Q.Give the full form of IAA?

IAA (auxin) is naturally synthesised in plants from the amino acid tryptophan.

Auxins are a class of plant growth hormones, the most important naturally occurring one being indole-3-acetic acid (IAA). IAA is biosynthesised in plant tissues (especially shoot and root apices) from the amino acid tryptophan through a series of enzymatic steps. Auxins promote cell elongation, apical dominance, rooting, and various tropic responses.

Gibberellin (a) and Ethylene (b) are themselves other classes of plant hormones, not precursors of auxin. Phenylalanine (c) is a different amino acid, precursor of other compounds (e.g. lignin, some secondary metabolites), not of IAA.

This statement is False — Auxin's textbook functions are growth promotion (cell elongation), apical dominance, root initiation, and parthenocarpy; flowering induction is not one of its characteristic roles.

Auxins (like IAA) are plant growth hormones with well-defined functions: they promote cell elongation in shoots and coleoptiles, help initiate rooting in stem cuttings, maintain apical dominance (suppression of lateral bud growth by the apical bud), induce parthenocarpic (seedless) fruit development, and prevent premature fruit/leaf drop at appropriate concentrations, besides being used commercially as a herbicide at high concentration. Promotion of flowering is instead most closely associated with gibberellins (which can induce bolting and flowering in many long-day/biennial plants) and the flowering signal molecule florigen; even the special case of induced flowering in crops like pineapple is achieved using ethylene, not auxin. Hence, auxin is not the hormone credited with promoting flowering.

IAA is a natural plant hormone, so the answer is (C).

Auxins occur as natural and synthetic types:

• Natural auxin: IAA (indole-3-acetic acid), produced by the plant itself in growing shoot and root tips.
• Synthetic auxins: NAA (naphthalene acetic acid) and 2,4-D (2,4-dichlorophenoxyacetic acid, used as a weedicide).

Ethylene glycol is not a plant hormone. Therefore the natural plant hormone among the options is IAA.

Auxin is responsible for cell elongation, so the answer is (B).

Roles of the main plant hormones:

• Auxin — promotes cell elongation, apical dominance, phototropism and root initiation.
• Cytokinin — promotes cell division and delays ageing.
• Abscisic acid — a growth inhibitor; promotes dormancy and stomatal closure.
• Ethylene — promotes fruit ripening and senescence.

Since the question asks about cell elongation, the answer is auxin.

Auxin is a multifunctional plant hormone that promotes cell growth, enforces apical dominance, and stimulates cambial activity — so all three listed effects are correct.

Auxins (e.g., IAA) are synthesized mainly in shoot and root apices and young leaves. Their documented physiological roles include:

• Promoting cell elongation and overall growth and development, by loosening the cell wall (acid growth theory) and stimulating cell division/differentiation.
• Maintaining apical dominance — auxin from the growing shoot tip suppresses the growth of lateral (axillary) buds; removing the tip (decapitation) releases lateral buds from this inhibition.
• Stimulating cambial activity, inducing division of cambial cells and helping differentiate secondary xylem and phloem.

Because auxin genuinely performs all three functions, none of the individual options (a), (b), or (c) alone captures its full role.

Auxins are the hormone group used to initiate rooting in stem cuttings, a widely used horticultural application of this growth regulator.

Among the five major classes of plant hormones:

• Auxins promote cell elongation, apical dominance, and — importantly — induce the formation of adventitious roots. Synthetic auxins such as indole-3-butyric acid (IBA) and naphthalene acetic acid (NAA) are widely applied as 'rooting hormones' to stem cuttings in horticulture and nurseries to promote rapid and profuse root development before the cutting is planted.
• Gibberellins (B) promote stem elongation, seed germination and flowering; they are not associated with root initiation in cuttings.
• Cytokinins (C) promote cell division and delay senescence, and are more associated with shoot/bud formation in tissue culture.
• Ethylene (D) promotes fruit ripening and senescence/abscission.

Since root initiation is specifically an auxin-mediated response, option (A) is correct.

F.W. Went's classic oat-coleoptile experiment led to the discovery and isolation of the plant hormone auxin.

F.W. Went's experiments (1928) built on earlier observations by Charles and Francis Darwin on phototropic bending of grass coleoptiles. Went cut off the tips of oat coleoptiles, placed them on agar blocks to let the growth-promoting substance diffuse into the agar, and then placed the agar blocks asymmetrically on decapitated coleoptile stumps. The stumps bent away from the side bearing the agar block, showing that a chemical substance from the tip promoted cell elongation. This substance was isolated and named auxin (from Greek 'auxein', to grow) — it is now known chemically as indole-3-acetic acid (IAA). Gibberellins, cytokinins and ethylene are distinct classes of plant hormones with different discovery histories.

IAA stands for Indole-3-Acetic Acid, a naturally occurring auxin (plant growth-promoting hormone).

IAA is the principal natural auxin of plants.

Indole-3-Acetic Acid (IAA) is the most common naturally occurring auxin in plants. It is synthesized mainly in shoot apices, young leaves and developing seeds, and it promotes cell elongation, apical dominance, root initiation, and various tropic (phototropic and geotropic) responses by regulating cell wall loosening and differential growth.

IAA (indole-3-acetic acid) and indole butyric acid (IBA) are naturally-occurring auxins isolated from plants; NAA and 2,4-D are synthetic auxin analogues.

Auxins were first isolated from human urine by F.W. Went, and later, Indole-3-acetic acid (IAA) was identified as a naturally occurring auxin produced by plants, mainly in the shoot and root apices, from where it migrates to the regions of its action. Indole butyric acid (IBA), also a natural auxin, was later isolated from plants too. In contrast, 2,4-Dichlorophenoxyacetic acid (2,4-D) and Naphthalene acetic acid (NAA) are synthetic auxins, widely used in agriculture/horticulture (e.g., 2,4-D as a herbicide) but not found naturally in plants.

Agent Orange is the code name for a herbicide made of the synthetic auxins 2,4-D and 2,4,5-T, used at high concentration to kill broad-leaved plants by disrupting their growth.

2,4-D (2,4-dichlorophenoxyacetic acid) and 2,4,5-T (2,4,5-trichlorophenoxyacetic acid) are synthetic auxin analogues. At low concentrations synthetic auxins promote normal growth responses, but at high concentrations they cause uncontrolled, abnormal growth that kills the plant - so they are widely used as selective herbicides against broad-leaved dicot weeds (monocots like cereals are relatively less sensitive). A roughly 1:1 mixture of 2,4-D and 2,4,5-T, code-named 'Agent Orange' (after the orange identification stripe on its storage drums), was used as a defoliant.

The other options are different plant hormones: ABA (abscisic acid) is the stress/dormancy hormone; auxin (IAA) is the natural auxin, not the synthetic herbicide mixture; NAA (naphthalene acetic acid) is a different synthetic auxin used mainly to induce rooting, not as 'Agent Orange'.

Tryptophan (the amino acid tryptophan) is the primary precursor of IAA (indole-3-acetic acid).

IAA, the principal natural auxin, is biosynthesised in plants mainly from the amino acid tryptophan.

Indole-3-acetic acid (IAA) is the major naturally occurring auxin in plants, involved in cell elongation, apical dominance, root initiation and tropic responses. It is synthesised biochemically in actively growing tissues (such as shoot apices and young leaves) primarily from the amino acid tryptophan, through pathways that convert tryptophan's indole ring structure into the IAA molecule.

NAA (Naphthalene Acetic Acid) is a synthetic (man-made) auxin, unlike IAA and IBA which occur naturally in plants.

Auxins are a group of plant growth hormones that promote cell elongation, apical dominance, and root initiation. They are classified based on their source:

• Natural auxins: Indole-3-acetic acid (IAA) and Indole-3-butyric acid (IBA) are synthesised by the plant itself, mainly in the shoot apex and young leaves.
• Synthetic auxins: Naphthalene Acetic Acid (NAA) and 2,4-Dichlorophenoxyacetic acid (2,4-D) are man-made chemical analogues of IAA, widely used in agriculture and horticulture (e.g., rooting hormone, weedicide, fruit thinning).

Gibberellic acid (GA) is not an auxin at all — it belongs to a separate hormone class, the gibberellins.