Transport of abscisic acid

The transport of abscisic acid in cotyledonary petiole sectionsof cotton seedlings was investigated using (2-¹⁴C)-ABA in adonor-receiver system. Using the intercept technique, a transportvelocity of 22.4 mm/hr was determined. This velocity was foundto be independent of the length of the petiole or of the concentrationof ABA applied. In a 24-hr period 70% of the applied ABA movedthrough the tissue sections. No polarity was found in any ofthe studies. The capacity of the transport system decreased with increasingage of the tissue and with increasing time of pre-exposure toABA. When metabolism of the tissue was restricted by cold temperature,low-O₂ tension or DNP treatment, transport was inhibited 80to 98%.The distribution of ABA in the petiole during transportwas also investigated. The results are discussed in relationto the well-studied transport of IAA and in relation to thephysiologicalrole of ABA. ¹ Present address: Stanford Research Institute-Irvine, 19722Jamboree Blvd., Irvine, Calif. 92664, U.S.A. (Received November 30, 1970; )     

Action of natural abscisic acid precursors and catabolites on abscisic acid receptor complexes

The phytohormone abscisic acid (ABA) regulates stress responses and controls numerous aspects of plant growth and development. Biosynthetic precursors and catabolites of ABA have been shown to trigger ABA responses in physiological assays, but it is not clear whether these are intrinsically active or whether they are converted into ABA in planta. In this study, we analyzed the effect of ABA precursors, conjugates, and catabolites on hormone signaling in Arabidopsis (Arabidopsis thaliana). The compounds were also tested in vitro for their ability to regulate the phosphatase moiety of ABA receptor complexes consisting of the protein phosphatase 2C ABI2 and the coreceptors RCAR1/PYL9, RCAR3/PYL8, and RCAR11/PYR1. Using mutants defective in ABA biosynthesis, we show that the physiological activity associated with ABA precursors derives predominantly from their bioconversion to ABA. The ABA glucose ester conjugate, which is the most widespread storage form of ABA, showed weak ABA-like activity in germination assays and in triggering ABA signaling in protoplasts. The ABA conjugate and precursors showed negligible activity as a regulatory ligand of the ABI2/RCAR receptor complexes. The majority of ABA catabolites were inactive in our assays. To analyze the chemically unstable 8'- and 9'-hydroxylated ABA catabolites, we used stable tetralone derivatives of these compounds, which did trigger selective ABA responses. ABA synthetic analogs exhibited differential activity as regulatory ligands of different ABA receptor complexes in vitro. The data show that ABA precursors, catabolites, and conjugates have limited intrinsic bioactivity and that both natural and synthetic ABA-related compounds can be used to probe the structural requirements of ABA ligand-receptor interactions.     

Transpiration-inhibiting abscisic acid analogs

Synthetic analogs of abscisic acid (ABA) and their inhibiting effect on transpiration rates of detached barley leaves are presented. By systematically varying the carbon skeleton of ABA, the influence of structural changes on biological activity was investigated. The results show that a properly substituted cyclohexane unit and a six-carbon side chain seem to be indispensable for high ABA-like activity, whereas the oxidation state of the terminal carbon atom in the side chain appears to be less essential. Thus, synthetic compounds have been created that exhibit biological activities comparable both in type and strength with ABA itself. On the basis of molecular models, a hypothesis of the geometric arrangement of essential substructural units is proposed.     

Transpiration-inhibiting abscisic acid analogs

Synthetic analogs of abscisic acid (ABA) and their inhibiting effect on transpiration rates of detached barley leaves are presented. By systematically varying the carbon skeleton of ABA, the influence of structural changes on biological activity was investigated. The results show that a properly substituted cyclohexane unit and a six-carbon side chain seem to be indispensable for high ABA-like activity, whereas the oxidation state of the terminal carbon atom in the side chain appears to be less essential. Thus, synthetic compounds have been created that exhibit biological activities comparable both in type and strength with ABA itself. On the basis of molecular models, a hypothesis of the geometric arrangement of essential substructural units is proposed.     

Role of abscisic acid in chilling tolerance of rice (Oryza sativa L.) seedlings. I. Endogenous abscisic acid levels

Changes of ABA levels in chilled rice (Oryza sativa L.) seedlings of two varieties were determined. On exposure to chilling, ABA concentration rapidly increased in the chilling-tolerant cultivar (cv. Tainung 67, TNG.67) but not in the chilling-sensitive cultivar (cv. Taichung Native 1, TN.1). Both detached shoots and roots of TNG.67 seedlings showed a significant ABA increase after exposure to chilling. TN.1 seedlings could not accumulate ABA under low temperature but well-watered status. Exogenous application of the ABA biosynthetic inhibitor, fluridone, reduced ABA accumulation, as well as survival ratio of chilled TNG.67 seedlings. Electrolyte leakage and leaf conductance were also increased by the inhibitor and the effects could be reversed by exogenously applied ABA. ABA concentrations in xylem sap of TNG.67 seedlings increased within 4 h after chilling, and this was temporally coincident with the reduction of leaf conductance. The roles of endogenous ABA in the tolerance of rice seedlings to chilling on a whole plant basis are discussed and suggested.     

Analysis of abscisic acid by high-performance liquid chromatography.

Methodology for the ready analysis of abscisic acid (ABA) in plant tissues based upon application of high-performance liquid chromatography (hplc) has been developed. The method involves isolation of the acid fraction, preparation of the methyl esters with diazomethane, and hplc using a combination procedure of two columns: (1) reversed-phase C₁₈, and (2) porous silica in the absorption mode. Only isocratic elution is required so the method is readily adaptable to laboratories having limited hplc capability. Measured recoveries are 70% and the use of an internal standard allows quantification of ABA levels to 1 ng/g of tissue with minimum absolute detectable levels of ABA of 20 ng. The method is illustrated by analysis of ABA concentration in potato tubers at various times postcutting.     

Potato cold hardiness development and abscisic acid. I. Conjugated abscisic acid is not the source of the increase in free abscisic acid during potato (Solanum commersonii) cold acclimation

Free and conjugated abscisic acid (ABA) levels in stem-cultured plantlets of potato ( Solanum commersonii Dun, PI 458317) during cold acclimation were measured. The levels of free and conjugated ABA were measured by an enzyme immunoassay (EIA) with rabbit anti-ABA-serum. The use of immunoglobulin G fraction purified from rabbit antiserum and the methylated form of ABA resulted in an improved measuring range (0.01 to 10 pmol ABA) and precision (slope of logit-log plot, −1.35) of EIA, compared to the use of antiserum and free ABA. Estimates of the EIA were consistent with those resulting from a commercial EIA. Under a 4/2°C (day/night) temperature regime, the potato plantlets increased cold hardiness from −5°C (warm-grown control) to −10°C by the 7th day. During the same period, there were two transitory increases in free ABA, the first one three-fold from 1.5 to 5.3 nmol (g dry weight)⁻¹ on the 2nd day and the second one five-fold from 1.5 to 7.6 nmol (g dry weight)⁻¹ on the 6th day. Each increase in ABA concentration was followed by an increase in cold hardiness. There was no significant change in conjugated ABA content (4.2±0.6 nmol [g dry weight]⁻¹) throughout the cold acclimation period. The lack of an interrelationship between levels of free and conjugated ABA suggested that the transitory increase in free ABA during cold acclimation was not a result of the conversion of conjugated ABA. The increase in free ABA due to biosynthesis of ABA during potato cold acclimation is discussed.     

Signalling of abscisic acid to regulate plant growth.

Abscisic acid (ABA) mediated growth control is a fundamental response of plants to adverse environmental cues. The linkage between ABA perception and growth control is currently being unravelled by using different experimental approaches such as mutant analysis and microinjection experiments. So far, two protein phosphatases, ABI1 and ABI2, cADPR, pH, and Ca2+ have been identified as main components of the ABA signalling pathway. Here, the ABA signal transduction pathway is compared to signalling cascades from yeast and mammalian cells. A model for a bifurcated ABA signal transduction pathway exerting a positive and negative control mechanism is proposed.     

Chemical biology of abscisic acid

Chemical biology is a discipline that utilizes chemicals to elucidate biological mechanisms and physiological functions. Various abscisic acid (ABA) derivatives have revealed the structural requirement for the perception by ABA receptors while biotin or caged derivatives of ABA have disclosed the localization of several ABA-binding proteins. Recently, selective ABA agonist has been used to identify ABA receptors. Furthermore, ABA biosynthesis and catabolic inhibitors have contributed to the identification of new ABA functions in plant growth and development. The physiological function of ABA in non-plant organisms has gradually been revealed. In this review, we discuss the development of small bioactive chemicals and their significance in ABA research.     

Cuticular penetration of abscisic acid

Summary Penetration of 2-¹⁴C abscisic acid (ABA) through enzymatically isolated cuticles from tomato fruit and from the upper epidermis of apricot, pear and orange leaves was assessed. Penetration was linear with time, greater as the undissociated than the dissociated ion, and greater through dewaxed than non-dewaxed cuticles. Significantly less (3–6 times) (2-¹⁴C)ABA penetrated the tomato fruit cuticle than NAA or 2,4-D. The leaf cuticles were less permeable than the tomato fruit cuticle. There was no evidence that the ABA was altered during transfer across the cuticle.     

Control of abscisic acid synthesis

The abscisic acid (ABA) biosynthetic pathway involves the formation of a 9-cis-epoxycarotenoid precursor. Oxidative cleavage then results in the formation of xanthoxin, which is subsequently converted to ABA. A number of steps in the pathway may control ABA synthesis, but particular attention has been given to the enzyme involved in the oxidative cleavage reaction, i.e. 9-cis-epoxycarotenoid dioxygenase (NCED). Cloning of a gene encoding this enzyme in maize was first reported in 1997. Mapping and DNA sequencing studies indicated that a wilty tomato mutant was due to a deletion in the gene encoding an enzyme with a very similar amino acid sequence to this maize NCED. The potential use of this gene in altering ABA content will be discussed together with other genes encoding ABA biosynthetic enzymes.     

A radioimmunoassay for abscisic acid

We have developed a radioimmunoassay (RIA) for abscisic acid (ABA) in the 0.1 ng to 2.5 ng range. Antibodies were obtained from rabbits immunized with ABA bound via its carboxyl group to bovine serum albumin. Cross-reactivity studies indicate that ABA esters are completely cross-reactive with ABA, while trans, trans abscisic acid (t-ABA) phaseic acid (PA) and dihydrophaseic acid (DPA) have much lower but significant cross-reactivities. Purification methods which reduce the levels of cross-reacting substances are described.Abbreviations RIA radioimmunoassay - DPA 4-dihydrophaseic acid - PA phaseic acid - GC gas chromatography - HPLC high performance liquid chromatography - TLC thin-layer chromatography - BSA bovine serum albumin - ABA abscisic acid - t-ABA trans, trans abscisic acid - IAA indoleacetic acid     

Abscission-promoting activities of abscisic acid and five abscisic acid analogs in cotton seedlings and explants

The abscission-promoting activities of abscisic acid (ABA) and 5 ABA analogs were examined in cotton (Gossypium hirsutum L. cv LG102) seedlings and cotyledonary node explants. The analogs tested included a series of acetylenic derivatives that differ in the oxidation state of the C-1 atom, a 2,3 dihydro-derivative of ABA and a 2,3 dihydro-derivative of an acetylenic analog with a C-1 carboxyl moiety. ABA and all five analogs were active in stimulating petiole abscission in explants. Following treatment with 100,µM ABA or analog, 50% abscission of explants was observed after 29 h and complete abscission occurred within 40 h. With one exception, none of the treatments resulted in an increase in explant ethylene production. Pretreatment of the explants with the ethylene antagonist silver thiosulfate completely abolished the abscission-promoting activities of ABA and all of the analogs. Daily application of ABA or any of the analogs had no effect on cotyledon abscission in intact seedlings. The implications of the results with respect to the development of a commercial ABA-like regulator as well as to ABA structure-activity studies are discussed.Mention of trademark or proprietary product does not constitute a guarantee or warranty of the product by the U.S. Department of Agriculture and does not imply its approval to the exclusion of other products that may also be suitable.     

Increased abscisic acid sensitivity and drought tolerance of Arabidopsis by overexpression of poplar abscisic acid receptors

Plant Cell, Tissue and Organ Culture (PCTOC) - Abscisic acid (ABA), a key plant hormone that regulates plant growth development and stress response, is recognized and bound by ABA Receptor...     

Inhibitors of abscisic acid 8'-hydroxylase

Structural analogues of the phytohormone (+)-abscisic acid (ABA) have been synthesized and tested as inhibitors of the catabolic enzyme (+)-ABA 8'-hydroxylase. Assays employed microsomes from suspension-cultured corn cells. Four of the analogues [(+)-8'-acetylene-ABA, (+)-9'-propargyl-ABA, (-)-9'-propargyl-ABA, and (+)-9'-allyl-ABA] proved to be suicide substrates of ABA 8'-hydroxylase. For each suicide substrate, inactivation required NADPH, increased with time, and was blocked by addition of the natural substrate, (+)-ABA. The most effective suicide substrate was (+)-9'-propargyl-ABA (K(I) = 0.27 microM). Several analogues were competitive inhibitors of ABA 8'-hydroxylase, of which the most effective was (+)-8'-propargyl-ABA (K(i) = 1.1 microM). Enzymes in the microsomal extracts also hydroxylated (-)-ABA at the 7'-position at a low rate. This activity was not inhibited by the suicide substrates, showing that the 7'-hydroxylation of (-)-ABA was catalyzed by a different enzyme from that which catalyzed 8'-hydroxylation of (+)-ABA. Based on the results described, a simple model for the positioning of substrates in the active site of ABA 8'-hydroxylase is proposed. In a representative physiological assay, inhibition of Arabidopsis thaliana seed germination, (+)-9'-propargyl-ABA and (+)-8'-acetylene-ABA exhibited substantially stronger hormonal activity than (+)-ABA itself.