gravitropism

Gravitropism is a directional growth response by plants to gravity, aligning growth so that roots tend to penetrate the soil (positive gravitropism) and shoots grow away from gravity (negative gravitropism). This orientation helps anchor plants and optimize access to water, nutrients, and light.

Perception of gravity occurs in specialized gravity-sensing cells called statocytes, located in the root cap for roots and in the endodermis of stems. Within these cells, dense starch-filled organelles called amyloplasts, or statoliths, settle under the influence of gravity. The resulting sedimentation is translated into a biochemical signal that alters the distribution of the plant hormone auxin, mediated by auxin transport proteins such as PINs.

In roots, gravity causes auxin to accumulate on the lower side of the organ. Because auxin inhibits cell elongation in roots, cells on the lower side grow more slowly than those on the upper side, and the root curves downward toward gravity. In shoots, the opposite pattern occurs: higher auxin concentration on the lower side promotes cell elongation, producing a bend upward away from gravity.

The starch-statolith hypothesis, associated with early work of Charles Darwin and others, describes the role of amyloplast sedimentation in gravity sensing, though other sensors and signaling pathways integrate additional environmental cues. Gravitropism interacts with phototropism and hydrotropism to shape overall plant architecture under varying light and moisture conditions.