In this article we will discuss about:- 1. Discovery of Abscisic Acid (ABA) 2. Quantitation of ABA 3. Occurrence 4. Metabolism 5. Mechanism of Action 6. Functions 7. Ion Movement 8. Fruit Growth and Ripening 9. Bud and Seed Dormancy 10. Abscission 11. Water Stress 12. Root Geotropism 13. Drought Resistance 14. Regulation of Enzyme Synthesis.
Contents:
- Discovery of Abscisic Acid (ABA)
- Quantitation of ABA
- Occurrence of ABA
- Metabolism of ABA
- Mechanism of Action of ABA
- Functions of ABA
- Ion Movement of ABA
- Fruit Growth and Ripening in ABA
- Bud and Seed Dormancy in ABA
- Abscission of ABA
- Water Stress of ABA
- Root Geotropism of ABA
- Drought Resistance of ABA
- Regulation of Enzyme Synthesis of ABA
1. Discovery of Abscisic Acid (ABA):
This phytohormone is a 15-carbon sesquiterpene and its synonyms are dormin and abscisin-II. It is universally distributed in plants. But synthetic and natural ABA’s are not identical. Cornus and Addicot (1963) isolated this compound from the shedding bolls of cotton plant and in 1965 its chemical structure was identified.
The application of ABA stimulates shedding of leaves and fruits and induces dormancy of buds and inhibition of seed germination. The naturally occuring ABA is 2—cis—4—trans structure (+). It is found in ferns, spores and diverse organs of angiosperm. It is mostly found in chloroplast (Fig. 20-12, 12A).
D.C. Walton (1980) has discussed biochemistry and physiology of this “youngster” among plant hormones.
One of the possible pathways could be its origin from zeaxanthin and violaxanthin (Fig. 20-12) (carotene) through degradation. The older view is that it arises from mevalonate.
This hormone is classified as an inhibitor and rarely it may be promotry as in pollen tube elongation. The hormone stimulates falling of leaves and fruits, bud dormancy and even senescence. ABA affects short-day plants. Recent studies have indicated that ABA causes stomatal closures.
The occurrence of ABA is made out using leaf abscission bioassay. Other bioassays include inhibition of coleoptile growth, inhibition of seed germination, etc.
2. Quantitation of ABA:
ABA is stable and has good electron-capturing properties and high UV absorbance and only a single species is known to be active. Gas chromatography equipped with an electron-capture detector is used to measure ABA levels in the 25-30 pg range. However, prepurification is required.
Tissues with high ABA levels, ethylene abscisate as an internal standard. High performance liquid chromatography (HPLC) alone or with a UV detector has also been used to measure ABA. Perhaps the most reliable and more specific method used is combined gas chromatography-mass spectrometry (GC-MS) where identity of chromatographic peaks can be undertaken from their characteristic mass spectra.
Single-ion current monitoring in which a single ion from the mass spectrum is used is employed. Further radioimmunoassay (RIA) has also been used to measure ABA and the technique is sensitive and helps in the quantification of several samples in a small time.
3. Occurrence of ABA:
ABA has high biological activity in several developmental processes. It’s in vivo function as a hormone has yet to be established. Once the widely accepted hormonal role of ABA in abscission and bud dormancy is seriously doubted today.
In fact convincing evidences have appeared against the role of ABA as an inhibitor in the light mediated growth regulation of internode elongation. There is a general acceptance that ABA contributes to the defence of the plant against stress conditions.
4. Metabolism of ABA:
Based on the analysis of various metabolites from different plant organs, e.g., endosperm, shoots and roots, seeds, metabolism of ABA has shown the following pathway:
ABA ” Phaseic acids”dihydrophaseic acid
In addition, from bean shoot epi-dpa has been produced from PA. However, when endogenous ABA is metabolised, the amount of epi-dpa is very small. When shoots are given ABA exogenously the level of epi-dpa is high. Apparently the metabolism of exo- and endogenous ABA differs.
Recently enzymes concerned with degradation of ABA to PA and dpa have been isolated. Further metabolites of ABA possibly hydroxy PA and hydroxy-dpa have also been isolated in some instances. The time taken for the metabolism of exogenously applied ABA varies.
ABA is synthesized via the early steps of the mevalonic acid pathway with two potential routes from isopentenyl pyrophosphate as shown in Fig. 20-12. The two possible routes are first via farnesyl pyrophosphate to ABA and second via carotenoids through a series of steps to ABA.
Most evidences support the synthesis of ABA from violaxanthin or a closely related xanthophyll in a wide variety of plant systems. There are two enantiomeric forms of ABA designated as R-ABA and S-ABA. S-ABA is the naturally occurring enantiomer.
There are evidences that the two enantiomers are metabolized at different rates and their products are not always the same. ABA can be metabolized in two different ways. It can be converted to abscisyl-β-D- glucopuranoside, which is a reversible reaction, or it can be irreversibly converted to 6′-hydroxymethyl ABA, phaseic acid, or 48-dihydrophaseic acid.
ABA can also be inactivated by the attachment of a glucose to the carboxyl group of ABA to form an ABA- glucose ester. Inactivation due to conjugation of ABA is similar to what occurs with IAA, gibberellins and cytokinin.
Most prevalent pathway through which ABA is widely metabolized in plant tissues involves the hyrdoxylation of one of the 6′ dimethyl groups of ABA followed by rearrangements to PA and reduction to DPA. In addition to the PA-DPA pathway, ABA is metabolized to its β’-D-glucose ester.
It remains to be verified whether ABA glucose ester can be a storage form to ABA. There is one report of a cell-free system capable of metabolizing ABA, e.g., particulate preparations from Echinocystis lobata liquid endosperm which converts ABA to PA in the presence of NADPH and O2 and is sensitive to CO2.
5. Mechanism of Action of ABA:
The exact mode of its action is not clear. One thinking is that it follows its effect on translation of RNA. ABA inhibits RNA synthesis. However, this inhibitory effect may be secondary. It does not appear to affect the de-repressing of DNA. The ABA interaction with its target site is in all probability very weak and not through covalent bonding.
Lemna plants are highly sensitive to ABA hence it has been widely used in biochemical and physiological action of ABA. At low concentrations ABA promotes frond number. But high concentrations are strongly inhibitory. When IAA is applied the inhibition increases. ABA tends to reduce its photosynthetic rate but dry weight and starch increase.
Similarly there is inhibition of DNA and RNA synthesis. Application of ABA strongly decreases potassium amount indicating that changes in membrane permeability occurred during this process. Some workers have also shown an interaction between ABA and gibberellins in the induction of a-amylase in aleurone layer of braley.
ABA decreases amylase synthesis whereas other proteins are not affected. There is a general acceptance that ABA inhibits continuous synthesis of a regulator RNA which is short lived. In one of the recent reports binding of ABA to subcellular fractions has been reported. Two binding sites have been shown.
6. Functions of ABA:
a. Seed Development and Germination:
In several seed species ABA level rises sharply and then falls during the development, e.g., in wheat, pea, soybean, barley, cotton, tomato, etc. In embryo when dry weight increases ABA is maximal. Origin, regulation of level and its role in the seed development are some of the aspects which have invited considerable interest in recent years.
In wheat evidence has been presented that fruit has high synthesizing capacity. Similarly detached tomato fruits also had increased ABA levels. It is not known whether the increase was due to de novo synthesis from MVA. Available evidences also suggest that ABA content of seed under water stress conditions is contributed by the leaves also.
Developing seeds can also metabolize ABA at different stages of development. In cotton it has been proposed that ABA synthesized in the ovule is transported to the embryo where it inhibits translation of specific mRNA species which otherwise be translated into ‘germination enzymes’. In this way vivipary or precocious germination is prevented. In corn and wheat ABA accumulation also prevents premature germination.
Exogenous ABA has been shown to prevent germination of most non-dormant seeds. It also inhibited enzymes involved in germination. At the end of the maturation process of most non-dormant seeds ABA levels are low.
7. Ion Movement of ABA:
ABA is also shown to affect ion movement in tissues other than guard cells. It inhibits proton extrusion and K. uptake that occurs during early stages of seed germination in several species. FC could reverse this inhibition. In Avena coleoptiles ABA has been reported to inhibit K+ and CI sup uptake.
ABA has been reported to inhibit the uptake of both ions and to promote and inhibit the movement of ions through roots. ABA also increases the exudation rate of decapitated plants and excised roots. There is a suggestion that the effect of ABA on water movement may be a consequence of its effect on ion movements rather than hydraulic conductivity.
8. Fruit Growth and Ripening in ABA:
The role of ABA in fruit growth and ripening has been described in several systems. Even in the later stages of wheat grain development when starch accumulation ceases and desiccation begins ABA level increases up to 60-fold or so.
Compared with seed coat and endosperm the amount of ABA in the embryo is much higher. We have no information whether the ABA in the grain is synthesized locally or is translocated from the flag leaf.
There are several suggestions regarding its roles which are, regulating assimilates, control of grain growth and ripening, and regulation of dormancy. In general, ABA seems to help in fruit ripening of tomato where high ABA concentrations coincided with stoppage of fruit growth and initiation of colouring. Some workers believe that ABA might be inducing ethylene production or vice versa.
Citrus fruits treated with ethylene had high amount of ABA. Similarly unripe tomato fruits sprayed with ABA underwent quick ripening. Possibly ABA synthesis takes place in the pericarp and seeds of the fruit. There are also reports available in other succulent fruits that the ABA level increases during the last phases of development.
In some ferns there is high ABA accumulation during spore ripening.
9. Bud and Seed Dormancy in ABA:
The general view is that bud and seed dormancy are controlled by ABA. The well advanced hypothesis that in birch the short day induced but dormancy is mediated by an increase in ABA levels of the leaves has not been confirmed. But even then growth rate and bud dormancy in woody plants are correlated with ABA levels in the bud and xylem sap.
No precise correlation exists with ABA level induction and maintenance of dormancy. Recent reports indicate that application of ABA exogenously inhibits lettuce seed germination possibly by converting GA4 to GA1. In Chenopodium album ABA does not appear to regulate dormancy. On the other hand, soybean seed had very high ABA levels which were correlated with growth rates.
10. Abscission of ABA:
No specific evidence shows a clear cut role of ABA in leaf abscission. On the other hand, water, deficit causes foliar abscission and increased ABA level. Evidently, ABA involvement in stress induce abscission seems unclear. Compared with IAA the transport of ABA is not affected under stress conditions. Apparently auxin was important in stress induced abscission.
There are several reports available where ABA has been connected with leaf and flower bud and fruit abscission. Even then most evidences do not prove the involvement of ABA in abscission. Most available evidences tend to correlate enhanced ABA level as a consequence of water stress of those organs resulting in the activation of the abscission zone.
A close examination shows that prior to abscission, flower and fruits show some amount of wilting. One needs to seek explanation that ABA is not a primary causal factor in the process of abscission.
11. Water Stress of ABA:
Role of ABA in protecting the plant against different stresses seems to gain firm hold. Its involvement in regulating water balance has been grossly well studied. Mesophytic plants which undergo wilting accumulate high levels of ABA and close their stomata. In hydrophytes, on the other hand, water stress does not increase ABA level even though stomata close.
Apparently either stomatal closure does not require ABA or ABA after inducing stomatal closure is translocated from the mesophyll cells. The interaction between ABA and CO2 on stomatal closure is still controversial. When stress increases there is new synthesis of ABA. This enhanced ABA synthesis may not be due to leaf water potential. There is also a view that changes in development induced by continued water stress may be mediated by increased ABA levels.
During stress period the water status is regulated by ABA either by inducing stomatal closure or by increasing water permeability of the root. Some investigators have shown the translocation of ABA via the phloem. Besides ABA, other regulators like farnesol also are concerned with reactions against stress.
12. Root Geotropism of ABA:
Works of Pilet and Wilkins have brought forth the possible involvement of ABA in root geotropism.
The recent thinking is that ABA, and possibly other inhibitors are produced in the root cap and move basipetally to the growing part of the root where they accummulate in the lower half of a root not in a vertical position, causing a positive geotropic response.
Most studies involved microsurgery, external use of ABA, etc. More data are needed regarding ABA levels and distribution in geotropically or light stimulated roots in comparison with non-stimulated roots. The kinetics of ABA distribution during the light-induced appearance and dark disappearance of georeactivity are known in Zea mays.
Inhibition of action:
ABA causes inhibition of GA action and promotes dormancy. It also inhibits the GA-stimulated ER synthesis and β-amylase.
Flowering:
It initiates flowering in some uninduced plants. Its effect is on short-day plants. This may be related to phytochrome controlled mechanism.
Stomatal closure:
ABA causes stomatal closure and its presence inhibits opening.
Moisture stress and ABA enhancement:
It appears that moisture stress triggers the ABA formation and the latter is accumulated in the guard cells. The guard cells lose water and then the stomata close.
Inhibition of frond growth:
Addition of ABA to medium inhibits growth of Lemna fronds.
Dormancy:
ABA causes dormancy in buds, tubers, seeds and several shoots.
13. Drought Resistance of ABA:
If ABA regulates stomatal opening in mesophytes in response to water stress, then possibly ABA may have some role in drought resistance in such plants. If it is true then it should be possible to breed plants with high ABA in an optimal manner. In fact only a few studies are available where serious attempts were made to correlate the ability of a plant to produce ABA with drought resistance.
Maize and sorghum cultivars which were drought-resistant accumulated more of ABA. Such correlations are not met within wheat cultivars. Since different plants overcome drought in different ways the ABA response may thus vary with species.
Some workers have reported correlations between the morphological characters in water-stressed wheat plants and those treated with ABA. A general lack of ABA also causes various morphological and biochemical changes, either directly or indirectly.
14. Regulation of Enzyme Synthesis of ABA:
ABA inhibits synthesis of various nucleic acid species. It is not clear whether the effect is direct or indirect since the effects on protein synthesis appear to be selective. One of the suggestions is that ABA may only directly affect the synthesis of those proteins whose synthesis is under hormonal control.
GA- promoted synthesis of a-amylase in barley aleurone layers is inhibited by ABA. ABA also inhibits other GA-promoted activities, e.g., proteases synthesis, ribonuclease, lecithin, ET, etc. ABA may be affecting mRNA synthesis.