Auxin is the plant hormone chiefly associated with bending toward light in growing shoots.
In a typical shoot, directional light causes auxin to become distributed unevenly, with more of it accumulating on the shaded side. In shoots, auxin promotes cell elongation there, so the shaded side grows faster and the stem bends toward the light. This response is called phototropism.
The best-known natural auxin is indole-3-acetic acid, or IAA. Auxin also influences roots, apical dominance, vascular development, fruit growth, and responses to gravity. Its effect depends on concentration and tissue: concentrations that promote elongation in shoots can inhibit elongation in roots. Auxin is therefore not simply a “growth chemical”; it coordinates different developmental responses throughout a plant.