The role of cis-carotenoids in abscisic acid biosynthesis

Abstract

Evidence has been obtained which is consistent with 9'-cis-neoxanthin being a major precursor of abscisic acid (ABA) in higher plants. A mild, rapid procedure was developed for the extraction and analysis of carotenoids from a range of tissues. Once purified the carotenoids were identified from their light-absorbance properties, reactions with dilute acid, high-performance liquid chromatography Rts, mass spectra and the quasiequilibria resulting from iodine-catalysed or chlorophyllsensitised photoisomerisation. Two possible ABA precursors, 9'-cis-neoxanthin and 9-cis-violaxanthin, were identified in extracts of light-grown and etiolated leaves (of Lycopersicon esculentum, Phaseolus vulgaris, Vicia faba, Pisum sativum, Cicer arietinum, Zea mays, Nicotiana plumbaginifolia, Plantago lanceolata and Digitalis purpurea), and roots of light-grown and etiolated plants (Lycopersicon, Phaseolus and Zea). The 9,9'-di-cisisomer of violaxanthin was synthesised but its presence was not detected in any extracts. Levels of 9'-cis-neoxanthin and all-trans-violaxanthin were between 20- to 100-fold greater than those of ABA in light-grown leaves. The levels of 9-cis-violaxanthin were similar to those of ABA but unaffected by water stress. Etiolated Phaseolus leaves contained reduced amounts of carotenoids (15-20% compared with light-grown leaves) but retained the ability to synthesise large amounts of ABA. The amounts of ABA synthesised, measured as increases in ABA and its metabolites phaseic acid and dihydrophaseic acid, were closely matched by decreases in the levels of 9'-cis-neoxanthin and all-trans-violaxanthin. In etiolated seedlings grown on 50% D2O, deuterium incorporation into ABA was similar to that into the xanthophylls. Relative levels of carotenoids in roots and light-grown and etiolated leaves of the ABA-deficient mutants, notabilis, flacca and sitiens were the same as those found in wild-type tomato tissues.

PSY3, a New Member of the Phytoene Synthase Gene Family Conserved in the Poaceae and Regulator of Abiotic Stress-Induced Root Carotenogenesis

“…However, the response mechanism may not involve increased flux to ABA because leaf ABA biosynthesis is limited not by the carotenoid precursor pool, but by NCED-mediated conversion of the xanthophyll precursors to ABA (Parry et al, 1990;Marin et al, 1996;Audran et al, 1998;Thompson et al, 2000a). Instead, increased carotenoid flux in leaves may relate to other processes, such as photosynthesis, temperature stress tolerance, and photoprotection (Davison, 2002;Rossel et al, 2002;Woitsch and Romer, 2003;Havaux et al, 2007).…”

“…Conversion of the downstream xanthoxin to ABA is not limiting as evidenced by the invariant levels of a requisite cytosolic enzyme activity in comparing normal and water-stressed leaves (Sindhu and Walton, 1987). Similarly, the upstream carotenoid levels are not thought to limit flux to ABA in leaves; leaf epoxycarotenoids are abundant (Parry et al, 1990), and transcripts for zeaxanthin epoxidase (ZEP), which converts zeaxanthin to violaxanthin, the precursor for epoxycarotenoids, were shown to be constant or deceased in leaves under drought stress (Audran et al, 1998;Iuchi et al, 2000;Thompson et al, 2000a).…”

The aba mutant of Arabidopsis thaliana is impaired in epoxy-carotenoid biosynthesis.

“…cis-Carotenoid isomers were characterized by rechromatography of iodine-treated trans-carotenoid isomers and by spectrophotometric analysis and predicted equilibrium stoichiometries (26,27). In addition, the identities of violaxanthin, neoxanthin, and antheraxanthin isomers were confirmed by their retention times in reverse-phase HPLC systems (16,17).…”

“…(v) A 1:1 molar correlation between decreases in trans-violaxanthin and 9'-cis-neoxanthin (Fig. 1) levels and concomitant increases in ABA and its catabolites has been shown for dark-grown water-stressed bean leaves (16,17). In contrast to higher plants, phytopathogenic fungi synthesize ABA by a direct pathway from farnesyl pyrophosphate (18, 19).…”

Eucalyptus ESTs involved in the production of 9-cis epoxycarotenoid dioxygenase, a regulatory enzyme of abscisic acid production

“…Due to the fact that no isomerization reaction occurs at the C 15 precursor level, the precursor carotenoids must be in the 9-cis configuration, and converted into ABA, which is, by definition, 2-cis,4-trans (Qin and Zeevaart, 1999). Evidence indicates that the conversion of xanthoxin into ABA is not a limiting factor in ABA biosynthesis, and this last step in the pathway is not regulated by water deficit, since the carotenoid substrate is abundantly available in photosynthetic tissues (Parry et al,1990). By elimination, 9-cis-epoxycarotenoid cleavage is the limiting step in the ABA biosynthesis pathway.…”