Discovery of abscisic acid

Abscisic acid was first isolated from cotton plants and was initially named Abscisin 2. In the same year, two scientists, Eagles and Wareing (1963), isolated an active substance of maple leaves and called it dormin. Abscisin 2 and dormin are the same substance which is now called abscisic acid. Abscisic acid is a sesquiterpene, made of three isoprene residues.  

Biosynthesis and transport of abscisic acid

• ABA is a sesquiterpene (15- Carbon compound), synthesised in almost all cells that contain chloroplast and other plastids. Biosynthesis of ABA occurs either from Mevalonic acid or from the oxidation of carotenoids. 
• The pathway of direct synthesis of ABA from mevalonic acid through farnesyl pyrophosphate in the water-stressed tissues is termed the Mevalonate pathway. The Mevalonic acid gets converted into isopentenyl pyrophosphate, a precursor of ABA that further converts into farnesyl pyrophosphate and results in abscisic acid formation.  
• Indirect synthesis of ABA takes place by oxidation of carotenoids or oxidized carotenoids(Xanthophyll). 
• Oxidative cleavage of Xanthophyll, either by a photo- oxidation process or by the action of lipoxygenase, produces a C-15 aldehyde xanthoxin. This aldehyde is then oxidized to produce ABA. Transport of ABA occurs through both xylem and phloem and through parenchyma cells outside vascular bundles.

Mechanism of action

• Membrane permeability

ABA affects the metabolism of lipids, an important component of the membrane, in many plants. It increases the synthesis of saturated fatty acids (such as palmitic acid) and decreases the synthesis of unsaturated fatty acids (such as linoleic and linolenic acid). Thus, changing the permeability and fluidity of the membrane. If the concentration of saturated fatty acids is more in the plasma membrane, then the van der Waal interaction between adjacent phospholipids increases, which decreases the membrane’s fluidity. If the concentration of unsaturated fatty acids is more, then, the fluidity and permeability of the membrane increase.

• Nucleic acid direct protein synthesis

• • ABA is involved in the suppression of gene expression and protein synthesis. This may be achieved by decreasing the m-RNA level during the transcription process or inhibiting the translation process.

• Decrease in the m-RNA may be brought about either by decreased RNA polymerase or increased ribonuclease activity. Increased ribonuclease activity stimulated by the secretion of abscisic acid induces senescence in plants.

• During a stressful environment, the ABA slows down or inhibits the growth of plants by blocking protein synthesis until the arrival of favourable environmental conditions.

• Dormancy

Abscisic acid is a potent inhibitor of seed germination.

• Growth inhibition

Synthetic abscisic acid retards seed germination and stops the growth of intact plants or individual plant organs. It counteracts the growth stimulatory effects of auxins, gibberellins and cytokinins. The effect of abscisic acid on seed germination can be reversed by treatment with gibberellic acid, kinetin or both.

• Moisture stress and stomatal closure

•  During moisture stress, the plant induces synthesis of ABA, and ABA then accumulates in the guard cells. Higher ABA concentration induces the opening of calcium ion channels on vacuole, plasma membrane and endoplasmic reticulum, leading to an influx of calcium ions. Thus, the cytosolic concentration of calcium increases.

• • The elevated level of calcium ions in the cytosol causes the opening of calcium-activated ion channels on the plasma membrane. The opening of anion channels permits large quantities of chloride and malate ions to escape from the cell, moving down their electrochemical gradient and leading to membrane depolarisation, triggering the opening of the voltage-gated potassium ion efflux channels and closing of voltage-gated potassium ions influx channels.

• ABA also inhibits the activity of H+-ATPase present on the plasma membrane, resulting in additional membrane depolarization. Large efflux of potassium ions and anions from guard cells leads to loss of guard cell turgor pressure, resulting in the closure of stomata.

• Roots and shoots growth

Under low water potential, ABA production increases. ABA then promotes root growth overshoot growth by suppressing the activities of ethylene and auxin.

• Vivipary

ABA deficient embryos may exhibit precocious germination and vivipary. Vivipary means the germination of mature seed within the fruit on maternal plants before the dispersal. It is primarily restricted to mangroves, where seed germination, while attached to the mother plants and seedlings, is shed, sticks into the mud below, and grows.

Conclusion

ABA is a phytohormone. It is also called stress or emergency hormone. During stressful conditions like low water content and high temperature, it induces seed dormancy and halts plant growth and development.  

It is synthesised in chloroplasts and other plastids from the precursor isopentenyl pyrophosphate. It is an inhibiting growth hormone. ABA induces seed dormancy by suppressing protein synthesis, by regulating m-RNA production during transcription by either decreasing the level of RNA polymerase or increasing ribonuclease activity. During low water potential, it induces root growth and inhibits shoot growth. It also regulates stomata closure during low moisture content to prevent internal water loss through transpiration. It is also termed the anti-gibberellin hormone, as it opposes the effect of gibberellin hormone. 

Gibberellin increases gene expression by inhibiting actinomycin D, which negatively regulates transcription and cycloheximide and fluorophenylalanine, negative translation regulators. It also breaks seed and bud dormancy, while ABA opposes its effects by suppressing protein synthesis and promoting seed dormancy.