Plant Growth Regulators (Plant Hormones or Phytohormones)
- Plant growth regulators (PGRs) are small, simple molecules that regulate plant growth.
- Based on their functions, PGRs are divided into two groups:
- Plant growth promoters: Promote activities like cell division, cell enlargement, tropic growth, pattern formation, flowering, fruiting, and seed formation. E.g., auxins, gibberellins, and cytokinins.
- Plant growth inhibitors: Involved in growth-inhibiting activities like dormancy and abscission, and respond to wounds and biotic/abiotic stresses. E.g., abscisic acid and ethylene (ethylene is largely a growth inhibitor but can fit either group).
1. Auxins
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Charles Darwin and his son Francis Darwin observed that coleoptiles of canary grass responded to unilateral illumination by growing toward the light source (phototropism). They concluded that the coleoptile tip caused the bending of the entire coleoptile.
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F.W. Went isolated auxin (Greek ‘auxein’: to grow) from the tips of oat seedling coleoptiles. Auxin was first isolated from human urine. Auxins are produced by growing apices of stems and roots, migrating to regions of action.
Types of Auxins:
- Natural: E.g., Indole-3-acetic acid (IAA) and indole butyric acid (IBA), isolated from plants.
- Synthetic: E.g., NAA (naphthalene acetic acid) and 2,4-D (2,4-dichlorophenoxyacetic acid.
Functions of Auxins:
- Initiate rooting in stem cuttings for plant propagation.
- Promote flowering (e.g., in pineapples).
- Prevent early fruit and leaf drop.
- Promote abscission of older leaves and fruits.
- Induce parthenocarpy (e.g., in tomatoes).
- Used as herbicides (e.g., 2,4-D kills dicot weeds without affecting mature monocots, used for weed-free lawns).
- Control xylem differentiation and aid cell division.
In higher plants, the growing apical bud inhibits lateral (axillary) bud growth, known as apical dominance. Removing shootanhão t tips (decapitation) promotes lateral bud growth, applied in tea plantations and hedge-making.
b) A plant with apical bud removed
2. Gibberellins
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Gibberellins are acidic PGRs. E. Kurosawa treated healthy rice seedlings with sterile filtrates of Gibberella fujikuroi, a fungus causing ‘bakane’ disease (foolish seedling) in rice, resulting in disease symptoms. The active substance was identified as gibberellic acid.
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There are over 100 gibberellins (GA1, GA2, GA3, etc.) in fungi and higher plants. Gibberellic acid (GA3 or Terpenes) is one of the first discovered and most studied.
Functions:
- Increase axis length, used to extend grape stalks.
- Elongate and improve fruit shape (e.g., apples).
- Delay senescence, allowing fruits to stay on trees longer for extended market periods.
- GA3 speeds up the malting process in the brewing industry.
- In sugarcane, gibberellin spraying increases stem length, boosting yield by up to 20 tonnes per acre.
- Spraying juvenile conifers with GAs hastens maturity, leading to early seed production.
- Promote bolting (internode elongation before flowering) in beets, cabbages, and plants with rosette habits.
3. Cytokinins
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F. Skoog and co-workers found that tobacco stem internodal segments’ callus proliferated only when the nutrient medium included vascular tissue extracts, yeast extract, coconut milk, or DNA. Skoog & Miller identified and crystallized the active substance, naming it kinetin.
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Cytokinins were discovered as kinetin (N6-furfurylamino purine, an adenine derivative) from autoclaved herring sperm DNA. Kinetin is not naturally occurring in plants. Zeatin (from corn kernels and coconut milk) is a natural cytokinin. Synthetic compounds also exhibit cell division-promoting activity.
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Natural cytokinins are synthesized in regions of rapid cell division (root apices, shoot buds, young fruits, etc.).
Functions:
- Play a role in cytokinesis.
- Promote new leaves, chloroplasts in leaves, lateral shoot growth, and adventitious shoot formation.
- Overcome apical dominance.
- Promote nutrient mobilization, delaying leaf senescence.
4. Ethylene (C2H4)
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Cousins confirmed that ripened oranges released a volatile substance hastening banana ripening. This substance was identified as ethylene, a simple gaseous PGR.
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Ethylene is synthesized in large amounts by tissues undergoing senescence and ripening fruits.
Functions:
- Influences horizontal seedling growth, axis swelling, and apical hook formation in dicot seedlings.
- Promotes senescence and abscission of leaves and flowers.
- Promotes fruit ripening, enhancing respiration rate during ripening (respiratory climactic).
- Breaks seed and bud dormancy, initiates germination in peanut seeds, and promotes potato tuber sprouting.
- Promotes rapid internode/petiole elongation in deep-water rice, keeping leaves/shoots above water.
- Promotes root growth and root hair formation, increasing absorption surface.
- Initiates flowering and synchronizes fruit-set in pineapples; induces flowering in mango.
- Widely used in agriculture.
The most widely used ethylene source is ethephon. Ethephon, absorbed in aqueous solution and transported within the plant, releases ethylene slowly. It hastens fruit ripening in tomatoes and apples, accelerates abscission in flowers and fruits (thinning cotton, cherry, walnut), and promotes female flowers in cucumbers, increasing yield.
5. Abscisic Acid (ABA)
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In the mid-1960s, three inhibitors—inhibitor-B, abscisin II, and dormin—were found to be chemically identical, now known as abscisic acid (ABA).
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ABA, a derivative of carotenoids, regulates abscission and dormancy.
Functions:
- Inhibits plant growth and metabolism.
- Inhibits seed germination.
- Stimulates stomatal closure in the epidermis.
- Increases plant tolerance to various stresses, earning it the name stress hormone.
- Supports seed development, maturation, and dormancy, aiding resistance to desiccation and unfavorable conditions.
Interactions of PGRs
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PGRs play individualistic or synergistic roles, which may be complementary or antagonistic. They interact to affect dormancy, abscission, flowering, senescence, vernalization, apical dominance, seed germination, and plant movements. In most cases, ABA acts as an antagonist to GAs.
Factors Influencing the Action of PGRs
- Intrinsic factor: Genomic control.
- Extrinsic factors: Light and temperature.