Phaseic acid

Qin, X. Q. and J. A. D. Zeevaart (2002). Overexpression of a 9-cA-epoxycarotenoid dioxygenase gene in Nicotiana plumbaginifolia increases abscisic acid and phaseic acid levels and enhances drought tolerance. Plant Physiol. 128(2) 544-551. 

For gas chromatography volatile derivatives such as the methyl ester of ABA (27, 29, 30) or trimethylsilylated ABA (20, 28) must be prepared. ABA is a molecule with a high electron affinity so that the methyl ester can be measured with a gas chromatograph equipped with an electron capture detector. Injections of as little as 5 pg of Me-ABA cause a detector response (27). Metabolites of ABA such as phaseic and dihydro-phaseic acid can also be measured by this method (33). 

ABA is metabolized via the unstable intermediate 6 -hydroxymethyl -ABA, more recently called hydroxyabscisic acid (HOABA) by Hirai et al. (45), to phaseic acid (PA). In certain plants the latter compound is further converted to 4 -dihydro-phaseic acid (DPA) which accumulates as the end product. The ABA-> PA DPA pathway (Figure 4) operates in beans (46), in pea seedlings (47), in ash seeds (48), in castor bean (49), and... 

ABA = abscisic acid DPA = 4 -dihydroxyphaseic acid PA= phaseic acid o-OH-BAP riboside = 6-(o-hydroxybenylmino)-9-B-D ribofuranosylpurine Raphanatin = 7-6-D glucopyranosylzeatin. 

ABA = abscisic acid IAA = indole-3 acetic acid DPA = dihydrophaseic acid PA = phaseic acid. 2GPC = gel permeation chromatography. 

ABA absorbs ultraviolet with maxima at 240 nm (e 2.1 x 104, a shoulder peak), 260 nm (e 2.6 x 104), and 320 nm (e 50) in an acidic methanol solution.590 Irradiation with UV with a wavelength shorter than 305 nm isomerizes the 2-(Z)-double bond to (E) to give an equilibrium mixture of ABA and its 2-( )-isomer with a ratio of 1 1, and also causes decomposition of ABA to unidentified compounds by the excitation of the 7t-7t transition of the side chain and the enone groups. ABA has a strong optical activity, and its specific optical rotation is +430° in an acidic methanol solution.591 In the optical rotatory dispersion (ORD)591 and the circular dichroism (CD) spectra,558 ABA shows a positive Cotton effect from 300 to 200 nm. Phaseic acid and dihydrophaseic acid and its epimer, which did not have the enone group, show a small specific optical rotation with a minus value and also a negative plain curve in the ORD. 

The catabolic inactivation of ABA would be achieved in a stepwise manner. The biological activity of 8 -hydroxy-ABA cannot be tested due to its instability, but several studies have suggested that the activity of 8 -hydroxy-ABA is about 20% of that of ABA.626 683 684 The activity of phaseic acid is low, about 5% of that of ABA, in many biological assays,548 and may be contributed by 8 -hydroxy-ABA present in a sample of phaseic acid. Dihydrophaseic acid and its epimer are almost inactive in assays tested, meaning that the catabolic inactivation is completed in the step reducing phaseic acid to dihydrophaseic acid and its epimer. 

Hydroxy-ABA (53) has been found in the leaves of Vicia faba6SS and other plants. 9 -Hydroxy-ABA (54) has been found in Brassica napus siliques along with its isomerized catabolite neophaseic acid (55),686 which had been formerly known as spi-phaseic acid.687 The occurrence of 7 - and 9 - hydroxy-ABAs is restricted to several plants at present, suggesting that these hydroxy-ABAs are minor catabolites of ABA. ABA 7 - and 9 -hydroxylases would be different from ABA 8 -hydroxylase since 7 - and 9 -hydroxy-ABAs are not detected in the products of ABA 8 -hydroxylase reaction. 

Phaseic acid (305) and dihydrophaseic acid (306a) are formed when (+)-abscisic acid (304) is fed to certain plants (e.g. tomato, bean, etc.). The stereochemistry of these compounds has recently been established131 by spectroscopic analysis (n.m.r. and i.r.) of the epimeric 4 -dihydrophaseic acids (306a and b) derived by reduction (NaBH4) of phaseic acid (305). 

HOABA is enzymatically or spontaneously isomerized to phaseic acid (PA, 25), followed by reduction to dihydrophaseic acid (DPA, 26), as shown in Figure (10) [1,7]. 

Mass spectrometry is the most recently introduced technique to be widely applied to carotenoid structural studies. Many spectra are recorded by Enzell et Recent advances include the recognition that a loss of 106 and 92 m.u. (rationalised as xylene and toluene) is characteristic of the polyene chain. The remaining fragments on either side of the polyene may be recognised by bisallylic fission. Ketonic carotenoids show characteristic fragmentation. MacMillan and Gaskin were able to detect 0.3 pg of abscisic acid or phaseic acid using... 

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]. 

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