathway in chloroplasts
Aust. J. Plant Physiol.
(1998) - et al.
Metabolism and physiology of abscisic acid
Annu. Rev. Plant Physiol. Plant Mol. Biol.
(1988) (+)-Abscisic acid 8′-hydroxylase is a cytochrome P450 monooxygenase
Plant Physiol.
(1998)- et al.
Evidence for a universal pathway of abscisic acid biosynthesis in higher plants from 18O2 incorporation patterns
Plant Physiol.
(1989)