Plant Hormones: Types, Functions, Diagram and Role in Plant Growth for NEET

Plant hormones, also known as phytohormones, are chemical substances present in extremely small quantities that regulate plant growth, development, reproduction, and survival. They act as internal signaling molecules that coordinate different physiological processes such as cell division, elongation, flowering, dormancy, and senescence.

Plants depend on both external factors (light, water, oxygen, minerals) and internal factors (plant hormones) for proper growth. While external factors provide resources, plant hormones ensure proper coordination and regulation of these processes.

Plant hormones are chemically diverse:

• Auxins → derived from indole compounds

• Gibberellins → derived from terpenes

• Cytokinins → derived from adenine

• Abscisic acid → derived from carotenoids

• Ethylene → gaseous hormone

These hormones are produced in almost all plant parts and transported to target tissues where they perform specific functions. They may act:

• Individually

• Synergistically (working together)

• Antagonistically (opposite effects)

They regulate key processes like phototropism, vernalisation, seed germination, dormancy, and overall plant development.

Functions of Plant Hormones

The plant hormones and their functions are central to plant life. They regulate almost every stage of development.

Major Functions:

• Control cell division and elongation

• Regulate flowering and fruit development

• Initiate seed germination

• Maintain dormancy and senescence

• Control abscission (leaf and fruit fall)

• Coordinate responses to environmental stimuli

Plant hormones act as master regulators ensuring balanced growth and survival under changing environmental conditions.

Types of Plant Hormones

The types of plant hormones are broadly classified based on their role:

1. Plant Growth Promoters

• Auxins

• Gibberellins

• Cytokinins

2. Plant Growth Inhibitors

• Abscisic Acid (ABA)

• Ethylene (acts as both promoter and inhibitor)

Plant Hormones At a Glance

Auxins (Major Plant Growth Hormones)

Auxins are one of the most important plant growth hormones responsible for elongation and directional growth.

Source:

• Growing tips of roots and shoots

Types:

• Natural: IAA (Indole-3-acetic acid), IBA

• Synthetic: 2,4-D, NAA

Functions of Auxins:

• Promote cell elongation in stems and roots

• Maintain apical dominance (inhibits lateral buds)

• Induce parthenocarpy (seedless fruits like tomato)

• Prevent premature fall of leaves and fruits

• Promote rooting in stem cuttings

• Stimulate flowering (e.g., pineapple)

• Help in xylem differentiation

• Used as herbicides (2,4-D)

Auxins are widely used in agriculture and horticulture practices.

Gibberellins

Gibberellins are growth hormones mainly responsible for stem elongation and reproductive development.

Key Features:

• More than 100 types identified (GA₁, GA₂, GA₃…)

• Found in higher plants and fungi

Functions:

• Promote bolting (rapid internode elongation)

• Induce parthenocarpy

• Reverse dwarfism

• Delay senescence

• Break seed dormancy

• Stimulate enzymes like amylase in germinating seeds

• Induce maleness in some plants

They play a crucial role in plant height and seed germination.

Cytokinins

Cytokinins are hormones involved in cell division and growth regulation.

Source:

• Root apices, shoot buds, young fruits

Movement:

• Basipetal (from top to bottom)

Types:

• Natural: Zeatin, isopentenyladenine

• Synthetic: Kinetin, benzyladenine

Functions:

• Promote cell division (cytokinesis)

• Stimulate lateral and adventitious shoot growth

• Overcome apical dominance

• Promote chloroplast formation

• Delay leaf senescence

• Help in nutrient mobilization

They are widely used in plant tissue culture techniques.

Abscisic Acid (ABA)

Abscisic acid is a growth-inhibiting plant hormone often called the stress hormone.

Key Role:

• Antagonist to gibberellins

Functions:

• Induces abscission of leaves and fruits

• Inhibits seed germination

• Promotes senescence

• Induces seed dormancy (important for storage)

• Causes stomatal closure during water stress

ABA helps plants survive under adverse environmental conditions.

Ethylene (Unique Gaseous Hormone)

Ethylene is the only gaseous plant hormone and has both promoting and inhibiting effects.

Source:

• Ripening fruits

• Senescing tissues

Functions:

• Accelerates fruit ripening

• Controls epinasty (downward bending of leaves)

• Breaks seed and bud dormancy

• Promotes senescence and abscission

• Stimulates root hair formation

• Induces femaleness in plants

• Helps in apical hook formation

Ethylene is widely used in agriculture for controlled fruit ripening.

Role of Plant Hormones in Plant Growth

The role of plant hormones is highly coordinated and complex.

Key Roles:

• Maintain balance between growth and inhibition

• Regulate developmental stages

• Help plants respond to environmental changes

• Control agricultural productivity

Interaction Example:

• Auxin suppresses lateral buds

• Cytokinins promote lateral growth. Together they regulate plant shape

Additional Plant Hormones

Apart from the five major hormones, other important regulators include:

• Brassinosteroids

• Jasmonates

• Salicylic acid

• Strigolactones

These also influence plant defense and development.

Final Verdict from Experts

Plant hormones are essential chemical regulators that control every aspect of plant life. These hormones work together to ensure proper growth, development, reproduction, and survival of plants.