Transcript Document

Abscisic Acid and Its
Role in Seed Dormancy
By: Laura Kenney
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Introduction
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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
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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
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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)
2)
3)
4)
5)
Regulates growth and stomate
opening
Stress hormone
Regulates seed dormancy
Regulates bud dormancy
Acts as an antagonist with auxin,
cytokinin, and gibberellins
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Seed Dormancy
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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
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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 Coatimposed Dormancy
1)
2)
3)
4)
5)
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
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Embryo Dormancy
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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
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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
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External factors such as
dehydration, light, and cold can
break seed dormancy
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ABA and Seed
Dormancy
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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 nondormant at maturity
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Measuring ABA
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ABA is measured using gas
chromatography, HPLC, and
immunoassays
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Recent Advances
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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
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RNA extraction, Western blots,
Northern blots and mutants were
used to analyze the seeds and
seedlings of Arabidopsis
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Results
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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
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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
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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
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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
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ABA is the main hormone involved
in seed dormancy
Seed dormancy is necessary to
ensure the survival of seedlings
and to maximize seed dispersal
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