EARLY MECHANISMS OF GRAVITY-SENSING IN PLANTS. J. Hauslage, N. Greuel and M. Braun. Gravitational Biology Group, Institute of Plant Molecular Physiology and Biotechnology, University of Bonn, Kirschallee 1, D-53115 Bonn, Germany.

Gravity is the constant factor that provides plants with reliable information for oriented growth. Any changes in the genetically programmed orientation of the different plant organs are sensed by specialized cell types, the statocytes, in which heavy particles, so-called statoliths, sediment in the direction of gravity. Statolith sedimentation induces a physiological signalling cascade that eventually results in the gravitropic response, namely, the correction of the growth direction. The early steps of gravity sensing and the gravireceptor-activation process have been investigated in a lower plant model cell type, the rhizoid of the green alga Chara, and in higher plant roots under microgravity conditions during parabolic flights and sounding-rocket flights. It was shown for Chara rhizoids that actin and associated proteins play a central role in the gravity-induced sedimentation process. The actomyosin system directs sedimenting statoliths to specific graviperception sites on the plasma membrane. Parabolic flight experiments aboard the airbus A300 Zero-G have shown that statoliths activate gravireceptors at the plasma membrane by directly interacting with the membrane-bound molecules, but not by exerting mechanical pressure on a mechanoreceptor. Similar findings were obtained with primary roots of higher plants; however, it was found that actin is not essentially required for the gravity-sensing process. Inhibitor treatments and experiments in microgravity suggest that actin has a fine-tuning function in statocytes of higher plants. Actin effectively modulates the statolith-receptor interaction to ensure a most beneficial response and to avoid unfavorable responses to only transient stimulation. Experiments under microgravity conditions have significantly contributed to the breakthrough that has been made in the understanding of plant gravity sensing and gravitropic reorientation processes. Supported by DLR 50WB0515 on behalf of Bundesministerium für Wirtschaft und Technologie.