How do auxin hormones cause a plant stem to bend toward light?

The process by which a plant stem bends toward light is known as phototropism. Contrary to the idea that this is a "stereotyped" or fixed mechanical response, it is a dynamic hormonal process driven by the unequal distribution of auxin hormones.12

When a plant is exposed to light from one direction, auxins—specifically indoleacetic acid (IAA)—are redistributed so that greater quantities accumulate on the side of the stem away from the light.12

This accumulation triggers the following sequence:
* Cell Elongation: Auxins act on the chemical bonds of the plant cell walls, allowing them to become irreversibly deformed or "loosened".12
* Growth Differential: As water enters these cells and new cell-wall material is synthesized, the cells on the shaded side elongate more rapidly than those on the illuminated side.12
* Curvature: This uneven growth rate causes the stem to curve or bend toward the light source.2

This mechanism demonstrates how plants actively regulate their growth in response to environmental stimuli.1