NCERT grounding
The authoritative text for this subtopic is NCERT Biology Class 11, Chapter 13 — Plant Growth and Development, Section 13.4.3.1 (Auxins). The section opens by tracing the discovery to Charles Darwin's 1880 coleoptile experiments and F.W. Went's 1926 isolation of auxin from oat tips. It defines auxin both narrowly (IAA specifically) and broadly (any natural or synthetic compound with similar growth-regulating properties), lists physiological effects from cell elongation to weedicide action, and closes with the key statement that 2,4-D does not affect mature monocotyledonous plants — the sentence directly tested in NEET 2024 Q.115.
"Auxin was isolated by F.W. Went from tips of coleoptiles of oat seedlings."
NCERT Biology Class 11, Chapter 13, §13.4.2
Discovery of auxins
The story of auxin spans nearly half a century of incremental experiment. Understanding the sequence — observation, isolation, bioassay — is essential because NEET frequently frames questions around the experimental logic, not just the conclusion.
Timeline of auxin discovery
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1880
Darwin's Coleoptile Experiment
Charles and Francis Darwin showed that canary grass coleoptile tips bend toward unilateral light. When the tip was covered or removed, bending stopped — proving a transmittable influence originating at the tip.
Phototropism clue -
1913
Boysen-Jensen's Diffusion
Showed that the tip's influence could pass through a gelatin block but not mica — confirming a diffusible chemical, not a physical signal.
Chemical nature confirmed -
1926
Went's Avena Curvature Test
F.W. Went placed oat coleoptile tips on agar blocks, then placed the blocks asymmetrically on decapitated stumps in darkness. The curvature produced was proportional to auxin concentration — establishing the first quantitative bioassay for IAA.
NEET 2016 Q.50 -
1934
Chemical Identification: IAA
Kögl and Haagen-Smit identified the active compound as Indole-3-Acetic Acid (IAA) — first isolated from human urine, then confirmed in plants. The Greek root auxein ("to grow") gave the class its name.
IAA = natural auxin
Chemistry and sites of synthesis
The term auxin is used in two senses. In the strict sense it refers to IAA (Indole-3-Acetic Acid), the principal natural auxin in higher plants. In the broader sense it applies to any compound — natural or synthetic — that produces IAA-like growth effects at low concentration.
IAA is an indole compound derived from the amino acid tryptophan. The indole ring system is central to its biological activity; structural analogues that preserve this core often retain auxin activity.
Shoot apex
Primary
Site of IAA biosynthesis
The apical meristem of shoots is the dominant source. IAA produced here moves basipetally to drive elongation in the sub-apical zone.
Core NCERT factYoung leaves
Secondary
Expanding leaf primordia
Expanding young leaves produce significant IAA, contributing to the basipetal flux that maintains apical dominance over axillary buds.
Synthesis siteDeveloping seeds
Tertiary
Post-fertilisation source
Developing seeds are a rich IAA source during fruit growth. This IAA prevents premature abscission of the young fruit from the plant.
Fruit retention linkPolar transport
Auxin does not move passively through the plant. Its transport is strictly unidirectional — a property termed polar auxin transport (PAT) — and is maintained by asymmetrically localised efflux carrier proteins of the PIN family on the plasma membrane.
Shoot axis
Basipetal
Apex → Base direction
- Auxin moves from the shoot apex downward toward the roots
- Maintains high auxin concentration in sub-apical elongation zone
- Sustains apical dominance over lateral buds
- Independent of gravity — same direction whether plant is upright or inverted
Root axis
Acropetal
Base → Tip direction
- Auxin that enters roots from the shoot moves toward the root tip
- High auxin at root tips inhibits elongation (roots are more sensitive)
- Concentration gradient underlies positive gravitropism of roots
- Lower auxin on upper side → upper root cells elongate more → root bends down
Physiological effects
Cell elongation — the acid growth hypothesis
The primary action of auxin at the cellular level is to promote cell elongation, not cell division. The accepted mechanistic explanation is the acid growth hypothesis:
Acid growth hypothesis — step by step
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Step 1
Auxin binds receptor
IAA binds to TIR1 receptor (F-box protein), triggering downstream signalling that activates plasma membrane H⁺-ATPase.
-
Step 2
H⁺ pumped into wall
The activated proton pump acidifies the cell wall space (apoplast pH drops from ~6 to ~5).
-
Step 3
Expansins activated
Acidic pH activates expansins — proteins that disrupt hydrogen bonds between cellulose microfibrils, loosening the cell wall matrix.
-
Step 4
Water influx and elongation
Reduced wall pressure allows water entry by osmosis; the vacuole expands and the cell elongates irreversibly — producing measurable growth.
Apical dominance
In most higher plants the apical bud suppresses the growth of lateral (axillary) buds — a phenomenon called apical dominance. IAA produced at the shoot apex moves basipetally and maintains lateral buds in a dormant state by keeping local auxin concentration above the threshold for bud activation.
Removal of the shoot tip (decapitation) eliminates the source of IAA. Lateral buds are released and begin to grow. This principle is commercially exploited in tea cultivation (repeated pruning induces lateral bushiness) and hedge-making.
Root initiation
At low concentrations, auxin promotes adventitious root formation on stem cuttings — the basis of vegetative propagation in horticulture. The naturally occurring IBA (Indole-3-Butyric Acid) is particularly effective and is the active ingredient in commercial rooting powders. NAA (Naphthalene Acetic Acid) is also used for this purpose.
Commercial rooting auxin
Indole-3-Butyric Acid — naturally occurring in plants and widely used in rooting powders. Applied to the basal cut end of stem cuttings to initiate adventitious roots. More stable than IAA in soil and growing media.
Prevention of abscission
Auxin produced by developing seeds and young fruits prevents the premature formation of the abscission zone at the fruit stalk. As seeds mature and IAA production declines, the abscission zone forms and fruit drop occurs naturally. Exogenous auxin application can delay this drop — commercially important for extending the harvest window of apple and citrus crops (NAA spray).
The relationship is concentration-dependent and developmental stage-specific: auxin promotes abscission of older, mature leaves and fruits (where endogenous ethylene is rising), but prevents abscission of young fruits and leaves. This distinction is a consistent NEET trap.
Parthenocarpy
Auxin can substitute for the hormonal signal normally provided by fertilisation. Application of auxin to unpollinated flowers induces parthenocarpy — the development of seedless fruits without fertilisation. The canonical example in the NCERT text is tomato. Gibberellins can also induce parthenocarpy; the two hormones together are used commercially in seedless grape and banana production.
Synthetic auxins and weedicide applications
Figure 1. 2,4-D causes uncontrolled growth and death in broad-leaved dicot weeds, while mature monocotyledonous plants (grasses and cereals) remain unaffected. This selectivity is the basis of its use as a lawn weedicide and in cereal agriculture. (NEET 2021 Q.124; NEET 2024 Q.115)
Four synthetic auxins are relevant to NEET. Two are used in agriculture and horticulture; two are herbicides.
2,4-D
Herbicide
2,4-Dichlorophenoxyacetic acid
Kills dicot weeds by inducing uncontrolled, disorganised growth. Does not affect mature monocots. Used in cereal fields and lawn care.
NEET 2021 Q.124 · 2024 Q.115NAA
Horticultural
Naphthalene Acetic Acid
Prevents premature fruit drop in apple (pre-harvest spray). Also used to promote adventitious rooting in cuttings.
Fruit retentionIBA
Rooting agent
Indole-3-Butyric Acid
Natural plant auxin used commercially in rooting powders for vegetative propagation of cuttings. More chemically stable than IAA.
Root initiationIAA (natural)
Natural
Indole-3-Acetic Acid
The primary natural auxin in higher plants. Isolated first from human urine, confirmed in oat coleoptile tips by Went (1926). Assayed via Avena curvature test.
NEET 2016 Q.50Worked examples
A gardener applies a chemical to a lawn that kills broad-leaved weeds but spares the grass. The chemical is a synthetic auxin at high concentration. Identify the compound and explain the mechanism of selectivity.
Compound: 2,4-D (2,4-Dichlorophenoxyacetic acid). At herbicide concentrations, 2,4-D causes uncontrolled disorganised growth in dicotyledonous plants — meristematic activity is so greatly stimulated that resources are exhausted and the plant dies. Mature monocotyledonous plants (grasses) are insensitive to 2,4-D at these concentrations because their leaf architecture (parallel-veined, narrow) and auxin receptor expression differ from dicots. The selective toxicity makes 2,4-D safe for cereal crops and lawn grass — exactly the observation stated in NCERT §13.4.3.1 and tested in NEET 2021 Q.124 and 2024 Q.115.
A student decapitates a young tomato plant. After one week, multiple lateral shoots emerge vigorously. Explain, with reference to the relevant plant growth regulator.
Apical dominance and auxin. The shoot apex continuously produces IAA, which undergoes basipetal (apex-to-base) polar transport. At the axillary buds, high local auxin concentration inhibits bud activation. Decapitation removes the source of IAA; auxin levels at the axillary buds fall below the inhibition threshold; cytokinins (produced in root apices) are no longer counter-balanced by IAA, and the lateral buds begin to grow. Multiple lateral branches emerge, increasing plant bushiness. This is exploited in tea cultivation and hedge trimming.
Pineapple growers want to synchronise flowering across their entire crop. Which combination of plant hormones can be used and what is the basis?
Auxin and Ethylene (NEET 2019 Q.77 Answer: (1)). Auxin application to pineapple plants promotes flowering. The mechanism involves auxin stimulating ethylene production; ethylene then acts as the direct trigger for floral initiation in pineapple. Commercial application uses NAA (a synthetic auxin) or ethephon (an ethylene-releasing compound) to achieve uniform, synchronised flowering across large plantations, ensuring uniform fruit harvest timing.
Common confusion & NEET traps
IAA — natural auxin
C₁₀H₉NO₂
Indole-3-Acetic Acid
- Produced in shoot apex, young leaves, developing seeds
- First isolated from human urine (Kögl 1934)
- Quantified by Avena curvature test (Went 1926)
- Relatively unstable — rapidly inactivated by IAA oxidase
- Low herbicidal activity at physiological concentrations
2,4-D — synthetic auxin
C₈H₆Cl₂O₃
2,4-Dichlorophenoxyacetic acid
- Not produced in plants — entirely synthetic
- Highly resistant to plant-produced auxin oxidase
- Accumulates to toxic levels in dicot tissues
- Selective herbicide: kills dicots, spares mature monocots
- Also used in tissue culture media at low concentration