Oh

COOH O'

Oxidation COOH

Oxidation COOH

ABA inactivation by conjugation with monosaccharides a

4'-Dihydrophaseic acid (DPA) Phaseic acid (PA)

ABA inactivation by conjugation with monosaccharides a

4'-Dihydrophaseic acid (DPA) Phaseic acid (PA)

As far as is currently known, ABA is a derivative of chloroplastic xanthophylls. These are not synthesised via the mevalonate pathway, but originate from the chloroplastic DOXP pathway (1-desoxy-D-xylulose-5-phosphate pathway; Lichtenthaler 1999). Key enzymes of ABA biosynthesis are zeaxanthin epoxidase (ZEP), encoded for by the gene ABA2, and 9-cz's-epoxycarotenoid dioxygenase (NCE), encoded for by the gene VP 14. NCE can cleave the xanthophylls: all-frans-violaxanthin, all-frans-neoxanthin, 9'-cz's-neoxanthin (Cutler and Krochko 1999).

Figure 1 shows the metabolism of (+)-ABA at the cellular level. The first steps of ABA biosynthesis, until cleavage of the 9'-epoxycarote-noids by the dioxygenase, occur in the plastids. The cleavage product xanthoxin after export from the plastid (by an unknown mechanism) is metabolised in the cytoplasm to (+)-ABA. The bold arrows show the main route and fine arrows the shunt via ABA-alcohol, whilst the pathway via xanthoxic acid is still disputed (dotted line). The fast import of ABA from the apoplasm into the cell is induced by the neutral pH of the cytoplasm (after Cowan 2000).

The main pathway of ABA degradation as shown in Fig. 2 starts with the hydroxylation of the 8'-carbon, followed by a cyclisation to phaseic acid which is converted by a reductase to dihydrophaseic acid. The three degradation products are physiologically inactive. Hydroxylation of the 7'-carbon is also known, but similar to the direct reduction of the ABA keto group to l',4'-ABA diol is quantitatively insignificant. The formation of conjugates of ABA and its derivatives, in particular with glucose, is physiologically relevant, as these compounds are water-soluble and are exported from the cytoplasm into the vacuole (after Taiz and Zeiger 2000).

ABA is already a stress signal at the cellular or subcellular level. The biochemical structure of ABA sensors has not yet been elucidated.

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