Plant Growth Regulators: Definition, Types, and Mechanism

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What are Plant Growth Regulators?

The plant growth which is irreversible, progressive and unlimited is mainly because of the fact that specialized meristem regions are capable of having an indefinite cell division when required and are regulated by internal factors.

Genetic makeup and other physiological factors namely hormones are responsible for the condition of unlimited growth of the plant.

Unique set of hormones are well developed which regulates the development of the plant in a way where it balances the growth of the plant and it retains it by a gradual process.

Plant Growth Regulators (PGR) are hormones which acts on cells to promote and inhibit when stimulated.

Plant Growth Regulators and hormones are similar terms, hormones generally cover a broad portion of compounds and Plant Growth Regulators’s are plant hormones or phytohormones which are specialized in cell division, elongation, growth of a plant body.

Hormones are simple molecules having varied diverse composition.

They are required in minimal amount for the growth of organism

Plant Growth Regulators History

The discovery was accidental and had many experimentations while concluding its actions.

Charles Darwin and Francis Darwin observed the plant response to light and uneven growth towards light was observed. Between 1950 – 1960 a group of hormones were discovered and were named as “classical five” – Auxin, Gibberellin, Abscisic Acid, Ethylene and Cytokinin.

Following this many natural and synthetic regulators were discovered later.

Characteristics of Plant Growth Regulators

• Hormones are pleiotropic in nature

Example: Auxin helps in cell division, elongation, apical dominance etc.

• Several hormones act for same responses leads to reduced response for a particular stimuli / action

• Small concentration is required

• Hormones are synthesized at different parts of the plant body and are transported to the site of action

• Hormone action is time specific and precise

• Hormones can be stimulator or inhibitor

Types of Plant Growth Regulators

Plant hormones are of 2 types:

1. Growth promoters: promote cell division, cell elongation, pattern formation, seed and fruit development

2. Responser: Biotic and Abiotic cues response and for stressors

Plant Growth Regulators and Their Functions

I. Auxin

• Naturally occurs as Indole 3 Acetic Acid.

• Synthesis: Leaf primordia and young leaves

• Transportation: flows through phloem

• Induces overall polarity of root shoot polarity

• Basipetal transportation of hormones from tips of leaf to the leaf and stem and similarly, root tips → root – shoot differentiation zone.

• Apical dominance reduces the Abscission in apical region

• Auxin are signal mediated multiple level coordinated developmental processes

II. Cytokinin

• Promote cell division

• Transported to shoot from root through Xylem

• Activates lateral bud formation

• Pro – vascular tissue formation

• Positive geotropism is observed

• Inhibitors of root development and control meristem activity

III. Ethylene

• Simple hydrocarbon which is volatile

• Present in Seeds of all plants

• Transportation by: Diffusion

• Production sites: shoot apices, nodal region of stem

• Ripening process in climacteric fruits

• High production during stress

• Ethylene promotes leaf abscission

IV. Abscisic Acid

• Present throughout the plan body

• Aids in Abscission triggered by ethylene

• Transportation: Xylem and Phloem. Abundant in Phloem

• When water is in excess, ABA transports to stomata through xylem which induces stomatal closure

• High production during seed protein synthesis and seed development

• Break dormancy in seeds

V. Gibberellins

• Tetracyclic diterpenoids

• In seed and fruit development

• Growth of bud, Leaves, Internodes

• Transportation via Phloem

• Stem and leaf elongation also in root elongation and pollen tube growth

• Secretes enzymes

Plant Growth Regulators Citations


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