Update on Development Gravitropism in Higher Plants

Since 1806, we have known that plant organs use gravity as a guide for growth (Knight, 1806). The gravity-directed growth process, called gravitropism, dictates upward shoot growth to ensure a proper positioning of the leaves for efficient photosynthesis and gas exchange. It also directs roots to grow downward in soil, where they can reach out to take up the water and mineral ions required for plant growth and development. Gravitropism has an important impact on agriculture. It allows plants to compete for the limited resources available in their immediate environment and ensures that crop shoots resume upward growth after prostration by the action of wind and rain (Fig. 1). Consequently, plants can keep their seeds away from soil moisture and pathogens and are more amenable to mechanical harvesting. At the end of the 19th century, Ciesielski (1872) and Darwin (1880) demonstrated that a structure at the tip of the roots, the cap, is essential for root gravitropism. They postulated that the root cap could perceive a change in root-tip orientation within the gravitational field (gravistimulus). Graviperception would then produce a physiological signal that, upon transmission to the elongation zone, would promote a differential cellular elongation on opposite flanks, which is responsible for the development of a curvature. The resulting curvature would allow the root tip to resume growth along a gravitropically more acceptable vector. These important early observations along with the proposed model for gravity perception marked the beginning of numerous studies that extended throughout the entire 20th century and helped us to gain a better understanding of the various physiological and molecular processes underlying gravitropism. In this Update we discuss our current knowledge of the gravitropic response of higher plants, with a special emphasis on roots.