Introduction

Abscisic acid is a phytohormone or a type of plant hormone. Its main function is to inhibit the growth of plants. Phytohormone is a type of biochemical substance that is produced in the plant. These biochemical substances regulate the formation of various organs, including the growth and development of plants (3).

Plant hormones are organic compounds that are synthesized in specific tissue cells. This hormone is secreted in the form of fluids and carried to certain places through circulation affecting the function of certain cells.

The complexity of the anatomy of a plant’s body is much less than that of animals. Therefore, hormones in the plant body are mainly used for growth, movement, germination, etc. Plants produce certain chemicals which disrupt the growth hormone activity and inhibit plant growth. Such substances are called growth inhibitors. Abscisic acid is one such plant growth inhibitor. The following is a discussion of how it acts as a growth inhibitor in plants (1) & (5).

What is Abscisic acid?

The growth and development of plants depend upon certain hormones in their bodies. Besides, there are some hormones that inhibit plant growth. Abscisic acid is a type of hormone that obstructs a plant’s growth. It is translocated by phloem tissue. In some plants, the same is done by the xylem. Because of its function of inhibition, it is also called the stress hormone (2).

Discovery of abscisic acid

In 1940, scientists discovered a substance from leaves known as dormins. There a narrow and clear zone of cells is formed in the stalks just below the leaves and ripe fruits, known as the abscission zone.

Cells located above this zone begin to divide, causing various organic compounds to form, and the tissues to soften in a special way so that the leaves and ripe fruits begin to lose their own weight from the plant. This process is called abscission (3)

Many scientists believe that abscission can be increased or decreased by auxins. Some substances are produced in plants that help to increase or decrease the abscission in them. A substance is formed below the stalks in the fruits of cotton containing abscisin I and abscisin II which accelerates the formation of abscission.

In 1963, scientist Addicott discovered a substance that encouraged the abscission of cotton fruit. This substance is known as abscisin II. In 1965, scientists proved that the three groups had discovered the same plant hormone. Later this plant hormone is known as abscisic acid (1) & (2).

Why abscisic acid is called a stress hormone?

This plant hormone plays a great role in the plant body. It stimulates the stomata closure of the epidermis of the leaves. It also increases the tolerance of plants to different types of stress. So abscisic acid is called the stress hormone (2).

Properties of abscisic acid

1. This hormone was first discovered as a regulator of leaf and fruit dropping.

2. Abscisic acid is most commonly found in fruits and seeds.

3. It is synthesized in leaves.

4. These acids transfer from the leaf through the phloem to the top of the stem.

5. Another name for this acid is dormin.

6. The formula of the acid is C15H20O4.

7. This hormone is essential for the survival of the plant in any adverse environment.

8. It is also called a stress hormone.

9. The acid is a weak organic acid. It is present in young buds or dormant seeds.

10. It is synthesized in almost all cells that contain chloroplast (3) & (4).

Name of a few plants & the location of Abscisic acid in their organs

The following is a list of some plants that contain this growth inhibitor hormone in different organs of their body.

Distribution of abscisic acid

This acid is present in dicot plants, monocot plants, and ferns. Besides, various fruits contain a large amount of this growth inhibitor hormone. It is usually obtained from the lower parts of the fruits, especially in the lower part of the ovary. The acid concentration increases when the fall of the fruit is approaching (1).

Biosynthesis of abscisic acid

The biosynthesis of this acid has still not been fully explained. One group suggests that this acid is synthesized via mevalonic acid. Moreover, many people suggested that this hormone is formed by the way of the breakdown of carotenoids. That is biosynthesis of this hormone takes place by two pathways (3) & (5).

1. By mevalonic acid

This pathway is followed by gibberellin biosynthesis. The biosynthesis of this acid from mevalonic acid by farnesyl has been described in many examples especially in the water deficit tissues. Lack of water increases the formation of abscisic acid. Basic reactions to this acid formation are similar to the compounds of gibberellins and carotenoids (2).

Mevalonic acid → Isopentenyl pyrophosphate → farnesyl pyrophosphate → abscisic acid

2. By broken down or oxidation of carotenoids

Many people suggested that these phytohormones are formed by the breakdown of carotenoids. This is because their structure is similar to carotenoids. Here this stress hormone takes place by the oxidation of carotenoids or xanthophyll. This pathway is indirect biosynthesis (1).

Mechanism

The process of abscisic acid can be understood in the following ways

• This hormone may interfere with or destroy the biosynthesis of other growth factor hormones.
• It can encourage the formation of some hydrolytic enzymes.
• These phytohormones inhibit the synthesis of RNA and proteins.
• Abscisic acid along with cytokinins, and gibberellins can complete for some enzymes because its effect is opposite to the effects of all the above hormones (4) & (6).

Functions

There are 5 main classes of plant hormones. These hormones control different aspects of the development of each plant. But of these five hormones, it is the phytohormone that inhibits plant growth. The functions of this acid are explained below.

1. These hormones regulate the seed development process. It inhibits the germination of seeds.

2. Plant growth and metabolism is inhibited by this hormone,

3. The primary role of this hormone is to regulate the abscission and dormancy of plants.

4. It inhibits the action of gibberellin, so it is called anti-gibberellin.

5. This hormone helps reduce the evaporation of plants.

6. It encourages dormancy in buds.

7. The process of flowering is inhibited by this acid (ABA).

8. This hormone is applied to prevent the germination of potatoes, onions, etc.

9. It also reduces the effect of auxin and cytokinins.

10. Abscisic acid controls the closing stomata in dry conditions.

11. It also inhibits shoot growth and encourages the storage of proteins in plant seeds.

12. This plant hormone inhibits cell division in the vascular cambium, adapting to dry weather by suspending the primary and secondary growth of plants.

13. It inhibits fruit ripening.

14. This acid acts on endodermis to prevent the growth of roots in salty areas (2) & (6).

Functional group of Abscisic acid

Abscisic acid is an isoprenoid plant hormone. It is synthesized in the 2-C-methyl-D-erythritol-4-phosphate pathway. This acid plays a major role in the growth process of plants and in the response of plants to adaptive stress. A new group of nucleocytoplasmic PYR/ PYL/RCAR is marked as an abscisic acid receptor. This hormone is a 15-C sesquiterpene compound. It is composed of three isoprene residues. The functional group of abscisic acid is cyclohexane with keto, one hydroxyl group, and a side chain with a carboxylic group (1) & (2).

Q&A

1. Which functional group would you predict is part of abscisic acid?

The carboxyl functional groups are known as carboxylic acids or organic acids. These acids can donate H⁺ ions (proton) to a solution due to a highly polar oxygen atom of the functional group.

2. What is abscisic acid?

Abscisic acid is a type of plant hormone or phytohormone that inhibits the plant’s growth. It is a weak organic acid that is present in young dormant seeds.

3. What does abscisic acid do?

• It is a growth inhibitor hormone of plants.
• The hormone regulates the abscission and dormancy of plants and inhibits the germination of seeds.
• This acid controls the closing stomata in dry conditions.
• It inhibits the action of gibberellin and reduces the effect of auxin and cytokinins.

4. Where is abscisic acid produced in plants?

Abscisic acid is produced in the roots of the plant as well as the young buds or dormant seeds. It is translocated by phloem tissue from roots to stem and leaves. In some tissues, this hormone is transferred by the xylem.

5. Where is abscisic acid produced?

It is a plant hormone. This hormone is produced in the roots of the dicot and monocot plants. Mainly it is transferred by phloem tissue. Fruits and seeds contain a large amount of this acid.

Written By: Manisha Bharati

References

1. B. Powar and G. R. Chatwal. Biochemistry, B. SC (general & honors course) and M. Sc. Himalaya publishing house, Chapter: Plant growth substances. Page no- 284 to 285.

2. B Agarwal and V. K. Agarwal. Unified Botany, B.Sc. second Year. Shiva Lal Agarwal & Company Publications, Indore. Chapter: plant hormone. Page no- 209 to 210.