weight of sedimented statoliths by lateral centrifugation did not enhance the gravitropic
response. However, graviperception was terminated within seconds when the contact of
statoliths with the plasma membrane was interrupted by inverting gravistimulated cells.
This result provides evidence that graviperception in characean rhizoids relies on di-
rect contact allowing as-yet unknown components on the statoliths’ surface to interact
with membrane-bound receptors rather than on pressure or tension which is caused by the
weight of the statoliths (Limbach et al. 2005). A mechanoreceptor was postulated to rep-
resent the receptor because the gravitropic responses of many plant organs seem to obey
the sine law of gravitropism (Galland 2002). The pressure that statoliths exert on the re-
ceptor at different gravistimulation angles would explain the sinusoidal dependency, but
the observation that the number of statoliths which settle on the receptor area in
characean rhizoids decreases with the steepness of the angle can equally well account for
this dependency.
7.11 Graviperception in the Statolith-based System of Loxodes
In order to show that Müller vesicles function as gravisensory organelles in Loxodes,
these structures were destroyed by laser ablation within individual cells. Observation of
the manipulated cells revealed that they were no longer able to orient to the gravity vec-
tor, though their vitality and swimming velocity were unaffected (Hemmersbach et al.
1998). As discussed earlier in this chapter, it has been proposed that Loxodesperceives
gravity through bending of the ciliary complex (Figure 7.4), thus inducing changes in the
membrane potential. Such changes modify the activity of the body cilia, affecting
the swimming behaviour (Fenchel and Finlay 1984, 1986). It is still unknown whether the
gravity-induced “pull” of the BaSO 4 -granulum is directly transduced via ion channels
in the plasma membrane, or whether it requires second messengers. Besides a cellu-
lar gravisensor, it is postulated from isodensity experiments that Loxodes also has a
protoplast-based perception mechanism (Figure 7.4). In order to determine the minimum
acceleration that is necessary to induce a graviresponse, Loxodes, Paramecium,and
Euglenawere exposed to increasing acceleration steps from microgravity to 1.5 g, or
vice versa, on a centrifuge microscope in space. With this experimental approach, thresh-
old values for gravitaxis have been determined: Loxodes0.15 g, Paramecium: 0.3
g(Hemmersbach et al. 1996a), Euglena: 0.16 gand 0.12 g(Häder et al. 1996,
1997). The existence of such thresholds implies that gravitaxis is the result of a physio-
logical signal transduction chain.
7.12 Graviperception in the Protoplast-based Systems of ParameciumandEuglena
Due to the lack of distinct statoliths in most protists, it was proposed that the mass of the
whole cytoplasm causes a mechanical load on the lower cell membrane, thus stimulating
mechano-(gravi)-sensitive ion channels. This sensory mechanism was termed the stato-
cyst hypothesis, which corresponds to the protoplast-pressure theory (for review, see
Machemer and Bräucker 1992). Thus, in this model, mechano-sensitivity is a prerequi-