Auxin and acid growth.
- Auxin stimulates proton pumps in the plasma membrane.
- The proton pumps lower the pH in the cell wall.
- The acidity activates expansin enzymes that loosen the wall's fabric.
- Cell wall-loosening enzymes cut cross-linking polysaccharides.
- With the cellulose in the cell wall loosened, the cell can elongate under turgor pressure.