Isoprenoid abscisic acid

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... 

Two compounds common in plant metabolism are believed to be precursors of isoprenoid cytokinins in plants adenosine-5 -monophosphate (AMP) and A -isopentenylpyrophos-phate (iPP). As a final product of the mevalonate pathway, the latter substance serves also as a precursor for a wide spectrum of metabolites including some other plant hormones, as abscisic acid, gibberellins and brassinosteroids. The hypothetical scheme of reactions resulting in the formation of iPA, Z and DHZ is given in Fig. 2. The enzyme of entry into isoprenoid cytokinin formation is A -isopentenylpyrophosphate 5 -AMP-A -iso-pentenyltransferase (EC 2.5.1.8, trivially named cytokinin synthetase ). This enzyme activity was first detected in a cell-free preparation from the slime mould Dictyostelium discoideum [7,8]. Later the enzyme from higher plants (cytokinin-independent tobacco callus [9,10] and immature Zea mays kernels [11]) was described and the data were recently summarised in [12], The enzyme is very specific as far as the substrate is concerned [13,14] only the nucleotide AMP can be converted and only iPP (with a double bond in A position) may function as a side chain donor. 

Poly(c -l,4-isoprene) belongs to the family of polyisoprenoids, which are the most structurally diverse and abundant natural products known, with more than 23,000 primary and secondary metabolites. This huge family comprises, for example, sterols which display not only structural functions (control of biological membrane fluidity) but also hormonal functions (steroid hormones). Key phyto-hormones, such as abscisic acid, gibberellins and cytokinins, are isoprenoids too. Moreover, isoprenoids are used in protein prenylation, which is a key step in the activation and the localization of metabolic enzymes in many organisms. The first common step of all isoprenoid biosynthesis pathways is the formation of isopentenyl diphosphate (IPP). ... 

More than 150 Ci3-isoprenoids, in which 2-butyl-1,1,3-trimethylcyclohexane as a partial structure of abscisic acid (section 3.2.1) and of p-carotene (section 7.1) forms the basic skeleton, are referred to as megastigmanes. Megastigmanes such as P-ionone belong to the most important pleasantly smelling degradation products of P-carotene in the flowers of many plants. Smaller metabolites of carotenoids, including 2,6,6-trimethyl-2-cyclohexenone, 2,4,4-trimethylcyclohexene-3-carbaldehyde and 5,5,9-trimethyl-l-oxabicyclo[4.3.0]-3-nonen-2-one, may also contribute to the fragrances of flowers. 

Plants produce a great variety of products based on a branched C5 budding block. Some of these are primary metabolites, such as steroids, and side chains of enzyme prosthetic groups, and are similar or identical to compounds synthesized by animals. Some are plant hormones, such as abscisic acid and gibberellins. Certain C40 carotenoids of plants are the ultimate source of vitamin A, essential for animal nutrition. However, the majority of the isoprenoid or terpenoid compounds synthesized by plants are secondary metabolites which are uniquely plant products. 

The source of the seed s abscisic acid is not yet clear. Some, or all in certain seeds, may be derived from the mother plant but in at least one case T, sativum) it has been demonstrated that the embryo and endosperm can synthesize this substance from supplied mevalonate [119]. It has been suggested also that as seeds suffer increasing water stress when they begin to dehydrate the ABA levels rise as they do in wilting leaves (e.g. [118]). This is not likely in all cases, however, since in wheat kernels for example, the ABA levels increase before any appreciable water loss occurs [147]. Synthesis of ABA is considered to follow the route for isoprenoid production from mevalonate (see [118] for further details). A suggested alternative is that ABA is not synthesized in its own right but that it arises from the carotenoid violaxanthin via the production of xanthoxin. This is a debatable point it now seems unlikely [118], especially in seeds. 

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