phototropisme

Phototropisme, or phototropism in English, is the growth response of plant organs to the direction of light. In most plants, shoots exhibit positive phototropism, bending toward a light source to maximize photosynthesis, while roots often show negative phototropism or a weaker response. The phenomenon is especially evident in germinating seedlings and in coleoptiles. Early experiments by Charles Darwin and later researchers established that the tip of the shoot is essential for bending toward light.

The mechanism begins with blue light perception by photoreceptors called phototropins, primarily phot1 and phot2, located in the plasma membrane of cells in the shoot tip. Upon blue-light exposure, phototropins undergo autophosphorylation and trigger a signaling cascade that alters the distribution of the hormone auxin between the illuminated and shaded sides of the stem. This creates an asymmetric auxin gradient across the shoot.

Auxin is redistributed toward the shaded side, where higher concentrations promote cell elongation. This differential growth causes the stem to bend toward the light. The directional transport of auxin is mediated by PIN-FORMED (PIN) proteins and AUX1/LAX influx carriers, whose localization and activity are regulated by light.

Phototropism interacts with other growth responses, including gravitropism and shade avoidance, and can be modulated by light intensity, duration, and timing. It is a foundational example of how light perception is translated into growth through signaling pathways and hormone redistribution, influencing plant architecture and efficiency of light capture.