New developments in abscisic acid perception and metabolism
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Abscisic acid (2‐ cis ,4‐ trans ‐abscisic acid) is a plant hormone that has an asymmetric carbon atom. We tried to separate the enantiomers of native abscisic acid by HPLC using a phenyl column and a chiral mobile phase containing γ‐cyclodextrin. The optimum mobile phase conditions were found to be 0.8% (w/v) γ‐cyclodextrin, 4% (v/v) acetonitrile, and 20 mM phosphate buffer (pH 6.0). It was found that ( R )‐abscisic acid was earlier detected than ( S )‐abscisic acid. Since γ‐cyclodextrin is hardly retained on a phenyl column, it was suggested that ( R )‐abscisic acid formed a more stable complex with γ‐cyclodextrin than the ( S )‐abscisic acid. Abscisic acid in an acacia honey sample was successfully enantioseparated with the proposed method and only ( S )‐abscisic acid was detected. A biologically inactive 2‐ trans ,4‐ trans ‐abscisic acid, which was prepared by irradiation of abscisic acid with a light‐emitting diode lamp at 365 nm, was partially enantioseparated by the proposed method. Since the irradiation of ( S )‐abscisic acid‐induced cis ‐to‐ trans isomerization to produce one 2‐ trans ,4‐ trans ‐abscisic acid enantiomer, it is reasonable that racemization did not proceed during the cis ‐to‐ trans isomerization. ( S )‐Abscisic acid and probably ( S )‐2‐ trans ,4‐ trans ‐abscisic acid were detected in a honey sample, where the peak area of ( S )‐abscisic acid was 7 times larger than that of ( S )‐2‐ trans ,4‐ trans ‐abscisic acid.
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Excised embryonic bean axes (Phaseolus vulgaris, var. White Marrowfat) rapidly metabolize 2-(14)C-(+/-)-abscisic acid to two compounds, M-1 and M-2, which have very low growth-inhibitory activity. Chemical tests indicate the M-1 and M-2 are not previously described abscisic acid metabolites. M-2 accumulates in the axes and evidence is presented for the hypothesis that abscisic acid --> M-1 --> M-2. Zeatin, which partially reverses the abscisic acid-mediated growth inhibition of axes, neither decreases abscisic acid uptake nor causes any major changes in its metabolism. It was observed that axes transferred from abscisic acid-containing solutions to buffer resume control rates of fresh weight increase while still containing considerable quantities of abscisic acid.
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Tomato shoots that had been (a) fed (�)-[2H9]abscisic aldehyde via the xylem or (b) fed H218O together with (�)-[2H9]abscisic aldehyde via the xylem or (c) exposed to 18O2 and fed (�)-[2H9]abscisic aldehyde, were then wilted. The abscisic acid present was isolated, methylated and resolved into (+)- and (-)- methyl abscisate. These methyl abscisate samples were then examined by negative ion chemical ionisation (methane) gas chromatography/mass spectrometry. The undeuteriated (+)-abscisic acid contained no 180 from H218O but did contain one 18O from 18O2. No 18O from either of these sources was present in the undeuteriated (-)-abscisic acid. It was not possible to discount the xanthophyll hypothesis for the origin of stress-induced abscisic acid on the basis of these experiments. Both (+)- and (-)- multiply deuteriated abscisic acid contained one and two 18O atoms from H218O but none from 18O2. It is postulated that this multiply deuteriated (�)-abscisic acid is formed by a separate enzyme system from that which forms endogenous stress-induced (+)-abscisic acid. On the basis of the low incor- poration of abscisic aldehyde into abscisic acid, it is suggested that the endogenous precursor of stress- induced abscisic acid is an as yet unidentified structure and that abscisic aldehyde competes with it.
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In vivo measurements of the enzymatic hydroxylation of the phytohormone (+)-abscisic acid to hormonally inactive (–)-phaseic acid in corn cell cultures revealed that (+)-abscisic acid 8′ hydroxylase activity was induced by (+)-abscisic acid treatment. This induction was blocked by the protein synthesis inhibitor cycloheximide and by the transcription inhibitor cordycepin. Following an induction treatment with abscisic acid, the amount of induced enzyme was measured by addition of both cycloheximide, to prevent further induction, and fresh abscisic acid as substrate for the induced enzyme. Phaseic acid production was related to the amount of enzyme induced. The experimental system was optimized for studying enzyme induction and the substrate specificity of the induced enzyme. The induced 8– hydroxylase was specific for (–)-abscisic acid, whereas the low basal activity also hydroxylated (–)-abscisic acid at the 7′ position to form 7′-hydroxy abscisic acid. After induction, the 8′ hydroxylase was rapidly degraded with a half-life of approximately 2 h. Phaseic and (′)-abscisic acid were weak inducers; however, the unstable oxidation product 8′-hydroxy abscisic acid exhibited 42% of the activity of (+)-abscisic acid. When corn cells were placed under water stress by increasing concentrations of mannitol in the culture medium, 8′ hydroxylase induction was suppressed. The experimental system described will be useful for further studies of the physiological and hormonal factors that modulate abscisic acid metabolism and for testing potential enzyme inhibitors and hormone analogues.
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The main activities of abscisic acid in seeds are abscisic acid synthesis,catabolism,transport and response.Abscisic acid levels,the specific enzyme and the transcription factor in signal transduction pathway of abscisic acid,the relation between abscisic acid and dormancy of seeds are reviewed in this paper.
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The closure of stomata by abscisic acid was examined in several species of plants through measurements of CO(2) and H(2)O exchange by the leaf. The onset of closure was very rapid, beginning at 3 minutes from the time of abscisic acid application to the cut base of the leaf of corn, or at 8 or 9 minutes for bean, Rumex and sugarbeet; rose leaves were relatively slow at 32 minutes. The timing and the concentration of abscisic acid needed to cause closure were related to the amounts of endogenous abscisic acid in the leaf. Closure was obtained in bean leaves with 8.9 picomoles/cm(2). (+)-Abscisic acid had approximately twice the activity of the racemic material. The methyl ester of abscisic acid was inactive, and trans-abscisic acid was likewise inactive. The effects of stress on levels of endogenous abscisic acid, and the ability of very small amounts of abscisic acid to cause rapid closure suggests that stomatal control is a regulatory function of this hormone.
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