explain the role of auxin in elongation growth by stimulating proton pumping to acidify cell walls

Control and Coordination – Plant Hormones (Topic 15, Cambridge IGCSE/A‑Level Biology 9700)

1. Syllabus overview – the six major plant hormones

2. Hormone interactions – key synergistic and antagonistic relationships (AO1)

3. Auxin – detailed focus (the acid‑growth model)

3.1 Biosynthesis & polar transport (PAT)

• Biosynthesis: Tryptophan → indole‑3‑pyruvic acid (TAA1/TAR) → IAA (YUCCA flavin‑monooxygenases).
• Polar auxin transport:

  • Efflux carriers – PIN‑family proteins (e.g., PIN1, PIN2) asymmetrically positioned on the plasma membrane.
  • Influx carriers – AUX1/LAX family.
  • ABCB (MDR/PGP) transporters assist loading/unloading.

  This creates auxin gradients essential for tropic responses and apical dominance.

3.2 Signalling cascade

Rapid (non‑genomic) branch – seconds to minutes

1. Auxin binds the plasma‑membrane receptor ABP1 (or triggers a rise in cytosolic Ca²⁺ and cGMP).
2. Second messengers activate a protein‑kinase cascade that phosphorylates plasma‑membrane H⁺‑ATPases.
3. SAUR (Small Auxin Up‑RNA) proteins inhibit PP2C phosphatases, sustaining H⁺‑ATPase phosphorylation.
4. Result – immediate increase in proton extrusion (first step of acid‑growth).

Genomic (long‑term) branch – minutes to hours

1. Perception: IAA binds the nuclear TIR1/AFB + SCF complex.
2. De‑repression: SCF‑mediated ubiquitination → degradation of Aux/IAA repressor proteins.
3. Transcriptional activation: Freed ARF (Auxin‑Response Factor) proteins bind AuxREs → up‑regulation of SAUR, GH3, and H⁺‑ATPase isoforms.

Feedback regulation (AO1)

• Auxin‑induced expression and phosphorylation of PIN proteins remodel efflux routes, reinforcing or redirecting gradients.
• SAUR proteins provide a positive feedback loop for H⁺‑ATPase activity.
• High auxin levels induce GH3 enzymes that conjugate IAA to amino acids, attenuating the signal.

3.3 Acid‑growth mechanism – how proton pumping drives cell elongation

1. H⁺‑ATPase activation – Phosphorylated H⁺‑ATPases are inserted into the plasma membrane and hydrolyse ATP:

   ATP + H₂O → ADP + Pᵢ + energy

   The energy drives extrusion of H⁺:

   H⁺cyt → H⁺apo

2. Apoplastic acidification – pH of the cell wall falls from ≈ 6.5 to ≈ 5.0 within 2–5 min.
3. Activation of wall‑loosening proteins (pH‑dependent):

   • Expansins – disrupt hydrogen bonds between cellulose microfibrils and matrix polysaccharides.
   • Pectin‑methylesterases – modify calcium‑cross‑linked pectin.
   • Endoglucanases & Xyloglucan endotransglycosylases (XTH) – remodel hemicellulose network.

4. Cell‑wall loosening → irreversible expansion – Turgor pressure (P) drives wall extension. The rate follows the Lockhart equation:

    dV/dt = m (P – Y) 

   where m = wall extensibility, Y = yield threshold.

3.4 Physiological contexts (AO1)

• Phototropism: Light causes lateral PIN relocation → higher auxin on the shaded side → acid‑growth → shoot bends toward light.
• Gravitropism (roots): Gravity‑induced PIN3/7 relocalisation sends auxin to the lower flank → inhibition of elongation there (high auxin + ethylene) → root bends downwards.
• Apical dominance: Apex‑produced IAA moves basipetally, suppresses cytokinin synthesis in the stem, keeping lateral buds dormant.
• Root‑hair development: Local auxin maxima at epidermal cells stimulate H⁺‑ATPase, loosening the wall for hair protrusion.
• Fruit set & growth: Developing seeds release auxin → acid‑growth in pericarp → rapid cell expansion.
• Stress integration: Drought or salinity can alter PIN localisation, modifying auxin gradients; cross‑talk with ABA adjusts growth rates.

4. Experimental evidence for the acid‑growth model (AO2)

• Exogenous IAA applied to pea stem segments produces a measurable drop in apoplastic pH within 2–5 min (detected with pH‑sensitive dyes or microelectrodes).
• Orthovanadate, a specific H⁺‑ATPase inhibitor, abolishes the IAA‑induced pH fall and prevents elongation, confirming the central role of proton pumping.
• tir1/afb double mutants in Arabidopsis show reduced H⁺‑ATPase transcription, lower wall acidification and impaired phototropic curvature.
• Transgenic lines over‑expressing SAUR genes display constitutively active H⁺‑ATPases and increased elongation even without added auxin.

5. Sample AO2 (data‑interpretation) questions

1. Question: A mutant pea stem segment lacking functional PIN1 shows a reduced curvature when exposed to unilateral light. Explain the result in terms of auxin transport and the acid‑growth mechanism.
2. Question: In an experiment, the apoplastic pH of a cucumber hypocotyl drops from 6.4 to 5.1 after 3 min of IAA application. Predict what will happen to the rate of elongation after 10 min and justify your answer using the Lockhart equation.
3. Question: A researcher treats Arabidopsis seedlings with orthovanadate and then adds excess IAA. No elongation occurs. Explain why orthovanadate blocks the response, linking it to the rapid signalling branch.

Mini‑data set (for question 2) and brief analysis guide

Analysis guide: The rapid fall in pH activates expansins, increasing wall extensibility (m) in the Lockhart equation. As m rises while turgor pressure (P) stays constant, the term m(P‑Y) increases, giving a higher elongation rate. The data show a progressive increase in relative growth that matches the predicted effect.

6. AO3 – practical investigation: “Auxin‑induced acid‑growth in pea stems”

Objective: To demonstrate that IAA stimulates proton extrusion, lowers apoplastic pH and promotes cell elongation, and to evaluate the effect of an H⁺‑ATPase inhibitor.

7. Summary – sequence of events in auxin‑induced cell elongation

8. Key points to remember (AO1)

• Auxin activates H⁺‑ATPases via a rapid, non‑genomic pathway (ABP1 → Ca²⁺/cGMP → kinase → SAUR‑PP2C inhibition) and a slower genomic pathway (TIR1/AFB → ARF‑mediated transcription of H⁺‑ATPase genes).
• Proton extrusion acidifies the cell wall, activating expansins and other wall‑modifying enzymes – the core of the acid‑growth hypothesis.
• Wall loosening allows turgor pressure to produce irreversible cell expansion; the rate is described by the Lockhart equation.
• Polar auxin transport creates the gradients required for phototropism, gravitropism, apical dominance and many other developmental processes.
• Cross‑talk with other hormones (cytokinin, GA, ABA, ethylene, brassinosteroids) integrates growth with development and stress responses.
• Experimental approaches (pH measurement, H⁺‑ATPase inhibitors, mutant analysis) provide strong evidence for the acid‑growth model and are typical of exam‑style questions.