What is the primary difference in the discovery of auxins and gibberellins?

The discovery of auxins originated from observations of phototropism in plant coleoptiles, where Charles Darwin and F.W. Went identified a growth-promoting substance from the plant's own tissues. In contrast, gibberellins were first identified in the context of a plant disease (bakanae disease in rice) caused by a fungus, *Gibberella fujikuroi*, by E. Kurosawa. This highlights that auxins were initially recognized as endogenous plant regulators, while gibberellins were first isolated as a fungal metabolite affecting plant growth.

How do auxins cause cell elongation, and what is the 'acid growth hypothesis'?

Auxins promote cell elongation primarily by increasing the extensibility of the cell wall. The 'acid growth hypothesis' explains this mechanism: auxins stimulate proton pumps (H+-ATPases) in the plasma membrane to pump protons into the cell wall. This lowers the pH of the cell wall, activating pH-sensitive enzymes like expansins. These enzymes loosen the cellulose microfibrils and other components of the cell wall, making it more flexible. With a loosened cell wall, the cell can then take up water and expand due to turgor pressure, leading to elongation.

Can gibberellins be used to overcome dwarfism in plants? If so, how?

Yes, gibberellins are highly effective in overcoming genetic dwarfism in many plant species. Dwarfism in such plants is often due to a deficiency in endogenous gibberellin synthesis or a reduced sensitivity to gibberellins. When treated with exogenous gibberellins, particularly GA3, these dwarf plants exhibit rapid and significant stem elongation, growing to a height comparable to their normal counterparts. This is because gibberellins promote both cell elongation and cell division in the internodes, leading to increased stem length.

What is apical dominance, and how do auxins regulate it?

Apical dominance is a phenomenon where the growth of the apical (terminal) bud inhibits the growth of lateral (axillary) buds along the stem. Auxins, produced in the actively growing apical meristem, are the primary regulators of this process. High concentrations of auxin transported downwards from the apical bud suppress the development of lateral buds. If the apical bud is removed (decapitation), the source of high auxin is eliminated, releasing the lateral buds from inhibition and allowing them to grow, leading to a bushier plant.

What are some practical applications of auxins and gibberellins in agriculture?

Auxins have several agricultural uses: promoting rooting in stem cuttings (e.g., NAA, IBA), inducing parthenocarpy (seedless fruit) in tomatoes (e.g., IAA), and acting as selective herbicides (e.g., 2,4-D) to kill broad-leaved weeds. Gibberellins are used to increase fruit size and elongate bunches in grapes (GA3), accelerate malting in the brewing industry (GA3), promote bolting and flowering in certain plants, and overcome dormancy in seeds to ensure uniform germination.

How do auxins and gibberellins differ in their primary mode of transport within the plant?

Auxins exhibit a unique characteristic called polar transport, meaning they move actively and unidirectionally, primarily from the shoot apex downwards (basipetal) through parenchyma cells, not through the vascular tissues like xylem or phloem. This directed transport is crucial for establishing auxin gradients. In contrast, gibberellins are generally transported more broadly throughout the plant via the vascular system (xylem and phloem), allowing for more systemic distribution from their sites of synthesis to target tissues.

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Auxins and Gibberellins

Biology

NEET UG

Version 1Updated 21 Mar 2026

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Auxins and gibberellins represent two fundamental classes of plant growth regulators (PGRs), also known as phytohormones, which orchestrate a vast array of developmental processes in plants. Auxins, primarily indole-3-acetic acid (IAA), are renowned for their role in cell elongation, apical dominance, root initiation, and fruit development, exhibiting polar transport within plant tissues. Gibberel…

Quick Summary

Auxins and Gibberellins are two vital classes of plant growth regulators (PGRs) that orchestrate plant development. Auxins, primarily Indole-3-acetic acid (IAA), are synthesized in shoot apices and young leaves.

They are known for promoting cell elongation, leading to phototropism and gravitropism. Key roles include establishing apical dominance, initiating adventitious roots in cuttings, preventing premature fruit and leaf drop, and inducing parthenocarpy.

Synthetic auxins like 2,4-D are used as herbicides. Auxins exhibit polar transport, moving unidirectionally.

Gibberellins, a diverse group with Gibberellic Acid (GA3) being prominent, are synthesized in young leaves, seeds, and roots. Their most striking effect is dramatic stem elongation, especially in dwarf varieties, and inducing bolting in rosette plants.

They are crucial for breaking seed dormancy and promoting germination by stimulating hydrolytic enzyme synthesis. Gibberellins also enhance fruit growth and can influence flowering. Both hormones are essential for agricultural applications, improving crop yield and quality, and represent fundamental aspects of plant physiology.

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Key Concepts

Apical Dominance and its Release

Apical dominance is a classic example of hormonal control over plant architecture. The actively growing…

Gibberellins and Seed Germination

Gibberellins play a critical role in breaking seed dormancy and initiating germination, particularly in…

Parthenocarpy Induction by Auxins

Parthenocarpy refers to the development of fruit without the process of fertilization, resulting in seedless…

Auxins: — IAA (natural), NAA, 2,4-D (synthetic).

Auxin Synthesis: — From Tryptophan.

Auxin Transport: — Polar (basipetal).

Auxin Effects: — Cell elongation, Apical dominance, Root initiation, Parthenocarpy, Prevent abscission, Xylem differentiation, Herbicide (2,4-D).

Gibberellins: — GA3 (most common).

Gibberellin Synthesis: — Mevalonic acid pathway.

Gibberellin Effects: — Stem elongation (dwarf plants), Seed germination (

\alpha

-amylase), Bolting, Fruit growth, Flowering.

Discovery: — Auxins (Darwin, Went), Gibberellins (Kurosawa, *Gibberella fujikuroi*).

All Apples Are Always Ready Particularly Herbicidal: Auxins, Apical dominance, Abscission prevention, Adventitious roots, Rooting, Parthenocarpy, Herbicides (2,4-D).

Giant Grapes Germinate Boldly: Gibberellins, Grape size, Germination (seed), Bolting.

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Biology

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Cytokinins, Ethylene and ABA