Abscisic acid ABA biosynthesis

Ethylene Burst, Abscisic Acid (ABA) Biosynthesis and Hydrogen Peroxide as the Phytotoxic Principle... 

Since the publication of the previous edition of the book Comprehensive Natural Products Chemistry in 1999,547 research on abscisic acid (ABA, 1) has progressed especially concerning its biosynthesis, catabolism, and signal transduction using molecular biology techniques. In this chapter, descriptions of these subjects have been extensively revised and the chemistry of ABA is described in brief although the essential information has been retained. 

Terpenoids, which are also known as isoprenoids, constitute the most abundant and structurally diverse group of plant secondary metabolites, consisting of more than 40,000 different chemical structures. The isoprenoid biosynthetic pathway generates both primary and secondary metabolites that are of great importance to plant growth and survival. Among the primary metabolites produced by this pathway are phytohormones, such as gibberellic acid (GA), abscisic acid (ABA), and cytokinins the carotenoids, such as chlorophylls and plastoquinones involved in photosynthesis the ubiquinones required for respiration and the sterols that influence membrane stmcture (see also Steroid and Triterpene Biosynthesis) (Fig. 1). Monoterpenoids (CIO), sesquiterpenoids (Cl5), diterpenoids (C20), and... 

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]. 

Cytokinins have also been reported to increase PA biosynthesis in some plant systems, while application of abscisic acid (ABA) leads to the contrary situation (see review in Smith, 1985a). For instance, benzyladenine increased putrescine and ADC activity in cucumber cotyledons. By contrast, ABA decreased putrescine and ADC activity (Suresh et ai, 1978 In rice seedlings, total PAs increased threefold after treatment with kinetin, but declined with ABA (Mukho-padhyay et ai, 1983). It has been proposed that the regulation of PA biosynthesis by ABA in the aleurone layers of barley seeds is exerted through inhibition of the conversion of putrescine to spermidine (Lin, 1985). 

Hartung, W., Gimmler, H. Heilmann, B. (1982). The compartmentation of abscisic acid, of ABA - biosynthesis, ABA - metabolism and ABA conjugation. In Plant Growth Substances 1982, ed. P.F. Wareing, pp. 325-334. London Academic Press. 

A (1996) Molecular identification of zeaxanthin epoxidase of Nicotiana plumbaginifolia, a gene involved in abscisic acid biosynthesis and corresponding to the ABA locus of Arabidopsis thaliana , EMBO J, 15, 331-42. 

Effects on Abscisic Acid, One of the most clearly defined effects of water stress (decreased turgor) on hormones is the enhancement of ABA biosynthesis in leaves (Wright and Hiron 1972, Milborrow 1974, Zeevaart 1980). The response is rapid and the increase in ABA content is large (Kriedemann et al. 1972, Milborrow 1974, Wright and Hiron 1972, Hiron and Wright 1973). Water stress-induced enhancement of ABA content also occurs in roots (Walton et al. 1976) but the effect is small in some cases (Milborrow and Robinson 1973). 

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