ABA is a plant hormone involved in processes like seed dormancy and germination, as well as responses to environmental stresses. It was first isolated in 1965 from cotton fruit and given different names before being termed abscisic acid. ABA is a sesquiterpene compound composed of three isoprene residues and a cyclohexane ring. It is synthesized from carotenoids in chloroplasts and transported throughout the plant. ABA signaling involves membrane and cytosolic receptors that regulate ion transport and gene expression changes related to closing stomata and inhibiting growth under drought conditions.

1. ABSCISIC ACID
(ABA)

2. 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 abscisin1

3. Okhuma and calleagues isolated another similar
substance from young cotton fruit and termed at 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.
Okuma and colleagues (1955) 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 COMPOSITION
ABA is a 15-C Sesquiterpene
compound
Composed of three isoprene
residues
Cyclohexane ring with keto, one hydroxyl
group, a side chain with a terminal
carboxylic group

6.  The orientation of carboxylic
group at carbon 2 determines
the cis and trans isomers of
ABA
 Cis-Abscisic acid (biologically
active)
 Trans-Abscisic acid (biologically
inactive) Nearly all the naturally
occurring ABA is in the cis form

7. OCCURRENCE AND DISTRIBUTION
 ABA is a ubiquitous plant hormone in vascular plants
 In bryophytes it has been found in mosses but not in liverworts.
 Some fungi synthesize ABA as secondary metabolite A 15-C compound called
lunularic acid has been found in algae and liverworts
 ABA is synthesized in all types of cells that contain chloroplasts or other
plastids It occurs predominantly in mature green leaves
 ABA has been detected in all major organs or living tissues from root caps to
Apical buds
 Phloem sap, xylem sap and in nectar

8. ABA TRANSPORT IN PLANT
 Externally applied ABA-distributed in all directions
 Cell to cell transport is slow
 ABA synthesized in root cap transported to central vascular tissue
 Transported mostly in its free form
 Transported in a conjugated form as ABA-B-D-glucosyl ester
 Redistribution of ABA-pH gradient
 At low pH protonated or un-dissociated form (ABAH)
 At high pH dissociated form ABA

10. Initial stages occur in the plastids, where
isopentenyl pyrophosphate (IPP) is converted
to the Cao 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 catalysed by
the enzymes aldehyde oxidases involving
xanthoxic acid as intermediates.
The enzymes aldehyde
oxidases require Mo as
cofactor

11. Biosynthesis of
ABA

12. ABA
PH
(Ca2+)
K+
H+ Pump
K+ Out
Prevent
Stomatal Opening
Prevent
Stomatal Closing
ABA Signalling in stomatal
Guard Cell
ROS pathway
IP3, cADPR
pathways

13. Blue light stimulates hydrogen ion and ATP Proton pump providing the membrane potential
needed for guard cell to import potassium ion and sugar as a result water enter by Osmosis by
aquaporin channel the water soil in guard cell separate opening the pores

14. At night or under condition of water stress that stimulate ab production potassium ion other
solid exit guard cells. this cause water to leave deflating guard cells and closing the pore

16. ABA SIGNAL
TRANSDUCTION PATHWAY
Abscisic acid elicits two response
1. Rapid response
2. Gradual responses
These fast and slow responses use different receptors
There are multiple ABA receptors which are soluble and membrane bound
Three classes of ABA receptors
1. Plasma membrane localized (G proteins)
2. Plastid localized enzyme
3. Cytosolic ligand binding proteins (START domain superfamily)

17.  Soluble START proteins are principle ABA receptors that function in stomatal
Closing and germination
 First experiment perform in Arabidopsis
 PY (pyrabactin) is a synthetic compound similar to ABA
 Genes conferred insensitivity to PY cloned called PYR1
 In Arabidopsis PYR and PYL are START domain proteins and ABA dependent
inhibitors of one class protein phosphatase type 2(PP2C)
 ABI1 and ABI2 member of PP2C

19. DEVELOPMENTAL
AND
PHYSIOLOGICAL
ROLE OF ABA

20.  Inhibit stomatal opening.
 Stress hormones
 Root and shoot growth-ABA Promotes Root Growth and Inhibits Shoot Growth at
Low Water Potentials
 Dormancy and germination (inhibit GA)
 Hydraulic conductivity
 ABA Promotes Leaf Senescence independent of ethylene.
 ABA Accumulates in Dormant Buds
 ABA Promotes Desiccation Tolerance in the Embryo 9. ABA exhibit vivipary-the precocious germination of
seeds in the fruit while still
 Attached to the plant. Vivipary is a feature of many ABA-deficient seeds.
Function