Gravitropie

Gravitropie, or gravitropism, is the directional growth response of plants to gravity. In most land plants, roots show positive gravitropism, growing toward the gravitational pull, while shoots exhibit negative gravitropism, growing away from gravity. This alignment helps establish a functional root system and an upright shoot.

Perception and signal transduction occur in specialized gravity-sensing cells called statocytes. In roots, statocytes are located in the columella cells of the root cap; in shoots, in the endodermis. Within these cells, dense starch-containing organelles called amyloplasts act as statoliths. When the plant is reoriented, amyloplasts sediment toward the lower side, triggering a signaling cascade that involves calcium signaling and changes in the cytoskeleton. This signal leads to a redistribution of the plant hormone auxin, mediated by PIN family transporters, toward the lower side of the organ.

Growth response results from the differential distribution of auxin. In roots, higher auxin concentration on the lower side inhibits cell elongation there, causing the root to bend downward toward gravity. In shoots, higher auxin on the lower side promotes elongation, bending the shoot upward away from gravity. The combined effect aligns growth with the vertical orientation.

Gravitropism interacts with other cues such as light and touch, and its magnitude varies with developmental stage and environmental conditions. It is a fundamental mechanism shaping root architecture and shoot orientation, and it is studied in contexts from basic plant biology to space biology.