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

Sourav PanNovember 1, 2024

Answered

Auxin’s Role in Elongation Growth: Acidifying Cell Walls

1. Auxin Signaling: Auxin, a plant hormone, triggers a signaling cascade that ultimately leads to cell elongation. This process involves both cell-surface and intracellular auxin signaling pathways.
2. Activation of Proton Pumps: Auxin stimulates the activation of plasma membrane H+-ATPases (proton pumps), which are responsible for pumping protons (H+) from the cytosol into the cell wall matrix (apoplast).
3. Cell Wall Acidification: The increased proton pumping leads to a rapid acidification of the cell wall, decreasing the apoplastic pH. This acidic environment is crucial for the subsequent steps in cell elongation.
4. Wall Loosening: The lowered pH in the cell wall activates wall-loosening processes, which involve the cleavage of load-bearing bonds between cell wall polysaccharides (e.g., cellulose microfibrils, pectins, and xyloglucans). This loosening allows for cell expansion.
5. Cell Elongation: As the cell wall is loosened, the cell can irreversibly elongate due to turgor pressure. Auxin also promotes the synthesis of new cell wall polymers, which are inserted into the wall to support continued growth.

Key Findings and Mechanisms:

• TMK-based cell-surface auxin signaling rapidly activates PM H+-ATPases, leading to cell wall acidification (within 10 seconds) [Nature, 2021].
• Acid Growth Theory: Auxin-induced cell elongation is coupled with cell wall acidification, which activates wall-loosening processes [Plant Physiol., 1992].
• Regulation of Proton Pumps: Auxin activates PM H+-ATPases by phosphorylation during hypocotyl elongation [Plant Physiol., 2012].
• Feedback Mechanism: Cell wall extension leads to an increase in cytosolic calcium, which inhibits H+-ATPase proton pumps and causes protoplast alkalization, potentially regulating the elongation process [PMC].

References:

• Lin, W., et al. (2021). TMK-based cell-surface auxin signalling activates cell-wall acidification. Nature, 599, 278–282.
• Rayle, D. L., & Cleland, R. E. (1992). The Acid Growth Theory of auxin-induced cell elongation is alive and well. Plant Physiol., 99, 1271–1274.
• Takahashi, K., et al. (2012). Auxin activates the plasma membrane H+-ATPase by phosphorylation during hypocotyl elongation in Arabidopsis. Plant Physiol., 159, 632–641.