Metabolites of abscisic acid

Roles of Metabolites of Abscisic Acid. Nothing is known about the physiological role of PA and DPA in plants, although these two metabolites of ABA have been tested in several bioassays recently. In the cotton explant abscission assay PA had one-tenth of the activity of ABA (19). PA and DPA were equally effective in inhibiting a-amylase secretion by barley aleurone layers treated with glbberellin A3 DPA had approximately one-tenth of the activity of ABA in this system (74). The effect of PA on growth of bean embryos was negligible (75). 

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... 

The plant hormone abscisic acid (110) may be a degraded carotenoid or a sesquiterpenoid. Studies using [2/ - H,2- C,3/ ]-, [2S- H,2- C,3R]-, and [2- C,3I. 5S- H]-mevalonic acid show that either route is possible." Xanthoxin (111) may be involved also, it being formed in turn from violaxanthin (108). Two new metabolites of abscisic acid have been detected. 

Satellite Metabolites and Synthetic Derivatives of Abscisic Acid as Potential Microbial Product... 

Nigaki alcohol (18) has been identified by spectroscopic and chemical means as a constituent of Picrasma ailanthoides Planchon. Latia luciferin (19) has been synthesized in a stereoselective manner. A key step in this synthesis involves the addition of lithium dimethylcuprate to an enol phosphate derived from a 8-keto-ester to form an a,/3-unsaturated ester. Dehydro-/8-ionilideneacetic acid (20), an important intermediate in the synthesis of abscisic acid, has been prepared, as have the two nor-abscisic acid derivatives (21). The metabolite (22) of abscisic acid has been identified in the seeds of Robinia pseudacacia... 

Metabolite C , derived from metabolism of (+)-abscisic acid by tomato shoots, has been assigned structure (27) and on methylation rearranged to the methyl... 

Metabolite C was first recognised by metabolic studies using [2- C]abscisic acid synthesised in the normal way from [2- C]bromoacetic ester. t-Butyl chromate oxidation of a-ionone provides an efficient synthesis of abscisic acid (Scheme 3). A large number of compounds related to abscisic acid have been synthesised to check their biological activity. In general, conventional synthetic methods were used. The Reformatsky reaction may be used as an alternative to the Wittig reaction for the synthesis of a-ionylidene acetic ester. 

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. 

Zeevaart JAD (1980) Changes in the levels of abscisic acid and its metabolites in excised leaf blades of Xanthium strumarium during and after water stress. Plant Physiol... 

The synthesis of optically active carotenoidshas been extended to include the preparation of important possible carotenoid metabolites such as (4-)-abscisic acid (126), (-)-xanthoxin (127), (-)-loliolide (128), (-)-actinidiolide (130), and (-)-dihydroactinidiolide (129), all from one starting compound... 

The relationship between catabolite repression by glucose and induction of cellulase by sophorose has been studied in T. viride by Nisizawa and co-workers (36, 37). The induction by sophorose (10 M) was competitively repressed by glucose and other metabolites such as pyruvate. Since glucose was an effective repressor when added one hour after the previous addition of actinomycin D, it was concluded that the repression takes place at the translational level. Previous work indicated (26) that the sophorose induction led to the formation of a cellulase component designated FII, which is the source of cellulase II discussed below. In higher plants indoleacetic acid (38) and abscisic acid (39) have been shown to stimulate cellulase production. 

Furthermore, many carotenoid metabolites exist that have distinct functions. One such example shown in Fig. 3 is retinal, the chromophore of visual pigments (rhodopsins) and the light-driven proton pump, bacteriorhodopsin. Other examples are the plant hormone, abscisic acid, or volatile compounds that contribute to the fragrance of roses, for example. 

Last are Car precursors for important metabolites. Only three examples shall be given. The first example is retinal (Fig. 3), which is the chromophore of the visual pigment rhodopsin (23) and is derived from P,P -carotene. Because the latter cannot be synthesized by mammals, they need it to be supplied as provitamin A. Retinal derivatives are also required for other regulatory functions. The second example is abscisic acid (Fig. 3), which is the plant hormone involved in the shedding of leaves in fall and in fruit ripening it is derived from violaxanthin. Finally, certain fragrances of roses are not synthesized directly, but they are breakdown products of the flowers Cars. 

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... 

In structural terms these fungal metabolites are analogous to the iridoid monoterpenoids cf. Chapter 1). Recent studies on the biosynthesis of cyclonerodiol,cyclonerotriol, and abscisic acid (25) are described in Chapter 6. 

Abscisic acid (ABA) (65), an important plant-growth regulator in higher plants, has been identified, for the first time, as an authentic metabolite of a fungus Cercospora rosicola). Further research on the constituents of bird s nest fungi cf. Vol. 7, p. 95 Vol. 8, p. 111) has established that the bicyclofarnesane lactones (66a—c) are produced by still cultures of Mycocalia reticulata Petch. An... 

Degraded Carotenoids. Several new natural products have structures similar to carotenoid end-groups and thus may be considered as degraded carotenoids. A new abscisic acid (31) metabolite from seeds of Robinia pseudacacia has been identified as the 3-hydroxy-3-methylglutaryl ester of hydroxyabscisic acid (32). 

Abscisic acid (ABA) 3-1 was originally detected because of its growth inhibitory properties. It is now known to play an important role in the control of a-amylase synthesis, and regulation of stomatal aperture during water stress. Phaseic acid (PA) 3-3 is an important metabolite of ABA. Over a hundred derivatives of ABA are known, activity correlations have been reviewed, and the difficulty of drawing firm conclusions due to differences in uptake, metabolism and sequestration between the different molecules assayed has been discussed [16-20]. In many correlations, racemates have been used, and it is possible that each enantiomer may be active, have a different type of activity, and/or interfere with the action of the other enantiomer. 

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). ... 

Secondary metabolites, produced by pathways derived from primary metabolic routes, are numerous and widespread, especially in higher plants. More than 20,000 were known in 1985 (Hartmann, 1985), and at least 1000 additional compounds, are described each year. In practice, the difference between the primary and secondary metabolites is fuzzy. Plant hormones such as gibberellic acid, indoleace-tic acid (auxin), ethylene, kinetin, and abscisic acid, as well as compounds involved in plant cell wall structure such as cinnamic acid and its polymeric derivative, lignin, are intermediate between primary and secondary metabolism (Birch, 1973). In some instances, compounds normally considered primary metabolites may accumulate in large amounts and behave in a manner usually associated with secondary metabolites. Entities such as shikimic acid and squalene, which initially were considered secondary metabolites, were subsequently shown to be important intermediates in the formation of primary metabolites (phenylalanine, tyrosine and tryptophan, and steroids, respectively). 

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. 

Several carotenoid metabolites have important functions. The most important of these to consider is vitamin A, which is a metabolite of /8-carotene. This compound plays a key role in vision and in other biological reactions. The role of vitamin A in animals has been reviewed by Pitt (1971). Trisporic acid, also a metabolite of j8-carotene, is important in sexual reproduction in Mucorales, a group of fungi (Bu Lock et al., 1976). Sporopol-lenin, found in the outer layer (exine) of both spores and pollen, is considered a carotenoid polymer (Krinsky, 1971). It has been proposed that abscisic acid, an important plant growth regulator, may be a carotenoid metabolite, but a direct biosynthetic pathway from GGPP seems more probable (Burden and Taylor, 1976). 

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