American Journal of Botany 100(1): 143–152. 2013.

Because of their sessile nature, plants have evolved elaborate sensory and signaling systems that enable them to adjust to rap-idly changing environmental conditions. Among the various environmental signals that a plant encounters during its life-time, gravity is one stimulus that remains constant and, as a re-sult, has played a pivotal role in driving the evolution of plants on earth ( Volkmann and Baluška, 2006). The most obvious manifestation of how gravity has shaped the development of a plant is through the upright growth of its aboveground organs and the downward growth of its roots. This biological phenom-enon popularly known as gravitropism involves the coordinated activity of different cell and tissue types wherein sensing of gravity and the growth response occur in spatially distinct re-gions. In roots, for example, the centrally located columella cells in the root cap are proposed to be the major site of gravit