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1. BIOSYNTHESIS OF ABA

2. DISCOVERY First identified while studying about a compound responsible for abscission of cotton fruits. By Frederick T. Addicott . It was originally called abscisin II because it was thought to play major role in abscission of fruits. Some other group called dormin cause they thought it has an important role in bud dormancy. Later the name abscisic acid (ABA) was coined to these compounds. It is supposed to involve in the abscission process.

3. ABA is one of the classes of plant hormone It is regarded as the stress hormone It inhibits growth and stomatal opening, particularly when the plant is under environmental stress condition Another important function is its regulation of seed maturation and dormancy ABA is synthesized in vascular tissues of leaves Description

4. Chemical formula: C 15 H 20 O 4 Structure and Chemical formula of ABA Naturally, it is a sesquiterpene (15-C)

5. ABA biosynthesis takes place in chloroplast and other plastids. It is mostly synthesized from carotenoids . Initial stages occur in the plastids, where Isopentenyl diphosphate (IPP) is converted to the C40 Xanthophyll Zeaxanthin . Zeathanthin is further modified to 9-cis- neoxanthin , which is cleaved by the enzyme NCED (9-cis- epoxycarotenoid dioxygenase ) to form the C15 inhibitor, Xanthoxin . Xanthoxin is then converted to ABA in the cytosol . Biosynthesis of ABA

6. Biosynthesis of ABA Isopentyl diphosphate (IPP) Zeaxanthin 9’-cis- violaxanthin All trans Violaxanthin Antheraxanthin Neoxanthin Xanthoxin 9’-cis- neoxanthin Zeaxanthin epoxidase (ZEP) Plastid 9’ cis epoxy carotinoids dehydrogenase (NCED) Cytosol

7. Biosynthesis of ABA Isopentyl diphosphate (IPP) Zeaxanthin 9’-cis- violaxanthin All trans Violaxanthin Antheraxanthin Neoxanthin Xanthoxin 9’-cis- neoxanthin Zeaxanthin epoxidase (ZEP) Plastid 9’ cis epoxy carotinoids dehydrogenase (NCED) Xanthoxin Abscisic aldehyde Short chain dehydrogenase (SDR) Cytosol Abscisic acid (ABA) Abscisic aldehyde oxidase (AAO)

8. ABA occurrence and distribution Phloem sap, xylem sap and in nectar ABA has been detected in all major organs or living tissues from root caps to apical buds It occurs predominantly in mature green leaves ABA is synthesized in all types of cells that contain chloroplast or other plastids A 15-C compound called lunularic acid has been found in algae and liverworts Some fungi synthesize ABA as secondary metabolite In bryophytes it has been found in mosses but not in liverworts ABA is a ubiquitous plant hormone in vascular plants

9. ABA transport At high pH dissociated form (ABA - ) At low pH protonated or undissociated form (ABAH) Redistribution of ABA- pH gradient Transported in a conjugated form as ABA- beta-D- glucosyl aster Transported mostly in its free form ABA synthesized in root cap transported to central vascular tissue Cell to cell transport is slow Externally applied ABA distributed in all directions

10. Promotes stomatal closing Induces bud dormancy and seed dormancy Promotes desiccation tolerance in the embryo Inhibits precocious germination and vivipary Promotes root growth and inhibits shoot growth at low water potentials Promotes leaf senescence independent of ethylene Functions of abscisic acid

11. ABA binds to its receptors ABA-binding induces the formation of reactive oxygen species, which activate plasma membrane Ca2+ channels. ABA increases the levels of cyclic ADP-ribose and IP3, which activate additional calcium channels on the tonoplast . The influx of calcium initiates intracellular calcium oscillations and promotes the further release of calcium from vacuoles.. The rise in intracellular calcium blocks K+(in) channels. The rise in intracellular calcium promotes the opening if Cl -out (anion) channels on the plasma membrane, causing membrane depolarization. The plasma membrane proton pump is inhibited by the ABA-induced increase in cytosolic calcium and a rise in intracellular pH, further depolarizing the membrane. Membrane depolarization activates K+ (out) channels. K+ and anions to be released across the plasma membrane are first released from vacuoles into the cytosol . Promotes stomatal closing

12. Growth of the seed suspended- dormant seeds Primary dormancy and secondary dormancy Controlled by the ratio of ABA to GA Embryo dormancy is due to the presence of inhibitors, especially ABA as well as the absence of growth promoters, such as GA The loss of embryo often associated with the sharp drop in the ratio of ABA to GA Induces seed dormancy

13. Dormant bud- shortened internodes Modified leaves known as the Bud scales Most of the trees- buds formed late summer and remain dormant throughout winter Levels of endogenous ABA is high Herbaceous plants eg - potato Induces bud dormancy

14. An important function of ABA- developing seed desiccation tolerance Levels of ABA in seeds peak during embryogenesis During the mid to late stages of seed development, specific mRNAs accumulate in embryos These mRNAs encode so called late embryogenesis Abundant (LEA) proteins Synthesis of many LEA proteins induced by ABA treatment Promotes desiccation tolerance in the embryo

15. When immature embryos are removed from the seeds and placed in culture medium they germinates precociously ABA added to the culture medium inhibits precocious germination This result, in combination with the fact that the level of endogenous ABA s high during mid to late seed development, suggest that ABA is the natural constraint that keeps developing embryos in their embryogenic state During seed development, embryo of monocot and dicot plants may fail to complete maturation and germination while the immature seed is still attached to the maternal tissue. This phenomenon is referred to as vivipary or preharvest sprouting In maize, several viviparous ( vp ) mutants have been selected in which the embryos germinate directly on the cob while still attached to the plant Vivipary in the ABA-deficient mutants can be partially prevented by treatment with exogenous ABA Inhibits precocious germination

16. ABA has different effects on the growth of roots and shoots and the effects are strongly dependent on the water status of the plant Maize: Two types of seedlings are used 1. Wild type seedlings with normal ABA levels 2. AN ABA-deficient viviparous mutant This suggests that: 1. Endogenous ABA promotes shoot growth in well watered plants 2. Endogenous ABA acts as a signal to reduce shoot growth only under water stressed conditions Shoot and root growth

17. The detoriative process that naturally terminate their functional life referred to as “senescence” Expression of senescence associated genes (SAGs) increases Hydrolytic enzymes- proteases, ribonucleases , lipases ABA involved in senescence of leaves but not the abscission of leaves Leaf scenesence

18. Abscisic Acid Applications Decrease Stomatal Conductance and Delay Wilting in Drought-stressed Chrysanthemums. All applications of ABA effectively delayed wilting by reducing stomatal conductance ( gS ). Shelf life was extended from 1.2 to 4.0 days depending on the concentration of ABA. Spray applications of 500 mgL–1 ABA to six additional chrysanthemum cultivars increased shelf life from 1.6 to 3.8 days following drought stress. ABA treatment also allowed severely drought-stressed chrysanthemums to recover and remain marketable after rewatering . Growers can treat chrysanthemums with ABA to reduce water use during shipping and to delay wilting if plants are not adequately watered during retailing. APPLICATION OF ABSCISIC ACID IN ORNAMENTAL CROPS Waterland et.al . (2010)

19. To help plants conserve water and reduce water needs, stomatal closure can be induced by applying abscisic acid (ABA). ABA drenches caused stomatal closure in salvia ( Salvia splendens Sellow ex Schult .) ‘Bonfire’ within three hours of application. ABA applications can be used to extend the shelf life of salvia, but the lowest possible dose should be used to minimize leaf abscission. Kim et.al . (2008)

20. Addicott FT, Carns JW, Cornforth JL, Lyon BV, Milborrow K, Ohkuma G et al. Abscisic acid: A proposal for the redesignation of abscisin II ( dormin ). In: F. Wightman and G. Setterfield (ed.) Biochemistry and Physiology of Plant Growth Substances. Runge Press, Ottawa, Canada, 1968, 1527-1529. Kumar M, Singh VP, Arora A, Singh N. The role of abscisic acid (ABA) in ethylene insensitive Gladiolus (Gladiolus grandiflora Hort.) flower senescence. Acta Physiologiae Plantarum . 2014; 36(1):151-159 Waterland , N. L., Finer, J. J., & Jones, M. L. (2010). Abscisic acid applications decrease stomatal conductance and delay wilting in drought-stressed chrysanthemums.  HortTechnology ,  20 (5), 896-901. Kim, J., & van Iersel , M. W. (2008). ABA drenches induce stomatal closure and prolong shelf life of Salvia splendens . In  Southern Nursery Assn. Res. Conf  (Vol. 53, pp. 107-111). References

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