Abscisic acid, role

Hartung, W., Radin, J.W. Hendrix, D.L. (1988). Abscisic acid movement into the apoplastic solution of water-stressed cotton leaves Role of apoplastic pH. Plant Physiology, 86, 908-13. 

Morgan, J.M. (1980). Possible role of abscisic acid in reducing seed set in water stressed wheat plants. Nature, 285, 655-7. 

Phytohormones such as ethylene, salicylic acid (SA), JA, and abscisic acid (ABA) regulate responses of plants to stresses via action referred as signaling crosstalk. Moreover, reactive oxygen species (ROS), the toxic byproducts of aerobic metabolism, play the important role of signaling molecules. Usually, the defensive responses of plants depend on the interaction (positive or negative) between phytohormone signaling pathways rather than on the independent contribution of each of them. " ... 

Carotenoid catabolic products also have a physiological role in the plant. Oxidative cleavage of carotenoids by carotenoid cleavage dioxygenases (CCDs) generates apocarotenoids [21]. Apocarotenoids serve the plant as antifungal agents or in the synthesis of flavor or aroma of flowers and fruits. A well-known downstream product of an apocarotenoid is abscisic acid (ABA), a phytohormone in plants [21]. 

Abe H, Yamaguchi-Shinozaki K, Urao T, Iwasaki T, Hosokawa D, Shinozaki K. 1997. Role of Arabidopsis MYC and MYB homologs in drought- and abscisic acid-regulated gene expression. Plant Cell 9 1859-1868. 

Pastori GM, Foyer H. 2002. Common components, networks, and pathways of crosstolerance to stress. The central role of redox and abscisic acid-mediated controls. Plant Physiol 129 460-468. 

The Role of Abscisic Acid as a Stress Hormone. The role of ABA in water stress has been studied most extensively, although there is also evidence for involvement of ABA in other stresses such as salinity, mineral deficiency, high osmotic concentration, and water-logging. A review on the role of ABA in stress phenomena has appeared recently (18). 

Roles of Metabolites of Abscisic Acid. Nothing is known about the physiological role of PA and DPA in plants, although these two metabolites of ABA have been tested in several bioassays recently. In the cotton explant abscission assay PA had one-tenth of the activity of ABA (19). PA and DPA were equally effective in inhibiting a-amylase secretion by barley aleurone layers treated with glbberellin A3 DPA had approximately one-tenth of the activity of ABA in this system (74). The effect of PA on growth of bean embryos was negligible (75). 

Such questions have been raised for the catabolic products of auxins, abscisic acid, conjugates of auxins, and gibberellin, and for the hormonal role of cytokinins and auxins in tRNA molecules, but are not reviewed in this article (17). A satisfactory answer to this question requires elucidation of the cellular mechanisms (discussed in this paper) that are believed to be primarily under hormonal regulation. 

There are two important classes of allelochemicals synthesized by oxidative cleavages of tetraterpene carotenoids. One is the plant hormone abscisic acid (ABA, 31) that plays important roles in growth and development of plants, especially in seed development and dormancy.17 Dry dormant seeds contain relatively large amounts of ABA, particularly in the seed coats. ABA and phenolic allelochemicals in the seed coats are easily released into the environment when the seeds are imbibed, resulting in inhibition of seed germination and seedling growth of plants in the vicinity. Both ABA and the phenolic compounds are rapidly broken down in the soil, and therefore the inhibition is short-lived. 

Desikan, R., Griffiths, R., Hancock, J., and Nedl, S. (2002). A new role for an old enzyme Nitrate reductase-mediated nitric oxide generation is required for abscisic acid-induced stomatal closure in Arabidopsis thaliana. Proc. Natl. Acad. Sci. U.S.A. 99, 16314—16318. 

Hradilik, J. and Cisarova, H. (1975b). The role of abscisic acid (ABA) in achenes of dormant cumin. Acta Univ. Agric. XXIII. 4 748-753. 

On the basis of genetical and physiological studies it was concluded that plants hormones like abscisic acid and gibberelline play an important role in the regulation of dormancy and germination. They suggested molecular techniques for further investigation related to these two phenomena. 

PLD plays important roles in the response to wounding and other stresses through mediating the actions and production of stress hormones [140]. PLDa is specifically involved in the actions of abscisic acid and ethylene in plant senescence and in the control of water loss [140, 141]. Different stresses in plants lead to different expression patterns for PLD isozymes. 

Other functions of carotenoids are briefly mentioned here. These include phototropism and phototaxis, reproduction, camouflage colour, precursor role of neoxanthin (38, Fig. 16) for the plant growth hormone abscisic acid and the ant repellent grasshopper ketone [18,41], and cell-cell communication [49], In recent years the function of zeaxanthin (15) and lutein (25) in the human retina has received much attention, cf. the section on Health aspects. 

The sesquiterpene abscisic acid is also derived from isopentenyl pyrophosphate. Abscisic acid counteracts the effects of most other plant hormones. That is, it inhibits germination, growth, budding, and leaf senescence. Physiological evidence suggests a role for abscisic acid in ion and water balance. 

Some biological roles of abscisic acid. The involvement of other hormones is indicated as (+) if the hormone... 

Abscisic acid (ABA) 3-1 was originally detected because of its growth inhibitory properties. It is now known to play an important role in the control of a-amylase synthesis, and regulation of stomatal aperture during water stress. Phaseic acid (PA) 3-3 is an important metabolite of ABA. Over a hundred derivatives of ABA are known, activity correlations have been reviewed, and the difficulty of drawing firm conclusions due to differences in uptake, metabolism and sequestration between the different molecules assayed has been discussed [16-20]. In many correlations, racemates have been used, and it is possible that each enantiomer may be active, have a different type of activity, and/or interfere with the action of the other enantiomer. 

The phytohormone abscisic acid (ABA) plays an important role in many physiological processes in plants. This hormone is necessary for regulation of several events during late seed development [54,70]. Furthermore, ABA is crucial for the response to environmental stresses such as desiccation, salt and cold [10,39]. 

---