Abscisic acid

Abscisic Acid (ABA), also known as abscisin II and dormin, is a plant hormone. It functions in many plant developmental processes, including abscission and bud dormancy. ABA-mediated signalling also plays an important part in plant responses to environmental stress and plant pathogens.^[2][3] The plant genes for ABA biosynthesis and sequence of the pathway have been elucidated.^[4][5] ABA is also produced by some plant pathogenic fungi via a biosynthetic route different from ABA biosynthesis in plants.^[6]

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Abscisic acid owes its names to its role in the abscission of plant leaves. In preparation for winter, ABA is produced in terminal buds.^{[citation needed]} This slows plant growth and directs leaf primordia to develop scales to protect the dormant buds during the cold season. ABA also inhibits the division of cells in the vascular cambium, adjusting to cold conditions in the winter by suspending primary and secondary growth.

Abscisic acid is also produced in the roots in response to decreased soil water potential and other situations in which the plant may be under stress. ABA then translocates to the leaves, where it rapidly alters the osmotic potential of stomatal guard cells, causing them to shrink and stomata to close. The ABA-induced stomatal closure reduces transpiration thus preventing further water loss from the leaves in times of low water availability.

Several ABA mutant Arabidopsis thaliana plants have been identified – both those deficient in ABA production and those insensitive to its action. ABA-deficient plants show defects in seed dormancy, germination, stomatal regulation and some mutants show stunted growth and brown/yellow leaves.^[7] These mutants reflect the importance of ABA in seed germination and early embryo development.

Biosynthesis

Abscisic Acid (ABA) is an isoprenoid plant hormone, which is synthesized in the plastidal 2-C-methyl-d-erythritol-4-phosphate (MEP) pathway; unlike the structurally related sesquiterpenes, which are formed from the mevalonic acid-derived precursor farnesyl diphosphate (FDP), the C₁₅ backbone of ABA is formed after cleavage of C₄₀ carotenoids in MEP. Zeaxanthin is the first committed ABA precursor; a series of enzyme-catalyzed epoxidations and isomerizations, and final cleavage of the C₄₀ carotenoid by a dioxygenation reaction yields the proximal ABA precursor, xanthoxin, which is then further oxidized to ABA.^[4]

• Synthesized in green fruit and seeds at the beginning of the wintering period
• Mobile within the leaf and can be rapidly translocated from the roots to the leaves by the transpiration stream in the xylem.
• Produced in response to environmental stress, such as heat stress, water stress, salt stress.
• Synthesized in all plant parts, e.g. roots, flowers, leaves and stems

Effects

• Induces stomatal closure, reducing transpiration to prevent water loss.^[8]
• Inhibits fruit ripening
• Responsible for seed dormancy by inhibiting cell growth – inhibits seed germination
• Inhibits the uptake of Kinetin
• Downregulates enzymes needed for photosynthesis.^[9]

References

1. ^ Abscisic Acid Chemical Name
2. ^ Zhu JK. (2002). "Salt and drought stress signal transduction in plants". Annu Rev Plant Biol. 53: 247-273. PMID 12221975.
3. ^ Seo M, Koshiba T (2002). "Complex regulation of ABA biosynthesis in plants". Trends Plant Sci. 7: 41-48. PMID 11804826.
4. ^ ^{a b} Nambara E, Marion-Poll A. (2005). "Abscisic acid biosynthesis and catabolism". Annu Rev Plant Biol. 56: 165-185. PMID 15862093.
5. ^ Milborrow BV (2001). "The pathway of biosynthesis of abscisic acid in vascular plants: a review of the present state of knowledge of ABA biosynthesis". J Exp Bot. 52: 1145-1164. PMID 11432933.
6. ^ Siewers V, Smedsgaard J, Tudzynski P. (2004). "The P450 monooxygenase BcABA1 is essential for abscisic acid biosynthesis in Botrytis cinerea.". Appl Environ. Microbiol. 70: 3868-3876. PMID 15240257.
7. ^ NASC - Arabidopsis Stock Centre
8. ^ Zhang, J., U. Schurr, and W.J. Davies, Control of Stomatal Behaviour by Abscisic Acid which Apparently Originates in the Roots. Journal of Experimental Botany, 1987. 38(7): p. 1174.
9. ^ Plant, A.R., GENE EXPRESSION REGULATED BY ABSCISIC ACID AND ITS RELATION TO STRESS TOLERANCE. Annu. Rev. Plant Physiol. Plant Mol. Biol, 1994. 45: p. 113-141.