physiology

2. ABSCISC ACID
(ABA)
(Growth Inhibitor)

3. Discovery
A strongly growth inhibiter substance
 F. T. Addicott
1963
 Young cotton fruit
 Named as Abcision II
 Name changed to Ascisic Acid

4.  Eagles and Wareing
1963-64
 Birch leaves (Betula pubescence L.)
 Named as “Dormin”
 Cornforth et al., 1965
 Abscisic Acid

5. Chemical Nature
• 15-C sesquiterpene compound
• Molecular formula (C15H20O4)
• Cyclohexane ring
• Resembles with violaxanthin and neoxanthin (carotenoids)
• Cis and trans isomeric form
• Cis ABA are biologically active

6. Occurrence and distribution of ABA
in plants
Occurrence
• Vascular plants
• Bryophytes (Mosses)
• Fungi
• Algae (ABA activity like Lunularic acid)

7. Distribution
• Roots
• Stems
• Buds
• Leaves
• Fruits
• Seeds
• Nectars

8. ABA found in plants parts in different
conc.
• 20-200 ng/f. wt.
• 10-20 micro g (higher conc.) in avocado fruit
• 100 time more ABA in developing seeds
• Decreased in mature seeds
• 50 time more in water stress in leaves

9. ABA conc. is regulated by
 Its synthesis
 Degradation
 Compartmentation
 Transport
ABA found in two form
 Free form
 Conjugated form e.g. ABA-B-D-glucosyl ester

10. ABA transport in plant
ABA transport take place in plants as
• Externally applied ABA get into every tissues
• Cell to cell transport is very low
• Non-polar
• Through xylem and phloem
• From root as basipetally (base to tip)
Movement of ABA in plant depend on pH gradient
• At low pH (6.3) ABA in protonated form (ABAH) readily absorbed
• At high pH (7.2) ABA in Dissociated form (ABA-) not readily
absorbed
NOTE: Mostly ABA transported in free form in plants

11. Detection and Estimation of ABA
1. Bioassay Methods
• Stimulation of stomatal closure
• Inhibition of wheat germination
• Inhibition of GA-induced alpha-amylase

12. 2. Physico-chemical Methods
• HPLC
• GC
• MS
NOTE: Using such method even10-13g ABA in plant can be
detected

13. Physiological effects of ABA
1. Stomatal closing
Opening and closure of the stomatal pore is
mediated by changes in the turgor pressure of the two guard
cells. The turgor pressure of guard cells is controlled by
movements of large quantities of ions and sugars into and
out of the guard cells.

14. 2. Inducing bud dormancy
ABA from leaves can have a hormonal role in the
induction of bud dormancy. In dormancy induction, additional
factors are necessary for the development of the typical
resting bud. ABA could also be involved in the maintenance
of bud dormancy.

15. 3. Desiccation tolerance in seed embryo
ABA have role in desiccation tolerance in seed embryo

16. 4.Inducing seed dormancy
During seed maturation, ABA levels increase
dramatically. This inhibits germination and turns on
the production of proteins that enable the embryo to
survive dehydration during seed maturation. The
breaking of dormancy is associated with a decline in
the level of ABA.

17. 5. Tuberization
• A stem tuber forms from thickened rhizomes or stolons. The top
sides of the tuber produce shoots that grow into typical stems and
leaves and the under sides produce roots. They tend to form at the
sides of the parent plant and are most often located near the soil
surface.

18. 6.Senescence of leaves
As in the final stage of fruit ripening, ABA
accelerates the biochemical changes characteristic of the process.
7.Abscission of leaves
ABA is a plant growth regulator which is involved in abscission
of buds, leaves, and flowers in different plants.

19. 8.Fruit ripening
ABA coming from the plastids triggers ethylene
stimulation and promotes the metabolism of fruit
ripening.

20. 9.Resistance to frost injury
Injuries to plants caused by low
temperatures (chilling temperatures) well above
their freezing point are called as chilling
injuries while injury caused by freezing
temperatures (below the freezing point of
water) that results in ice formation in plant
tissues is called as freezing or frost injury.

21. 10.Falling of fruit
ABA trigger falling of fruit.

22. 11.Inhibition of GA-induced α-amylase
 ABA inhibit GA-depending α-amylase synthesis by
inhibiting transcription of alpha-amylase synthesis by
inhibiting the transcription of α-amylase mRNA

23. 12. Increasing root : shoot ratio
 ABA decrease shoot growth
 Shoot length
 Shoot braches
 ABA increase root growth
 Root branching
 Root hair formation

24. 13.Inhibition of germination in vivipary
Vivipary is the condition whereby the embryo (the young
plant within the seed) grows first to break through the seed coat
then out of the fruit wall while still attached to the parent plant.
A
B
A
Vivipary

25. • Factors stimulation ABA synthesis
 Heat stress
Increase in temperature
 Water stress
Less or higher availability of water
 Salt stress
Addition of salt in soil or water

26. Heat Stress and ABA
synthesis
ABA produced under heat
stress for heat tolerance in
plants.

27. Water Stress and ABA synthesis
ABA produced under water stress in plants
 Drought stress/tolerance

28. Biosynthesis of ABA in Plants
Biosynthesis of ABA take place in
• Initial steps in plastid (chloroplast)
• Final steps in cytosol

30. Degradation/inactivation of ABA in
plants
Two ways
1. Oxidation
 ABA oxidized to phaseic acid (PA)
 Phaseic acid to dihydro-phaseic acid (DPA)
2. Conjugation as Glucosides
 1st free ABA inactivated
 2nd inactivated conjugate with glucose
 3rd conjugate to form ABA-beta-glucosyl ester (ABA-GE)
 ABA-GE accumulate in vacuole