Fototropism

Fototropism, or phototropism, is the growth response of plants to directional light. In most land plants, shoots exhibit positive phototropism, bending toward a light source to maximize photosynthesis, while roots typically display negative phototropism, growing away from light. The response is strongest to blue light and relies on a defined light perception and signaling system.

Blue light is detected by phototropin receptors, mainly phot1 and phot2. When activated, these receptors initiate a signaling cascade that redistributes the plant hormone auxin toward the shaded side of the organ, aided by auxin transport proteins such as PINs. The higher auxin concentration on the shaded side promotes cell elongation, causing the shoot to bend toward the light. In roots, auxin accumulation on the illuminated side tends to inhibit elongation, producing a bend away from the light.

Auxin transport and growth are modulated by factors including the circadian clock and environmental conditions. The phototropic response is rapid and can be reversed by changing light direction. The phenomenon was first described by Charles and Francis Darwin in seedlings, with later work by Wilhelm Went linking the response to auxin.

Phototropism enhances light capture for photosynthesis and interacts with other tropic responses, such as gravitropism and heliotropism, contributing to the overall architecture and adaptive efficiency of plants.