Geotropisme

Geotropisme, or gravitropism, is the directional growth response of a plant to gravity. In most land plants, roots exhibit positive geotropism (they grow downward toward gravity), while shoots show negative geotropism (they grow upward away from gravity).

Perception of gravity occurs in specialized cells called statocytes, located in the root cap (columella) and in the shoot endodermis. Within these cells, dense amyloplasts, or statoliths, settle in response to gravity and initiate signaling that reorients growth. The signal involves plant hormones, especially auxin, and the polar transport proteins of the PIN family that redistribute auxin to the lower side of the organ.

The growth response results from differential cell elongation driven by auxin distribution. In roots, auxin accumulates on the lower side, inhibiting elongation there and causing the root to bend downward. In shoots, higher auxin on the lower side promotes elongation, causing the shoot to bend upward. The gravitropic response is rapid and can be modulated by light, temperature, and moisture.

Geotropism is essential for proper root anchorage, nutrient and water uptake, and seedling establishment. It interacts with other directional growth responses, such as phototropism, to guide plants in three-dimensional environments. In microgravity, gravitropic responses are reduced or absent, illustrating gravity’s central role in typical plant development.