Abscisic Acid

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Abscisic Acid

• James Jordan
• April 3 2002

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History And Discovery of Abscisic Acid

• Abscisic acid or ABA was found identified 1953 by
  Bennet-Clark and Kefford as an inhibitory
  compound.
• In 1963 the substances was crystallized and
  purified. It was then named Abscisic Acid by
  Ohkuma.

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Biosynthesis

• The biosynthesis of ABA involves 5 steps
• 1) Break down of Violaxanthin which is a
  carotenoid.
• 2) It is isomerized and then split via an
  isomerase reaction followed by an oxidation
  reaction

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Biosynthesis cont

• 3) One molecule of xanthoxin is produced from one
  molecule of violaxanthin.
• 4) The one molecule of xanthoxin produced is
  unstable and spontaneously changed to ABA
  aldehyde

• 5) More oxidation forms the ABA.
• The molecule is then activated in two ways
• 1) The attachment of a glucose molecule to ABA
  can form an ABA-glucose ester

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Biosynthesis cont

• Or
• 2) More oxidation can occur causing ABA to form
  phaseic acid and dihydrophaseic acid.
• The transport of ABA can occur in both xylem and
  phloem tissues. It is not transported polar.

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Functions of ABA

• 4. Inhibits the affect of gibberellins on
  stimulating synthesis of a-amylase.
• 5. Has some effect on induction and maintenance
  of dormancy.
• 6. Induces gene transcription especially for
  proteinase inhibitors in response to wounding
  which may explain an apparent role in pathogen
  defense

• ABA can alter many pathways in plants.
• Stimulates the closure of stomata
• Inhibits shoot growth.
• 3. Induces seeds to synthesize storage proteins.

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Seed Dormancy

• There are five factors that effect coat imposed
  dormancy
• Prevention of water uptake
• Mechanical constraint,
• Interference with gas exchange,
• Retention of inhibitors
• Inhibitor production.

• There are 2 types of seed dormancy coat imposed
  dormancy and embryo dormancy.
• Coat dormancy is imposed on the embryo from its
  surrounding tissues.

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Seed Dormancy cont

• The GA is needed to break down the storage
  reserves for food.

• Embryo dormancy is due to the presence of
  inhibitors such as ABA.
• Also due to the absence of Gibberellins.
• Loss of this dormancy is given by a drop in the
  ABAGA ratio.
• This loss of dormancy is caused by a increase in
  the amount of GA available to the seed.

• ABA promotes and regulates the storage of
  proteins in seeds.
• ABA can also keep the mature embryo in a dormant
  state.

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How ABA is Measured

• ABA is measured in a variety of different ways.
• Coleoptile growth.
• Inhibition of germination.
• Inhibition of amylase.
• Stomata closure.
• GC and mass spec.

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Recent Advances

• Mass conversion of ABA to phaseic acid via
  8'-hydroxy ABA.
• This is done by clone the gene that codes for ABA
  8'-hydroxylase an enzyme that regulates ABA
  catabolism

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Bio tech

• The cloning of the ABA 8'-hydroxylase gene will
  cause plants to improve productivity, performance
  and plant architecture