Abscisic Acid

• James Jordan• April 3 2002

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.

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

Biosynthesis con’t• 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

Biosynthesis con’t

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.

Functions of ABA ABA can alter many

pathways in plants.

1. Stimulates the closure of stomata

2. Inhibits shoot growth.

3. Induces seeds to synthesize storage proteins.

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

Seed Dormancy

• There are 2 types of seed dormancy: coat imposed dormancy and embryo dormancy.

• Coat dormancy is imposed on the embryo from its surrounding tissues.

• There are five factors that effect coat imposed dormancy:

• Prevention of water uptake

• Mechanical constraint, • Interference with gas

exchange, • Retention of inhibitors• Inhibitor production.

Seed Dormancy con’t• 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 ABA:GA ratio.

• This loss of dormancy is caused by a increase in the amount of GA available to the seed.

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

•ABA promotes and regulates the storage of proteins in seeds.

• ABA can also keep the mature embryo in a dormant state.

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.

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

•

Bio tech

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