to sediment upon gravistimulation by lateral centrifugation and to initiate the
gravitropic response (Braun 1997).
The almost stimulus-free environment of space was also used to study the
gravisensitivity of the gravity-sensing mechanisms and to determine gravitropic
thresholds in characean rhizoids. Gravisensitivity was first examined in detail
during the Spacelab mission IML-2 aboard the Columbia Space Shuttle in 1994.
The result of the application of different lateral accelerations was observed by videomicroscopy in the slow-rotating centrifuge microscope facility (NIZEMI, Friedrich
et al. 1996). The threshold values for gravisensitivity was found to be in the range of
0.1 g and subsequent experiments performed on TEXUS missions and with laser
tweezers further narrowed down the minimal molecular force for a lateral displacement (sedimentation) of a statolith towards the gravisensitive plasma membrane area
to be in a range of 2 Â 10
À14 N (Limbach et al. 2005).
And finally, parabolic plane flight experiments were paramount to the very first
characterization of a plant gravireceptor activation mechanism. Microgravity experiments provided clear evidence that the membrane-bound gravireceptor molecules in
characean rhizoids (of yet unknown nature) are activated upon direct contact with
statoliths.
Several reports supporting the idea that characean green algae are the closest
relatives of land plants (Turmel et al. 2003; Lewis and McCourt 2004) additionally
increase the attractiveness of the characean cell types as model systems for elucidating also the gravitropic sensing mechanism in higher plants. Considering the
results from characean unicellular model systems and the evidence for actininteractions with statoliths in higher plant statocytes obtained from microgravity
studies, it is tempting to speculate that actin might play similar roles in the early
processes of gravity sensing in higher plants (Volkmann et al. 1991; Hou et al.
2004). However, since disrupting the actin cytoskeleton in statocytes of higher
plants does not prevent gravity sensing and the graviresponse at all, actomyosin
forces may not be required for the sensory process per se; in fact, the findings suggest
that actomyosin may rather have a fine-tuning function by acting as a guiding system
and damping modulator for sedimenting statoliths. Thus, actin could ensure an
adequate graviresponse by avoiding unfavorable and inappropriate responses to
only transient changes in the orientation of the organ with respect to the gravity
vector, e.g. when a corn stalk is bending in strong winds.
Nevertheless, there are indications that actomyosin-statolith interactions in higher
plant statocytes rely on the same actin-associated proteins and that the mode of
gravireceptor activation is very similar relying also on direct contact with higher
plant statoliths (unpublished results) rather than on direct or indirect (via actin)
mechanical interactions which was postulated by authors since several decades
(e.g. see Sievers et al. 1991b; Driss-Ecole et al. 2000; Perbal et al. 2004).
The progress that has been made since the flight of TEXUS 21 in the unravelling
of gravitropic signalling pathways further underlines the significance of microgravity research on single cell model systems. Experiments in microgravity and utilization of gravitational and acceleration forces have turned out to be valuable research
methods which have contributed greatly to our current knowledge of how plants use
62
4 Gravitropism in Tip-Growing Rhizoids and Protonemata of Characean Algae
gravitropic response (Braun 1997).
The almost stimulus-free environment of space was also used to study the
gravisensitivity of the gravity-sensing mechanisms and to determine gravitropic
thresholds in characean rhizoids. Gravisensitivity was first examined in detail
during the Spacelab mission IML-2 aboard the Columbia Space Shuttle in 1994.
The result of the application of different lateral accelerations was observed by videomicroscopy in the slow-rotating centrifuge microscope facility (NIZEMI, Friedrich
et al. 1996). The threshold values for gravisensitivity was found to be in the range of
0.1 g and subsequent experiments performed on TEXUS missions and with laser
tweezers further narrowed down the minimal molecular force for a lateral displacement (sedimentation) of a statolith towards the gravisensitive plasma membrane area
to be in a range of 2 Â 10
À14 N (Limbach et al. 2005).
And finally, parabolic plane flight experiments were paramount to the very first
characterization of a plant gravireceptor activation mechanism. Microgravity experiments provided clear evidence that the membrane-bound gravireceptor molecules in
characean rhizoids (of yet unknown nature) are activated upon direct contact with
statoliths.
Several reports supporting the idea that characean green algae are the closest
relatives of land plants (Turmel et al. 2003; Lewis and McCourt 2004) additionally
increase the attractiveness of the characean cell types as model systems for elucidating also the gravitropic sensing mechanism in higher plants. Considering the
results from characean unicellular model systems and the evidence for actininteractions with statoliths in higher plant statocytes obtained from microgravity
studies, it is tempting to speculate that actin might play similar roles in the early
processes of gravity sensing in higher plants (Volkmann et al. 1991; Hou et al.
2004). However, since disrupting the actin cytoskeleton in statocytes of higher
plants does not prevent gravity sensing and the graviresponse at all, actomyosin
forces may not be required for the sensory process per se; in fact, the findings suggest
that actomyosin may rather have a fine-tuning function by acting as a guiding system
and damping modulator for sedimenting statoliths. Thus, actin could ensure an
adequate graviresponse by avoiding unfavorable and inappropriate responses to
only transient changes in the orientation of the organ with respect to the gravity
vector, e.g. when a corn stalk is bending in strong winds.
Nevertheless, there are indications that actomyosin-statolith interactions in higher
plant statocytes rely on the same actin-associated proteins and that the mode of
gravireceptor activation is very similar relying also on direct contact with higher
plant statoliths (unpublished results) rather than on direct or indirect (via actin)
mechanical interactions which was postulated by authors since several decades
(e.g. see Sievers et al. 1991b; Driss-Ecole et al. 2000; Perbal et al. 2004).
The progress that has been made since the flight of TEXUS 21 in the unravelling
of gravitropic signalling pathways further underlines the significance of microgravity research on single cell model systems. Experiments in microgravity and utilization of gravitational and acceleration forces have turned out to be valuable research
methods which have contributed greatly to our current knowledge of how plants use
62
4 Gravitropism in Tip-Growing Rhizoids and Protonemata of Characean Algae