Plant Growth Regulators — Explained

Detailed Explanation

Plant Growth Regulators (PGRs), also known as phytohormones, are endogenous organic compounds that, at very low concentrations, significantly influence physiological processes in plants. These chemical messengers are crucial for coordinating growth, development, and responses to environmental stimuli. They are broadly categorized into two groups based on their primary functions: plant growth promoters and plant growth inhibitors.

I. Plant Growth Promoters

These PGRs are generally involved in cell division, cell enlargement, pattern formation, tropic growth, flowering, fruiting, and seed formation.

A. Auxins

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Discovery — The concept of a growth-promoting substance was first proposed by Charles Darwin and his son Francis Darwin in 1880, observing the phototropism of canary grass coleoptiles. F.W. Went, in 1928, isolated auxin from the tips of oat coleoptiles, naming it 'auxin' (from Greek 'auxein' meaning 'to grow').
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Chemical Nature — The most common natural auxin is Indole-3-acetic acid (IAA). Other natural auxins include Indole-3-butyric acid (IBA). Synthetic auxins like Naphthalene acetic acid (NAA) and 2,4-Dichlorophenoxyacetic acid (2,4-D) are also widely used.
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Physiological Effects

* Cell Elongation: Auxins promote the elongation of cells, particularly in stems and coleoptiles, by increasing cell wall plasticity (acid growth hypothesis). * Apical Dominance: The apical bud grows preferentially, inhibiting the growth of lateral (axillary) buds.

This is due to auxin produced by the apical meristem. Removal of the apical bud (decapitation) promotes lateral bud growth. * Root Initiation: Auxins promote root initiation in stem cuttings. However, high concentrations can inhibit root growth in intact plants.

* Parthenocarpy: Application of auxins can induce the development of fruits without fertilization, leading to seedless fruits (e.g., tomatoes). * Abscission Prevention: Young leaves and fruits are prevented from premature shedding by auxins.

As they mature, the auxin concentration decreases, leading to abscission. * Weedicides: Synthetic auxins like 2,4-D are widely used as selective herbicides to kill dicotyledonous weeds without affecting monocotyledonous crops.

* Xylem Differentiation: Auxins play a role in the differentiation of xylem elements.

B. Gibberellins (GAs)

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Discovery — The existence of gibberellins was first reported by E. Kurosawa in 1926 while studying the 'bakanae' (foolish seedling) disease in rice, caused by the fungus *Gibberella fujikuroi*. The active substance was later isolated and named gibberellin.
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Chemical Nature — There are over 100 types of gibberellins, denoted as GA1, GA2, GA3, etc. GA3 (Gibberellic acid) is the most commonly studied and biologically active form.
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Physiological Effects

* Stem Elongation: GAs cause a significant increase in stem length, especially in genetically dwarf plants (e.g., dwarf peas, maize). This effect is due to increased cell elongation and cell division.

* Bolting: In rosette plants (e.g., cabbage, beet), GAs induce bolting, which is the sudden elongation of the internodes just prior to flowering. * Seed Germination: GAs break seed dormancy and promote germination, particularly in cereal grains.

They stimulate the synthesis of $alpha$-amylase in the aleurone layer, which breaks down stored starch into sugars for the embryo. * Fruit Growth: GAs can increase the size of fruits, such as grapes, and improve their shape (e.

g., apples). * Malting Process: GAs are used in the brewing industry to speed up the malting process. * Juvenility: GAs can promote juvenility in conifers, leading to early seed production.

C. Cytokinins

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Discovery — The discovery of cytokinins originated from experiments by F. Skoog and C. Miller in the 1950s, who found that cell division in tobacco pith callus required an active substance from degraded DNA. This substance was later identified as kinetin (a modified adenine), a synthetic cytokinin. The first natural cytokinin, zeatin, was isolated from corn kernels and coconut milk by Letham in 1963.
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Chemical Nature — Cytokinins are derivatives of adenine (a purine). Kinetin is synthetic, while zeatin is a natural cytokinin. Other natural cytokinins include isopentenyladenine.
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Physiological Effects

* Cell Division: Cytokinins are essential for cell division (cytokinesis) in plant tissues, especially in the presence of auxins. * Morphogenesis: The ratio of auxin to cytokinin in tissue culture determines the differentiation of roots and shoots.

A high auxin:cytokinin ratio promotes root formation, while a low ratio favors shoot development. * Lateral Bud Growth: Cytokinins promote the growth of lateral buds, counteracting apical dominance induced by auxins.

* Delay Senescence: They delay the aging (senescence) of leaves by promoting nutrient mobilization. * Chloroplast Development: Cytokinins promote chloroplast development in leaves.

II. Plant Growth Inhibitors

These PGRs are primarily involved in dormancy, abscission, and responses to stress. Ethylene, while often an inhibitor, also has promoter-like effects.

D. Ethylene

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Discovery — H. Cousins (1910) confirmed that a volatile substance released from ripening oranges accelerated the ripening of unripe bananas, later identified as ethylene.
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Chemical Nature — Ethylene is a simple gaseous hydrocarbon ($C_2H_4$). It is the only gaseous plant hormone.
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Physiological Effects

* Fruit Ripening: Ethylene is a key hormone for fruit ripening, especially in climacteric fruits (fruits that continue to ripen after harvest, e.g., bananas, apples, tomatoes). It increases the respiration rate during ripening.

* Senescence and Abscission: It promotes senescence (aging) of leaves and flowers and accelerates abscission (shedding) of leaves, flowers, and fruits. * Epinasty: Downward bending of leaves due to faster growth on the upper side of the petiole.

* Root Growth and Root Hair Formation: Ethylene promotes root growth and the formation of root hairs, increasing the absorption surface area. * Triple Response: In dicot seedlings, ethylene causes a 'triple response' to mechanical stress: inhibition of stem elongation, increased radial swelling of the stem, and horizontal growth of the hypocotyl.

* Flowering: Can induce flowering in some plants (e.g., pineapple) and synchronize fruit set.

E. Abscisic Acid (ABA)

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Discovery — ABA was discovered independently by three groups of scientists in the 1960s and was initially named abscisin II and dormin. F.T. Addicott isolated abscisin from cotton bolls, and P.F. Wareing isolated dormin from sycamore leaves.
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Chemical Nature — ABA is a carotenoid derivative.
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Physiological Effects

* Seed Dormancy: ABA induces and maintains seed dormancy, ensuring seeds germinate only under favorable conditions. It acts antagonistically to gibberellins in this regard. * Stomatal Closure: ABA is a 'stress hormone.

' Under water stress, it signals guard cells to close stomata, reducing transpiration and conserving water. * Abscission: It promotes the abscission of leaves, flowers, and fruits, especially under stress conditions.

* Growth Inhibition: ABA generally inhibits plant growth and metabolism. * Bud Dormancy: Induces dormancy in buds, particularly in temperate regions during winter.

III. Interactions of Plant Growth Regulators

PGRs rarely act in isolation. Their effects are often a result of complex interactions, either synergistic (where two or more PGRs work together to produce an effect greater than the sum of their individual effects) or antagonistic (where one PGR opposes the action of another).

Auxin-Cytokinin — Antagonistic in apical dominance (auxin promotes, cytokinin breaks). Synergistic in cell division in tissue culture (both required).
Gibberellin-ABA — Antagonistic in seed dormancy (ABA promotes, GA breaks) and stomatal closure (ABA promotes, GA has minor opposing effects).
Auxin-Ethylene — Auxin can induce ethylene production, and high ethylene can inhibit auxin transport, leading to complex feedback loops.

IV. Common Misconceptions and NEET-Specific Angle

Misconception — All auxins promote root growth. Correction: While auxins are used to initiate roots in cuttings, high concentrations can inhibit root elongation in intact plants. The optimal concentration for root growth is much lower than for shoot growth.
Misconception — Ethylene only causes ripening. Correction: While prominent in ripening, ethylene also promotes senescence, abscission, and has roles in root growth and the triple response.
Misconception — ABA is solely a 'bad' hormone. Correction: ABA is crucial for plant survival under stress, inducing dormancy and stomatal closure, which are vital adaptive mechanisms.

NEET-Specific Angle: Questions often focus on specific applications (e.g., 2,4-D as a herbicide, GA for malting), antagonistic/synergistic relationships, the gaseous nature of ethylene, and the stress-response role of ABA. Experimental setups (like Went's experiment or tissue culture ratios) are also frequently tested. Remembering the specific examples of plants where a particular PGR effect is prominent (e.g., bolting in cabbage, seedless tomatoes) is highly beneficial.