lateral centrifugal forces in a range of 0.1 g were sufficient to trigger a movement of
statoliths towards the membrane-bound gravireceptors. From these results, the
molecular forces acting on a single statolith in lateral direction were determined to
be in a range of 2 Â 10
À14 N (Limbach et al. 2005), whereas forces required to move
statoliths towards the apex were several orders of magnitude higher. All these studies
show that by acting differently on statoliths in the different regions of the cells,
actomyosin forces ensure that statoliths are actively kept in an area close to the tip,
where they can serve as sedimenting particles in the gravitropic signalling pathway.
They are free to sediment quickly upon gravistimulation and are close enough to the
tip to interfere with the tip-growing process (Braun 2002).
Upon gravistimulation the actomyosin and gravitational forces acting on the
statoliths are no longer well balanced and statoliths move towards the lateral cell
flanks. Statoliths in both cell types, however, do not simply follow the gravity vector
and sediment onto the lateral cell flank; in fact, actomyosin forces direct sedimenting
statoliths to specific gravisensitive regions of the plasma membrane (Braun 2002),
the only areas, where the gravitropic signalling cascade is elicited triggering the
reorientation of the growth direction (Fig. 4.8). Only when rhizoids are reoriented
horizontally sedimenting statoliths simply follow the gravity vector and settle
onto the lower cell flank 10–35 μm above the tip. However, when rhizoids are
gravistimulated at any other angles, sedimenting statoliths do not simply follow the
gravity vector, but are (even in inverted rhizoids) actively redirected towards the
same belt-like plasma membrane area above the tip where statoliths sediment in
horizontally stimulated cells. Centrifugation studies have confirmed that only statolith sedimenting in this specific membrane area were able to trigger graviperception
and initiate the positive graviresponse (Braun 2002).
Gravistimulating tip-upward growing protonemata at any angle causes a
stimulation-angle independent, actin-mediated acropetal displacement of sedimenting
statoliths into the apical dome (Fig. 4.8) where they settle onto the gravisensitive
plasma membrane which in protonemata is only a small area very close to the tip,
5–10 μm behind the tip (Braun 2002). Pushing statoliths onto any other plasma
membrane area by centrifugation failed to initiate a gravitropic response.
4.4 Gravireceptor Activation Requires Well-Concerted
Action of Gravity and Actomyosin Forces
Although the nature of the membrane-bound gravireceptor molecules at the specific
areas of the apical plasma membrane in rhizoids and protonemata is still unknown
and the immediate downstream physiological processes still need to be clarified, the
process of graviperception, the transformation of the physical stimulus of statolith
sedimentation into a physiological response, is well understood. There is good
experimental evidence from centrifugation, laser-tweezer experiments and parabolic
flight experiments that statoliths have to be in contact with the cell-type specific
4.4 Gravireceptor Activation Requires Well-Concerted Action of Gravity. . .
57
statoliths towards the membrane-bound gravireceptors. From these results, the
molecular forces acting on a single statolith in lateral direction were determined to
be in a range of 2 Â 10
À14 N (Limbach et al. 2005), whereas forces required to move
statoliths towards the apex were several orders of magnitude higher. All these studies
show that by acting differently on statoliths in the different regions of the cells,
actomyosin forces ensure that statoliths are actively kept in an area close to the tip,
where they can serve as sedimenting particles in the gravitropic signalling pathway.
They are free to sediment quickly upon gravistimulation and are close enough to the
tip to interfere with the tip-growing process (Braun 2002).
Upon gravistimulation the actomyosin and gravitational forces acting on the
statoliths are no longer well balanced and statoliths move towards the lateral cell
flanks. Statoliths in both cell types, however, do not simply follow the gravity vector
and sediment onto the lateral cell flank; in fact, actomyosin forces direct sedimenting
statoliths to specific gravisensitive regions of the plasma membrane (Braun 2002),
the only areas, where the gravitropic signalling cascade is elicited triggering the
reorientation of the growth direction (Fig. 4.8). Only when rhizoids are reoriented
horizontally sedimenting statoliths simply follow the gravity vector and settle
onto the lower cell flank 10–35 μm above the tip. However, when rhizoids are
gravistimulated at any other angles, sedimenting statoliths do not simply follow the
gravity vector, but are (even in inverted rhizoids) actively redirected towards the
same belt-like plasma membrane area above the tip where statoliths sediment in
horizontally stimulated cells. Centrifugation studies have confirmed that only statolith sedimenting in this specific membrane area were able to trigger graviperception
and initiate the positive graviresponse (Braun 2002).
Gravistimulating tip-upward growing protonemata at any angle causes a
stimulation-angle independent, actin-mediated acropetal displacement of sedimenting
statoliths into the apical dome (Fig. 4.8) where they settle onto the gravisensitive
plasma membrane which in protonemata is only a small area very close to the tip,
5–10 μm behind the tip (Braun 2002). Pushing statoliths onto any other plasma
membrane area by centrifugation failed to initiate a gravitropic response.
4.4 Gravireceptor Activation Requires Well-Concerted
Action of Gravity and Actomyosin Forces
Although the nature of the membrane-bound gravireceptor molecules at the specific
areas of the apical plasma membrane in rhizoids and protonemata is still unknown
and the immediate downstream physiological processes still need to be clarified, the
process of graviperception, the transformation of the physical stimulus of statolith
sedimentation into a physiological response, is well understood. There is good
experimental evidence from centrifugation, laser-tweezer experiments and parabolic
flight experiments that statoliths have to be in contact with the cell-type specific
4.4 Gravireceptor Activation Requires Well-Concerted Action of Gravity. . .
57