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 sulfonamidehypocotyl 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 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 C40carotenoid 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.
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
ABA is synthesised 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 AOG. 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.
Acts on endodermis to prevent growth of roots when exposed to salty conditions
Delays cell division
Dormancy inducer - It is used to induce dormancy in the seeds.
used as anti - transpirant - In drought prone areas, water stress is serious problem in agriculture production. so sprays of ABA are suggested that cause partial closure of stomata for few days, to reduce transpirational loss of water
Signal cascade
In the absence of ABA, the phosphatase ABI1-INSENSITIVE1 inhibits the action of SNF1-related protein kinases . ABA is perceived by the PYRABACTIN RESISTANCE 1 and PYR1-like membrane proteins. On ABA binding, PYR1 binds to and inhibits ABI1. When SnRK2s are released from inhibition, they activate several transcription factors from the ABA RESPONSIVE ELEMENT-BINDING FACTOR family. ABFs then go on to cause changes in the expression of a large number of genes. Around 10% of plant genes are thought to be regulated by ABA.
In fungi
Like plants, some fungal species have an endogenous biosynthesis pathway for ABA. In fungi, it seems to be the MVA biosynthetic pathway that is predominant. One role of ABA produced by these pathogens seems to be to suppress the plant immune responses.
In animals
ABA has also been found to be present in metazoans, from sponges up to mammals including humans. Currently, its biosynthesis and biological role in animals is poorly known. 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. Many biological effects in animals have been studied using ABA as a nutraceutical or pharmacognostic drug, but ABA is also generated endogenously by some cells when stimulated. There are also conflicting conclusions from different studies, where some claim that ABA is essential for pro-inflammatory responses whereas other show anti-inflammatory effects. Like with many natural substances with medical properties, ABA has become popular also in naturopathy. While ABA clearly has beneficial biological activities and many naturopathic remedies will contain high levels of ABA, some of the health claims made may be exaggerated or overly optimistic. In mammalian cells ABA targets a protein known as lanthionine synthetase C-like 2, triggering an alternative mechanism of activation of peroxisome proliferator-activated receptor gamma. LANCL2 is conserved in plants and was originally suggested to be an ABA receptor also in plants, which was later challenged.
Measurement of ABA concentration
Several methods can help to quantify the concentration of abscisic acid in a variety of plant tissue. The quantitative methods used are based on HPLC and GC, and ELISA. Recently, 2 independent FRET probes have been developed that can measure intracellular ABA concentrations in real timein vivo.
