proposal, called the gravitational pressure model, originated with studies in single intern-
odal cells of Chara.Charainternodes display a gravity-dependent polarity of cytoplas-
mic streaming such that the downward velocity of streaming is 10% faster than the up-
ward stream. However, when positioned horizontally, left and right streaming velocity of
the internodes occur at the same rate. A series of studies in the 1990s suggest that this
change in the polarity of streaming is a physiological response to gravity and thus reflec-
tive of gravity sensing (Wayne and Staves 1996). Because of the lack of sedimentable or-
ganelles in the internodes, the gravitational pressure model proposed that the cell per-
ceives gravity by sensing the weight of the entire protoplast. Differential tension between
the upper and lower part of the cell then activates receptors in the plasma membrane, ini-
tiating events relevant to the gravity response (reviewed in Staves 1997).
Although the gravitational pressure model seems to adequately explain the gravity-
induced changes in cytoplasmic streaming in giant internodal cells of Chara, the debate
continues as to its applicability to gravity sensing in higher plants. One interpretation
made by proponents of the gravitational pressure model is that amyloplasts contribute to
gravity sensing by simply adding to the weight of the protoplast. They argue that if grav-
ity sensing was dependent on the mass of the protoplast, then media of higher density
should inhibit gravitropism. Indeed, when rice roots were grown in aqueous media of
higher density due to the addition of various solutes, gravitropism was inhibited without
affecting amyloplast sedimentation (Staves et al. 1997). However, these data have yet to
be confirmed in other plant species. Instead, one report on protonemata of the moss
Ceratodon purpureuscontradicts the earlier studies with rice. In plastid-containing pro-
tonemata of Ceratodon, robust upward bending was shown to proceed despite growth in
high-density media (Schwuchow et al. 2002).
It has also been suggested that both plastid and protoplast pressure-based sensing may
operate in regulating gravitropism. Indeed, although sedimenting plastids appear to accel-
erate a gravity response in Arabidopsisinflorescence stems, they are not required to elicit
this response (Weise and Kiss 1999). There are earlier examples in the literature that sup-
port the gravitational pressure model for gravity sensing (Kiss 2000), but it seems that
most of the cell biological and genetic studies in the last five years continue to point to
sedimenting plastids as a primary mechanism to explain gravity perception in higher
plants (Blancaflor and Masson 2003; Morita and Tasaka 2004).
1.5 The cytoskeleton in gravity perception
Although much of the experimental evidence to date continues to favor the starch-
statolith hypothesis, it remains unclear how the physical falling of amyloplasts in the
gravity-sensing cells is translated into a biochemical or physiological signal that eventu-
ally leads to differential organ growth. One cellular structure that has been implicated in
modulating gravitropism is the cytoskeleton. Although recent review articles have
touched on this topic quite extensively (e.g., Baluˇska and Hasenstein 1997; Volkmann et
al. 1999; Blancaflor 2002; Perbal and Driss-Ecole 2003), we cover this area again briefly
in light of new evidence pointing to a more complex role for the cytoskeleton in gravity
sensing than was previously proposed.