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ABSCISIC ACID Dr. Uttam Kumar Kanp ABSCISIC ACID Dr. Uttam Kumar Kanp Contents: 1. Discovery of ABA 2. Chemical Nature 3. Bioassay of ABA 4. Physiological roles of Abscisic Acid 5. Biosynthesis of AB A in Plants 6. Degradation/Inactivation of AB A in Plants 7. Occurrence and Distribu tion of ABA in Plants and 8. ABA Transport in Plant. 1. Discovery of ABA: In 1963, a substance strongly antagonistic to growth was isolated by F. T. Add icott from young cotton fruits and named Abscisin II. Later on, this name was changed to Abscisic acid (ABA). The chemical name of abscisic acid whose structure is given in fig. 1. Is [3-methyl 5-1′ (1′- hydrox y, 4′-oxy-2′, 6′, 6′-trimethyl-2- cyclohexane-l-yl)-cis, trans-2,4- penta-dienoic acid]. Eagles and Wareing (1963, 64), at the same time pointed out the presence of a substance in birch leaves (Betula pubescens, a decidu ous plant) which inhibited growth and induced dormancy of buds and, therefore, named it ‘dormin’. But, very soon as a result of the work of Cornfort h et. al. (1965), it BOTANY: SEM – V, PAPE R-C12T: PLANT PHYSIOLOGY, UNIT -5: PLANT GROWTH REGULATORS – ABSCISIC ACID was found to be identical with absc isic acid. 2. Chemical Nature of ABA: Abscisic acid is a 15-C sesquiterpene compound (molecular formula C15H20O4) composed of three isoprene residues and having a cyclohexane ring with keto and one hydroxyl group and a side chain with a terminal car boxylic group in its structure. ABA resembles terminal portion of some carotenoids such as violaxanthin and neoxanthin (see Fig. 17.31) and appears to be a breakdown product of such carotenoids. Any change in its molecular structure results in loss of activity. ABA occurs in cis and trans -isomeric forms that are decided by orientation of -COOH group around 2nd carbon atom in the molecule (Fig. 1.). Almost all naturally occurring ABA in plants exist in cis form that is biologically active and the name absci sic and usually refers to this form. Trans-ABA is inactive form but can be inter convertible with cis - ABA. Due to the presence of a chiral centre (asymmetric carbon atom) at l’ -position, ABA also occurs in two enantiomeric forms called S and R or (+) and (-) forms respectively which can be distinguished from each other by their optical rotatory dispersion curves. In the S or (+) form, which is dextrorotatory, the -OH at 1′ position faces below the plane of the ring and is shown by dashed bond line while in R or ( -) form, which is laevorotatory, it faces above the plane of the ring and is shown by a thick wedge shaped bond line (Fig. 2.). BOTANY: SEM – V, PAPE R-C12T: PLANT PHYSIOLOGY, UNIT -5: PLANT GROWTH REGULATORS – ABSCISIC ACID 2. Although both S and R enantiome rs of cis-ABA are biologically active, the R form is not active in fast responses of ABA such as stomatal closure. Unlike cis and trans forms of ABA, the S and R forms are not inter- convertible in plant. Commercially available synthetic sample of ABA is a racemic mixture of equal amounts of both S and R forms. 3. Bioassay of ABA: A number of different bioassays have been developed to detect and estimate ABA in plant extracts which are based on, Acceleration of the abscission of cotyledonary petioles in explants of 14 day old cotton seedlings; Inhibition of IAA-induced straight growth of Avena (oat) coleoptile; Inhibition of GA-induced synthesis of a -amylase in aleurone layer of germinating barley (Hordeum); Inhibition of growth of duckweed Lemna minor. Inhibit ion of germination of isolated wheat (Triticum) embryos and, Stimulation of stomatal closure. Of all above bioassays, the last one that is based on stimulation of stomatal closure has fast response of guard cells and is comparativ ely more sensitive. It can detect as low as 10-9 M ABA in plant BOTANY: SEM – V, PAPE R-C12T: PLANT PHYSIOLOGY, UNIT -5: PLANT GROWTH REGULATORS – ABSCISIC ACID extracts. Preliminary purification measures are however, necessary in all bioassays. Physicochemical Methods: The old bioassay methods have now been replaced with more accurate and reliable modern physicochemical methods of sep aration and quantification of the hormone. These include high performance liquid chromatography (HPLC) and gas chromatography (GC) which are then followed by mass spectrometry (MS) to pro vide proof of structure. Such methods can dete ct as low as 10-13g ABA in plant extracts. Prelimina ry purification steps including thin layer chro- ma tography (TLC) are also required in such methods. Immunoassay Methods: Like other plant hormones (aux ins, gibberellins and cytokinins), highly sens itive and specific immunoassays are also applied for quantification of ABA which can detect as low as 10-13g ABA in partly purified or crude extracts. In immunoassay, specific anti hormonal antibodies are obtained by injecting the hormone (in this case ABA) in mice or rabbit. These antibodies are then used to react with the hormone in a cuvette assa y. 4. Physiological Role s of Abscisic Acid (ABA): 1. Stomatal Closing: The role of ABA in causing stomatal closure in plants undergoing water - stress is now widely recognised. It has been suggested by various workers that in response to the water-stress, the permeability of the chloroplast membranes of mesophyll cells to ABA is greatly increased. As a result, the ABA synthesized and stored in mesophyll chloroplasts diffuses out into the cyt oplasm. It then moves from one mesophyll cell to another through plasmodesmata and finally BOTANY: SEM – V, PAPE R-C12T: PLANT PHYSIOLOGY, UNIT -5: PLANT GROWTH REGULATORS – ABSCISIC ACID reaches to the guard cells where it causes closing of stomata. Fresh biosynthesis of ABA continues in mesophyll chloroplasts during period of water -stress. When wate r potential of the plant is restored (i.e., increased), the movement of ABA into the guard cells stops. ABA disappears from the guard cells a little later. The application of exogenous ABA to leaves of normal plants causes closing of stomata within a few m inutes. It has been suggested that ABA causes closing of stomata by inhibiting the ATP-mediated H +/K + ions exchange pumps in guard cells. 2. Bud Dormancy: Inducing bud dormancy in some temperate zone trees such as birch (Betula pubescens), Acer, Fraxinus etc. ; Inducing dormancy of seeds which require stratification (i.e., exposure to low temp, for germination); 3. Process of tuberization; 4. Senescence of leaves; 5. Fruit ripening; 6. Abscission of leaves, flowers and fruits 7. Increasing the resistance of temperate zone p lants to frost injury, 8. Inhibition of GA-induced synthesis of a -amylase in aleurone layers of germinating barley; 9. Inhibition of precocious germination and vivipary and 10. Increase in root : shoot ratio at low water potentials .(The role of ABA in causing absc ission of leaves, flowers and fruits is controversial. 11. ABA may stimulate abscission of organs only in a few species that too probably through increase in production of ethylene. The primary hormone causing abscission is not ABA but ethylene). BOTANY: SEM – V, PAPE R-C12T: PLANT PHYSIOLOGY, UNIT -5: PLANT GROWTH REGULATORS – ABSCISIC ACID 5. Biosynthesis of ABA in Plants: Extensive studies done by research ers with ABA deficient mutants of tomato, Arab idopsis and other plants have clearly shown that ABA is synthesized in higher plants not from simple terpenoid precursors directly through 15-C farnesyl diphosphate (FPP), but indirectly through carotenoid pathway as breakdown product of 40-C xanthophyll such as violaxanthin or neo xan thin (Fig. 3). 3. BOTANY: SEM – V, PAPE R-C12T: PLANT PHYSIOLOGY, UNIT -5: PLANT GROWTH REGULATORS – ABSCISIC ACID The initial steps of ABA biosynthesis take place in chloroplasts or other plastids while final steps occur in cytosol. Violaxanthin is synthesized from zeaxanthin (also a 40 -C xanthophyll) in a reaction that is catalysed by the enzyme zeaxanthin epoxidase (ZEP). This enzyme is encoded by ABA 1 locus of Arabidopsis. Violaxanthin is converted into 9 ′-cis-neoxanthin. The latter is then cleaved into a 15-C compound xanthoxal (previously called xanthoxin) and a 25-C epoxy aldehyde in the presence of the enzyme 9 ′-cis- epoxycarotenoid dioxygenase (NCED). (This enzyme can also catalyse cleavage of violaxanthin into xanthox al and a 25-C allenic apoaldehyde). Xanthoxal (xanthoxin) is finally converted into ABA in cytosol via two oxidation steps catalysed by the enzymes aldehyde oxidases involving abscisyl aldehyde (and/or possibly xanthoxic acid) as intermediates. The enzymes aldehyde oxidases require Mo as cofactor. 6. Degradation/Inactivation of ABA in Plants: There are two ways in which ABA is degraded or inactivated in plants (Fig. 4). Oxidation to Phaseic Acid: This is the main route of degradation of ABA in plants. ABA is o xidised to phaseic acid (PA) with a subsequent reduction of the keto group on the cyclohexane ring to form dihydro phaseic acid (DPA). BOTANY: SEM – V, PAPE R-C12T: PLANT PHYSIOLOGY, UNIT -5: PLANT GROWTH REGULATORS – ABSCISIC ACID 4. In some cases, DPA may be further metabolized to form 4 ′-glucoside of DPA. Both dihydrophaseic acid and its glucosid e are metabolically inactive. Phaseic acid is also inactive or exhibits greatly reduced activity in most bioassays but not in all. Like ABA, phaseic acid (PA) can also induce stomatal closure in some species and is as effective in inhibiting GA induced synthesis of a-amylase in aleurone layers of germinating barley. Conjugation as Glucosides: Free ABA can also be inactivated in plants by covalent conjugation with some simple sugar molecule such as glucose to form ABA -p-D- glucosyl ester (ABA-GE). The latter accumulates in vacuole.
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