Control and Coordination in Plants – Role of Auxin in Elongation Growth
Control and Coordination in Plants
Objective
Explain the role of auxin in elongation growth by stimulating proton pumping to acidify cell walls.
1. Introduction to Auxin
Auxin (indole‑3‑acetic acid, IAA) is a plant hormone that regulates many aspects of growth and development, most notably cell elongation in shoots and roots. Its distribution is highly polar, moving from the tip of the plant downwards through the apoplast.
2. The Acid‑Growth Hypothesis
The acid‑growth hypothesis states that auxin promotes cell elongation by lowering the pH of the cell wall (apoplast), activating wall‑loosening enzymes and allowing the wall to expand under turgor pressure.
3. Mechanism of Auxin‑Induced Proton Pumping
Auxin perception – Auxin binds to the TIR1/AFB receptor complex in the cytoplasm, leading to degradation of Aux/IAA repressor proteins.
Signal transduction – The degradation releases ARF transcription factors, which up‑regulate genes encoding plasma‑membrane H⁺‑ATPases.
Activation of H⁺‑ATPases – The newly synthesized H⁺‑ATPases are phosphorylated and inserted into the plasma membrane, increasing their activity.
Proton extrusion – Active H⁺‑ATPases pump protons from the cytosol into the apoplast:
Apoplastic acidification – The pH of the cell wall falls from \overline{6}.5 to \overline{5}.0.
Wall‑loosening enzymes – Low pH activates expansins and pectin‑methylesterases, which disrupt hydrogen bonds between cellulose microfibrils and matrix polysaccharides.
Cell expansion – With the wall softened, turgor pressure (\$P\$) drives the cell to enlarge. The rate of elongation can be described by the Lockhart equation:
\$\frac{dV}{dt}=m(P-Y)\$
where \$m\$ is the wall extensibility, \$P\$ is turgor pressure and \$Y\$ is the yield threshold.
4. Summary of the Sequence of Events
Step
Event
Resulting Change
1
Auxin binds TIR1/AFB
Aux/IAA degradation
2
ARF activation
Up‑regulation of H⁺‑ATPase genes
3
H⁺‑ATPase phosphorylation & insertion
Increased proton pumping
4
Proton extrusion into apoplast
Apoplastic pH drops to \overline{5}.0
5
Acid activation of expansins
Cell‑wall loosening
6
Turgor‑driven expansion
Cell elongation
5. Experimental Evidence
Application of exogenous IAA to pea stem segments causes a rapid drop in apoplastic pH measured with pH‑sensitive dyes.
Inhibitors of H⁺‑ATPase (e.g., orthovanadate) block IAA‑induced elongation, confirming the necessity of proton pumping.
Mutant Arabidopsis lines lacking functional TIR1 receptors show reduced H⁺‑ATPase activity and impaired elongation.
6. Key Points to Remember
Auxin stimulates the synthesis and activation of plasma‑membrane H⁺‑ATPases.
Proton extrusion acidifies the cell wall, activating wall‑loosening proteins.
The softened wall allows turgor pressure to drive irreversible cell expansion.
The process is rapid (minutes) and reversible, providing plants with a flexible growth control mechanism.