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

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

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

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

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

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

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

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

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

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

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

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

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

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.

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

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

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

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

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.

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