The role of Abscissic acid in Water Stress by 761BJA

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									The role of Abscissic acid in
       Water Stress


        By: Sinthuja J
     Abscissic Acid (ABA)
• ABA is a single compound unlike the
  auxin, gibberellins, and cytokinins.
• It was called “abscisin II” originally
  because it was thought to play a major
  role in abscission of fruits.
• It is a ‘stress hormone’ involved in plant
  response to environmental stress.
               History of ABA
• ABA was discovered in 1963, by group of
    scientists working in 3 countries.
•   One of theses groups headed by Philip Wareing
    was working on bud dormancy in maple trees.
    They called the substance that promotes bud
    dormancy as dormin.
•   The Other two groups were working on
    abscission. One group led by Van Steveninck
    and the Other group let by Frederic Addicott.
•   These groups agreed to call the compound
    Abscissic Acid.
3-dimensional structure of ABA
                Chemical Structure




• ABA is a 15-carbon terpenoid compund derived from the
    terminal portion of carotenoids.
•   It has 15 carbon atoms, an aliphatic ring with one
    double bond, 3 methyl groups and an unsaturated chain
    that has a terminal carboxyl group.
         Measurement of ABA
• Bioassays
  – Coleoptile growth (looks for growth inhibition)
  – Inhibition of germination
  – Stomata closures
• Physical Method
  – Gas chromatography
  – HPLC
  (More reliable than bioassays)
• Immunoassay
           Biosynthesis and Metabolism
• ABA is a naturally occurring compounds in plants. It is
    synthesized in almost all cells.
•   The initial steps take place in chloroplasts and other
    plastids.
•   Because it is synthesized partially in the chloroplasts, it
    makes sense that biosyntheses primarily occurs in the
    leaves.
•   The production of ABA levels rise in response to stress
    such as water loss and freezing temperature.
•   It is believed that biosynthesis occurs indirectly
    through the production of carotenoids.
•   ABA transport occur in both xylem & phloem and there
    is no polarity. It can also be translocated through
    paranchyma cells.
•   ABA is capable of moving both up and down the stem
           Function of ABA

• Prevent seed germination
• Inhibits shoot growth during
  environmental stress
• Stimulate stomata closure (e.g. response
  to water)
• Cause Abscission (buds, leaves, petals,
  flowers, and fruits)
                     Water Stress
• When there is a Water Stress

  -The initial responses of plants
          – Stomata close, reduction in growth, decrease photosynthesis
            and vacuole shrinks


  -If stress continue
        - decrease protein synthesis, limit size and number of
          leaves.
      Mechanism of response

• Early warning system
  – Roots produce ABA, partial closing of stomata
• Accumulation of ABA in leaves
  – Stomates close
• Decreased photosynthesis
  – Lowers chloroplasts pH.
Areas of Extended Water Stress

• Deserts

• Physiological drought

• Exposed rock surfaces
• ABA level of leaves rise 50-fold in response to water
  stress, inducing rapid stomatal closure

• Stomates remain closed until ABA levels drop again.

• ABA production increase in root in respond to dry
  condition. It is transported to the shoot via the xylem

• Under normal conditions the xylem sap is slightly acidic,
  favoring the uptake of ABA by the mesophyll cells.

• During Water stress, the xylem sap becomes slightly
  alkaline, favoring the dissociation of ABA to ABA-.
      -As a result, less ABA is taken up by the mesophyll
        cells and more reaches the guard cells.
              Recent advances
• ARS Plant physiologist Kay Walker-Simmons and
  Research Council of Canada, discovered features of the
  ABA molecule that affect wheat and barley plants.
• Lopez-Molina and Sebastien, Ph.D., both postdoctoral
  fellows at Rockefeller University showed that ABA inhibit
  growth of newly germinated Arabidopsis plants for up to
  30-days.
• North Central Soil Conservation Research Laboratory
  tested possibility of ABA in flowers of drought plants.
              Recent advances
           (Kay Walker-Simmons )
• ABA slows seed germination and improves wheat’s
    tolerance to altering ABA
•   Plants normally produce an enzyme that breaks down
    ABA.
•   Abrams modified the part of the ABA molecule that’s
    broken down by the enzyme.
•   Then she and Simmons demonstrated that chemically
    blocking the enzyme allows ABA to stay active longer.
•   This would provide the extra delay needed to improve
    wheat survival during dry conditions.
            Recent advances
      (Lopez-Molina and Sebastien)
• ABA activates a recently isolated Arabidopsis protein
    called ABI5.
•   This protein is essential to the newborn plant’s ability to
    protect itself against dry condition.
•   Strains of Arabidopsis was genetically engineered to
    produce an excess of the protein
•   Plants overproducing ABI5 were found to loses water
    less rapidly only when ABA was present.
•   Therefore the researchers concluded, ABA must Activate
    ABI5.
             Recent advances
        (NCSC Research Laboratory)
• Dry condition during flower development causes large yield losses in
    wheat.
•   Under dry conditions, pollen in wheat flowers becomes sterile so the
    fertilization does not occur
•   The stress level and ABA content of flower parts in wheat plants
    exposed to water stress during the early stages of flower
    development was measured.
•   Flower parts containing the pollen are far less stressed by lack of
    water than are leaves and remain turgid even when leaves are
    wilted.
•   By keeping the leaves turgid as the soil dried, the leaves, rather
    than roots or floral tissues, are the likely source of ABA that
    accumulates in the flowers during drought.
•   These results suggest a factor other than ABA causes pollen sterility
    in water-stressed wheat.
Potential relevance to Biotechnology

• Seed dormancy
• Water stress
  – By controlling ABA level
  – e.g if the weather dries up seedlings could
    die. If the seeds waited until more water was
    available before germinating, they would be
    less likely to suffer from environmental stress.
            Farmers Fortune
Every Fall
      they plant their seeds
The nature rolls the die
      Will it rain enough to support
Is the hope
      hopes are on scientist who may one day
Reduce the risks of Water stress

								
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