Auxin and acid growth.

1. Auxin stimulates proton pumps in the plasma membrane.

2. The proton pumps lower the pH in the cell wall.

3. The acidity activates expansin enzymes that loosen the wall's fabric.

4. Cell wall-loosening enzymes cut cross-linking polysaccharides.

5. With the cellulose in the cell wall loosened, the cell can elongate under turgor pressure.