Abscisic acid ring

Abscisic acid (ABA) is a 15-carbon acid, related in structure to one end of a carotene molecule [83], Four stereoisomers exist, differing in the orientation of the carboxyl group and the sidechain attachment to the ring. The natural ABA is the c/y-(+)-isomer shown in Fig. le. It is made from zeaxanthin via xanthoxin, probably in plastids (see Chapter 8). ABA can be made in all parts of a plant, with the leaves and the root cap being sites of extensive synthesis. It can be metabolized into phaseic acid, which is active in some, but not all ABA-sensitive processes [83]. 

Xanthoxin is an advanced biosynthetic precursor of ABA and thus its quantification in higher plants is of interest. The aldehyde group is readily coupled directly to carrier proteins via reduction of a Schiff s base formed by reaction with protein amino-groups (Fig 3). Antiserum to such a conjugate has been produced and shows specificity for compounds having the same functionality on the cyclohexane ring [73]. Phaseic acid is a metabolite of abscisic acid and specific monoclonal antibodies have been produced to the... 

Two new compounds, blumenol B (41 R = OH)andC(41 R = H), have been isolated from the leaves of Podocarpus blumei Endl. These co-occur with vomifoliol (42 R = OH) and on the basis of o.r.d. data they have been shown to have the same absolute stereochemistry at the ring chiral centre as abscisic acid in the revised form (40). The dehydro-derivative (42 R = H) of blumenol C has been identified as a constituent of Greek tobacco. 

Epoxidation of zeaxanthin by zeaxanthin epoxidase (ZE) would result in the production of violaxanthin via antheraxanthin. From that substrate, the enzyme neoxanthin synthase (NXS) would yield neoxanthin opening the cyclohexenyl 5-6 epoxide ring in violaxanthin [38]. Neoxanthin would be the last product of carotenoid biosynthesis in green parts of the plant, and it would derive in the abscisic acid (ABA) synthesis pathway. The accumulation of neoxanthin and violaxanthin in flowers results in wildtype yellow petals. A defective mutation in the gene encoding CRTR-B2 prevents formation of these xanthophylls, resulting in the white-flower phenotype [18]. 

With the increased chain length and additional double bond, the number of possible ways that cyclization can occur is also increased, and there exists a wide variety of cyclic sesquiterpenes, for example, abscisic acid, fumagillin, germacrene, dendrolasin, and trans-y-monocyclo famesol. In addition to common six-membered ring systems such as in zingiberene, a constituent of the oil from ginger, cyclization of one end of the chain to the other end can lead to macrocyclic rings such as humulene. 

Lutein, the most abimdant carotenoid in photosynthetic tissues, is produced via hydrox-ylation of the s-ring of zeionoxanthin catalyzed by CYP97C1 [58]. Crn 707Al and CTm07A2 are critical for the control of seed dormancy and germination in Arabidopsis by hydroxylating abscisic acid at the C-8 position for cataboUsm of this carotenoid-related phytohormone [59]. 

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