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 Physical Method Immunoassay 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 -If stress continue -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