Abscisic Acid and Its Role in Seed Dormancy

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Abscisic Acid and Its Role in Seed Dormancy

• By Laura Kenney

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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

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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

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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

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Functions

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

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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

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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

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Mechanisms for Coat-imposed Dormancy

1. Prevention of water uptake
2. Seed coat is too tough for the radicle to break
   through
3. Limit oxygen supply to the embryo
4. Prevents the release of germination inhibitors
   from the seed
5. Contains growth inhibitors such as ABA

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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

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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

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Loss of Dormancy

• External factors such as dehydration, light, and
  cold can break seed dormancy

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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

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Measuring ABA

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

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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.

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Procedure

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

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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

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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

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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

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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.

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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