Planta (Berl.) 107, 315--324 (1972) 9 by Springer-Verlag 1972

Uptake and Metabolism of 3H-Gibberellin A1 by Barley Aleurone Layers: Response to Abscisic Acid R o n n N a d e a u , L a w r e n c e R a p p a p o r t , a n d Charles F. S t o l p Department of Vegetable Crops, University of California, Davis, California, U.S.A. Received April 4 / June 2, 1972

Summary. When barley aleurone layers were incubated with 3It-Gibberellin A 1 (3It-GA1), the hormone was converted to 3It-GA-X (not identified), all-GAs and two other compounds tentatively identified as 3H-GAl-glucoside, and aH-GAs-glucoside. Uptake and metabolism of the aH-GAx were markedly enhanced by simultaneous treatment with abscisic acid (ABA). Uptake of aH-GA1 from the medium containing ABA was linear over a 24-h period, whereas in the absence of ABA, uptake of aH-GAx leveled off after 5 h. After 24 h, aleurones treated with aI-I-GA1 and aH-GA1 plus ABA, had taken up 9 and 24%, respectively, of the original aH-GA1 provided. Metabolism of aH-GA1 proceeded at a linear rate in the presence of ABA. The amount of aH-GAl-metabolites which had accumulated by the end of a 24-h incubation appeared to be linearly correlated to the logarithm of the ABA concentration. Gibberellins As and-As-glucoside did not reverse GAl-enhanced synthesis of e-amylase. Introduction A b s c i s i c acid (ABA) can " o v e r c o m e " a n u m b e r of p l a n t responses m e d i a t e d b y t h e gibberellins (GAs) ( A d d i c o t t a n d L y o n , 1969; Chrispeels a n d Varner, 1967; W a r e i n g et al., 1968; a n d Cherry, 1968). Various mechanisms h a v e been p r o p o s e d for t h e a c t i o n of A B A . Studies of t h e m e t a b o l i s m of 3H-GA1 in g e r m i n a t i n g b e a n seeds ( N a d e a u a n d R a p p a port, 1972) led to t h e h y p o t h e s i s t h a t A B A c o u n t e r a c t s responses t o GAs b y m e d i a t i n g their conversion to biologically i n a c t i v e forms, t h u s reducing t h e a v a i l a b l e t i t e r of free GAs. To s t u d y t h e possible r e l a t i o n between A B A - e n h a n c e d G A m e t a bolism a n d G A - e n h a n c e d response, t h e fate of aH-GA1 was s t u d i e d in b a r l e y aleurone layers. W e f o u n d t h a t A B A , which inhibits aH-GA1e n h a n c e d synthesis of g - a m y l a s e in aleurones, m a r k e d l y p r o m o t e d t h e a c c u m u l a t i o n of 3H-GA1 m e t a b o l i t e s during a 24-h i n c u b a t i o n period.

Materials and Methods Hormone~. 1,2-3H-gibberellin A1 (aI-I-GA1) was prepared by selective hydrogenation of GAz with tritium gas by New England Nuclear Corp., Boston, Mass. U.S.A., according to the method of Kende (1967). The product was purified by thin layer chromatograph (TLC), using Silica-gel glass-fiber sheets (Chrom

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A R 500, Mallinckodt Chemical Works, St. Louis, MO, U.S.A.) impregnated with AgNO 3 (Nadeau and Rappaport, 1972). The ~H-GA 1 h a d a specific activity of 5 Ci/mmole (1.3 • 107 dpm/t~g, 5.0 • 106 cpm/~g). Pure (S)-(-~)abscisic acid (ABA) was a gift of Hoffmann La Roche, Nutley, N J , U.S.A. Incubation, and Assay o/c~-Amylase. The procedures of Chrispeels a n d Varner (1967) were used to prepare isolated Meurones. The embryos and distal tips were excised from barley seeds (Hordeum vulgare L. cv. Himalaya). The remaining "halfseeds" were surface-sterilized in 1% sodium hypochlorite for 20 rain, washed 3 times with sterile water, and placed on moistened sand in Petri dishes for 72 h. Then under aseptic conditions, the half-seeds were cut along the suture, and the aleurone layers were separated from the starchy endosperm. The aleurone layers were incubated in a buffer solution containing 0.002 M acetate and 0.02 M CaC12 a t p H 4.8, plus the hormones at concentrations indicated later. To assure t h a t replicate flasks contained the same concentration of 8H-GA1, a stock solution was prepared with the hormone dissolved in the necessary a m o u n t of buffer. 2-ml portions of this solution were transferred to each incubation flask, and the solutions were autoclaved for 20 rain j u s t prior to the addition of the aleurone layers. Earlier experiments h a d shown t h a t the hormones were unaffected b y the sterilization process. I n each experiment, lots of 10 isolated aleurone layers were placed in 25-ml Erlenmeyer flasks containing the buffer solution. The flasks were placed on a reciprocating shaker, and their contents incubated for various lengths of time a t 25~ Assays of a-amylase activity were done according to the method of Jones and Varner (1967). Uptake o/ 3H-GAr Periodically, during time-course incubation experiments, 15-~zl samples were withdrawn from the incubation medium of each flask. These samples were placed in scintillation vials containing Bray's solution (Bray, 1960) and were counted until a m a x i m u m error of 1% was obtained. Extraction of Radioactive Compounds from Aleurones. The medium was decanted at the end of an incubation period, and the aleurones layers were washed twice (10 s each time) with buffer to remove surface radioactivity. The aleurones were then boiled 5 min in 5 ml of 80 % ethanol, and after this homogenized in a glass tube with a teflon pestle. Extracts were decanted from the residues a n d further homogenized in 5 ml of 80 % ethanol. The concentrated extracts were assayed for total radioactivity and t h e n reduced to near dryness in preparation for TLC. Thin-layer Chromatography. The solvents were: A, isopropanol: 6 N N H 4 0 H (5:1); a n d ]3, the organic phase of butanol: acetic acid: water (4:4:1). The concent r a t e d extracts were diluted with 100 tzl of solvent A, streaked onto 5 • 20-cm strips of ChromAR-500, and developed in solvent A for 2 h. Peaks of radioactivity were located with a radiochromatogram scanner. Usually, non-radioactive standards were also spotted on t h e C h r o m A R strips, and were visualized b y spraying with 5% H~SOt in 95% ethanol and heating. The radioactive zones were cut out, and the compounds eluted exhaustively with 80 % ethanol. The eluates were then analyzed b y scintillation counting in Bray's solution. Gas-liquid Chromatography (GLC). Products were identified b y comparison with authentic standards on a gas chromatograph equipped with flame ionization detectors a n d an effluent splitter. Stainless steel columns (3.5 m m • 183 cm), packed with 2% SE-30 or 2% QF-1 on Gas-Chrom Q (Applied Science Lab., State College, PA, U.S.A.) were used. The radioactive peaks were detected b y collecting column effluent samples a t 0.5-min intervals and analyzing them b y liquid scintillation counting. Retention times of aH-GA1 and metabolites were compared with those of mass pe~ks from eoin]eeted cold standards. The compounds were aI1 derivatized to produce the methyl ester-trimethyl silyl ethers (Me-TMS).

Barley Aleurone: Gibberellin and ABA

317

Results Aleurone layers, whether in the presence or absence of ABA, converted aH-GA1 to four metabolites (Fig. 1). Arranged in order of decreasing polarity, these were: I, aH-GA-X (unknown structure); II, aI-I-GAs-glucoside (tentative): I I I , SH-GAl-glucoside (tentative); IV, all-GAs (Fig. 1). Compounds I I and I V were identified by comparing their TLC and GLC (SE-30 and QF-1) properties with those of authentic

standards (Nadeau and l~appaport, 1972). Also, treatment of II with 1 N H2SO 4 at 90 ~ for 4 h gave the expected ~H-GAs-ketone. Metabolites I I and I V had been previously identified in extracts of Phaseolus vulgaris seeds that had been incubated in aH-GA1 (Nadeau and Rappaport, 1972). Compound I I I displayed chromatographic behavior similar to t h a t of II. For example, on SE-30 at 265 ~ II-Me-TMS and III-Me-TMS had retention times of 22.5 min and 21.5 min, respectively. However, acid hydrolysis of I I gave the Wagner-Meerwein rearranged GA s derivative (Schreiber et al., 1970), whereas under the same conditions, I I I yielded the corresponding GA 1 analogue. A puzzle in this work is the identity of I, which was produced in highest yield (Fig. 1). Neither acid nor base (1 N H2SO 4, 90 ~ 4 h; 1 N NaOH, 90 ~ 4 h) treatment of I liberated a "free"gibberellin-like compound. Most surprising is the fact that on ChromAR electrophoresis, using as buffer pyridine : acetic acid :H20 (10:1:90) at p H 6.0, I migrated further toward the anode than did GA 1 or GAs-glucoside, both of which are monoearboxylic. Hence, it is likely that I contains more than one carboxyl group. Sehreiber et al. (1970) described a GA derivative, generated by alkali treatment of GAs-glucoside, which is dicarboxylie and has the same Rf as 3H- GA-X in Solvent A. We prepared some of this "dicarboxylic acid" and compared it to aH-GA-X by TLC in solvent B. The Rr of aH-GA-X was 0.37, whereas that of the GAs-glucoside derivative was 0.70. Alcurone layers incubated in aH-GA1 accumulated radioactivity rapidly for 5 h and then, after a slight but reproducible reversal in uptake, continued to accumulate radioactivity at a low rate (Fig. 2). However, with ABA in the incubation medium, the initial rate of uptake of aH-GA 1 was maintained over the 24-h course of the experiment. The comparative figures for uptake of aH-GA1 after 24 h in the absence and presence of ABA were, respectively, 9 and 24%. Fig. 2 shows that the aleurones absorbed 3H-GA1 from the external medium at the same rate during the initial 5 h, whether or not ABA was present. However, from the 5th through 24th h, the slopes diverged and it is apparent t h a t in medium containing only aIt-GA~, the amount of aH-GA1 in the medium remained relatively constant. I n contrast, in a

318

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Barley Aleurone: Gibberellin and ABA

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m e d i u m containing aH-GA1 plus A B A , t h e 3H-GA1 d i s a p p e a r e d from t h e m e d i u m a t a s t e a d y rate. A b e t t e r u n d e r s t a n d i n g of this result came from a n e x p e r i m e n t in which t h e levels of s H - G A 1 a n d r a d i o a c t i v e m e t a b o l i t e s in t h e aleurones were m e a s u r e d as a f u n c t i o n of i n c u b a t i o n time. F i v e lots of aleurones were i n c u b a t e d in m e d i u m containing ~H-GA 1 a n d ABA. A f t e r each i n c u b a t i o n p e r i o d (4, 8, 14, I9, a n d 24 h) 3H-GAI a n d r a d i o a c t i v e m e t a bolites were e x t r a c t e d with 80 % e t h a n o l from one of the lots of aleurones. As can be seen in Fig. 3, the level of e x t r a c t e d sIt-GA1 was v i r t u a l l y c o n s t a n t over time, whereas t h e level of r a d i o a c t i v e m e t a b o l i t e s increased in a linear m a n n e r . To d e t e r m i n e w h e t h e r the conversion p r o d u c t s of 3H-GAi can l e a k o u t of t h e aleurone cells, we a n a l y z e d t h e m e d i a of 3H-GA1- a n d aH-GA1 + A B A - t r e a t e d aleurones after 24 h of incubation. Only 3H-GA1 was f o u n d in t h e media, irrespective of t h e presence of ABA. W e also f o u n d t h a t 3H-GA1 can m o v e o u t of t h e aleurones b y i n c u b a t i n g t h e m in SH-GAI for 15 h a n d t h e n transferring t h e m to identical m e d i u m containing cold G A 1. I n b o t h t r e a t m e n t s only aH-GA1 l e a k e d o u t of t h e layers d u r i n g t h e n e x t 9h. The effect of A B A c o n c e n t r a t i o n on GA~-enhaneed synthesis of c~-amylase, a n d on m e t a b o l i s m of SH-GA1, was s t u d i e d b y i n c u b a t i n g 6 lots of 10 aleurones in solutions containing 1.4 • 10 .7 M 3H-GA1 plus A B A a t 0, 1 . 9 • .9 , 1 . 9 • -s, 1 . 9 • .7 , 1 . 9 • -s or 1 . 9 • -sM. Atter 24 h, e - a m y l a s e in t h e m e d i a was measured, a n d t h e 3H-GA1 a n d m e t a -

320

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bolites were extracted from the aleurones with ethanol. Metabolite formation increased with increasing logarithm of ABA concentration (Fig. 4). ~-Amylase production was inhibited in a linear manner over the entire range of ABA concentration. The possibility that a metabolite of aH-GA1 might act as an inhibitor of ~-amylase synthesis was tested by incubating aleurones in GA sglucoside, separately and together with GAr No inhibitory effect on the secretion was noted. I n fact, the glucoside slightly enhanced the promoting effect of GA~ on secretion of the enzyme. However, the most abundant metabolite, 3H-GA-X, has not been tested in this way. Discussion

A novel feature of this investigation is that ABA markedly enhanced uptake and metabolism of 3H-GA1 in barley aleurone layers, a tissue in which GAs promote enzyme secretion (Paleg et a l . , 1964; Pollard, 1970; Jones, 1971). Conversion to the four products occurred both in the presence and absence of ABA, although there was much more conversion in its presence. Metabolites I and I I I were not found in extracts of germinating bean seeds that had been incubated for 24 h in 8H-GA1 (Nadeau and Rappaport, 1972), although I was the major metabolite in aleurones.

Barley Aleurone" Gibberellin and ABA

321

The structure of aH-GA-X is not known, but it is evident that a glucosylating enzyme system must be present for the formation of compounds I I and I I I , and a hydroxylating system for I I and IV. Thus the simplest but unconfirmed explanation for the action of ABA in these experiments is that addition of ABA to the incubating medium enhances synthesis of the necessary enzymes. Some evidence for ABA-enhanced synthesis was presented b y Cherry (1968), who showed t h a t development of beet root invertase was enhanced by ABA. Also, Walton and Sondheimer (1968) reported that ABA increased the activity of phenylalanine ammonia lyase in bean leaf discs. Associated with the question of the mechanism by which ABA influences the enzymes responsible for GA 1 metabolism is the problem of the role of metabolism in regulation of biochemical and physiological proceesses. Is metabolism simply a means to remove surplus GAs, and/or does it have special significance in controlling development ? Although the answer is not known, current experiments are being designed with this question in mind. Whatever role(s) ABA has in metabolism of 3H-GA1 in the Meurone system, it seems obvious t h a t it is affecting either 1) uptake of aH-GA1, or 2) enzyme synthesis or activity, or both. The experiments reported herein do not discriminate sufficiently between uptake and metabolism to be certain whether one or both processes are influenced by ABA. However, the data of Figs. 2 and 3 favor the view t h a t there is an influence on metabolism. During the initial 5 h incubation period ABA- and non ABA-treated aleurones absorb 3H-GA 1by diffusion from the medium. At this time it appears t h a t a diffusion equilibrium is reached, i.e., ~H-GA1 molecules enter and exit the aleurone layers at the same rate. I n medium containing 3H-GA1 and ABA, GA metabolites are steadily formed in, and become localized within the aleurone layers. Thus 3H-GA1 steadily disappears from the medium. In medium containing no ABA, GA metabolism in the tissue progresses more slowly, accounting for the relatively level absorption curve. Evidence that ABA can influence synthesis or activity of enzymes is summarized in the review by Addieott and Lyon (1970), as well as in some recent papers in which ABA was reported to inhibit synthesis of nucleic acid or protein, or both (van Overbeek et al., 1968; Shih and Rappaport, 1970; G]ew, 1968) and to inhibit activity of commercial ~-amylase in vitro (Hemberg, 1967). The data of Fig. 2 indicate that some all-GA 1is present in the alenrone layers during a 24-h incubation period. Musgrave et al. (1972) observed t h a t when aleurones are first incubated in 3H-GA 1 and then transferred to a medium containing the same concentration of GA1, some 3H-GA1 leaks

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from the aleurones into the external medium. Whether free stI- GA 1 within the aleurones is available for enhancement of protein synthesis is not known. Therefore, to determine whether GA metabolism plays a major functional role in plant regulation, it appears t h a t is will be necessary to find where the 8It-GA1 localizes in the alenrone layer. I t would also be desirable to know the target site of the hormone and the site of conversion as well. Unfortunately, research aimed at identifying the site of GA binding in aleurone layers has been unrewarding (Ginzburg and Kende, 1968; Musgrave st al., 1972). The hypothesis that accumulation of metabolites within aleurone layers results in inhibition of GA 1 action could be tested by assaying all metabolites to determine whether they can inhibit GA~-enhanced protein synthesis or secretion. Two of the four metabolites shown in Fig. 2, namely SH-GA s and 3H-GAs-glucoside, are virtually inactive at physiological concentrations in the barley half-seed assay as well as in several other assays (Crozier et al., 1970). I n competition experiments, we did not find any inhibition of 3H-GAl-enhanced release of ~-amylase b y simultaneously supplied GAs-glucoside. Neither metabolites I nor i I I were available for bioassay, so it is not possible to reach a conclusion at present concerning GA 1 metabolites as feedback inhibitors of enzyme production. Despite the lack of information concerning sites of action and conversion, the data in this paper indicate that a significant percentage of the SH-GA1 taken up is converted in the presence of ABA. Wareing et al. (1968) found t h a t ABA treatment of corn leaves resulted in a marked decrease in level of endogenous GAs. Although they suggested t h a t ABA m a y play a role in facilitating intereonversion of free and bound forms of GAs, the major thrust of their interpretation was that ABA inhibits biosynthesis of GAs. Smith and Sadri (1970) reported that ABA did not inhibit synthesis of GAs by the fungus ~ ' u s a r i u m monili/orme. Although this finding does not rule out the possibility t h a t ABA inhibits GA biosynthesis in vascular plants, the results of the present study strongly favor the concept t h a t ABA plays a major role in modulating supplies of GAs in tissues via its effects on GA uptake and metabolism.

This work was supported by USPHS Grant GM 12885 and NSF Grant GB 1335. We acknowledge with appreciation samples of GAs and GAs-glucoside provided by Drs. K. Schreiber, Institut fiir Kulturpflanzenforschung, Gatersleben, GDR; J. MacMillan, School of Chemistry, University of Bristol, England; and N. Takahashi, Department of Agricultural Chemistry, University of Tokyo, Japan. A sample o5 GA~ was supplied by Imperial Chemical Industries, Alderley Park, Cheshire, England.

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References Addicott, F. T., Lyon, J. L. : Physiology of abscisic acid and related substances. Ann. Rev. Plant Physiol. 20, 139-164 (1969). Bray, G. A. : A simple, efficient liquid scintillator for counting aqueous solutions in a liquid scintillation counter. Analyt. Biochem. 1, 279-285 (1960). Cherry, J. H. : Regulation of invertase activity in washed sugar beet tissue. In: Biochemistry and physiology of plant growth substances (F.Wightman, G. Setterfield, eds.), p. 417431. Ottawa: Runge Press 1968. Chrispeels, M. J., Varner, J. E.: Hormonal control of enzyme synthesis: On the mode of action of gibberellic acid and abscisin in aleurone layers of barley. Plant Physiol. 42, 1008-1016 (1967). Crozier, A., Kuo, C. C., Durley, P~. C., Pharis, R. D. : The biological activity of 26 gihberellins in nine bioassays. Canad. J. Bot. 48, 867 877 (1970). Ginzburg, C., Kende, H.: Studies on the intracellular localization of radioactive gibberellin. I n : Biochemistry and physiology of plant growth substances (F. Wightman, G. Setterfield, eds.), p. 333-340. Ottawa: Runge Press 1968. Glew, 1~. H. : Developmental aspects of lipid metabolism in the developing and germinating castor bean seed. Doctoral dissert., University of California, Davis (1968). I-Iemberg, T.: Abscisin I I as an inhibitor of ~-amylase. Acta chem. scand. 21, 1665-1666 (1967). Jones, R . L . : Gibberellic acid enhanced release of fl-l,3-glucanase from barley aleurone cells. Plant Physiol. 47,412~$16 (1971). Jones, l~. L., Varner, J. E. : The bioassay of gibberellins. Planta (Berl.) 72, 155-161 (1967). Kende, H. : Preparation of radioactive GA 1 and its metabolism in dwarf peas. Plant Physiol. 42, 1612-1618 (1967). Lang, A.: Gibberellins: structure and metabolism. Ann. Rev. Plant Physiol. 21, 537-570 (1970). ~usgrave, A., Kays, S.E., Kende, H.: Uptake and metabolism of radioactive gibberellins by barley aleurone layers. Planta (Berl.) 102, 1-10 (!972). Nadeau, R., Rappaport, L.: metabolism of gibberellin A 1 in germinating bean seeds. Phytochem. l l , 1611-1616 (1972). Overbeek, J. van, Loeffler, J. E., mason, M. I. R. : Mode of action of abseisic acid. I n : Biochemistry and physiology of plant growth substances (F. Wightman, G. Setterfield, eds.), p. 1593-1607. Ottawa: Runge Press 1968. Paleg, L., Aspinall, D., Coombe, B., Nicholls, P. : Physiological effects of gibberellic acid. VI. Other gibberellins in three test systems. Plant Physiol. 39, 286-290 (1964). Pollard, C. J. : Initiation of responses in aleurone layers by gibberellie acid. Biochim. biophys. Aeta (Amst.) 222,501-507 (1970). Sehreiber, K., Weiland, J., Sembdner, G.: Isolierung yon Gibberellin-As-0(3)fi-D-glucopyranosid aus Friichten yon Phaseolu8 coccincus. Phytochem. 9, 189-198 (1970). Shih, C. Y., Rappaport, L.: Regulation of bud rest in tubers of potato, Solanum tuberosum L. VII. Effects of abscisic and gibberellie acids on nucleic acid synthesis in excised buds. Plant Physiol. 45, 33-36 (1970). Smith, 0. E., Sadri, H. A. : Effect of abscisic acid on gibberellin in biosynthesis in .Fusarium monili]orme. Plant Cell Physiol. l l , 345-348 (1970).

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Walton, D. C., Sondheimer, E. : Effects of abscisin II on phenylalanine ammonialyase activity in excised bean axes. Plant Physiol. 43, 467-469 (1968). Wareing, 1). F., Good, J., Manuel, J. : Some possible physiological roles of abscisie acid. In: Biochemistry and physiology of plant growth substances (F. Wightman, G. Setterfield, eds.), p. 1547-1560. Ottawa: Runge Press 1968. L. Rappaport Department of Vegetable Crops Mann Laboratory University of California Davis, CA 95616, U.S.A.

Uptake and metabolism of (3)H-Gibberellin A 1 by barley aleurone layers: Response to abscisic acid.

When barley aleurone layers were incubated with (3)H-Gibberellin A1 ((3)H-GA1), the hormone was converted to (3)H-GA-X (not identified), (3)H-GA8 and ...
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