Plant Growth Regulators: Overview, Types, Its Role In Plant Growth

What Are Plant Growth Regulators?

Plant growth regulators are either natural or artificial substances that can manipulate all phases of plant growth and development, including cell division, cell elongation, and cell differentiation. They have a crucial role in controlling physiological activities such as flowering, fruiting, and responses to stress, and are important tools in agriculture and horticulture in optimising plant health and productivity.

Classification of Plant Growth Regulators

The plant growth regulators are classified into two main categories, i.e., growth promoters and growth inhibitors

Growth Promoters

Growth promoters enhance the growth of the plant. Below are the types of plant growth regulators:

• Auxins are a class of phytohormones mainly influencing plant growth through cell elongation and cell differentiation.

• Gibberellins are hormones that affect the growth and development of plants, including stem elongation and seed germination.

• Cytokinins are hormones that enhance cell division and shoot development; they have an antiaging effect in plants.

Growth Inhibitors

Growth inhibitors slow or stop the growth of the plant. Below are the types of plant growth inhibitors:

• Abscisic acid is a plant hormone that mediates responses to plant stress and dormancy in seeds.

• Ethylene is a gaseous phytohormone that tends to affect several growth events of plants, primarily fruit ripening and the abscission of leaves.

Discovery Of Plant Growth Regulators

Early Observations: Details regarding plant hormones and their role in growth were finalised by the end of the 19th and beginning of the 20th centuries.

• Auxins: Its existence was shown by Charles Darwin and his son Francis, who showed its role in phototropism.

• Gibberellins: It was isolated from the fungus Gibberella fujikuroi which caused abnormal growth in rice.

• Cytokinins: It was isolated from coconut milk since it allows cell division.

• Abscisic Acid: It was isolated for its role in the abscission of leaves and other organs and its response to different stresses.

Types of Plant Growth Regulators and Their Functions

The different types of plant growth regulators and their functions are:

Auxin

Function And Mechanism

• Cell Elongation: It causes elongation through the loosening of cell walls.

• Root Development: Required for root initiation and growth

• Tropisms: Channels growth responses of plants to environmental stimuli, such as light, and gravity.

Key Examples

• Indole-3-acetic Acid, IAA: The most ubiquitous naturally occurring auxin that is implicated in many growth events.

Gibberellins

Function And Mechanism

• Stem Elongation: The cell will start to elongate and divide into stems.

• Seed Germination: Break dormancy of seed and lead to germination

• Flowering: affects flowering in some plants

Key Examples

• Gibberellic acid, GA: The most famous Gibberellin used in promoting growth and production yield.

Cytokinins

Function And Mechanism

• Cell Division: Induce mitosis in plant cells.

• Shoot Development: Promote shoot growth and leaf expansion.

• Delay Senescence: Extend the life of leaves and other plant organs.

Key Examples

• Zeatin: Produced in maize and induces cell division.

• Kinetin: An artificial cytokinin used to study tissue culture and plant growth.

ABA

Function And Mechanism

• Stress Response: It helps the plants fight drought and other stress conditions.

• Seed Dormancy: It causes dormancy and prevents the seed from germinating prematurely.

Key Examples

• ABA in Drought Tolerance: It enhances the water-retention ability of the plant and decreases the rate of transpiration.

Ethylene

Function and Mechanism

• Fruit Ripening: It speeds up fruit ripening.

• Leaf Abscission: It causes the fall of leaves and flowers.

• Commercial Fruit Ripening: Used to synchronize and control the ripening process of fruits like bananas and tomatoes.

Mechanism of Action of PGRs

The signal transduction pathways of different PGRs:

Applications Of Plant Growth Regulators

The applications of PGRs are explained below:

Agriculture

Used to enhance crop yield and productivity.

• Growth enhancement: Size and quality of crops enhanced.

• Disease resistance: Plants are more resistant to diseases.

Examples Of PGRs Used In Agriculture

• Gibberellins: Used on plants to increase fruit size and seed size.

• Cytokinin: Used on crops to promote yield and quality.

Ornamental Plants

• Flowering: The extent and time of flowering can be induced.

• Aesthetics: Modify the shape of plants and the appearance of foliage.

Environmental Management

PGRs are involved in maintaining and restoring the environment.

• Promotion of Growth: This accelerates the growth of reforested areas.

• Stress Tolerance: This promotes the survival of plants on degraded soils.

Differences Between Growth Promoters & Growth Inhibitors

The difference between growth promoters and growth inhibitors is:

Plant Growth Regulators NEET MCQs (With Answers & Explanations)

Important topics for NEET are:

• Types of Plant Growth Regulators

• Mechanism of Action

Practice Questions for NEET

Q1. Name the plant growth regulator, which, upon spraying on sugarcane crops, increases the length of the stem, thus increasing the yield of sugarcane crops.

1. Cytokinin

2. Gibberellin

3. Ethylene

4. Abscisic acid

Correct answer: 2) Gibberellin

Explanation:

• The plant growth hormone gibberellin helps in the elongation of stems in crops such as sugarcane.

• When applied to sugarcane plants, it promotes the development of taller stems, resulting in increased yields.

• This hormone is frequently utilized in agriculture since it is essential for encouraging cell elongation.

• Farmers can increase crop yield and have better outcomes with crops that need vertical growth by utilizing gibberellins.

Hence, the correct answer is option 2) Gibberellin.

Q2. The cells in watermelon may increase in size by up to 3,50,000 times. This phenomenon is called

1. Hypotrophy

2. Hypertrophy

3. Hyperplasia

4. Hypoplasia

Correct answer: 2) Hypertrophy

Explanation:

Hypertrophy is the increase in the size of cells leading to the enlargement of a tissue or organ. HypertrophyHypertrophy typically occurs as a response to increased demand or stress. For instance, muscle hypertrophy takes place when muscle fibres grow larger due to exercise, especially through strength training. While physiological hypertrophy is generally healthy, pathological hypertrophy, such as that occurring in the heart due to high blood pressure, can impair function and lead to complications.

Hence, the correct answer is option 2) Hypertrophy.

Q3. Growth promoting hormone is

1. Auxin

2. Gibberellin

3. Cytokinin

4. All of the above

Correct answer: 4) All of the above

Explanation:

The PGRs can be broadly divided into two groups based on their functions in a living plant body:

• Growth promoting such as mediating cell division, cell enlargement, pattern formation, tropic growth, flowering, fruiting, and seed formation.

• Growth inhibiting such as mediating responses to wounds and stresses of biotic and abiotic origin.

Auxin, gibberellin, and cytokinins are growth-promoting hormones.

Hence the correct answer is option 4) All of the above.

Also Read:

• MCQs on Plant Growth Regulators

• Phases of Plant Growth

• Factors Affecting Plant Growth

Recommended Video On 'Plant Growth Regulators'