Phototropie

Phototropie, or phototropism, is a directional growth response of organisms to light, most notably in plants. In plants, it refers to shoots bending toward a light source (positive phototropism) and, to a lesser extent, roots bending away from light (negative phototropism). This enables more efficient light capture for photosynthesis and helps seedlings orient themselves in their environment.

The response is largely driven by blue light receptors called phototropins, especially phot1 and phot2. When activated by blue light, phototropins initiate signaling cascades that cause an asymmetric distribution of the plant hormone auxin (IAA). Auxin accumulates on the shaded side of the growing shoot, promoting cell elongation there and producing a bend toward the light. This process involves auxin transport proteins of the PIN family and signaling components such as NPH3 and RPT2, which relay the light signal to the growth machinery. In roots, phototropic responses are more variable and can lead to bending away from light or to situational adjustments that support nutrient and moisture foraging.

Historical understanding began with observations by Charles Darwin and his wife in the 1880s, who noted that oat coleoptiles bent toward unilateral light. In 1928, Frits Went demonstrated that a mobile growth-promoting substance, later identified as auxin, mediates the effect. Molecular dissection followed in the late 20th century, with the identification of phototropins in the 1990s, establishing the blue-light sensing mechanism at the core of phototropism.

Phototropism interacts with other plant responses to light and stress, and it has implications for agriculture and horticulture, including optimization of light exposure in controlled environments.