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

Definition Of Plant Growth Regulators (PGRs)

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.

Types Of Plant Growth

It is possible to divide the nature and patterns of growth into several different types. Being able to understand and know these types in detail is key to understanding how plants grow and develop and so how they may be managed or manipulated for agricultural and horticultural purposes.

Primary Growth

• This growth takes place in the tips of roots and shoots and involves cell division by mitosis of meristematic cells.

• This causes an increase in the length of the plant, both upwards and downwards.

• The elongation of stems and roots takes place by primary growth.

Secondary Growth

• Secondary growth is the increase in the diameter of the plant body.

• It takes place due to the division of the secondary meristem such as the vascular cambium and cork cambium.

• This type of growth causes the thickening of stems and roots and provides support to the plant by giving rise to structures that can provide structural support and greater vascular transport.

Unlimited Growth

• Unlimited growth refers to the continuous growth of plants from germination until death.

• Plants that show unlimited growth are indefinite in size and will continue to grow as long as conditions are favourable.

• Examples: Most trees and perennial plants are indeterminate. They continue to make new tissues and organs throughout their life and, in doing so, continue to grow in size.

Limited Growth

• In limited growth, the growth of plant parts comes to an end once it reaches a particular size.

• This is characteristic in most plants where growth is limited to certain phases or organs.

• Examples: Most annual plants and some plant organs, such as flowers and fruits, have indeterminate growth, which stops when a maximum size is reached.

Vegetative Growth

• Vegetative growth is the growth of stems, leaves and branches which are non-reproductive structures.

• This stage of growth is significant in developing the form and structure of the plant body and also in laying the groundwork that will support future reproductive growth.

Reproductive Growth

• The transition to the reproductive phase is marked by the formation of flowers, which produce seeds.

• This is a very significant stage in the life cycle of a plant species to generate offspring via sexual reproduction.

Conditions That Influence Plant Growth

• Availability of Water: Cell activities and overall plant physiology

• Availability of Nutrients: Macronutrients N, P, and K and micronutrients Fe, Mn, and Zn have a role in growth and development.

• Light: Currently Influences photosynthesis and plant photoreceptors

• Temperature: Influences the activity of enzymes and metabolic rate

• Soil Conditions: Affects the root growth pH, texture, structure, and availability of nutrients

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

Below are the types of plant growth regulators:

Auxins

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

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

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

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

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

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.

Abscisic Acid (ABA)

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

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

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

Function And Mechanism

• Fruit Ripening: It speeds up fruit ripening.

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

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

Mechanism Of Action

Signal Transduction Pathways:

• Auxins: Acting on receptors, changes gene expression and causes cell elongation.

• Gibberellins: Activating transcription factors that stimulate growth.

• Cytokinins: Inducing signalling cascades that promote cell division.

• Abscisic Acid: Turning on stress response genes and closing stomata.

• Ethylene: Modify enzyme activities that control fruit ripening and senescence.

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.

Recommended Video On 'Plant Growth Regulators'