Geotropie

Geotropie, or gravitropism, is the directional growth response of plant organs to gravity. In most vascular plants, roots exhibit positive geotropism by growing toward the gravitational pull, while shoots show negative geotropism by growing away from gravity. This organization helps secure the plant in the soil and position shoots for optimal light exposure.

Perception: Gravity is detected by specialized gravity-sensing cells called statocytes, located in the root cap (columella) and in the shoot apex. Within these cells, dense starch-filled plastids, known as amyloplasts or statoliths, settle under gravity and initiate a signaling cascade that affects hormone transport and distribution.

Mechanism: The gravity signal is transduced into an asymmetric distribution of the growth regulator auxin. Through polar auxin transport, higher auxin levels accumulate on the lower side of a bending organ: in roots, this inhibits elongation on the lower side, producing downward bending; in shoots, it promotes elongation on the lower side, producing upward bending. The cytoskeleton and intracellular signaling molecules participate in the response.

Timing and variability: Geotropic responses develop as tissues expand and can adjust with developmental stage and environmental conditions. Experimental observations, including clinostat experiments and microgravity studies, show that altering the gravity vector changes growth direction and curvature.

Geotropie is a fundamental aspect of plant physiology, interacting with phototropism and other tropisms to shape plant form and spatial orientation. See also gravitropism, statolith, auxin.