Abscisic acid (ABA) is a plant hormone that controls various processes in response to environmental stresses like drought. It is produced in leaves and fruits, especially under stress conditions. ABA promotes stomatal closure, abscission of leaves and fruits, bud and seed dormancy, and inhibits shoot growth under water stress. It was first identified and characterized in 1963 from studies on cotton fruit abscission. ABA is synthesized in chloroplast-containing cells and transported throughout the plant.

1. ABSCISIC ACID
(ABA)
DR. R. RENUKA, M.SC.,M.PHIL.,PH.D.
ASSOCIATE PROFESSOR OF BIOCHEMISTRY
V.V.VANNIAPERUMAL COLLEGE FOR WOMEN
VIRUDHUNAGAR, TAMILNAD, INDIA.

2. Abscisic acid (ABA)
• ABA is a natural plant growth inhibitor produced by aged
leaves and fruits and also when plants are under stress
condition. So, it is also called as Stress Hormone.

3. Abscisic acid (ABA)
• Abscisic acid (ABA) is the major phytohormone that controls plant’s
ability to survive in harsh, changing environment.
• ABA promotes abscission of leaves and fruits; hence it is this action that
gave rise to the name of this hormone ‘abscisic acid’.
• ABA is a naturally occurring compound in plants. ABA is found in leaves
(where it is partially synthesized), stems, and green fruits.
• It is generally associated with negative-feedback interactions or stress-
related environmental signals such as drought, freezing temperatures
and environmental pollutants

4. DISCOVERY OF ABA
• In 1963, abscisic acid was first identified and characterized by Frederick Addicott
and his associates in University of California. They were studying compounds
responsible for the abscission of cotton fruits. Two compounds were isolated and
called abscisin I and abscisin II.
• Two other groups in UK at about the same time discovered the same compound.
One group headed by Philip Wareing was studying bud dormancy in woody plants
and named the substance responsible for bud dormancy as ‘Dormin’ The other
group led by Van Steveninck was studying abscission of flowers and fruits from
lupine.
• Each group found their compounds within a few days of each other. When they
compared notes, they found they had isolated the same compound.
• Eventually, they decided to fix on the name abscisic acid.

5. CHEMICAL STRUCTURE OF ABA
• ABA is 15-C sesquiterpene compound; Mol. Formula is C15H20O4.
• Composed of three isoprene residues
• Having cyclohexane ring with keto and one hydroxyl group and a side chain
with a terminal COOH group.

6. Site of synthesis of ABA
ABA is an ubiquitous plant hormone in vascular plants
It is synthesized in all types of cells that contain chloroplast or other
plastids.
It occurs predominantly in mature leaves.
It has been detected in all major organic or living tissues from root
cap to apical buds..

7. Transport of ABA in plants
• Externally applied ABA can be distributed in all directions.
• Synthesized ABA is exported from roots in the xylem and from leaves
in the phloem.
• Cell to cell transport is slow.
• Transport is mostly in its free form and rarely in a conjugated form as
ABA-ß-D-glucosyl ester.
• Redistribution of ABA among plant cell compartments is controlled
by pH gradients.

8. Physiological role of ABA
Stimulates the closure of stomata.
used as anti - transpirant - In drought prone areas , water stress is serious problem in
agriculture production. so sprays of ABA are suggested that cause partial closure of
stomata for few days , to reduce transpirational loss of water
Induce bud dormancy and seed dormancy.
Delays fruit ripening and softening.
Promotes root growth and inhibits shoot growth in water stress conditions.
 Downregulates enzymes needed for photosynthesis.
Induces seeds to synthesize storage proteins.
Inhibits (counteracts) the effect of gibberellins on stimulating de novo synthesis of
𝛾-amylase.
Inhibit cell division
Promotes leaf senescence
Increase in ABA in response to wounding Induces gene transcription especially for
proteinase inhibitors, so it may be involved in defense against insects and pathogens.

9. Closing of stomata
• Stomatal closure decreasing transpiration and prevent water loss.

10. Abscission
• ABA promotes abscission of leaves and fruits (in contrast to auxin, which
inhibits abscission). It is, in fact, this action that gave rise to the name
abscisic acid.

11. Apical dominance

12. Bud dormancy

13. Seed dormancy
• The shoot buds of temperate tree species become dormant in
autumn season. Similarly, freshly harvested seeds of apple and peach
do not germinate. The potato tubers fail to sprout soon after
harvesting. All these are caused by accumulation of large amount of
ABA in these organs. The concentration of ABA decreases and
concentration of gibberellins increase in spring or after the rain as
the ABA is metabolized away with water. Therefore, shoot buds
sprout and the seeds start germinating in spring. By this way, ABA
prevent the premature germination and helps the plant make sure
that it’s seeds germinate in the right time of the year.

14. Inhibits precocious germination and
vivipary
• During seed development, embryos of monocot and dicot plants
may fail to complete maturation and germinate while the immature
seed is still attached to the maternal tissue. This phenomenon is
referred to as vivipary or preharvest or precocious sprouting.
• ABA inhibits precocious germination.
• The level of endogenous ABA is high during mid- to late seed
development which suggested that ABA is the natural constraint
that keeps developing embryos in their embryogenic state.
• Vivipary in ABA deficient mutants can be partially prevented by
treatment with exogenous ABA.

15. Promotes desiccation tolerance in the
embryo
• These proteins prevent the seeds from drying out.

16. Effect of Drought
• A large quantity of ABA accumulates in plant during
drought or water deficiency. It has a biological
advantage for the plant. ABA closes the stomata. It
prevents further loss of water,. Thus plant is able to
conserve available moisture and withstand the
drought.