Abscisic acid
Abscisic acid is a plant hormone. ABA functions in many plant developmental processes, including seed and bud dormancy, the control of organ size and stomatal closure. It is especially important for plants in the response to environmental stresses, including drought, soil salinity, cold tolerance, freezing tolerance, heat stress and heavy metal ion tolerance.
Discovery
In the 1940s, Torsten Hemberg, while working at the University of Stockholm, found evidence that a positive correlation exists between the rest period and the occurrence of an acidic ether soluble growth inhibitor in potato tubers.In 1963, abscisic acid was first identified and characterized as a plant hormone by Frederick T. Addicott and Larry A. Davis. They were studying compounds that cause abscission of cotton fruits. Two compounds were isolated and called abscisin I and abscisin II. Abscisin II is presently called abscisic acid.
In plants
Function
ABA was originally believed to be involved in abscission, which is how it received its name. This is now known to be the case only in a small number of plants. ABA-mediated signaling 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.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. A close linear correlation was found between the ABA content of the leaves and their conductance on a leaf area basis.
Seed germination is inhibited by ABA in antagonism with gibberellin. ABA also prevents loss of seed dormancy.
Several ABA-mutant Arabidopsis thaliana plants have been identified and are available from 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 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.
Homeostasis
Biosynthesis
Abscisic acid 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, 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. via abscisic aldehyde.Abamine has been designed, synthesized, developed and then patented as the first specific ABA biosynthesis inhibitor, which makes it possible to regulate endogenous levels of ABA.
Locations and timing of ABA biosynthesis
- Synthesized in nearly all plant tissues, e.g., roots, flowers, leaves and stems
- Stored in mesophyll cells where it is conjugated to glucose via uridine diphosphate-glucosyltransferase resulting in the inactivated form, ABA-glucose-ester
- Activated and released from the chlorenchyma in response to environmental stress, such as heat stress, water stress, salt stress
- 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 leaves to the roots in the phloem
- Accumulation in the roots modifies lateral root development, improving the stress response
- ABA is synthesized in almost all cells that contain chloroplasts or amyloplasts
Inactivation
ABA can be catabolized to phaseic acid via CYP707A or inactivated by glucose conjugation via the enzyme uridine diphosphate-glucosyltransferase. Catabolism via the CYP707As is very important for ABA homeostasis, and mutants in those genes generally accumulate higher levels of ABA than lines overexpressing ABA biosynthetic genes. In soil bacteria, an alternative catabolic pathway leading to dehydrovomifoliol via the enzyme vomifoliol dehydrogenase has been reported.Effects
- Antitranspirant - Induces stomatal closure, decreasing transpiration to prevent water loss.
- Promotes root growth during periods of low humidity.
- 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.
- Acts on endodermis to prevent growth of roots when exposed to salty conditions
- Promotion of plant antiviral immunity