gravisensitive membrane areas in both cell types in order to trigger graviperception
and to induce the gravitropic signalling cascade (Braun 2002; Limbach et al. 2005).
Lateral movements of statoliths alone, which do not eventually lead to a contact with
the plasma membrane, fail to induce a curvature response.
Experiments performed during parabolic flights on board of the A300 Zero-G
aircraft ultimately revealed the mechanism of gravireceptor activation in characean
rhizoids (Limbach et al. 2005). Statoliths which were weightless during the 22-s
microgravity phases but still in contact with the plasma membrane were able to
activate the putative membrane-bound gravireceptors. Thus, it is ruled out that the
pressure exerted by the weight of statoliths is required for receptor activation; it is the
direct contact with the gravisensitive membrane that initiates the gravitropic signalling pathway.
Furthermore, control experiments on ground have demonstrated that increasing
the weight of sedimented statoliths by lateral centrifugation did not enhance or
accelerate the gravitropic response (Limbach et al. 2005), but graviperception was
terminated within seconds after the contact of statoliths with the plasma membrane
was lost by inverting gravistimulated rhizoids for a few seconds. These results
provide clear evidence that graviperception in characean rhizoids relies on direct
contact allowing yet unknown components on the statoliths´ surface to interact with
membrane-bound receptors rather than on pressure or tension exerted by the weight
of statoliths (Limbach et al. 2005).
4.5 Calcium and Cytoskeletal Forces Govern the Positive
and the Negative Gravitropic Response Mechanisms
In characean rhizoids and protonemata, the actomyosin system that plays a crucial
role in statolith sedimentation and in the activation of gravireceptors is also essentially involved in the fundamentally different graviresponse mechanisms in both
cell types. The smooth downward curvature response of rhizoids has been described
as ´bending by bowing´ (Fig. 4.3) since it is the result of reduced growth rates of the
lower apical cell flank (Sievers et al. 1979; Hodick 1994; Braun and Limbach 2006).
Only in this belt-like gravisensitive plasma membrane area sedimentation of statoliths was shown to result in a drastically reduced concentration of cytoplasmic-free
calcium, most likely caused by the inhibition of calcium channels and followed
by a locally limited reduction of exocytosis of cell-wall material at the lower cell
flank (Fig. 4.9; Sievers et al. 1979; Braun and Richter 1999; Braun 2002). The
position of the Spitzenkörper (indicated by spectrin-labelling in Fig. 4.10) in the
graviresponding rhizoid was always found to be fixed in its central position in the
apical dome. And accordingly, also the tip-most position of the calcium gradient did
not change at all during bending (Fig. 4.10). This led us to conclude that the center of
maximal growth at the cell tip is not affected during the positive graviresponse in
rhizoids (Braun 2002).
58
4 Gravitropism in Tip-Growing Rhizoids and Protonemata of Characean Algae
and to induce the gravitropic signalling cascade (Braun 2002; Limbach et al. 2005).
Lateral movements of statoliths alone, which do not eventually lead to a contact with
the plasma membrane, fail to induce a curvature response.
Experiments performed during parabolic flights on board of the A300 Zero-G
aircraft ultimately revealed the mechanism of gravireceptor activation in characean
rhizoids (Limbach et al. 2005). Statoliths which were weightless during the 22-s
microgravity phases but still in contact with the plasma membrane were able to
activate the putative membrane-bound gravireceptors. Thus, it is ruled out that the
pressure exerted by the weight of statoliths is required for receptor activation; it is the
direct contact with the gravisensitive membrane that initiates the gravitropic signalling pathway.
Furthermore, control experiments on ground have demonstrated that increasing
the weight of sedimented statoliths by lateral centrifugation did not enhance or
accelerate the gravitropic response (Limbach et al. 2005), but graviperception was
terminated within seconds after the contact of statoliths with the plasma membrane
was lost by inverting gravistimulated rhizoids for a few seconds. These results
provide clear evidence that graviperception in characean rhizoids relies on direct
contact allowing yet unknown components on the statoliths´ surface to interact with
membrane-bound receptors rather than on pressure or tension exerted by the weight
of statoliths (Limbach et al. 2005).
4.5 Calcium and Cytoskeletal Forces Govern the Positive
and the Negative Gravitropic Response Mechanisms
In characean rhizoids and protonemata, the actomyosin system that plays a crucial
role in statolith sedimentation and in the activation of gravireceptors is also essentially involved in the fundamentally different graviresponse mechanisms in both
cell types. The smooth downward curvature response of rhizoids has been described
as ´bending by bowing´ (Fig. 4.3) since it is the result of reduced growth rates of the
lower apical cell flank (Sievers et al. 1979; Hodick 1994; Braun and Limbach 2006).
Only in this belt-like gravisensitive plasma membrane area sedimentation of statoliths was shown to result in a drastically reduced concentration of cytoplasmic-free
calcium, most likely caused by the inhibition of calcium channels and followed
by a locally limited reduction of exocytosis of cell-wall material at the lower cell
flank (Fig. 4.9; Sievers et al. 1979; Braun and Richter 1999; Braun 2002). The
position of the Spitzenkörper (indicated by spectrin-labelling in Fig. 4.10) in the
graviresponding rhizoid was always found to be fixed in its central position in the
apical dome. And accordingly, also the tip-most position of the calcium gradient did
not change at all during bending (Fig. 4.10). This led us to conclude that the center of
maximal growth at the cell tip is not affected during the positive graviresponse in
rhizoids (Braun 2002).
58
4 Gravitropism in Tip-Growing Rhizoids and Protonemata of Characean Algae