phototropism

Phototropism is the directional growth of a plant or plant part in response to light, most commonly resulting in bending toward a light source. In most land plants, shoots exhibit positive phototropism, bending toward light to maximize photosynthesis, while roots often show negative phototropism, bending away from light.

Blue light is the primary trigger for phototropic bending. Photoreceptors called phototropins, especially phot1 and phot2, perceive blue light and initiate a signaling cascade that redistributes the plant hormone auxin toward the shaded side of the organ. The higher auxin concentration on the shaded side promotes cell elongation there, causing the organ to curve toward the light. The early steps involve activation of proton pumps in the plasma membrane, leading to cell-wall acidification and enhanced expansion.

Auxin transport underlies the directional response. Polar auxin transport, mediated by PIN-FORMED (PIN) efflux carriers and AUX1/LAX influx carriers, moves auxin from the illuminated side toward the shaded side, creating an asymmetric growth stimulus. This system integrates signals from phototropins and other receptors to modulate the strength and orientation of bending.

Phototropism interacts with other light responses and developmental cues. While blue-light photoreceptors are central for directional bending, cryptochromes and other light receptors influence broader growth and timing, including circadian regulation. The phenomenon is widely studied in seedling emergence and plant architecture.

Historical studies by Darwin and his son established the basic observation of bending toward light, with subsequent molecular work identifying phototropins and auxin redistribution as core mechanisms. Understanding phototropism helps explain how plants optimize light capture and competitive growth in fluctuating environments.