Question
How does auxin (IAA) cause phototropism in plants? Explain the mechanism with reference to the Cholodny–Went hypothesis and describe the structural changes that occur in the stem.
Solution — Step by Step
Phototropism is the directional growth of a plant organ in response to unilateral (one-sided) light. Stems show positive phototropism (grow towards light); roots show negative phototropism (grow away from light).
The response is mediated by auxin (indole-3-acetic acid, IAA), synthesised mainly in the shoot apex (apical meristem and young leaves).
When light strikes one side of a coleoptile or young stem, auxin migrates laterally from the illuminated side to the shaded side. This redistribution is driven by phototropins — light receptor proteins (phototropin 1 and 2, encoded by PHOT1 and PHOT2 genes) that activate lateral auxin transport proteins.
Result: the shaded side accumulates more IAA; the illuminated side has less.
Diagrammatically:
Light →
___________
| |
IAA → | STEM | ← IAA (more here)
|___________|
Less IAA More IAA
(light side) (dark side)Auxin promotes cell elongation by binding to receptor proteins and activating H⁺-ATPase pumps. These pumps acidify the cell wall (acid growth theory), activating expansins — enzymes that loosen the cellulose microfibrils. The cell then takes up water by osmosis and elongates.
Since the shaded side has more IAA, cells on that side elongate more than cells on the illuminated side. This differential elongation curves the stem towards the light source.
The sequence at the cellular level:
- Auxin binds TIR1 receptor (part of SCF-TIR1 ubiquitin ligase complex)
- This leads to degradation of AUX/IAA repressor proteins
- ARF (Auxin Response Factor) transcription factors are released
- ARFs activate auxin-responsive genes, including those encoding H⁺-ATPase
- Cell wall acidification → expansin activation → cell wall loosening → elongation
The shaded side undergoes more of this cascade → more elongation → curvature towards light.
Roots are much more sensitive to auxin than stems. The same auxin concentration that promotes stem elongation actually inhibits root elongation (roots have an optimal IAA concentration around 10⁻¹⁰ M vs 10⁻⁶ M for stems).
In phototropism, auxin reaching the lower/shaded side of a root inhibits that side, so the upper/illuminated side elongates more — root grows downward (negative phototropism combined with gravitropism).
Why This Works
The key insight is that auxin acts as a concentration-dependent growth regulator. It does not directly create movement — it creates a gradient. The gradient produces differential elongation, which produces curvature.
The Cholodny–Went hypothesis (proposed in 1927) elegantly explains why cutting off the shoot apex abolishes the phototropic response — no apex, no auxin source, no gradient.
Modern work has refined the picture: phototropin receptor proteins (not auxin itself) are the primary light sensors. They reorganise PIN auxin efflux carriers to redirect auxin flow. But the core idea — lateral auxin gradient → differential elongation → bending — remains correct.
Alternative Method
For CBSE questions asking you to “explain with diagram,” a simpler two-step answer is acceptable:
- Draw the coleoptile with arrows showing auxin migrating to the dark side
- State: more auxin on dark side → more cell elongation on dark side → stem bends towards light
This earns full marks in a 3-mark or 5-mark board question. The detailed molecular mechanism (TIR1, ARF, expansins) is more relevant for NEET preparation.
Common Mistake
A very common error: students write “auxin moves towards the light” — exactly backwards. Auxin moves away from the light (to the shaded side). The plant bends towards light precisely because the shaded side grows faster. If you get the direction of auxin movement wrong, the whole explanation falls apart.
NEET frequently asks: “What will happen if the shoot tip of a coleoptile is covered with an opaque cap?” Answer: no phototropic response, because auxin redistribution requires the tip to detect light. If the tip is intact but the rest is covered, bending still occurs below the tip. This is the classic Boysen-Jensen experiment result.