highly dynamic actin cytoskeleton that is regulated by the concerted action of numerous
associated proteins (Braun et al. 2004). Inhibitor studies have shown that disrupting the
actin cytoskeleton in rhizoids and in protonemata terminated the actomyosin-driven po-
larized growth and caused statoliths to drop into the tip or toward the nucleus, respec-
tively (Hejnowicz and Sievers 1981; Bartnik and Sievers 1988; Sievers et al. 1996). After
removing the drug, statoliths were quickly repositioned and gravity-oriented tip growth
was restored as soon as the actin cytoskeleton was rearranged and fully functional (Braun
2001).
Experiments conducted under microgravity conditions provided by parabolic flights
of sounding rockets (TEXUS, MAXUS) and during Space Shuttle missions (IML-2,
S/MM05), as well as experiments in simulated weightlessness provided by two-
dimensional (fast-rotating) and three-dimensional clinostats, have unravelled the specific
contributions of gravity and actomyosin to the interplay of forces that underlie the
statoliths-based gravity-sensing (susception) apparatus of Chararhizoids and protone-
mata (Buchen et al. 1993, 1997; Cai et al. 1997; Hoson et al. 1997; Braun et al. 2002).
When the influence of gravity was abolished during the microgravity phases of sounding
rocket flights (Volkmann et al. 1991; Buchen et al. 1993) and randomized during rotation
on clinostats (Hoson et al. 1997; Braun et al. 2002), actomyosin forces generated a dis-
placement of statoliths against the former direction of gravity. Thus, in vertically-oriented
rhizoids and protonemata at 1 g, the statoliths are kept in a dynamically stable equilib-
rium position by actomyosin forces which exactly compensate the effect of gravity on the
statoliths (Figure 7.3).
Detailed analysis of the movements of statoliths in microgravity and of statoliths
146 PLANT TROPISMS
Figure 7.2. Apical part of a Chararhizoid. Statoliths sediment onto the subapical plasma membrane upon
gravistimulation. The growth direction is reoriented by a reduction of elongation growth of the lower cell
flank. Diameter of the cell is 30 μm.