Abscisic acid functions

Along with certain other flavonoids and compounds such as abscisic acid, the jasmonates, and ethylene, it seems that the anthocyanins may function to mitigate the effects of general stress, and are therefore a useful component of the general adaptation syndrome ... 

Abscisic acid, as the name suggests, has been implicated in the control of abscission of leaves, flowers and fruits, as well as with the function of stomata in response to water stress (Figure 5.4). Abscission involves the synthesis of cellulase in the ageing process and it is thought that abscisic acid influences the rate at which this proceeds. 

Violaxanthin also functions as a precursor to the plant hormone abscisic acid. Compare the structure of the latter (Fig. 22-4) with those of carotenoids. Oxidative cleavage of violaxanthin or related epoxy-carotenoids initiates the pathway of synthesis of this hormone.142 143... 

Burnette, R.N., Gunesekera, B.M. and Gillaspy, G.E., 2003, An Arabidopsis inositol 5-phosphatase gain-of-function alters abscisic acid signaling. Plant Physiol. 132 1011-1019. 

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. 

The analogous or, ) - /,5-unsaturated ester on the other hand, did not undergo cyclization under these conditions [159], whereas the methyl ester of abscisic acid [Eq. (18)], which also contains an a,) -, 5-unsaturated ester function, undergoes cyclization upon reduction in MeCN (containing AcOH) [160]. 

Abscisic acid (ABA) levels in rice plants, 308,31Or levels in squash hypocotyls, 315/.316 Active component of brassins identification, 9,lQf pilot plant extraction, 6,7/,8 solvent partition and column chromatography, 8 Adventitious root(s) development, 233,234r,235 formation, 247 Agriculture, application of 24-epibrassinolide, 280-290 22-Aldehydes, synthesis of brassinosteroids, 47-50f a hormone function, description for brassins, 4... 

Other functions of carotenoids are briefly mentioned here. These include phototropism and phototaxis, reproduction, camouflage colour, precursor role of neoxanthin (38, Fig. 16) for the plant growth hormone abscisic acid and the ant repellent grasshopper ketone [18,41], and cell-cell communication [49], In recent years the function of zeaxanthin (15) and lutein (25) in the human retina has received much attention, cf. the section on Health aspects. 

Ethylene and abscisic acid have been shown to function via signalling pathways that involve protein phosphorylation. The abscisic acid pathway involves a signalling component called cyclic ADP-ribose (cADPR), which regulates calcium release in animal cells. 

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 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. 

The absolute stereochemistry of abscisic acid (57) has at last been established. Owing to the unusual bisallylic alcohol function, the previous assignment based on Mills rule was incorrect and this has now been reversed so that it is consistent with violaxanthin (54). Oritani et al. showed that ( —)-a-ionone (56) was converted in to (— )-abscisic acid with retention of configuration (their enantiomers... 

C 5H2203, Mr 250.34, cryst., mp. 85 - 86 °C, [alp - 56° (CHCI3), a cyclofamesane sesquiterpene formed by photochemical oxidation of violaxanthin. It has been detected in several marine and higher plants. Naturally occurring X. exists as a mixture of -isomers of the 2,3-double bond of the 2,4-pentadienal side chain. It is an inhibitor of plant growth and is involved in the winter rest phase of plants (senescence), in some cases it is also involved in the damage to trees X. is closely related structurally and in its biological activity to abscisic acid and presumably functions as an intermediate in the biosynthesis of it. 

---