The role of Abscissic acid in
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
3-dimensional structure of ABA
• 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
– Coleoptile growth (looks for growth inhibition)
– Inhibition of germination
– Stomata closures
• Physical Method
– Gas chromatography
(More reliable than bioassays)
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
• Because it is synthesized partially in the chloroplasts, it
makes sense that biosyntheses primarily occurs in the
• 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
• ABA is capable of moving both up and down the stem
Function of ABA
• Prevent seed germination
• Inhibits shoot growth during
• Stimulate stomata closure (e.g. response
• Cause Abscission (buds, leaves, petals,
flowers, and fruits)
• 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
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
• 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.
• 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
• North Central Soil Conservation Research Laboratory
tested possibility of ABA in flowers of drought plants.
(Kay Walker-Simmons )
• ABA slows seed germination and improves wheat’s
tolerance to altering ABA
• Plants normally produce an enzyme that breaks down
• 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.
(Lopez-Molina and Sebastien)
• ABA activates a recently isolated Arabidopsis protein
• 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
(NCSC Research Laboratory)
• Dry condition during flower development causes large yield losses in
• 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
• 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.
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