How Auxins Work

Auxin causes cell elongation by a mechanism described by the acid growth hypothesis.1. IAA binds to receptors in the target cell.
2. This stimulates proton pumps in the cell surface membrane to actively transport hydrogen ions (H⁺) out of the cytoplasm into the cell wall.
3. The pH of the cell wall falls to around 5.
4. The acidic conditions activate proteins called expansins, which break the hydrogen bonds between cellulose microfibrils and other wall polysaccharides.
5. The cell wall becomes looser and more flexible.
6. The cell takes up water by osmosis, and the pressure of the expanding vacuole stretches the loosened wall, so the cell elongates.Auxin also changes gene expression, which helps keep elongation going for longer.PhototropismWhen light shines on a shoot from one side, auxin is transported from the illuminated side to the shaded side of the shoot tip. The higher auxin concentration on the shaded side makes those cells elongate more, so the shoot bends towards the light.Geotropism in rootsIn a root lying horizontally, auxin accumulates on the lower side. Because root cells are inhibited by high auxin concentrations, the lower side elongates less than the upper side, so the root bends downwards. In a horizontal shoot, auxin also collects on the lower side, but there it stimulates elongation, so the shoot bends upwards.