Geotropism

Geotropism, or gravitropism, is a directional growth response of plant organs to gravity. In most plants, roots show positive geotropism by growing toward the gravitational pull, while shoots display negative geotropism by growing away from gravity. This orientation supports effective nutrient and water uptake, stable anchorage, and proper shoot and root architecture.

Mechanism and sensing: Gravity detection occurs in specialized cells called statocytes, located in the root cap and in the shoot’s internal tissues. Starch-containing organelles known as statoliths settle under gravity, providing directional information. This physical signal triggers a redistribution of the plant hormone auxin through PIN and AUX1/LAX transport proteins. In roots, higher auxin concentration on the lower side inhibits expansion there, while the upper side grows more, bending the root downward. In shoots, the opposite distribution causes greater elongation on the lower side, bending the shoot upward away from gravity. The coordinated differential growth results in gravitropic curvature that reorients the organ.

History and genetics: Early experiments by Charles and Francis Darwin helped establish gravitropism as a distinct response to gravity. Modern research links geotropic bending to genetic and molecular components of auxin signaling, transport, and cell expansion, with specific transporters and signaling factors playing key roles in different tissues.

Significance and applications: Geotropism is essential for seedling establishment and overall plant architecture. It interacts with other tropisms, such as phototropism, to optimize light capture and resource acquisition. In microgravity environments, gravitropism diminishes, illustrating gravity’s role in normal plant development.