obviously very different, the presented experimental data reveal that especially the early
phases of gravity sensing share common features. The gravisensory processes can be re-
duced to two principles: perception via intracellular statoliths and via the whole proto-
plast. Gravisensory ion channels and cascades of ubiquitous second messengers are pre-
dicted to operate in most gravity-dependent signaling pathways, and have been identified
in some cases. Finally, the cytoskeleton has been shown to play a master role in the com-
plex process of gravity sensing and graviorientation. One fascinating question, further
discussed in Chapters 8 and 9, will hopefully be answered in the near future: What will
be the impact of long-term exposure to microgravity conditions in multigeneration exper-
iments on the physiology of specialized gravisensory cells?
7.16 Acknowledgments
The authors thank the several space shuttle crews, the teams of EADS ST, Kayser-Threde,
Deutsches Zentrum für Luft- und Raumfahrt (DLR), NASA, Swedisch Space
Corporation (SSC), Novespace, and the European Space Agency (ESA) for their dedi-
cated work and for stimulating discussions. This work was financially supported by
Deutsches Zentrum für Luft- und Raumfahrt (DLR) on behalf of the Bundesministerium
für Bildung und Forschung (50WB9998 and 50WB0515).
7.17 Literature cited
Baba SA, Tatematsu R and Mogami Y. 1991. A new hypothesis concerning graviperception of sin-
gle cells, and supporting simulated experiments. Biological Sciences in Space 5 :290–291.
Barlow PW. 1995. Gravity perception in plants: a multiplicity of systems derived by evolution?
Plant Cell Environment 18 :951–962.
Bartnik E and Sievers A. 1988. In-vivoobservation of a spherical aggregate of endoplasmic retic-
ulum and of Golgi vesicles in the tip of fast-growing Chararhizoids.Planta176:1–9.
Bean B. 1984. Microbial geotaxis. In Membranes and Sensory Transduction, edited by G.
Colombetti and F. Lenci, pp. 163–98. London, New York: Plenum Press
Blancaflor EB. 2002. The cytoskeleton and gravitropism in higher plants. Journal of Plant Growth
Regulation21:120–136.
Block I, Briegleb W and Wohlfarth-Bottermann K-E. 1986. Gravisensitivity of the acellular slime
mold Physarum polycephalumdemonstrated on the fast-rotating clinostat. European Journal of
Cell Biology41:44–50.
Bonini NM, Gustin MC and Nelson DL. 1986. Regulation of ciliary motility by membrane poten-
tial in Paramecium: a role for cyclic AMP. Cell Motility and the Cytoskeleton 6 :256–272.
Bräucker R, Cogoli A and Hemmersbach R. 2001. Graviperception and graviresponse at the cel-
lular level. In Astrobiology: The Quest for the Conditions of Life, edited by Christa Baumstark-
Khan and G.G. Horneck, pp. 284–297. Berlin, Heidelberg: Springer-Verlag.
Braun M. 1996a. Immunolocalization of myosin in rhizoids of Chara globularisThuill. Proto-
plasma191:1–8.
Braun M. 1996b. Anomalous gravitropic response of Chararhizoids during enhanced accelera-
tions. Planta199:443–450.
Braun M. 1997. Gravitropism in tip-growing cells. Planta203: 511–519.