How It Works: Auxin and Phototropism

How It Works: Auxin and Phototropism

Plants cannot move to find light, but they can grow towards it using a process called phototropism. The mechanism depends entirely on the unequal distribution of a plant hormone called auxin.

Auxin is produced in the growing tip (apex) of a shoot. When light strikes the shoot from one side only, auxin is transported laterally away from the illuminated side and accumulates on the shaded side of the shoot. This uneven distribution is the key to tropism.

On the shaded side, where auxin concentration is higher, cells absorb more auxin and elongate more (auxin stimulates cell elongation by making cell walls more flexible, allowing them to expand when the cell takes up water by osmosis). On the illuminated side, where auxin concentration is lower, cells elongate less. Because one side of the shoot grows faster than the other, the shoot bends towards the light source — positive phototropism.

In roots, auxin has the opposite effect: high concentrations of auxin inhibit rather than promote cell elongation. This means the root side with more auxin grows less, causing the root to bend towards gravity (positive gravitropism) since gravity causes auxin to accumulate on the lower side of the root, where growth is then inhibited and the upper side grows more, bending the root tip downward.