Phaseic Acid Research Articles

Hormonal Regulation of α-Amylase Expression in Barley Aleurone Layers

The expression of alpha-amylase isozymes in barley (Hordeum vulgare L.) aleurone layers is known to be maximally induced between 12 and 20 hours after addition of the phytohormone, gibberellic acid (GA(3)). Addition of another hormone, abscisic acid (ABA), or its metabolite, phaseic acid, during this time period resulted in reduced alpha-amylase expression. Expression of the high isoelectric point alpha-amylase isozyme group was affected much more by both of these treatments than was expression of the low isoelectric point alpha-amylase isozyme group. Addition of either the translation inhibitor cycloheximide or the transcription inhibitor cordycepin prevented the decrease in alpha-amylase mRNA levels after ABA treatment. Cordycepin also prevented the decreases in alpha-amylase expression that result from phaseic acid treatment. Midcourse GA(3) removal experiments were performed to determine whether ABA treatment and the removal of GA(3) have analogous effects on alpha-amylase expression. It was found that cordycepin treatment also prevented decreases in alpha-amylase mRNA levels resulting from GA(3) removal. We conclude that the suppression of alpha-amylase expression caused by ABA or midcourse GA(3) removal is dependent on continuous RNA and protein synthesis.

The apoplastic pool of abscisic acid in cotton leaves in relation to stomatal closure

Suboptimal nitrogen nutrition, leaf aging, and prior exposure to water stress all increased stomatal closure in excised cotton (Gossypium hirsutum L.) leaves supplied abscisic acid (ABA) through the transpiration stream. The effects of water stress and N stress were partially reversed by simultaneous application of kinetin (N(6)-furfurylaminopurine) with the ABA, but the effect of leaf aging was not. These enhanced responses to ABA could have resulted either from altered rates of ABA release from symplast to apoplast, or from some "post-release" effect involving ABA transport to, or detection by, the guard cells. Excised leaves were preloaded with [(14)C]ABA and subjected to overpressures in a pressure chamber to isolate apoplastic solutes in the exudate. Small quantities of (14)C were released into the exudate, with the amount increasing greatly with increasing pressure. Over the range of pressures from 1 to 2.5 MPa, ABA in the exudate contained about 70% of the total (14)C, and a compound co-chromatographing with phaseic acid contained over half of the remainder. At a low balancing pressure (1 MPa), release of (14)C into the exudate was increased by N stress, prior water stress, and leaf aging. Kinetin did not affect (14)C release in leaves of any age, N status, or water status. Distribution of ABA between pools can account in part for the effects of water stress, N stress, and leaf age on stomatal behavior, but in the cases of water stress and N stress there are additional kinetinreversible effects, presumably at the guard cells.

Endogenous Gibberellins from Sporophytes of Two Tree Ferns, Cibotium glaucum and Dicksonia antarctica

Gibberellin A(1) (GA(1)), 3-epi-GA(1), GA(4), GA(9), 11alpha-hydroxyGA(12), 12alpha-hydroxyGA(12), GA(15), GA(17), GA(19), GA(20), GA(25), GA(37), GA(40), GA(58), GA(69), GA(70), and GA(71) have been identified from Kovats retention indices and full scan mass spectra by capillary GC-MS analyses of purified extracts from sporophytes of the tree fern, Cibotium glaucum. Abscisic acid, dihydrophaseic acid, an epimer of 4'-dihydrophaseic acid, and the epimeric ent-6alpha, 7alpha, 16alpha, 17-(OH)(4) and ent-6alpha, 7alpha, 16beta, 17-(OH)(4) derivatives of ent16, 17-dihydrokaurenoic acid, in addition to the epimeric 16alpha, 17- and 16beta, 17-dihydroxy-16, 17-dihydro derivatives of GA(12), were also identified in extracts of C. glaucum. An oxodihydrophaseic acid and a hydroxydihydrophaseic acid were also detected. In extracts of sporophytes of Dicksonia antarctica, GA(4), GA(9), 12alpha- and 12beta-hydroxyGA(12), GA(15), GA(25), and GA(37) were identified by the same criteria, as well as abscisic acid, phaseic acid, 8'-hydroxymethylabscisic acid and dihydrophaseic acid. This is the first time that GA(40) has been identified in a higher plant; it is also the first report of the natural occurrence of the two gibberellins, 11alpha- and 12beta-hydroxyGA(12). The total gibberellin (GA) content in C. glaucum (tall) was at least one order of magnitude greater than that of D. antarctica (dwarf) based on total ion current response in GC-MS and bioassay data. Abscisic acid was a major component of D. antarctica and the oxodihydrophaseic acid was a major component of C. glaucum.

The cyclization of 8′-hydroxy abscisic acid to phaseic acid in vivo

The cyclization of 8′-hydroxyabscisic acid to phaseic acid was investigated by feeding RS-[3′,5,7- 2D 6]abscisic acid to tomato ( Lycopersicon esculentum) shoots, the 1′- S-enantiomer was metabolized to dihydrophaseic acid-4′- O-β- d-glucopyranoside (DPAGS) which was isolated as the acetylated methyl ester and subjected to 500 MHz 1H NMR. The 2H atom from C-3′ of abscisic acid (ABA) was confined to the axial 3′- pro-S position of DPAGS (numbered as in ABA) and so the carbanion presumed to be formed at C-3′ during cyclization is protonated from the oxymethylene bridge, β-face. Enolization of the 3′-hydrogen atoms of phaseic acid in D 2O at pH 10.55 exchanged the axial hydrogen atom (α-face) preferentially. Thus the carbanion present at this position during cyclization is protonated from the opposite side. This suggests that the reaction that gives rise to phaseic acid is catalysed enzymically.

β- d-Glucopyranosyl phaseic acid from shoots of Lycopersicon esculentum

The glucose ester of phaseic acid has been isolated and characterized from tomato shoots ( Lycopersicon esculentum cv. Grosse Lisse) fed with RS-[2- 14C]ABA and has been shown to occur naturally. The metabolite was isolated by reverse and normal phase HPLC as the tetra-acetyl derivative and identified by a combination of 1H NMR (500 MHz), CIMS and TLC. Evidence for the natural occurrence of the glucose esters of 4′-dihydrophaseic and 4′- epi-dihydrophaseic acid was also obtained.

Abscisic Acid Biosynthesis in Leaves and Roots of Xanthium strumarium

RESEARCH ON THE BIOSYNTHESIS OF ABSCISIC ACID (ABA) HAS FOCUSED PRIMARILY ON TWO PATHWAYS: (a) the direct pathway from farnesyl pyrophosphate, and (b) the indirect pathway involving a carotenoid precursor. We have investigated which biosynthetic pathway is operating in turgid and stressed Xanthium leaves, and in stressed Xanthium roots using long-term incubations in (18)O(2). It was found that in stressed leaves three atoms of (18)O from (18)O(2) are incorporated into the ABA molecule, and that the amount of (18)O incorporated increases with time. One (18)O atom is incorporated rapidly into the carboxyl group of ABA, whereas the other two atoms are very slowly incorporated into the ring oxygens. The fourth oxygen atom in the carboxyl group of ABA is derived from water. ABA from stressed roots of Xanthium incubated in (18)O(2) shows a labeling pattern similar to that of ABA in stressed leaves, but with incorporation of more (18)O into the tertiary hydroxyl group at C-1' after 6 and 12 hours than found in ABA from stressed leaves. It is proposed that the precursors to stress-induced ABA are xanthophylls, and that a xanthophyll lacking an oxygen function at C-6 (carotenoid numbering scheme) plays a crucial role in ABA biosynthesis in Xanthium roots. In turgid Xanthium leaves, (18)O is incorporated into ABA to a much lesser extent than it is in stressed leaves, whereas exogenously applied (14)C-ABA is completely catabolized within 48 hours. This suggests that ABA in turgid leaves is either (a) made via a biosynthetic pathway which is different from the one in stressed leaves, or (b) has a half-life on the order of days as compared with a half-life of 15.5 hours in water-stressed Xanthium leaves. Phaseic acid showed a labeling pattern similar to that of ABA, but with an additional (18)O incorporated during 8'-hydroxylation of ABA to phaseic acid.

The Catabolism of (±)-Abscisic Acid by Excised Leaves of Hordeum vulgare L. cv Dyan and Its Modification by Chemical and Environmental Factors

Excised light-grown leaves and etiolated leaves of Hordeum vulgare L. cv Dyan catabolized applied (+/-)-[2-(14)C]abscisic acid ([+/-]-[2-(14)C]ABA) to phaseic acid (PA), dihydrophaseic acid (DPA), and 2'-hydroxymethyl ABA (2'-HMABA). Identification of these catabolites was made by microchemical methods and by combined capillary gas chromatographymass spectrometry (GC-MS) following high dose feeds of nonlabeled substrate to leaves. Circular dichroism analysis revealed that 2'-HMABA was derived from the (-) enantiomer of ABA. By selecting tissue samples in which endogenous catabolites were undetectable by gas chromatography, it was possible to identify unequivocally ABA catabolites by GC-MS without the need to employ deuteriated substrate to distinguish the (+/-)-ABA catabolites from the same endogenous compounds. Refeeding studies were used to confirm the catabolic route. The methyl ester of (+/-)-[2(14)C]-ABA was hydrolyzed efficiently by light-grown leaves of H. vulgare. Leaf age played a significant role in (+/-)-ABA catabolism, with younger leaves being less able than their older counterparts to catabolize this compound. The catabolism of (+/-)-ABA was inhibited markedly in water-stressed Hordeum leaves which was characterized by a decreased incorporation of label into 2'-HMABA, DPA, and conjugates. The specific, mixed function oxidase inhibitor, ancymidol, did not inhibit, dramatically, (+/-)-ABA catabolism in light-grown leaves of Hordeum whereas the 80s ribosome, translational inhibitor, cycloheximide, inhibited this process markedly. The 70s ribosome translational inhibitors, lincomycin and chloramphenicol, were less effective than cycloheximide in inhibiting (+/-)-ABA catabolism, implying that cytoplasmic protein synthesis is necessary for the catabolism of (+/-)-ABA in Hordeum leaves whereas chloroplast protein synthesis plays only a minor role. This further suggests that the enzymes involved in (+/-)-ABA catabolism in this plant are cytoplasmically synthesized and are ;turned-over' rapidly, although the enzyme responsible for glycosylating (+/-)-ABA itself appeared to be stable.

Abscisic acid metabolism in Ceratocystis coerulescens

The fungus Ceratocystis coerulescens Bakshi (strain RWD 390) has been shown to produce the plant hormone, abscisic acid (ABA). The production of ABA in defined liquid medium during a culture period of 50 days was measured by gas‐liquid chromatography. A considerable accumulation of ABA occurred in the stationary phase. Maximum ABA contents were 3.5 ng ml−1 in culture media and 218 ng (g dry weight)−1 in mycelial extracts.The ABA‐metabolizing capability of the fungus was investigated. Dihydrophaseic acid, and phaseic acid, ABA metabolites in higher plants, were not present in cultures of Ceratocystis coerulescens. When [2‐14C]‐ABA was fed to the fungus, the formation of [2‐14C]‐2‐trans, 4‐trans‐ABA and a second metabolite, less polar than ABA, was observed. This suggests a different metabolic pathway of ABA in the fungus.

Water stress and the catabolism of (�)-abscisic acid by excised leaves of Hordeum vulgare L. cv. Dyan

When excised, light-grown leaves of Hordeum vulgare were fed with (±)-[2-14C]-abscisic acid and stressed until they had lost 12% of their original fresh weight, marked changes in the distribution of radioactivity between abscisic acid and its catabolites were observed. Wilted leaves were less able than their turgid counterparts to transform (±)-[2-14C]-abscisic acid into 2′-hydroxymethyl abscisic acid, dihydrophaseic acid and water-soluble conjugates of abscisic acid. Water stress had little effect on the production of phaseic acid although refeeding studies with [14C]-phaseic acid showed that the step from phaseic acid to dihydrophaseic acid was inhibited in wilted leaves. Evidence was obtained which suggested that these changes did not result from dilution of applied, radiolabelled substrate by endogenous abscisic acid. The catabolites of (±)-abscisic acid were identified by capillary gas chromatography-mass spectrometry.

Abscisic acid synthesis and metabolism in barley leaves and protoplasts

Protoplasts isolated from barley ( Hordeum vulgare L. cv. Clipper) leaves contained abscisic acid (ABA). The ABA content of these protoplasts did not change when they were incubated for up to 6 h in media of decreasing osmotic potential. There was a substantial, but transient, increase in ABA in barley leaf segments during protoplast isolation. The magnitude of this increase was inversely dependent on the osmotic potential of the isolation medium. Maximum ABA content was recorded after 2 h of exposure to the sorbitol-containing medium. The subsequent decline was due to conversion of ABA to phaseic acid (PA) and to other metabolites. Barley mesophyll protoplasts were not able to metabolise ABA, PA or any of the other metabolites formed from ABA by intact leaf tissue.

Mode of Action of Abscisic Acid in Barley Aleurone Layers

As part of a continuing effort to elucidate the mode of action of abscisic acid (ABA) in barley (Hordeum vulgare L. cv Himalaya) aleurone layers, we have investigated the induction of several polypeptides by ABA in this tissue. There were nine ABA-induced polypeptides as observed by one-dimensional sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and considerably more (at least 16 spots) on a two-dimensional gel. These proteins started to show enhanced synthesis 2 to 4 hours after ABA treatment, and their synthesis continued for at least 48 hours. In vitro translation using total RNA isolated from ABA-treated aleurone layers indicated that translatable mRNA levels of these proteins essentially paralleled the levels of in vivo synthesized proteins. The most abundant of the ABA-induced proteins was a 29 kilodalton polypeptide which was also synthesized in tissue incubated without ABA. In vivo synthesis of this protein declined as ABA concentration was decreased, with 1 nanomolar ABA approaching control level. Cell fractionation experiments located the 29 kilodalton major ABA-induced protein in 1,000g and 13,000g pellets; most other induced proteins were in the 80,000g supernatant. The 29 kilodalton protein appeared to be sensitive to degradation by sulfhydryl type proteases. As expected, the induction of these proteins by ABA was suppressed by gibberellic acid. Phaseic acid, the first stable metabolite of ABA, suppressed the gibberellic acid-enhanced alpha-amylase synthesis but was unable to induce the ABA-induced proteins. None of the ABA-induced proteins were secreted into the incubation medium. A 36 kilodalton ABA-induced protein showed cross-reactivity with antibody against a barley lectin specific for glucosamine, galactosamine, and mannosamine.

Relationships among drought resistance, transpiration rates, and abscisic acid levels in three northern conifers.

Water potentials, transpiration rates and abscisic acid (ABA) levels in shoots of black spruce (Picea mariana (Mill.) BSP), white spruce (Picea glauca (Moench) Voss) and jack pine (Pinus banksiana Lamb.) seedlings were monitored during periods of drought and recovery from drought. Abscisic acid contents of shoots increased during the period of drought as water potentials decreased. The increase in levels of ABA was closely associated with a decrease in rates of transpiration. In the spruces, the levels of ABA peaked and then fell while plant water potentials continued to decrease, whereas in jack pine, the level of ABA rose throughout the drought treatment. After rewatering, the levels of ABA in all three conifers fell concurrent with a rise in transpiration rates. At the end of the three-day recovery period, ABA levels and transpiration rates in the spruces were either at or near control levels, whereas the concentration of ABA in jack pine remained approximately twice the control level, and transpiration was only 60% of the control rate. A compound tentatively identified as phaseic acid followed trends similar to those for ABA.

Abscisic acid metabolism ?vacuolar/extravacuolar distribution of metabolites

The biotransformation of abscisic acid (ABA) was studied in cell suspension cultures of Lycopersicon esculentum. The ABA was converted by the cells to phaseic acid, nigellic acid, dihydrophaseic acid, abscisic acid-β-D-glucopyranosyl ester (ABA-Glc) and other ABA and phaseic acid conjugates. Investigation of their cellular distribution showed that the conjugated forms were located only in the vacuoles whereas ABA and its acidic metabolites were found mainly in the extravacuolar fractions. Our results, together with a number of studies on the increase of ABA-Glc as a response to stress, allow us to propose that ABA-Glc is irreversibly compartmented in the vacuoles of plant cells.

ABA and IAA in rice seedlings under anaerobic conditions

The rice is important in plant science for its ability to germinate and grow with restricted or without oxygen availability. In this work we have investigated the variation of growth substances when anoxia was imposed to rice seedlings previously grown in air. An increase, in all the organs of a seedling and in particular in the fraction released in the medium, was observed for ABA (abscisic acid), PA (phaseic acid) and DPA (dihydrophaseic acid) quantities.Vice versa a reduction of total IAA (indol-3-ylacetic acid) was observed in seedlings. This was accompanied by its accumulation in roots. IAA was poorly released in aerobic conditions and anoxia has not changed this pattern.

Metabolism of Abscisic Acid in Guard Cells of Vicia faba L. and Commelina communis L.

Metabolism of abscisic acid (ABA) was investigated in isolated guard cells and in mesophyll tissue of Vicia faba L. and Commelina communis L. After incubation in buffer containing [G-(3)H]+/-ABA, the tissue was extracted by grinding and the metabolites separated by thin layer chromatography. Guard cells of Commelina metabolized ABA to phaseic acid (PA), dihydrophaseic acid (DPA), and alkali labile conjugates. Guard cells of Vicia formed only the conjugates. Mesophyll cells of Commelina accumulated DPA while mesophyll cells of Vicia accumulated PA. Controls showed that the observed metabolism was not due to extracellular enzyme contaminants nor to bacterial action.Metabolism of ABA in guard cells suggests a mechanism for removal of ABA, which causes stomatal closure of both species, from the stomatal complex. Conversion to metabolites which are inactive in stomatal regulation, within the cells controlling stomatal opening, might precede detectable changes in levels of ABA in bulk leaf tissue. The differences observed between Commelina and Vicia in metabolism of ABA in guard cells, and in the accumulation product in the mesophyll, may be related to differences in stomatal sensitivity to PA which have been reported for these species.

Interdependence of growth, water relations and abscisic acid level in Phaseolus vulgaris during waterlogging

The interdependence between changes in growth and water relations after waterlogging was investigated by recording simultaneously growth, transpiration, water potential, turgor, leaf diffusion resistance and abscisic acid content in Phaseolus vulgaris L. cv. bruine Noord‐Hollandse. Growth was inhibited immediately after flooding, whereas transpiration decreased gradually to a low level in about three days. The first two days after flooding a small increase in abscisic acid content in the leaves was observed which was accompanied by an increase in diffusion resistance. The increase in abscisic acid content could result from an inhibited export from the leaves. After the first two days a decrease in water potential and turgor was accompanied by a drastic increase in both abscisic acid content and diffusion resistance. This large increase in abscisic acid content occurred before the turgor had reached its minimum value. The change in diffusion resistance kept showing a lag of about one day with the change in abscisic acid content. The possibility is discussed that besides abscisic acid also its metabolite phaseic acid is involved in stomatal closure. After the formation of adventitious roots on the hypocotyl, abscisic acid level, diffusion resistance, water potential and turgor returned to the control values. Transpiration showed a slow recovery from the sixth day after flooding, whereas growth was inhibited for at least nine days. A remarkable similarity exists between our observations on the responses of bean plants to flooding and the well known responses to drought.

Abscisic Acid Metabolism in Relation to Water Stress and Leaf Age in Xanthium strumarium

Intact plants of Xanthium strumarium L. were subjected to a water stress-recovery cycle. As the stress took effect, leaf growth ceased and stomatal resistance increased. The mature leaves then wilted, followed by the half expanded ones. Water, solute, and pressure potentials fell steadily in all leaves during the rest of the stress period. After 3 days, the young leaves lost turgor and the plants were rewatered. All the leaves rapidly regained turgor and the younger ones recommenced elongation. Stomatal resistance declined, but several days elapsed before pre-stress values were attained.Abscisic acid (ABA) and phaseic acid (PA) levels rose in all the leaves after the mature ones wilted. ABA-glucose ester (ABA-GE) levels increased to a lesser extent, and the young leaves contained little of this conjugate. PA leveled off in the older leaves during the last 24 hours of stress, and ABA levels declined slightly. The young leaves accumulated ABA and PA throughout the stress period and during the 14-hour period immediately following rewatering. The ABA and PA contents, expressed per unit dry weight, were highest in the young leaves. Upon rewatering, large quantities of PA appeared in the mature leaves as ABA levels fell to the pre-stress level within 14 hours. In the half expanded and young leaves, it took several days to reach pre-stress ABA values. ABA-GE synthesis ceased in the mature leaves, once the stress was relieved, but continued in the half expanded and young leaves for 2 days.Mature leaves, when detached and stressed, accumulated an amount of ABA similar to that in leaves on the intact plant. In contrast, detached and stressed young leaves produced little ABA. Detached mature leaves, and to a lesser extent the half expanded ones, rapidly catabolized ABA to PA and ABA-GE, but the young leaves did not. Studies with radioactive (+/-)-ABA indicated that in young leaves the conversion of ABA to PA took place at a much lower rate than in mature ones. Leaves of all ages rapidly conjugated PA to PA-glucose ester. Furthermore, when half expanded leaves were stressed on the intact plant, their rate of ABA catabolism was enhanced, an effect not observed in the young leaves.In conclusion, young leaves on intact Xanthium plants produce little stress-induced ABA themselves, but due to import and a low rate of catabolism accumulate more ABA and PA than mature leaves.

ABSCISIC ACID TRANSPORT AND METABOLISM IN GRAPEVINE (VITIS VINIFERA L.)

SummaryRadioactive abscisic acid (ABA) was applied to leaves of potted grapevines (Vitis vinifera L.). During a 24‐h experimental period, the ABA was extensively metabolized within the leaf to phaseic acid (PA), 4′‐dihydrophaseic acid (DPA) and abscisyl‐β‐D‐glucopyranoside (ABAGE), together with a number of unknown metabolites. ABA was transported out of the leaf to the roots. The major metabolite in roots was DPA and, 24 h after application of ABA to the leaves, only 5% of the radioactivity in the roots was present as ABA. Very little ABAGE was formed by roots. Xylem sap, expressed from canes or collected by root pressure exudation, also contained labelled material, a high proportion of which was unmetabolized ABA.It is suggested that export of ABA from leaves to roots and subsequent transport back to the leaves via the xylem occur naturally in unstressed grapevines and that this forms an integral part of the stomatal control mechanism.

Metabolism of (±) 2‐[14C]‐abscisic acid in Lactuca sativa achenes

Achenes ofLactuca sativa L. cv. Grand Rapids were treated with (±) 2‐[14C]‐abscisic acid (ABA) at 105 ‐ or 2‐106 M for 6, 12, 24, 48 or 96 h in darkness at 24°C. They were then extracted in 80% ethanol. Two acidic diethyl ether phases which contained the free acids and the acids released after mild alkaline hydrolysis respectively, were analyzed as well as the radioactivity which remained in the final aqueous phase. For treatment durations between 6 and 96 h, the major part of the radioactivity was found in the free phase, in the form of ABA. For treatment durations up to 48 h, no radioactivity was detected at the Rf of phaseic acid or dihydrophaseic acid (free and hydrolysed phases). After 96 h culture on 105 M ABA, dihydrophaseic acid was present, but only in very small quantities. Two ABA metabolites were detected. One was characterized as β‐d‐glucopyranosyl abscisate since its Rf was the same as that of an authentic sample in three different solvent systems and also since it released ABA on mild alkaline hydrolysis. It increased steadily with time and represented the main metabolite. The other metabolite found in the aqueous phase after mild alkaline hydrolysis and extraction with ether at pH 3 was a very polar compound, resistant to alkaline hydrolysis in the presence of concentrated ammonia and to methylation. It was, however, metabolized by apple embryo, yielding essentially dihydrophaseic acid and an ester which released dihydrophaseic acid on mild alkaline hydrolysis. These results indicate that under the conditions tried, the metabolism of [14C]‐ABA by lettuce achenes leads almost exclusively to the formation of conjugates, oxidative metabolism of ABA being almost non‐existent. Separate analysis of the integuments and of the endosperm plus embryo after culture of whole achenes for 48 h in the presence of 105 M [14C]‐ABA showed that ABA metabolism occurred only in the endospermembryo tissue.

Mode of Action of Abscisic Acid in Barley Aleurone Layers

As a part of our effort to study the mode of action of abscisic acid (ABA) and its metabolites during seed germination, we have investigated the regulation of ABA metabolism in barley (Hordeum vulgare) aleurone layers and a few other plant tissues. The rate of conversion of [(3)H]ABA to [(3)H]phaseic acid (PA), the first stable metabolite of ABA, is enhanced by 2- to 5-fold in barley aleurone layers when the tissue is pretreated with ABA. However, the conversion of [(3)H]PA to [(3)H] dihydrophaseic acid (DPA), the next metabolite after PA, is not enhanced by pretreatment with either ABA or PA. The ABA enhancement of its own metabolism in barley aleurone layers is detectable with a pretreatment of ABA ranging from 10(-3) to 10(-4) molar. This apparent self-induction of ABA conversion to PA can be observed after the barley aleurone layers have been treated with 10(-5) molar ABA for as short as 2 hours, and is inhibited by the transcription inhibitor, cordycepin (3'-deoxyadenosine), or the translation inhibitor, cycloheximide. The self-induction of ABA conversion to PA also occurs in wheat aleurone layers, but not in other plant tissues that have been investigated, including corn root tips, barley embryos, barley, and soybean leaf discs. It is probably a phenomenon unique to the aleurone layers of some cereal grains. In view of the recent observations that ABA is able to induce new proteins in barley aleurone layers, we suggest that some of these ABA-induced proteins are involved in the conversion from ABA to PA in this tissue.