Fototropismi

Fototropismi is the directional growth of plant organs in response to light direction. It is most commonly observed as shoots bending toward light, a positive phototropism, while roots typically grow away from light, showing negative phototropism. This response helps optimize light capture for photosynthesis and influences plant architecture.

Molecular basis and mechanism: Phototropism is primarily mediated by blue-light receptors called phototropins, notably phot1 and phot2. When blue light is absorbed, phototropins activate signaling pathways that stimulate plasma membrane H+-ATPases, leading to cell-wall loosening and elongation on the shaded side of the organ. Concurrently, the plant hormone auxin is redistributed toward the shaded side via PIN transporters. The higher auxin concentration on the shaded side promotes greater elongation there, causing the organ to bend toward the light in shoots. In roots, the redistribution of auxin typically inhibits elongation on the illuminated side, resulting in bending away from the light. The exact pattern can vary with species and developmental stage.

Additional light receptors, including cryptochromes and UVR8, modulate phototropism by adjusting growth responses to light quality and intensity, though blue light detected by phototropins remains the primary trigger. Phototropic responses can occur rapidly and are coordinated with gravitropism and circadian rhythms to optimize light exposure.

History and significance: Early experiments by Darwin and Boysen-Jensen established the blue-light requirement for phototropism. Today, fototropismi is understood as a conserved, blue-light–driven mechanism that shapes plant form and behavior, with broad implications for agriculture and horticulture.