Pointing roots in the right direction: the role of auxin transport in response to gravity

The study of tropisms—the directed growth responses to external stimuli such as light, temperature, water, and gravity, is a major area of research in plant physiology. Tropic responses have been studied for over a century, beginning with the pioneering work of Darwin, when he observed that dark-grown grass seedlings grew toward a light source when illuminated from one side (Sailsbury and Ross 1992). Plant roots show a similar response to gravity. When a seed germinates, the roots penetrate the soil and grow downward. However, if a root is reoriented by 90° with respect to the gravitational field, the root responds by altering its direction of growth, curving until it is again vertical. Although gravitropism has been widely studied, relatively little is known about the molecules involved in sensing the gravitational signal and transducing it. One molecule that has been implicated in tropic responses is the phytohormone auxin. Auxin is clearly involved in asymmetric tropic growth as well as a plethora of other processes in plants including vascular development and root formation. Insight into the mechanism of auxin action in the gravitropic response has recently been gained by the analysis of mutants that are defective in the response to the gravity signal. In this issue, Luschnig et al. (1998) describe the cloning and characterization of an Arabidopsis gene involved in the response to the gravity signal. The ETHYLENE INSENSITIVE ROOT 1 (EIR1) gene is a member of a family of plant genes with similarity to bacterial membrane transporters, suggesting that EIR1 functions as an auxin efflux pump, strengthening the connection between auxin transport and gravitropism.