- Abscisic acid
Abscisic acid Identifiers Abbreviations ABA CAS number PubChem ChemSpider EC number MeSH ChEBI ChEMBL RTECS number RZ2475100 Beilstein Reference 2698956 3DMet Jmol-3D images Image 1 Properties Molecular formula C15H20O4 Molar mass 264.32 g mol−1 Appearance Colorless crystals Melting point
186-188 °C, 459-461 K, 367-370 °F
120 °C, 393 K, 248 °F (sublimes)
log P 1.896 Acidity (pKa) 4.868 Basicity (pKb) 9.129 Hazards S-phrases , (what is: /?)
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Abscisic acid (ABA), also known as abscisin II and dormin, is a plant hormone. ABA functions in many plant developmental processes, including bud dormancy. It is degraded by the enzyme (+)-abscisic acid 8'-hydroxylase.
ABA was originally believed to be involved in abscission. This is now known to be the case only in a small number of plants. ABA-mediated signalling also plays an important part in plant responses to environmental stress and plant pathogens. The plant genes for ABA biosynthesis and sequence of the pathway have been elucidated. ABA is also produced by some plant pathogenic fungi via a biosynthetic route different from ABA biosynthesis in plants.
Abscisic acid owes its names to its role in the abscission of plant leaves. In preparation for winter, ABA is produced in terminal buds. 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 by the Nottingham Arabidopsis Stock Centre - both those deficient in ABA production and those with altered sensitivity to its action. Plants that are hypersensitive or insensitive to ABA show phenotypes in seed dormancy, germination, stomatal regulation, and some mutants show stunted growth and brown/yellow leaves. These mutants reflect the importance of ABA in seed germination and early embryo development.
Pyrabactin (a pyridyl containing ABA activator) is a naphthalene sulfonamide hypocotyl cell expansion inhibitor, which is an agonist of the seed ABA signaling pathway. It is the first agonist of the ABA pathway that is not structurally related to ABA.
ABA has recently been shown to elicit potent anti-inflammatory and anti-diabetic effects in mouse models of diabetes/obesity, inflammatory bowel disease, atherosclerosis and influenza infection. In mammalian cells ABA targets a protein known as lanthionine synthetase C-like 2 (LANCL2), triggering an alternative mechanism of activation of peroxisome proliferator-activated receptor gamma (PPAR gamma).
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 via violaxanthin, and final cleavage of the C40 carotenoid by a dioxygenation reaction yields the proximal ABA precursor, xanthoxin, which is then further oxidized to ABA.
Abamine has been designed, synthesized, developed and then patented as the first specific ABA biosynthesis inhibitor, which makes it possible to regulate endogenous level of ABA. 
Location and timing of ABA biosynthesis
- Released during desiccation of the vegetative tissues and when roots encounter soil compaction.
- Synthesized in green fruits at the beginning of the winter period
- Synthesized in maturing seeds, establishing dormancy
- 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
- Antitranspirant - Induces stomatal closure, decreasing transpiration to prevent water loss.
- Inhibits fruit ripening
- Responsible for seed dormancy by inhibiting cell growth – inhibits seed germination
- Inhibits the synthesis of Kinetin nucleotide 
- Downregulates enzymes needed for photosynthesis.
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