generate a multicellular body, and proliferate. These cells elongate by polarized growth
or tip growth enabling them to extend rapidly and so penetrate easily all kinds of sub-
strates. When accidentally covered with sediment or sand, protonemata of Charaarise
from cells at the nodal complexes of the green thallus by asymmetric cell division. These
cells use negative gravitropic tip growth to grow back to the light. Light induces termi-
nation of tip growth and a depolarization of the cells. The cells undergo a complex pat-
tern of divisions that reconstitute the green thallus. This process ensures survival of the
plant in a difficult environment characterized by unpredictably changing conditions.
7.5 What makes a cell a biological gravity sensor?
Gravity is a weak but ubiquitous force that acts on masses. Therefore, work has to be done
in the form of moving a mass in the gravitational field in order to create sufficient energy
to activate a biological sensor. Theoretically, many cells possess organelles with suffi-
cient masses whose gravitationally induced movements could create sufficient energy.
However, most cell types of the various tissues usually actively prevent organelles from
sedimenting by keeping them in place via cytoskeletal anchorage. This genetically de-
fined high degree of cytoplasmic organization is quite stable, even against moderately in-
creased acceleration forces. Thus, organelles do not move in the gravitational field or
their movements are obviously not transduced into a physiological response. Therefore,
in addition to the presence of organelles with sufficient mass and size, a gravity-sensing
apparatus must exist in a gravisensitive cell type that facilitates sedimentation of specific
masses and mediates the activation of gravity-specific receptors (i.e., the transduction of
the physical stimulus into a physiological gravity perception signal).
As discussed in Chapters 1 and 2, little is known about the mechanism of gravity sens-
ing in higher plant statocytes, and identification of components of a gravity-sensor appa-
ratus is limited to starch-filled amyloplasts. These amyloplasts act as statoliths whose
gravity-directed sedimentation precedes graviperception and gravitropic curvature.
Several studies suggest the involvement of actin microfilaments that might act as trans-
ducers of tensional forces generated by the gravity-induced sedimentation of statoliths
(Sievers et al. 1991a; Yoder et al. 2001; Blancaflor 2002; Perbal and Driss-Ecole 2003)
to mechano-sensitive receptors in cortical ER membranes or in the plasma membrane of
higher plant statocytes (Ding and Pickard 1993; Kiss 2000). However, experimental evi-
dence for the role of the actin cytoskeleton in susception and perception of gravity re-
mains controversial. For example, treating statocytes with actin-disrupting drugs in-
creased the sedimentation rate of statoliths (Sievers et al. 1989) and actually enhanced the
gravitropic response (Hou et al. 2003, 2004; Yamamoto and Kiss 2002).
In light of this dilemma as to the identity and role(s) of the gravity perception machin-
ery in higher plants, the ease of manipulation afforded by single-cell systems opens up
new vistas in order to understand gravity-sensing and gravity-oriented growth. In the fol-
lowing, we summarise the data on single-cell systems that have led to an increasing un-
derstanding of the nature of the gravisusception apparatus and the complexity of compo-
nents (i.e., cytoskeletal elements, membrane compartments, ion channels, and receptor
proteins that need to interact to elicit a beneficial orientation response).