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Most of the effects first discovered for plant hormones were stimulatory. For example, IAA stimulates cellular elongation and cytokinins stimulate cell division. But near the end of the decade 1940s, Torsten Hemberg of Sweden reported that dormant buds of ash and potato contained inhibitors (rather than stimulators) that blocked the effects of IAA. When the buds germinated, the amount of these inhibitors decreased. Later, Eagles and Wareing (1963) isolated an inhibitor from the birch (Betula pubescens) leaves held under short day conditions. When this substance was reapplied to the leaves of birch seedlings, apical growth was completely arrested. As this substance induced dormancy, they named it as dormin. Later in 1965, Ohkuma et al isolated an inhibitor from cotton fruits and named it abscisin II. The same year, Cornforth and his associates isolated a growth inhibitor from sycamore and pointed out that both dormin and abscisin II are identical. Abscisin II is peculiar in that it is effective in much lower concentration than phenolic inhibitors and is accumulated under short day conditions. This compound was later named as abscisic acid (abbreviated as ABA)— an unfortunate name, because subsequent research has shown that ethylene rather than abscisic acid controls abscission.
Distribution and Biosynthesis
Abscisic acid occurs in angiosperms and gymnosperms but apparently not in liverworts. ABA in plants is made from carotenoids. Once synthesized, ABA moves throughout a plant in xylem, phloem and parenchyma. Like gibberellins and cytokinins, abscisic acid moves nonpolarly. There are no synthetic abscisic acids.
Chemistry
Abscisic acid (Fig. 32–25) is a sesquiterpene with molecular formula C15H20O4. Its molecule contains a corboxyl groups, 4 double bonds and an identical number of methyl groups.
Physiological Roles
1. Closure of stomata. It is a known fact that during drought, leaves synthesize large amounts of ABA which causes stomata to close. Thus, ABA acts as a messenger and enables plants to conserve water during drought. Because ABA-induced closure of stomata occurs within 1 to 2 minutes, this effect probably occurs independently of protein synthesis. As to its mechanism, ABA probably produces its effect by binding to proteins on the outer surface of the plasmalemma of guard cells. This renders the plasmalemma more positively-charged, thereby stimulating transport of ions (especially K+) from guard cells to epidermal cells. The loss of these ions causes water to leave guard cells (via osmosis) which then collapse, thus closing the stomatal aperture.
2. Delays seed dormancy. In many species, applying ABA delays seed germination. Similarly, in many other plants, the amount of ABA in their seeds decreases, when seeds germinate. Thus, it may be inferred that ABA controls seed dormancy in some cases. However, this conclusion may not be generalized, since germination of many seeds occurs without any changes in the amount of abscisic acid.
3. Controls bud dormancy. Bud dormancy was previously thought to be controlled solely by ABA. But when leaves are treated with radioactive ABA, no radioactivity has been detected in buds. This suggests that, besides ABA, bud dormancy is probably also influenced by cytokinins and IAA-induced synthesis of ethylene.
4. Counteracts the effects of other hormones. ABA counteracts the stimulatory/inhibitory effects of other hormones. For example,
(a) ABA inhibits cell growth promoted by IAA.
(b) ABA inhibits amylase produced by seed treated with gibberellin.
(c) ABA promotes chlorosis that is inhibited by cytokinins.
This may be due to the fact that ABA is a Ca2+ antagonist and its inhibition of the stimulatory effects of IAA and cytokinin may be due to its interference with Ca2+ metabolism. Although ABA usually inhibits growth, it is not toxic to plants as are inhibitors of RNA / protein synthesis. ABA often decreases gene acctivity, but there are instances of ABA stimulating genes. For example, ABA stimulates the synthesis of mRNAs for storage proteins in developing wheat grains.
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