which were displaced to different cell regions by optical laser tweezers or by centrifuga-
tion revealed the surprising complexity of forces by which actomyosin controls statolith
positioning (Braun et al. 2002). Individual acropetal and basipetal movements of sta-
toliths can be observed in both cell types, indicating that statoliths interact with the
mainly axially oriented actin microfilaments with opposite polarities. When statoliths
were centrifuged into the subapical region, statolith transport back to the original posi-
tion was observed which is not notably influenced by gravity (Sievers et al. 1991b; Braun
and Sievers 1993). Active transport is generated along actin microfilaments and statoliths
do not sediment onto the lower cell flank until they have reached the statolith region near
the tip, where sedimentation is not constrained by microtubules (Braun and Sievers
1994). Microtubules are excluded from the apical region of rhizoids and protonemata.
The actomyosin component of movement is always the strongest pointing toward the
statolith region. This ensures that statoliths are always kept in, or are retransported to,
their original position. In the statolith region itself, however, gravity plays the decisive
CHAPTER 7 SINGLE-CELL GRAVITROPISM AND GRAVITAXIS 147
Figure 7.3. Gravitropic phases in characean rhizoids and protonemata. In rhizoids, the position of the sta-
toliths (St) is balanced by net-basipetally acting actomyosin forces (Factin) and gravity (Fgravity). Upon grav-
istimulation, actomyosin forces guide the sedimenting statoliths toward the gravireceptors located in a belt-
like area of the plasma membrane 10 to 35 μm above the tip. The Spitzenkörper (SpK) remains arrested at
the tip and the calcium gradient (indicated by darker and lighter grey dotted areas) is always highest at the
tip. When statoliths contact the gravireceptors (GR), graviperception takes place and is followed by a local
reduction of cytosolic Ca2+that results in differential extension of the opposite cell flanks (double-headed ar-
rows). In upward-growing protonemata (lower row), the effect of gravity on statoliths is compensated by net-
acropetally acting actomyosin forces. Upon horizontal positioning, sedimenting statoliths are guided to the
gravireceptors which are located near the growth center at the tip. This causes a drastic shift of the calcium
gradient and the Spitzenkörper (SpK) upward where the new outgrowth occurs. The white arrows point to
the area of maximal calcium influx. MT, microtubule; MY, myosin; SpKc, center of the Spitzenkörper.