mal noise level (2 10 –21 J). However, for smaller species such as Euglenaor even
smaller systems such as human cells (lymphocytes), the signal-to-noise ratio may be lim-
iting. This ratio is even more critical if one considers threshold values in the range of 0.1
gto 0.3 gfor graviperception. Therefore, in these systems the involvement of sup-
porting amplifying structures, such as the microfilament system of Loxodes, may facili-
tate graviperception (Häder et al. 2005).
7.13 Signal Transduction Pathways and Graviresponse Mechanisms in the
Statolith-based Systems of Chara
Although several studies clearly indicate that gravireceptor molecules are located in the
subapical plasma membrane area of the statolith region (Braun 2002), the nature of the
membrane-bound gravireceptor and the immediate downstream physiological steps in-
volved in graviperception in rhizoids and protonemata remain to be clarified. Results ob-
tained from the local application of ions and channel blockers by means of microcapil-
laries suggest that membrane-potential changes might be among the earliest steps
following gravireceptor activation (Braun, unpublished results). However, more data have
been published in the last decade that illuminate the cellular and molecular processes that
underlie the gravitropic response.
The smooth, downward curvature response of a rhizoid is best described as “bending
by bowing,” whereas the response of a protonema was described as “bending by bulging”
(Braun 1996b), referring to the bulge that initially appears on the upper cell flank and in-
dicates the drastic upward shift of the growing tip. The Spitzenkörper (a tip-growth or-
ganizing complex consisting of endoplasmic reticulum, actin filaments, and a dense net-
work of vesicles) (Braun 1997) and, consequently, also the center of maximal growth are
displaced upon gravistimulation of protonemata by intruding statoliths (Figure 7.3).
Although rhizoids can be forced to respond to some extent like protonemata, this can only
be done by pushing statoliths aymmetrically into the apical dome with optical tweezers
or by centrifugal forces greater than 50 g(Braun 2002). There is evidence from cen-
trifugation experiments (Braun 1996b; Hodick and Sievers 1998) and from attaching par-
ticles to the surface of gravitropically responding rhizoids (Sievers et al. 1979) that the
position of the growth center at the cell tip is relatively stable and that the Spitzenkörper
is more tightly anchored by cytoskeletal forces in rhizoids than in protonemata.
The specific properties of the actin cytoskeleton, which have been shown to be respon-
sible for Spitzenkörper anchorage, are controlled by calcium. Interestingly, calcium im-
aging demonstrated a drastic shift of the steep, tip-high calcium gradient toward the upper
flank during initiation of the graviresponse in protonemata (Figure 7.3), but not in rhi-
zoids (Braun and Richter 1999). In accordance with this observation, dihydropyridine
fluorescence (indicating the tip-focused distribution of putative calcium channels) was
also displaced toward the upper flank in graviresponding protonemata, but not in rhizoids
(Braun and Richter 1999).
Calcium imaging studies in rhizoids conducted by Simon Gilroy and coworkers (un-
published results) indicated that the impact of statolith sedimentation in rhizoids is lim-
ited to a local decrease in the concentration of cytosolic calcium in the area where sta-