role as an additional passive transport component that contributes to the resting position-
ing of statoliths as long as the rhizoid tip points downward. Any changes in the orienta-
tion of the cells with respect to the direction or the amount of the acceleration results in
a disturbance of this balance and a displacement, namely, the lateral sedimentation of sta-
toliths (Figure 7.3).
In lateral directions, statolith position is only weakly controlled by the actomyosin sys-
tem in both cell types (Leitz et al. 1995; Buchen et al. 1997). Recently, the forces required
to move statoliths in the lateral direction were determined during the 13-minute micro-
gravity phases of two MAXUS rocket flights. In rhizoids growing vertically downward,
lateral acceleration forces in a range of 0.1 gwere sufficient to displace statoliths to-
ward the membrane-bound gravireceptors. Based on the known size of statoliths and the
density differences between statoliths and the surrounding cytoplasm, it was calculated
that molecular forces in a range of 2 10 –14N must be exerted on a single statolith in
the lateral direction to overcome the cytoskeletal bonds and induce sedimentation, thus
eliciting graviperception (Limbach et al. 2005).
The need for the complex actomyosin-based control of statoliths position for gravity
perception became fully comprehensible only after it was discovered that only the sub-
apical plasma membrane area of the statolith region, 10 to 35μm from the cell tip, is able
to trigger the gravitropic response upon statolith sedimentation (Braun 2002). Forcing
statoliths to sediment outside these areas by using optical laser tweezers or centrifugation
did not result in a gravitropic response (Braun 2002). After reorienting rhizoids by 90 de-
grees, the statoliths sediment mainly along the gravity vector and settle onto the lower
cell flank of the statolith region where graviperception takes place and the graviresponse
is initiated. However, when cells were stimulated at angles different from 90 degrees, sta-
toliths did not simply follow the gravity vector (Figure 7.3) (Hodick et al. 1998). Instead,
even in almost fully inverted cells, statoliths were actively redirected against gravity and
were guided to the small gravisensitive area of plasma membrane in this “statolith re-
gion” of the tip.
Gravistimulation of tip-upward-growing protonemata causes an actin-mediated
acropetal displacement of statoliths into the apical dome, where they sediment very close
to the tip (Figure 7.3) (Hodick et al. 1998). In contrast to rhizoids, the gravisensitive
plasma membrane area in protonemata is limited to an area 5 to 10 μm basal to the tip
(Braun 2002). During the upward bending of protonemata, the statoliths periodically sed-
iment along the gravity vector and leave the gravisensitive site, which deactivates the
gravireceptor. The periods when the statoliths exit the gravisensitive region are reflected
by phases of straight growth. Actomyosin-mediated transport of statoliths back to the
gravisensitive membrane area reinitiates the gravitropic response from time to time until
the vertical orientation is resumed (Figure 7.3). On one hand, these actin-mediated sus-
ception mechanisms guarantee a highly efficient readjustment of the growth direction
and, on the other hand, avoid inexpedient responses to transient stimuli.
Gravity sensing and negative gravitropic (upward) response mechanisms in other
single-celled systems like the protonemata of mosses, especially Ceratodon purpureus,
are less well-understood. It is thought that these cells use starch-filled amyloplasts as sta-
toliths. Should they be using the “protoplast pressure” mechanism for graviperception
discussed above (i.e., the density-difference between the cell ́s protoplast and the sur-
frankie
(Frankie)
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