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The phytohormone abscisic acid (ABA) plays regulatory roles in a host of physiological processes in all higher as well as in lower plants (Davies and Jones, 1991; Zeevaart and Creelman, 1988). Abscisic acid mediates stress tolerance responses in higher plants, is a key signal compound that regulates stomatal aperture and, in concert with other plant signaling compounds, is implicated in mediating responses to pathogens and wounding.

In seeds, ABA promotes seed development, embryo maturation, synthesis of storage products (proteins and lipids), desiccation tolerance, is involved in maintenance of dormancy (inhibition of germination), and apoptosis (Zeevaart and Creelman, 1988; Davies and Jones, 1991; Thomas, 1993; Bethke et al 1999). As well, ABA affects plant architecture, including root growth and morphology, and root-to-shoot ratios.

The molecular events involved in ABA signal transduction are the focus of numerous well-known laboratories world-wide (e.g. McCourt in Canada; Giraudat in France; Shinozaki in Japan).

Endogenous Enzymatic Breakdown of Abscisic Acid

ABA is exceedingly labile with a half-life of approximately eight hours. Once internalized, the molecule is susceptible to rapid enzymatic degradation. This rapid catalytic breakdown reduces the biological responses of ABA and makes the molecule less attractive as a commercially usable product. The natural ABA demonstrates an opening for ABA analogs towards various environmental markets.

Physiological Actions and Hormonal Functions

Abscisic Acid (ABA) influences almost all aspects of plant growth to a greater or lesser extent, through complex mechanisms rather than through simple linear signaling processes. The functional, spatial and temporal complexity of adaptation to continuous environmental fluctuations (not just stressful conditions!) is only beginning to be understood. ABA production and action is part of a network or web of interacting processes (Trewavas, 1991; Genoud and Metraux, 1999) that changes continuously in response to inputs from primary environmental sensors (e.g. phytochrome [Kraepiel et al, 1994]), the blue light receptor (Assmann and Shimazaki, 1999), putative low temperature-sensing ICE proteins (Thomashow, 1999) and putative osmosensor proteins (Shinozaki et al, 1998). The primary sensors mediate adaptive phenotypic plasticity both directly and by altering various signaling systems. Therefore, ABA action and metabolism is modulated not only by environmental signals but also by endogenous signals generated by metabolic feedback, transport, hormonal cross-talk and developmental stage, with cell-specific effects probably crucial.

Manipulation of ABA and/or metabolite levels has been described as a very promising means to improve productivity, performance and plant architecture (Zeevaart 1999; Cutler and Krochko 2000).

In developing oilseeds, ABA regulates the expression of many embryo-specific genes and the hormone has been suggested as a potential control signal during the reserve accumulation phase of B. napus seed development (Finkelstein et al., 1989). Seed storage protein regulation and lipid accumulation have been studied and a positive response to exogenous ABA addition has been demonstrated in many systems (Delisle and Crouch, 1989; Taylor et al, 1990; Holbrook et al, 1992). The modulation of very long chain fatty acid content by (+)-ABA and its metabolites has also been studied extensively at Plant Biotechnology Institute.

As reported by Holbrook et al. (1992), microspore-derived B. napus embryos treated with 10 µM (± )-ABA had a total fatty acid content 40% higher than controls, and levels of eicosenoic and erucic acids were increased 3-4 fold after ABA treatment, corresponding to the stimulation of 18:1 elongation. (+)-ABA is metabolized in plants principally via oxidation to (-)-phaseic acid (PA), via 8´ -HOABA. Zou et al. (1995 [48]) demonstrated that VLCFA fatty acid and oleosin synthesis were highly stimulated when microspore-derived embryos of B. napus were grown in the presence of (+)-ABA and 8´ -HOABA, while phaseic acid (PA), was biologically inactive.

Receptors for ABA have not yet been identified, and the location, number, and nature of ABA binding proteins in plant cells are poorly understood (Leung and Giraudat 1998). Significant advances are being made in ABA signal transduction (McCourt 1999).

Abscisic acid is known to have numerous hormonal functions and uses (Addicott, F.T., Abscisic Acid, New-York, Praeger, 1983). The following illustrate some of them:

• In Abscission: The early experiments with young fruit of cotton and with flowers and young fruit of lupins demonstrated that ABA was a hormone acting to accelerate abscission. Subsequent experiments showed that ABA is also a leaf abcission accelerating hormone in Coleus and Phaseolus. ABA is hormonally involved in abscission of buds, leaves, petals, flowers, and fruits in many, if not all, instances, as well as in dehiscence of fruits.
• In Bud Dormancy: There is evidence suggesting that ABA from leaves can have a hormonal role in the induction of dormancy. In dormancy induction, additional factors are necessary for the development of the typical restinf bud. ABA could also be involved in the maintenance of dormancy and in emergence from dormancy.
• In Seed Dormancy: In many species, ABA from the embryo, endosperm, seed coats, or surrounding tissues is responsible for prolonging dormancy and delaying germination.
• In elongation Growth: ABA is inhibitory in many systems such as coleoptiles, hypocotyls, and radicles. The ability of ABA to induce tuberization 9Wareing and Jennings, 1980) and to promote flowering in some species (El-Antably et al., 1967) appears related to its ability to inhibit elongation growth.
• In stomatal closure: Moving from the site of production in the mesophyll to the site of action in the guard cells, ABA promotes the loss of turgor that leads to stomatal closure.
• In Root Growth: ABA is implicated in the control of elongation, lateral root development, and geotropism, as well as in water uptake and ion transport by roots.
• In Fruit Ripening: ABA coming from the plastids promotes the metabolism of ripening.
• In Senescence: As in the final stage of fruit ripening, ABA accelerates the biochemical changes characteristic of the process.

Literary References

1. Himmelbach, A., M. Iten, E. Grill. Signaling of Abscisic Acid to Regulate Plant Growth. Philos. Trans R. Soc. Lond. B. Biol. Sci. 1998. 353(1374): 1439-1444.
2. Busk, P.K., M. Pages. Regulation of Abscisic Acid-Induced Transcription. Plant Molecular Biology. 1998. 37(3): 425-435.
3. Sheen, J. Mutational Analysis of Protein Phosphatase 2C Involved in Abscisic Acid Signal Transduction in Higher Plants. Proc. Natl. Acad. Sci. USA. 1998. 95(3): 975-980.
4. Wu, Y., J. Kuzma, E. Marechal, et al. Abscisic Acid Signaling Through Cyclic ADP-Ribose in Plants. Science. 1997. 278(5346): 2126-2130.
5. Sharp, R.E. et al. Endogenous ABA maintains Shoot Growth in Tomato independently of effects on Plant Water Balance: Evidence for an Interaction with Ethylene. J. Exp. Bot. 2000. 51(350): 1575-1584.
6. Taylor, I.B. et al. Control of Abscisic Acid Synthesis. J. Exp. Bot. 2000. (350): 1563-1574.

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