the plastids’ osmotic pressure during division through mechanisms related to those of
mechano-sensitive ion channels (Haswell and Meyerowitz 2006).
As of now, there has been no direct evidence for an involvement of these two potential
mechano-sensitive ion channels in gravity signal transduction (Haswell and Meyerowitz
2006). However, some of the eight remaining Arabidopsis MSLgenes are predicted to en-
code proteins targeted to other cellular compartments. Even more strikingly, some of these
are highly expressed in the root statocytes and are transcriptionally responsive to gravis-
timulation (Neal and Masson, unpublished data; Nawy et al. 2005). Careful functional
analysis of this outstanding set of genes, along with that of other potential plant ion chan-
nels with as-yet poorly defined properties and functions (Schulz et al. 2006), may soon
yield important new insights into the gravitropic response.
It remains quite possible that gravity reception involves mechanisms that do not rely
upon mechano-sensitive ion channels. It is quite exciting to note recent developments in the
study of model systems that involve single-cell gravitropic responses. As discussed by
Braun and Hemmersbach in Chapter 7 of this book, Chararhizoids sense gravity through
the sedimentation of BaSO 4 -containing statoliths. Elegant experiments with this system
have demonstrated that the statoliths must sediment onto sensitive membranes at a subapi-
cal region of the cell for the signal to be perceived and transduced into a curvature response
(Braun 2002). Interestingly, experiments involving short-term exposure to hypo- and hyper-
gravity have indicated that simple contact of statoliths with the sensitive membrane is suf-
ficient for gravity perception to occur; differential pressure or tension is not needed (Braun
2002; Limbach et al. 2005). This result led the authors to postulate that gravity signal trans-
duction might be triggered by molecular interaction between ligands carried by the sedi-
menting statoliths and receptors located at the sensitive membranes (Limbach et al. 2005).
A similar ligand-receptor model of gravity reception should not be excluded in higher
plants. In fact, experiments involving starch-deficient mutants of Arabidopsis(which
have lighter amyloplasts that do not appear to sediment), or seedlings exposed to drugs
that destabilize the actin filaments in shoot statocytes—thereby disabling amyloplast sed-
imentation (see Chapter 1)—have suggested that a few “rogue” sedimenting amyloplasts
might be responsible for the remaining gravitropic capability associated these systems
(Kiss et al. 1997; Palmieri and Kiss 2005; Saito et al. 2005). This interesting model could
be tested by subjecting higher plants to short-term hyper- and hypogravity treatments
similar to those performed on Chara (Limbach et al. 2005).
2.2.2 Inositol 1,4,5-trisphosphate seems to function in gravity signal transduction
A potential role for inositol 1,4,5-trisphosphate (InsP 3 ) in gravity signal transduction was
recently suggested from elegant physiological, biochemical, and genetic studies utilizing
both aboveground and root model systems. These experiments demonstrated both the ex-
istence of gravity-induced changes in InsP 3 levels in stimulated organs and a need for
wild-type levels of InsP 3 for full graviresponsiveness.
Biphasic changes in InsP 3 levels upon gravistimulation were first reported for pulvini
of maize and oat (Perera et al. 1999; Perera et al. 2001). In this system, transient fluctu-
ations in InsP 3 levels were observed in both upper and lower pulvinus halves within 10
to 15 sec of gravistimulation. This was followed by a long-term, sustained increase that