Chemical nature of abscisic acid:
Abscisic acid is a naturally occurring compound in plants. Abscisic acid (ABA) is an isoprenoid plant hormone, which is synthesized in the plastids by the 2-C-methyl-D-erythritol-4-phosphate (MEP) pathway; It is a sesquiterpenoid (15-carbon) which is produced partially via the mevalonic pathway in chloroplasts and other plastids. Since it is sythesized partially in the chloroplasts, biosynthesis of ABA primarily occurs in the leaves. The production of ABA is accentuated by stresses such as water loss and freezing temperatures.
Functions of Abscisic acid:
Some of the physiological responses of Abscisic acid are listed below:
(1) Antitranspirant: Stimulates the closure of stomata by decreasing transpiration to prevent water loss (water stress brings about an increase in ABA synthesis). In angiosperms and gymnosperms (but not in ferns and lycopsids), ABA triggers closing of stomata when soil water is insufficient to keep up with transpiration.
Mechanism: ABA binds to receptors at the surface of the plasma membrane of the guard cells.
The receptors activate several interconnecting pathways which converge to produce a rise in pH in the cytosol. Transfer of Ca2+ from the vacuole to the cytosol. These changes stimulate the loss of negatively-charged ions (anions), especially NO3-− and Cl−-− , from the cell and also the loss of K+ from the cell. The loss of these solutes in the cytosol reduces the osmotic pressure of the cell and thus turgor. The stomata close. ABA also promotes abscission of leaves and fruits (in contrast to auxin, which inhibits abscission). It is, in fact, this action that gave rise to the name abscisic acid. The dropping of leaves in the autumn is a vital response to the onset of winter when ground water is frozen — and thus cannot support transpiration — and snow load would threaten to break any branches still in leaf. Inhibits shoot growth but will not have as much affect on roots or may even promote growth of roots, induces seeds to synthesize storage proteins. It inhibits the affect of gibberellins on stimulating de novo synthesis of alpha-amylase. It also has some effect on induction and maintanance of dormancy. ABA inhibits seed germination in antagonism with gibberellins and induces gene transcription especially for proteinase inhibitors in response to wounding which may explain an apparent role in pathogen defense. Inhibits fruit ripening and is responsible for seed dormancy by inhibiting cell growth. It downregulates enzymes needed for photosynthesis.