Fototropie

Fototropie, or phototropism, is the directional growth of plant organs in response to directional light. In most plants, shoots exhibit positive phototropism and bend toward the light source, while roots typically display negative phototropism and grow away from light. This growth bias helps maximize light capture for photosynthesis and aids roots in exploring soil resources.

The blue light receptors known as phototropins, particularly phot1 and phot2, detect directional light and initiate signaling that redistributes the plant hormone auxin toward the shaded side of the organ. In shoots, higher auxin concentration on the shaded side promotes cell elongation there, causing the shoot to bend toward the light. In roots, the distribution and sensitivity of auxin lead to reduced elongation on the illuminated side, producing a bend away from the light. The differential growth is mediated by changes in cell wall extensibility through the action of proton pumps and the acid-growth mechanism.

Auxin transport proteins, such as the PIN family, mediate the lateral redistribution of auxin, and their activity is modulated by light signaling. Cryptochromes and other photoreceptors can influence phototropic responses, integrating other environmental cues like temperature and circadian rhythms with light signals.

Phototropism occurs across many land plants and is especially evident during early seedling development, though it continues to affect growth and architecture in mature organs. As a fundamental example of plant tropisms, phototropism contributes to optimal light harvesting and competitive advantage in natural and agricultural contexts.