1. Abscisic Acid and Its Role in Seed Dormancy
By: Laura Kenney

2. Introduction
ABA is a plant hormone in vascular plants and is also found in mosses and fungi
In plants ABA has been detected in every major organ or living tissue from the root cap to the apical bud and is synthesized in cells containing chloroplasts or amyloplasts

3. Discovery
In 1963 a substance that promotes the abscission of cotton fruits was identified and named abscisin II
At the same time a substance that promotes bud dormancy was purified from sycamore leaves and called dormin
Dormin and abscisin II are chemically identical and were renamed abscisic acid

4. Structure
ABA is a 15 carbon terpenoid compound derived from carotenoids
Naturally occurring ABA is in the cis form
The S enantiomer is involved in fast responses and both enantiomers are involved in long term responses

5. Functions Regulates growth and stomate opening Stress hormone
Regulates seed dormancy
Regulates bud dormancy
Acts as an antagonist with auxin, cytokinin, and gibberellins

6. Seed Dormancy
Enters a temporal delay in the germination process to give time for greater seed dispersal
Maximizes seedling survival by preventing germination under unfavorable conditions

7. Coat-imposed Seed Dormancy
Dormancy imposed on the embryo by the seed coat and other enclosing structures
Germination occurs readily in the presence of water and oxygen once the seed coat and other enclosing tissues have been damaged or removed

8. Mechanisms for Coat-imposed Dormancy
Prevention of water uptake
Seed coat is too tough for the radicle to break through
Limit oxygen supply to the embryo
Prevents the release of germination inhibitors from the seed
Contains growth inhibitors such as ABA

9. Embryo Dormancy
Dormancy that is inherent to the embryo and is not caused by the seed coat or surrounding tissues
Caused by the presence of growth inhibitors (ABA) and the absence of growth promoters (GA)
Seed dormancy is controlled by the ratio of ABA to GA

10. Primary and Induced Dormancy
Primary dormancy refers to seeds that are released from the plant in a dormant state
Induced dormancy refers to seeds that are initially released from a plant in a non-dormant state but are induced to go dormant if the conditions are unfavorable for germination

11. Loss of Dormancy
External factors such as dehydration, light, and cold can break seed dormancy

12. ABA and Seed Dormancy ABA added to culture medium prevents germination
ABA inhibits the synthesis of hydrolytic enzymes that are required for the breakdown of storage reserves in seeds
ABA deficient mutants are non-dormant at maturity

13. Measuring ABA
ABA is measured using gas chromatography, HPLC, and immunoassays

14. Recent Advances
Lopez-Molina et al found that the basic leucine zipper transcription factor ABI5 causes an enhanced response to exogenous ABA during germination, seedling growth and vegetative growth.
ABA regulates ABI5 accumulation and activity.
ABA delays germination and prevents vegetative growth by arresting development of mature germinated embryos.

15. Procedure
RNA extraction, Western blots, Northern blots and mutants were used to analyze the seeds and seedlings of Arabidopsis

16. Results
ABI5 transcript was undetectable in the absence of ABA but was induced after one day in the presence of ABA
ABA also induced accumulation of ABI5
In the absence of ABA, ABI5 was absent
Therefore, ABI5 is induced by ABA

17. Results
ABA treatment prevented the decrease in ABI5 level suggesting that ABA prevents ABI5 degradation
On removal of ABA, ABI5 is rapidly degraded
ABA applied to seeds within 60h poststratification maintained the germinated embryos in a arrested state
ABA applied outside the time frame failed to arrest growth

18. Results
ABA applied to seedlings allowed germination after 8 days but blocked further growth for at least a month
Once the ABA was removed the embryos resumed normal growth

19. Experimental Conclusions
Arabidopsis plants that have already germinated are still able to arrest growth if external environmental conditions are unfavorable. ABA and ABI5 are key players in this process.
ABA is more efficient as a early growth inhibitor than a germination inhibitor.
It is unclear whether ABI5 has a direct or indirect effect on cell cycle machinery when arresting growth upon ABA treatment.

20. Conclusion ABA is the main hormone involved in seed dormancy
Seed dormancy is necessary to ensure the survival of seedlings and to maximize seed dispersal