7.6 Gravity susception—the initial physical step of gravity sensing
A gravisensitive cell must provide the molecular and cellular conditions that facilitate the
activation of a receptor by gravity. The pure physical action that is induced by a change
of the direction or amount of acceleration resulting in the activation of a gravireceptor has
been termed susception. Sedimentation of masses is a prerequisite for any interaction of
gravity with cellular components. Two hypotheses have been put forward to explain how
gravity is sensed in a cell (Salisbury 1993; Sack 1997; Kiss 2000). Candidates for sedi-
mentable masses are either intracellular statoliths or the entire protoplast. In higher
plants, the statolith-based hypothesis (the starch-statolith theory) is favored since the dis-
covery of starch-filled amyloplasts (statoliths) in specialized columella and endodermal
cells of higher plants (reviewed in Sack 1997; see also Chapter 1). However, the absence
of obvious statoliths in several graviresponsive cells, including the internodal cells of
Chara, and the observation that starchless mutants of Arabidopsiscan sense gravity,
were decisive for the formulation of the alternative, protoplast-based hypothesis (the
protoplast-pressure theory, also discussed in Chapter 1; Pickard and Thimann 1966;
Wayne et al. 1990; Wayne and Staves 1996).
In the protoplast-pressure theory, the hydrostatic pressure of the entire protoplast is
suggested to trigger conformational changes of gravireceptor molecules at the plasma
membrane. The cell then perceives the direction of gravity by sensing the differential ten-
sion and compression between the plasma membrane and the extracellular matrix at the
top and at the bottom of the cell, respectively. In the following, we will provide evidence
that both models of gravity sensing are realized in single-cell systems.
7.7 Susception in the statolith-based systems of Chara
Characean rhizoids and protonemata are among the best-studied gravisensory cell types
in which the cytoskeleton-based susception apparatus is well-understood. In downward-
growing Chararhizoids, BaSO 4 -crystal-filled vesicles rather than starch-filled amylo-
plasts serve as statoliths. The high density of BaSO 4 and the vesicle size of 1 to 2 μm
make these statoliths ideal for indicating the direction of gravity to the cell. Removal of
statoliths from the tip abolishes gravitropic responsiveness (Sievers et al. 1991b), which
clearly indicates that gravity signaling is triggered by these intracellular sedimentable
particles. Thus, the sensory system for gravitropism is not obscured by alternative and re-
dundant mechanisms as appears to be the case for higher plants.
Statoliths in Chararhizoids do not passively fall into the tip. In fact, they are actively
kept in an area 10 to 35 μm above this region (Figure 7.2) (Hejnowicz and Sievers 1981;
Braun 2002). Myosin-like proteins, which were found attached to the surface of statoliths
(Braun 1996a), interact with predominately axially arranged actin microfilaments to pre-
vent statoliths from settling into the tip by exerting net-basipetal forces (Braun and
Wasteneys 1998). In tip-upward-growing protonemata of Chara, actomyosin forces pre-
vent statoliths from sedimenting toward the cell base by acting net-acropetally (Hodick
et al. 1998; Braun et al. 2002).
Polar organization and gravity-oriented tip growth were shown to be dependent on the