Acidgrowth

Acidgrowth, more commonly referred to as the acid growth hypothesis, is a concept in plant biology that describes how certain stimuli, particularly the plant hormone auxin, promote cell elongation by acidifying the cell wall region (the apoplast). The central idea is that a lower apoplastic pH activates wall-loosening processes, enabling turgor-driven expansion of cells.

Mechanism: Auxin signaling activates plasma membrane H+-ATPases, increasing proton extrusion into the apoplast and lowering its pH. The acidic environment activates wall-modifying proteins such as expansins, which loosen the cellulose–hemicellulose network. Additional enzymes that remodel pectin and cross-links in the wall can further adjust extensibility. Sustained water uptake maintains turgor pressure needed for expansion, linking chemical changes in the wall to mechanical growth.

Evidence and history: The acid growth hypothesis emerged in the 1970s as researchers connected auxin-induced growth with apoplastic acidification. Supporting evidence includes rapid apoplastic pH decreases following auxin treatment, increased wall extensibility in acidic conditions, and the stimulating effect of fusicoccin, a toxin that activates proton pumps. Later, expansins were identified as pH-dependent wall-loosening agents, providing a molecular basis for the mechanism. Pharmacological and genetic studies have shown that disrupting proton pumps or expansin function can attenuate acid growth.

Current understanding: Acid growth is a major contributor to many rapid elongation responses in plants but is not the sole mechanism of cell expansion. Growth results from an integrated network of hormone signaling, wall chemistry, turgor pressure, and ongoing wall synthesis and remodeling. Some tissues exhibit elongation with minimal apoplastic acidification, indicating additional pathways can drive growth.