Gravitropic Signaling in Plants

Abstract

Abstract The direction of gravity represents a fundamental signal that plants use to shape their growth and development. In the simplest description of this phenomenon, roots grow down and shoots grow up; however, closer examination reveals that different plant organs such as the individual branches of a tree, maintain different angles to gravity, their ‘gravitropic set‐point angle’. When displaced from this orientation, plant organs have the ability to redirect growth to regain their preferred angle through the process of gravitropism. The starch‐statolith hypothesis describes the cellular mechanism whereby plants can sense the direction of gravity and so regulate growth in response to this signal. The sedimentation of dense starch‐filled amyloplasts within specialised sensory cells is thought to generate an as yet undefined directional biochemical signal. This signalling system then modulates transport of the plant hormone auxin resulting in its accumulation and the localisation of growth that leads to the gravitropic response. Key Concepts: Plants sense the direction of gravity and regulate growth of roots and shoots using this information. Directional growth in response to gravity is known as gravitropism. Different organs can use gravitropism to maintain different genetically defined angles, or gravitropic set‐point angles, relative to the gravity vector. Sensing the direction of gravity is achieved by dense starch‐filled amyloplasts sedimenting in specialised sensory cells, called statocytes. The statocytes are the columella cells of the root cap and the endodermal cells in the shoot. Sedimentation of amyloplasts generates an initial biochemical signal within the statocytes that has yet to be defined. Signalling within the statocytes triggers redistribution of transporters for the hormone auxin to the plasma membrane on the lower side of the cell. Asymmetrical activity of auxin transport causes this hormone to accumulate on the lower side of the root or shoot. Auxin promotes cell elongation in shoots and inhibits this process in roots. Differential cell elongation between one side of an organ and the other causes the root to bend downwards and the shoot upwards.