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ABSCISIC ACID

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M
MANDEEP KAUR

This document provides information about abscisic acid (ABA), including its chemical structure, biosynthesis, roles in plants, and research findings. Some key points: - ABA is a plant hormone involved in processes like seed dormancy, stomatal closure, and leaf senescence. It has a cis-trans isomer structure and exists primarily in the cis form. - ABA is biosynthesized through direct and indirect pathways, with the indirect pathway being more common in plants. This pathway involves carotenoid precursors that are cleaved to form ABA. - Research has examined ABA's role in regulating strawberry fruit development and ripening. Studies show ABA levels change over fruit growth stages and that

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Mandeep Kaur Ph.D. Fruit Science ABSCISIC ACID
Chemical name Abscisic acid; (2-cis,4-trans)-5-(1-Hydroxy-2,6,6-trimethyl- 4- oxo-2-cyclohexen-1-yl)-3-methyl-2,4-pentadienoic acid Synonyms ABA, Dormin & Absicin II Molecular formula C₁₅ H₂₀ O₄ Molecular weight 264.32 g Appearance White crystals Purity 98% Melting point 183-186 ⁰C Loss on drying < 0.5% INTRODUCTION TO ABSCISIC ACID
HISTORY AND DISCOVERY  Liu and Carns (1965) isolated a substance in crystalline form, from mature cotton fruit which stimulated abscission of deblated cotton petioles and it was called abscisin 1.  Okhuma and colleagues isolated another similar substance from young cotton fruit and termed it as abscisin 2.  Eagles and Wareing published a study on extraction of an inhibitor that accumulated in birch leaves (Betula pubescens, a deciduous plant) held under short day conditions they termed it as ‘dormin’.  Okhuma and colleagues (1965) proposed the chemical structure of abscisin 2.  Conforth and its colleagues isolated dormin in pure form from methaonilic extract of sycamore leaves.  Later to eliminate the confusion caused by different names of the same substance the principle scientist decided to term it as abscissic acid.
CHEMICAL STRUCTURE  ABA is a 15-C Sesquiterpene compound (molecular formula C₁₅ H₂₀ O₄).  Composed of three isoprene residues.  Having a cyclohexane ring with keto & one hydroxyl group & a side chain with a terminal carboxylic group.  ABA resembles terminal portion of some carotenoids such as violaxanthin & neoxanthin & appears to be a breakdown product of such carotenoids.
 Light isomerizes ABA to a mixture of the cis & trans structures & this photolytic conversion is thought to account for the presence of the inactive isomer in plant extracts.  The orientation of carboxylic group at carbon 2 determines ‘cis’ and ‘trans’ isomers of ABA.  Cis-Abscisic acid: Biologically active.  Trans-Abscisic acid: Biologically inactive.  Nearly all the naturally occurring ABA in plants exist in the cis form, that is biologically active & name abscisic is usually refers to this form. ‘CIS’ & ‘TRANS’ ISOMERS OF ABA
OCCURRENCE  Abscisic acid has been found to be a ubiquitous plant hormone within the plant.  Abscisic acid has been detected in every major organ or living tissue from root cap to apical part.  It is synthesized in almost all cells that contain chloroplast or amyloplast.  ABA is transported by both the xylem and the phloem, but it is normally much more abundant in the phloem sap.  ABA synthesized in the roots moves basipetally & can also be transported to the shoot via the xylem.  Redistribution of ABA among plant cell compartments is controlled by pH gradients. TRANSLOCATION
Fig. Redistribution of ABA in the leaf resulting the alkalization of the xylem sap during water stress
Detection & Estimation of ABA Bioassay methods Physico-chemical methods Immunoassay methods
ABA Biosynthesis
ABA Biosynthesis Direct pathway (From isopentenyl diphosphate (C5) via farnesyl pyrophosphate (C15) to ABA) Current evidence indicates that this pathway operates in fungi. Indirect pathway (In which ABA is a cleavage product of carotenoids)
INDIRECT PATHWAY
DIRECT PATHWAY  Initial stages occur in the plastids, where isopentenyl pyrophosphate (IPP) is converted to the C₄₀ Xanthophyll - zeaxanthin.  Zeaxanthin is further modified to 9-cis- neoxanthin, which is cleaved by the enzyme NCED (9-cis epoxycarotenoid dioxygenase) to form the C₁₅ inhibitor, xanthoxal (previously called xanthoxin).  Xanthoxal is finally converted to ABA in the cytosol via two oxidation steps catalyzed by the enzymes aldehyde oxidases involving xanthoxic acid as intermediates.  The enzymes aldehyde oxidases require Mo as cofactor.
DEGRADATION/ INACTIVATION OF ABA IN PLANTS
ABA signal perception & transduction
Fig : Simplified model for ABA signaling in stomatal guard cells SIGNAL PERCEPTION TRANSDUCTION AND REGULATION OF PHYSIOLOGICAL PROCESSES
Developmental and Physiological role of ABA
ABA PROMOTES STOMATAL CLOSING
ABA PREVENT WATER LOSS
ABA INDUCES SEED DORMANCY  Growth of the seed suspended, it is known as ‘dormant seed’.  It is controlled by the ratio of ABA to GA.  Embryo dormancy is due to the presence of inhibitors, especially ABA, as well as the absence of growth promoters, such as GA.  The loss of embryo dormancy is often associated with a sharp drop in the ratio of ABA to GA.
 An important function of ABA in the developing seed is to promote the acquisition of desiccation tolerance.  During the mid- to late stages of seed development, specific mRNAs accumulate in embryos at the time of high levels of endogenous ABA.  These mRNAs encode so-called late-embryogenesis-abundant (LEA) proteins thought to be involved in desiccation tolerance.  Synthesis of many LEA proteins, or related family members, can be induced by ABA treatment of either young embryos or vegetative tissues.  Thus the synthesis of most LEA proteins is under ABA control. ABA PROMOTES DESICCATION TOLERANCE IN THE EMBRYO
ABSCISIC ACID
 ABA inhibits the synthesis of hydrolytic enzymes that are essential for the breakdown of storage reserves in seeds.  For example, GA stimulates the aleurone layer of cereal grains to produce a-amylase and other hydrolytic enzymes that break down stored resources in the endosperm during germination.  ABA inhibits this GA-dependent enzyme synthesis by inhibiting the transcription of a- amylase mRNA.  ABA exerts this inhibitory effect via at least two mechanisms:  VP1, a protein originally identified as an activator of ABA-induced gene expression, acts as a transcriptional repressor of some GA-regulated genes (Hoecker et al. 1995).  ABA represses the GA-induced expression of GAMYB, a transcription factor that mediates the GA induction of α-induction of α-amylase expression (Gomez-Cadenas et al. 2001). ABA INHIBITS GA-INDUCED ENZYME PRODUCTION
 When immature embryos are removed from their seeds and placed in culture midway through development before the onset of dormancy, they germinate precociously—that is, without passing through the normal quiescent and/or dormant stage of development.  ABA added to the culture medium inhibits precocious germination. This result, in combination with the fact that the level of endogenous ABA is high during mid- to late seed development, suggests that ABA is the natural constraint that keeps developing embryos in their embryogenic state. ABA INHIBITS PRECOCIOUS GERMINATION AND VIVIPARY
 ABA has different effects on the growth of roots and shoots, and the effects are strongly dependent on the water status of the plant.  Maize :  Two types of seedlings were used a) wild-type seedlings with normal ABA levels b) an ABA-deficient, viviparous mutant  This suggests that a) Endogenous ABA promotes shoot growth in well watered plants. b) Endogenous ABA acts as a signal to reduce shoot growth only under water stressed conditions SHOOT AND ROOT GROWTH
LEAF SENESCENCE  The deteriorative processes that naturally terminate their functional life referred to as “senescence”.  Expression of senescence associated genes (SAGs) increases.  Hydrolytic enzymes- proteases, ribonucleases, lipases.  ABA involved in “senescence of leaves” but not the abscission of leaves.
Research Findings
Fig 1: A detailed description of different fruit developmental stages, which includes early growth and late ripening phases. The early phase, which is further divided into seven stages, S1–S7, is characterized by gradual increase in fruit (receptacle and achene) size, weight, and firmness. The ripening phase, divided into RS1–RS5. For each stage, the corresponding days after pollination (DAP) are indicated. Fig 2: Changes in ABA contents of fruit (receptacle and achene) in different developmental stages. *Source: www.pnas.org/cgi/doi/10.1073/pnas.1812575115 Interlinked regulatory loops of ABA catabolism and biosynthesis coordinate fruit growth and ripening in woodland strawberry (Liao et al., 2018)
Abscisic acid plays an important role in the regulation of strawberry fruit ripening (Jia et al., 2011) Fig 1: Seven developmental visual stages of the strawberry cv. Fugilia: small green (SG), big green (BG), degreening (DG), white (Wt), initial red (IR), partial red (PR), and full red (FR), at about 7, 14, 18, 21, 23, 25, and 28 days after anthesis, respectively. Fig 2: Effects of ABA, DMSO, and fluridone on strawberry fruit development. Fluridone at 50 µM, DMSO at 50 mM & ABA at 0.5 µM, was injected into 2-week-old BG fruits on alternate days for 6 days (three times) using a 0.2-mL syringe. Control fruits were injected with distilled water, and phenotypes were investigated 1 week after treatment. Twenty fruits still attached to plants were selected for each treatment (n = 20). *Source: www.plantphysiol.org/cgi/doi/10.1104/pp.111.177311
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ABSCISIC ACID

  • 1. Mandeep Kaur Ph.D. Fruit Science ABSCISIC ACID
  • 2. Chemical name Abscisic acid; (2-cis,4-trans)-5-(1-Hydroxy-2,6,6-trimethyl- 4- oxo-2-cyclohexen-1-yl)-3-methyl-2,4-pentadienoic acid Synonyms ABA, Dormin & Absicin II Molecular formula C₁₅ H₂₀ O₄ Molecular weight 264.32 g Appearance White crystals Purity 98% Melting point 183-186 ⁰C Loss on drying < 0.5% INTRODUCTION TO ABSCISIC ACID
  • 3. HISTORY AND DISCOVERY  Liu and Carns (1965) isolated a substance in crystalline form, from mature cotton fruit which stimulated abscission of deblated cotton petioles and it was called abscisin 1.  Okhuma and colleagues isolated another similar substance from young cotton fruit and termed it as abscisin 2.  Eagles and Wareing published a study on extraction of an inhibitor that accumulated in birch leaves (Betula pubescens, a deciduous plant) held under short day conditions they termed it as ‘dormin’.  Okhuma and colleagues (1965) proposed the chemical structure of abscisin 2.  Conforth and its colleagues isolated dormin in pure form from methaonilic extract of sycamore leaves.  Later to eliminate the confusion caused by different names of the same substance the principle scientist decided to term it as abscissic acid.
  • 4. CHEMICAL STRUCTURE  ABA is a 15-C Sesquiterpene compound (molecular formula C₁₅ H₂₀ O₄).  Composed of three isoprene residues.  Having a cyclohexane ring with keto & one hydroxyl group & a side chain with a terminal carboxylic group.  ABA resembles terminal portion of some carotenoids such as violaxanthin & neoxanthin & appears to be a breakdown product of such carotenoids.
  • 5.  Light isomerizes ABA to a mixture of the cis & trans structures & this photolytic conversion is thought to account for the presence of the inactive isomer in plant extracts.  The orientation of carboxylic group at carbon 2 determines ‘cis’ and ‘trans’ isomers of ABA.  Cis-Abscisic acid: Biologically active.  Trans-Abscisic acid: Biologically inactive.  Nearly all the naturally occurring ABA in plants exist in the cis form, that is biologically active & name abscisic is usually refers to this form. ‘CIS’ & ‘TRANS’ ISOMERS OF ABA
  • 6. OCCURRENCE  Abscisic acid has been found to be a ubiquitous plant hormone within the plant.  Abscisic acid has been detected in every major organ or living tissue from root cap to apical part.  It is synthesized in almost all cells that contain chloroplast or amyloplast.  ABA is transported by both the xylem and the phloem, but it is normally much more abundant in the phloem sap.  ABA synthesized in the roots moves basipetally & can also be transported to the shoot via the xylem.  Redistribution of ABA among plant cell compartments is controlled by pH gradients. TRANSLOCATION
  • 7. Fig. Redistribution of ABA in the leaf resulting the alkalization of the xylem sap during water stress
  • 8. Detection & Estimation of ABA Bioassay methods Physico-chemical methods Immunoassay methods
  • 10. ABA Biosynthesis Direct pathway (From isopentenyl diphosphate (C5) via farnesyl pyrophosphate (C15) to ABA) Current evidence indicates that this pathway operates in fungi. Indirect pathway (In which ABA is a cleavage product of carotenoids)
  • 12. DIRECT PATHWAY  Initial stages occur in the plastids, where isopentenyl pyrophosphate (IPP) is converted to the C₄₀ Xanthophyll - zeaxanthin.  Zeaxanthin is further modified to 9-cis- neoxanthin, which is cleaved by the enzyme NCED (9-cis epoxycarotenoid dioxygenase) to form the C₁₅ inhibitor, xanthoxal (previously called xanthoxin).  Xanthoxal is finally converted to ABA in the cytosol via two oxidation steps catalyzed by the enzymes aldehyde oxidases involving xanthoxic acid as intermediates.  The enzymes aldehyde oxidases require Mo as cofactor.
  • 15. Fig : Simplified model for ABA signaling in stomatal guard cells SIGNAL PERCEPTION TRANSDUCTION AND REGULATION OF PHYSIOLOGICAL PROCESSES
  • 19. ABA INDUCES SEED DORMANCY  Growth of the seed suspended, it is known as ‘dormant seed’.  It is controlled by the ratio of ABA to GA.  Embryo dormancy is due to the presence of inhibitors, especially ABA, as well as the absence of growth promoters, such as GA.  The loss of embryo dormancy is often associated with a sharp drop in the ratio of ABA to GA.
  • 20.  An important function of ABA in the developing seed is to promote the acquisition of desiccation tolerance.  During the mid- to late stages of seed development, specific mRNAs accumulate in embryos at the time of high levels of endogenous ABA.  These mRNAs encode so-called late-embryogenesis-abundant (LEA) proteins thought to be involved in desiccation tolerance.  Synthesis of many LEA proteins, or related family members, can be induced by ABA treatment of either young embryos or vegetative tissues.  Thus the synthesis of most LEA proteins is under ABA control. ABA PROMOTES DESICCATION TOLERANCE IN THE EMBRYO
  • 22.  ABA inhibits the synthesis of hydrolytic enzymes that are essential for the breakdown of storage reserves in seeds.  For example, GA stimulates the aleurone layer of cereal grains to produce a-amylase and other hydrolytic enzymes that break down stored resources in the endosperm during germination.  ABA inhibits this GA-dependent enzyme synthesis by inhibiting the transcription of a- amylase mRNA.  ABA exerts this inhibitory effect via at least two mechanisms:  VP1, a protein originally identified as an activator of ABA-induced gene expression, acts as a transcriptional repressor of some GA-regulated genes (Hoecker et al. 1995).  ABA represses the GA-induced expression of GAMYB, a transcription factor that mediates the GA induction of α-induction of α-amylase expression (Gomez-Cadenas et al. 2001). ABA INHIBITS GA-INDUCED ENZYME PRODUCTION
  • 23.  When immature embryos are removed from their seeds and placed in culture midway through development before the onset of dormancy, they germinate precociously—that is, without passing through the normal quiescent and/or dormant stage of development.  ABA added to the culture medium inhibits precocious germination. This result, in combination with the fact that the level of endogenous ABA is high during mid- to late seed development, suggests that ABA is the natural constraint that keeps developing embryos in their embryogenic state. ABA INHIBITS PRECOCIOUS GERMINATION AND VIVIPARY
  • 24.  ABA has different effects on the growth of roots and shoots, and the effects are strongly dependent on the water status of the plant.  Maize :  Two types of seedlings were used a) wild-type seedlings with normal ABA levels b) an ABA-deficient, viviparous mutant  This suggests that a) Endogenous ABA promotes shoot growth in well watered plants. b) Endogenous ABA acts as a signal to reduce shoot growth only under water stressed conditions SHOOT AND ROOT GROWTH
  • 25. LEAF SENESCENCE  The deteriorative processes that naturally terminate their functional life referred to as “senescence”.  Expression of senescence associated genes (SAGs) increases.  Hydrolytic enzymes- proteases, ribonucleases, lipases.  ABA involved in “senescence of leaves” but not the abscission of leaves.
  • 27. Fig 1: A detailed description of different fruit developmental stages, which includes early growth and late ripening phases. The early phase, which is further divided into seven stages, S1–S7, is characterized by gradual increase in fruit (receptacle and achene) size, weight, and firmness. The ripening phase, divided into RS1–RS5. For each stage, the corresponding days after pollination (DAP) are indicated. Fig 2: Changes in ABA contents of fruit (receptacle and achene) in different developmental stages. *Source: www.pnas.org/cgi/doi/10.1073/pnas.1812575115 Interlinked regulatory loops of ABA catabolism and biosynthesis coordinate fruit growth and ripening in woodland strawberry (Liao et al., 2018)
  • 28. Abscisic acid plays an important role in the regulation of strawberry fruit ripening (Jia et al., 2011) Fig 1: Seven developmental visual stages of the strawberry cv. Fugilia: small green (SG), big green (BG), degreening (DG), white (Wt), initial red (IR), partial red (PR), and full red (FR), at about 7, 14, 18, 21, 23, 25, and 28 days after anthesis, respectively. Fig 2: Effects of ABA, DMSO, and fluridone on strawberry fruit development. Fluridone at 50 µM, DMSO at 50 mM & ABA at 0.5 µM, was injected into 2-week-old BG fruits on alternate days for 6 days (three times) using a 0.2-mL syringe. Control fruits were injected with distilled water, and phenotypes were investigated 1 week after treatment. Twenty fruits still attached to plants were selected for each treatment (n = 20). *Source: www.plantphysiol.org/cgi/doi/10.1104/pp.111.177311