GRAVITROPIC SIGN REVERSAL - A FUNDAMENTAL FEATURE OF THE GRAVITROPIC PERCEPTION OR RESPONSE MECHANISMS IN SOME PLANT ORGANS

The direction of the gravitational response of some plant organs reverses during development. For instance, the apical hook of an etiolated hypocotyl is formed because the youngest part of the elongation zone is positively gravitropic while the more mature elongating region is negatively gravitropic. Likewise, the youngest nodes of some trailing plants such as Oplimenus hirtellus are negatively gravitropic, but the same nodes later in their developmental progression are positively gravitropic. The sign of the gravitropic response in some organs not only changes developmentally, but can also be controlled by light. The positive gravitropic phase of hypocotyls and nodes is absent in light-grown plants. At least in the case of Oplimenus nodes, the light exposure changes the sign of the gravitropic response of some of the older nodes from negative to positive. Likewise the older nodes of light-grown plants can change their gravitropic response from positive to negative when transferred to darkness. This ability of an organ to reverse the direction of its gravitropic response has been given the term gravitropic sign reversal and it is argued that the process is so widespread that it must be a consequence of some property of either the gravitropic perception or the gravitropic response mechanism. The fact that an organ can reversibly alter the direction of the gravitropic response has considerable implications for models of gravitropism.