LESSON 4:Phytohormones and Growth Regulators in Plants

Scense Me
May 3
2 min read

Whether it is a seedling pushing through the soil or a fruit ripening on a branch, every movement a plant makes is orchestrated by complex chemical messengers known as Plant Growth Regulators (PGRs) or Phytohormones. Derived from the Greek word hormaein, meaning "to excite," these natural molecules work in minute concentrations to produce substantial physiological changes.

What Defines a Phytohormone?

Unlike animals, plants do not have specialised glands for hormone production. Instead, these organic compounds are produced naturally in tissues like apical meristems or young leaves and are often transported to remote sites to control growth and development.

The Five Major Classes of Phytohormones

Plant hormones are generally categorised into two functional groups: Growth Promoters and Growth Inhibitors.

1. Auxins: The Master Regulators

First isolated by F.W. Went in 1926, Auxins are essential for structural development.

Primary Function: Regulates cell elongation and maintains apical dominance (where the main central stem grows more strongly than side stems).
Key Habit: It diffuses away from light toward the shaded side of a plant, causing uneven growth that makes the plant bend toward the sun—a process called phototropism.

2. Gibberellins: The Growth Stimulators

Discovered through the "foolish seedling" disease in rice, Gibberellins are terpenes that affect both cell division and elongation.

Primary Function: Promotes stem elongation, breaks seed dormancy, and increases fruit size.
Practical Use: They are often used commercially to produce larger, seedless grapes and to improve the shape of fruits like apples.

3. Cytokinins: The Rejuvenators

Derived from adenine, these hormones are the primary drivers of cell division (mitosis).

Primary Function: Promotes shoot formation, delays leaf senescence (ageing), and encourages lateral bud development.
The Balance: The ratio of Cytokinin to Auxin determines whether a plant tissue will develop into roots or shoots.

4. Abscisic Acid (ABA): The Stress Hormone

Often acting as a general growth inhibitor, ABA helps plants survive adverse conditions.

Primary Function: Stimulates the closure of stomata to prevent water loss during drought and maintains seed dormancy.
Role: It acts as an antagonist to growth promoters like Gibberellins to ensure seeds only germinate under favourable conditions.

5. Ethylene: The Gaseous Signal

Ethylene is unique as the only gaseous plant hormone.

Primary Function: Induces fruit ripening, encourages leaf and flower senescence, and assists seedlings in pushing past soil impediments (the "triple response").
Impact: Because it is a gas, Ethylene produced by one plant can affect nearby plants, causing synchronised ripening.

Beyond the Big Five

Modern plant science identifies several other critical regulators:

Brassinosteroids: Essential steroid hormones for cell expansion and seed development.
Salicylic Acid: Best known for mediating a plant's immune response to pathogen infections.
Strigolactones: Signalling compounds that inhibit shoot branching and promote symbiotic interactions with soil microbes.

Understanding these "invisible conductors" is key to advancing sustainable agriculture and food security. By manipulating these natural pathways, we can improve crop resilience, yield, and quality.

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