Adv. Qxxe Rex Vol. 27, No. 5, pp. 915-919,2001 0 2001 COSPAR. Published by Elsevier Science Ltd. All rights reserved Pergamon www.elsevier.com/locate/asr PII: SO273-1177(01)00189-2 Printed in &eat Britain 0273-1177/O] $20.00 + 0.00 AUTONOMIC STRAIGHTENING OF GRAVITROPICALLY CURVED CRESS ROOTS IN MICROGRAVITY B. StankoviC1.4, F. Antonsen2, A. Johnsson2, D. Volkmann3, F. D. Sack4 ‘Wisconsin Center for Space Automation and Robotics, University of Wisconsin, Madison, Wl, USA ‘Dept. of Physics, Informatics and Mathematics, Norwegian Univ. of Science and Technology, Trondheim, Norway ‘Botanisches Institut, Universitiit Bonn, Bonn, Germany ‘Department of Plant Biology, Ohio State University, Columbus, OH, USA ABSTRACT The typical response of plant organs to gravistimulation is differential growth that leads to organ bending. If the gravitropic stimulus is withdrawn, endogenous compensation of the graviresponse and subsequent straightening occur in some plants. For instance, autonomic straightening of Lepidium roots occurs when gravitropically-curved roots are rotated on a clinostat (Stankovie et al., 1998a). To determine whether endogenous compensation of the graviresponse also occurs in space, microgravity-grown cress roots were laterally centrifuged in-flight and then returned to microgravity using Biorack hardware on a shuttle mission (STS-81). The cress roots were centrifuged at 4 different g-doses (O.lxg and Ixg for 15 or 75 min). All four treatments yielded varying degrees of root curvature. Upon removal from the centrifuge, roots in all four treatments underwent subsequent straightening in microgravity. This straightening resulted from a loss of gravitropic curvature in older regions of the root and the coordinated alignment of new growth. These results show that both microgravity and clinostat rotation on Earth are equivalent in stimulus withdrawal with respect to the induction of endogenous compensation of the curvature. Cress roots are the only plant organ shown to undergo compensation of the curvature in both microgravity and on a clinostat. The compensation of graviresponse in space rules out the hypothesis that the endogenous root straightening (“autotropism”) represents a commitment to a pre-stimulus orientation with respect to gravity and instead suggests that there is a default tendency towards axiality following a withdrawal of a g-stimulus. 0 2001 COSPAR. Published by Elsevier Science Ltd. All rights reserved. INTRODUCTION The reorientation of most plant organs results in gravitropic curvature that normally persists for the life of the organ. This curvature is due to differential growth that at some point becomes stabilized and long- lasting. However, following the withdrawal of a gravity stimulus, gravitropic curvature can be lost, a phenomenon known as autotropic or autonomic straightening (reviewed in StankoviC et al., 1998b). A g- stimulus can also be withdrawn by placement of the plant in microgravity in spaceflight. Autotropic straightening has been demonstrated in ground-based studies using a clinostat in several organs and plants including Lepidium (cress) roots (Larsen, 1957; Mirza et al., 1984; Stankovii et al., 1998a). Cress roots straighten on a clinostat (1 rpm) through a combination of a loss of gravitropic curvature and the alignment of new growth closer to the prestimulus vertical. Such alignment is only one of several possible fates since in theory the growth that occurs in the absence of a g-stimulus could be random or could reference persistent or past internal signal distributions (Figure 1A-D). The observation that the root returns closer to the prestimulus vertical suggests that there is an inherent tendency for disoriented organs to revert to a previous equilibrium orientation with respect to gravity (Figure lE-F). Without any g-stimulus, Lepidium roots in microgravity have been shown to have an initial development period characterized by a high degree of spontaneous, random growth (Johnsson et al., 1996). 915