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. 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 – 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. 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..

Location and timing of ABA biosynthesis

 * Released during desiccation of the vegetative tissues and when roots encounter soil compaction.
 * 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.
 * Inhibits fruit ripening
 * Responsible for seed dormancy by inhibiting cell growth – inhibits seed germination
 * Inhibits the uptake of Kinetin
 * Downregulates enzymes needed for photosynthesis.