center by differentially regulating the myosin-mediated anchorage and the activity of
actin-associated proteins along the shifting calcium gradient (Braun and Richter
1999). Gravistimulation experiments have shown that protonemata tend to reorient
towards the former growth axis after only short gravistimulation phases. The position of the newly established growth axis induced by the upward shift of the calcium
gradient appears to be rather labile before it is anchored by actin and associated
proteins (Braun and Richter 1999; Braun 2001).
4.6 The Impact of Research in Microgravity for Unraveling
Plant Gravitropic Signalling Pathways
It all started with a simple experiment. Chara rhizoids growing in an agar-filled
vacuum-tight chamber on a small microscope payload was put on a TEXUS rocket
(TEXUS 21) and launched to an altitude of approx. 360 km. With the beginning of
the 6-min microgravity phase of the parabolic flight, statoliths moved away from the
tip (Fig. 4.7). For the first time the video-images recorded during the microgravity
phase provided direct proof that actomyosin exerts forces on statoliths in a gravity
sensing cell (Sievers et al. 1991a; Volkmann et al. 1991; Buchen et al. 1993). When
the actin cytoskeleton was destroyed by applying cytochalasin D shortly before
launch of another TEXUS rocket, statoliths settled into the very tip of the rhizoids
and did not move at all during the subsequent microgravity phase. It was the first
direct proof for the intimate interaction of statoliths with the actin cytoskeleton and
unambiguous evidence was obtained for the complex and well-balanced positioning
of statoliths in characean rhizoids and protonemata by gravity and actomyosin forces
(Buchen et al. 1993). The subsequent studies with rhizoids in microgravity, on
clinostats and on centrifuges complemented by laser tweezers studies resulted in
the first detailed description of a cytoskeleton-based plant gravity-sensing apparatus
(Fig. 4.8; Braun 2002).
In the absence of directional accelerations aboard the Space Shuttle rhizoids
developed and grew out from the green thallus nodes in random orientation
(Braun 1997). Except for the position of the statoliths, microgravity-developed
rhizoids looked identical and showed the same structural polarity as the 1-g controls.
This demonstrated that the gravity-sensing cells follow their genetic program and do
not require gravity as an environmental cue for development and morphogenesis.
Graviresponsiveness was also the same in 1-g and space-grown cells, although the
latter had never experienced gravitational forces in their life.
The position of statoliths in microgravity-developed rhizoids (IML-2 Space
Shuttle mission) at a greater distance from the cell tip was like that found in rhizoids
at the end of the 6-min microgravity phase of TEXUS sounding rocket flights.
Although in microgravity the statoliths were positioned further away from the tip,
they were still kept in the microtubule-depleted apical zone, where statoliths are able
4.6 The Impact of Research in Microgravity for Unraveling Plant. . .
61
actin-associated proteins along the shifting calcium gradient (Braun and Richter
1999). Gravistimulation experiments have shown that protonemata tend to reorient
towards the former growth axis after only short gravistimulation phases. The position of the newly established growth axis induced by the upward shift of the calcium
gradient appears to be rather labile before it is anchored by actin and associated
proteins (Braun and Richter 1999; Braun 2001).
4.6 The Impact of Research in Microgravity for Unraveling
Plant Gravitropic Signalling Pathways
It all started with a simple experiment. Chara rhizoids growing in an agar-filled
vacuum-tight chamber on a small microscope payload was put on a TEXUS rocket
(TEXUS 21) and launched to an altitude of approx. 360 km. With the beginning of
the 6-min microgravity phase of the parabolic flight, statoliths moved away from the
tip (Fig. 4.7). For the first time the video-images recorded during the microgravity
phase provided direct proof that actomyosin exerts forces on statoliths in a gravity
sensing cell (Sievers et al. 1991a; Volkmann et al. 1991; Buchen et al. 1993). When
the actin cytoskeleton was destroyed by applying cytochalasin D shortly before
launch of another TEXUS rocket, statoliths settled into the very tip of the rhizoids
and did not move at all during the subsequent microgravity phase. It was the first
direct proof for the intimate interaction of statoliths with the actin cytoskeleton and
unambiguous evidence was obtained for the complex and well-balanced positioning
of statoliths in characean rhizoids and protonemata by gravity and actomyosin forces
(Buchen et al. 1993). The subsequent studies with rhizoids in microgravity, on
clinostats and on centrifuges complemented by laser tweezers studies resulted in
the first detailed description of a cytoskeleton-based plant gravity-sensing apparatus
(Fig. 4.8; Braun 2002).
In the absence of directional accelerations aboard the Space Shuttle rhizoids
developed and grew out from the green thallus nodes in random orientation
(Braun 1997). Except for the position of the statoliths, microgravity-developed
rhizoids looked identical and showed the same structural polarity as the 1-g controls.
This demonstrated that the gravity-sensing cells follow their genetic program and do
not require gravity as an environmental cue for development and morphogenesis.
Graviresponsiveness was also the same in 1-g and space-grown cells, although the
latter had never experienced gravitational forces in their life.
The position of statoliths in microgravity-developed rhizoids (IML-2 Space
Shuttle mission) at a greater distance from the cell tip was like that found in rhizoids
at the end of the 6-min microgravity phase of TEXUS sounding rocket flights.
Although in microgravity the statoliths were positioned further away from the tip,
they were still kept in the microtubule-depleted apical zone, where statoliths are able
4.6 The Impact of Research in Microgravity for Unraveling Plant. . .
61