Abscissic Acid (ABA)
Abscissic acid (ABA) is a ubiquitous plant hormone that inhibits growth of vascular plants. It has been detected in mosses but appears to be absent in liverworts. Several fungal genera make abscissic acid as a secondary metabolite. In algae and liverworts, a compound similar to abscissic acid called lunularic acid appears to play physiologic role similar to that of abscissic acid. A major function of abscissic acid in plants is probably to cause stomata’s to close when water stress occurs or when carbon dioxide level increases in guard cells
History of Discovery
For many years, plant physiocologists suggested that phenomenon of seed or bud dormancy is caused by some inhibitory compound. Torsen Hemberg in 1949 first obtained evidence that bud dormancy might be due to a growth inhibitor. He found that dormant buds of potato tubers and ash trees contained growth inhibitors and level of these inhibitors decreased when dormancy was broken. In 1950s many compounds different from auxin and inhibiting growth was separated from different plant extracts. These compounds form an inhibitory zone on paper chromatograms and were named beta-inhibitors.
At the same time a substance that promoted bud dormancy was purified from sycamore leaves and was called dormin. When dormin was chemically identified, it was found to be identical to abscisin II. The compound was renamed as abscissic acid because of its involvement in abscission process. For many years ABA was considered to be responsible for abscission but later, ethylene was found to be major hormone causing abscission.
Chemical Nature of Abscissic Acid
Chemically, abscissic acid resembles the terminal portion of some carotenoid molecules. The 15 carbon atoms of abscissic acid configurate an aliphatic ring with one double bond, two methyl groups and an unsaturated chain with a terminal carboxyl group. ABA exists in cis- and trans- isomers and the orientation of carboxyl group at C-2 determine the cis-and trans- form. Nearly all the naturally occurring ABA is in cis- form.
Biosynthesis & Distribution of Abscissic Acid
The ABA is synthesized in almost all cells containing chloroplasts or amyloplasts. Within plants, ABA has been detected in every major organ or living tissue from the root cap to the apical bud.
The first involves mevalonic acid, which is precursor of all terpenoids in plants. This pathway is called direct pathway. During this pathway the 15-carbon ABA molecule is formed from mevalonic acid and a 15-carbon precursor, farnesyl pyrophosphate.
The second pathway known as indirect pathway involves formation of ABA by cleavage of a 40-carbon carotenoid. The indirect pathways involved formation of xanthoxin, which is converted to ABA to supplied the shoots. It is breakdown product of carotenoid such as violaxanthin.
Transport of Abscissic Acid
ABA is transported by both xylem and phloem but is much more abundant in phloem sap. It also carried by parenchyma cells present outside vascular bundles. When radioactive ABA is applied to a leaf, it is transported both up the stem and down towards the roots. Most of the radioactive ABA is found in the roots within 24 hours. If phloem is destroyed by a stem girdle, ABA accumulation in root is prevented, indicating that hormone is transported in the phloem sap.
It is also found that ABA concentration increases in chloroplasts during light and during dark it is higher in apoplast. This distribution of ABA, suggests its role in the regulation of stomatal apertures.
Physiological Effects of Abscissic Acid
Abscissic acid causes many physiological responses in higher plants. These include:
In woody species, low temperature induces dormancy in buds and these stop growth temporarily. P. Warening and his colleagues in 1964 suggested that ABA induces dormancy. They found that if ABA is directly applied to the non-dormant buds, their growth is inhibited.
Dry dormant seeds usually contain higher ABA levels than non-dormant seeds. The role of ABA in seed dormancy is also indicated by applying ABA exogenously. It induces dormancy, for example lettuce seeds which require red light to germinate when illuminated in the presence of ABA do not germinate. ABA inhibits the synthesis of hydrolytic enzymes that are essential for the breakdown of storage reserves in the seeds.
Effect on Growth
ABA inhibit growth in the seedlings induced by auxins, thus acting as a growth inhibitor. ABA blocks H+ secretions promoted by auxin that causes loosening of cell wall and increase the rate of cell elongation.
Role in Stomatal Closure
ABA is characterized as stress hormone because of its role in freezing, salt and water stress. Under stress the ABA concentrations can increase up to forty times, more than any hormone in response to an environmental change. ABA is very effective in closing stomata and conserving water under water stress conditions. Guard cells appear to have specific ABA receptors in the outer surfaces of their plasma membrane. ABA may cause stomata closure by modulating opening of ion channels and the activity of proton pump.
Effect on Water Uptake
If ABA is applied to root tissue, it stimulates water flow and ion flux, suggesting that ABA regulates turgor not only by decreasing transpiration but also by increasing water influx into the roots. ABA decreases resistance to flow of water by increasing ionic uptake that results in increase in water potential gradient between the soil and the root. ABA also induces root growth, stimulate formation of lateral roots and suppresses leaf growth. This lead to reduction in leaf area and increase in absorption area of roots which helps in increased water uptake during water stress condition.
Promotion of Abscission & Senescence
ABA was originally isolated as a hormone causing abscission. But later on ethylene was found to cause abscission. However, ABA is clearly involved in promotion of senescence. This involvement may be indirectly responsible for ethylene synthesis and stimulation of abscission, as senescence is the last developmental stage prior to death or an organ or whole organism. Experiments with excised leaves suggested that ABA greatly accelerate the senescence.
Regulation of Protein Synthesis
ABA after protein synthesis under certain conditions such as heat shock adaptation to cold temperatures and salt tolerance. ABA controls transcription leading to synthesis of new proteins. ABA induces a gene, encoding glycine-rich protein and gene promoter in wheat.