Abscisic Acid: Definition, Meaning, Overview, Function

What Is Abscisic Acid In Plants?

Abscisic acid, ABA, is one of the phytohormones that play a vital role in regulating various physiological processes in plants. Sometimes referred to as 'stress hormone' owing to its involvement in plant responses to environmental stresses like drought, cold, and salinity. ABA is a sesquiterpenoid compound with a unique chemical structure. It is naturally dextrorotatory (+), and the synthetic ABA is a racemic mixture. ABA acts both as an inhibitor of plant growth and as a modulator of abscission and dormancy.

Why Is Abscisic Acid Called A Stress Hormone?

ABA is considered a stress hormone because:

• Its synthesis is induced by various environmental stresses, including drought, water logging, and high salinity.

• It enables the plants to adapt to adverse external conditions by appropriately altering their internal environment.

• For example, during drought conditions it induces closing of stomata to reduce water loss.

Functions Of Abscisic Acid (ABA)

Abscisic acid occupies a center-stage position in many developmental and physiological processes:

Stomatal Closure

ABA promotes stomatal closure in the event of high salinity and water stress, thus reducing the loss of water through transpiration. ABA combines with other phytohormones like jasmonates, nitric oxide and signalling molecules to trigger stomatal closure.

Seed Dormancy

ABA induces seed dormancy, making them tolerant to desiccation and other growth-unfavorable conditions, hence facilitating their storage for a longer period.

Root Growth Regulation

ABA helps in regulating the expression of genes and biosynthesis of lipids and storage proteins involved in maintaining root growth and water uptake under nitrogen deficiency and drought conditions.

Signal Transduction

ABA is involved in the signal transduction pathway that comes into play in response to stress. ABA interacts in this process with other phytohormones like jasmonates and nitric oxide, which are signaling molecules.

Dehydration and Osmoprotection

ABA induces the synthesis of dehydrins, osmoprotectants, and protective proteins that confer dehydration tolerance on plants.

ABA vs Gibberellins

The difference between abscisic acid and gibberellins is:

Applications of ABA (Agriculture & Research)

The major application of abscisic acid can be seen in the fields of agriculture and research:

• Enhance drought resistance

• Improving seed storage

• Regulating stress tolerance genes

• Studying hormone signaling pathways

Human Stress Hormones

Stress hormones are those which get secreted during stress and emergency conditions. These hormones take on to launch a set of stress reactions, namely the "fight or flight" response. The three main human stress hormones are:

• Adrenaline: Secreted by the adrenal medulla, it increases the rate of heartbeat, rate of respiration, alertness, dilation of pupils, piloerection, and sweating.

• Noradrenaline: It is also released from the adrenal medulla, leads to increased alertness and prepares the body physically for an action.

• Cortisol: Produced by the adrenal cortex, it maintains water balance and blood pressure, has immunosuppressive effects, and leads to the regulation of gluconeogenesis, lipolysis, and proteolysis.

Abscisic Acid NEET MCQs (With Answers & Explanations)

Important topics for NEET are:

• Functions of Abscisic acid in Plants

• Abscisic acid vs Gibberellins

Practice Questions for NEET

Q1. Which one of the following acids is a derivative of carotenoids?

1. Indole-3-acetic acid

2. Gibberellic acid

3. Abscisic acid

4. Indole butyric acid

Correct answer: 3) Indole butyric acid

Explanation:

Abscisic acid is a mildly acidic dextrorotatory cis sesquiterpene growth hormone that functions as a general growth inhibitor by counteracting other hormones (auxins, gibberellins, and cytokinins) or reactions mediated by them. The hormone is formed from mevalonic acid, or xanthophyll. It is a derivative of carotenoids. Abscisic acid (ABA) regulates various physiological processes, including seed dormancy, stomatal closure, and stress responses. It plays a key role in plant adaptation to environmental stresses such as drought, salinity, and cold. ABA inhibits cell division and elongation, thereby slowing down overall plant growth. Its levels increase under water-deficient conditions, signaling guard cells to close stomata and reduce transpiration. The biosynthesis of ABA occurs in plastids and cytoplasm, primarily through the oxidative cleavage of carotenoids.

Hence, the correct answer is option 3) Abscisic acid.

Q2. The precursor of ABA is

1. Tryptophan

2. Succinyl Co A

3. Methionine

4. Mevalonic acid

Correct answer: 4) Mevalonic acid

Explanation:

It indirectly contributes to the biosynthesis of this hormone, as carotenoids, mainly from xanthophylls, and violaxanthin, lead to its formation. The initial stage in this case is the fragmentation of C40 carotenoids into xanthoxin and subsequent conversion to ABA. The pathway mediated by mevalonic acid is also present but contributes less to the overall production of ABA than does the carotenoid route. Consequently, it would be fair to say that mevalonic acid is not as key to ABA formation as the xanthophylls.

Hence, the correct answer is option 4) Mevalonic acid.

Q3. Which one of the following growth regulators is known as the'stress hormone?

1. Abscisic acid

2. Ethylene

3. GA3

4. Indole acetic acid

Correct answer: 1) Abscisic acid

Explanation:

Abscisic acid is a mildly acidic dextrorotatory cis sesquiterpene growth hormone that functions as a general growth inhibitor by counteracting other hormones (auxins, gibberellins, and cytokinins) or reactions mediated by them. It is also called the stress hormone because the produc­tion of the hormone is stimulated by drought, waterlogging, and other adverse environmental conditions. It plays a key role in regulating plant responses to environmental stress, particularly in managing water loss. ABA production is stimulated under stress conditions such as drought, waterlogging, and extreme temperatures, helping plants conserve water and adapt. By promoting stomatal closure and inhibiting growth under unfavorable conditions, ABA helps plants survive and withstand environmental challenges.

Hence, the correct answer is option 1) abscisic acid.

Also Read:

• MCQs on Abscisic Acid

• Phases of Growth in Plants

• Plant Growth Regulators

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