Plant responses to gravity.

Tropisms are directed growth-mediated plant movements which allow plants to respond to their environment. Gravitropism is the ability of plants to perceive and respond to the gravity vector and orient themselves accordingly. The gravitropic pathway can be divided into three main components: perception, biochemical signaling, and differential growth. Perception of the gravity signal occurs through the movement/sedimentation of starch-filled plastids (termed statoliths) in gravity sensing cells. Once perceived, proteins interact with the settling statoliths to set a cascade of plant hormones to the elongation zones in the roots or shoots. Plant growth regulators that play a role in gravitropism include auxin, ethylene, gibberellic acid, jasmonic acid, among others. Differential growth on opposing sides of the root or shoot allow for the plant to grow relative to the direction of the perceived gravity vector. In this review, we detail how plants perceive gravity and respond biochemically in response to gravity as well as synthesize the recent literature on this important topic in plant biology. Keywords: auxin, gravitropism, gravity perception, plant growth regulators, space biology, statolith.