The same second messengers have been implicated in the modulation of gravity sig-
nal transduction in aboveground organs as in roots, including cytoplasmic pH (Johannes
et al. 2001) and InsP 3 (Perera et al. 1998, 1999, 2001). Furthermore, several proteins have
been identified as potential gravity signal transducers in shoots (Yamauchi et al. 1997;
Wyatt et al. 2002; Morita et al. 2006). Yet, there has been no indication of relocalization
of auxin transporters within the statocytes in aboveground organs, even though evidence
for asymmetric downward transport of auxin across gravistimulated organs also exists in
these systems (Li et al. 1991; Kaufman et al. 1995; Philippar et al. 1999; Friml et al.
2002; Long et al. 2002; Abas et al. 2006). Below, we briefly discuss how these pieces of
the gravity signal transduction puzzle might fit together to promote pathways that ulti-
mately lead to the curvature of roots and aboveground organs.
2.2.1 Do mechano-sensitive ion channels function as gravity receptors?
As discussed in Chapter 1, several researchers have proposed that membrane-associated
mechano-sensitive ion channels might function as gravity receptors in plant statocytes.
The sedimentation of, and/or pressure/tension exerted by amyloplasts would trigger the
opening of such channels at sensitive membranes. This would allow for a flux of Ca2+
ions within the statocytes, serving as second messengers to trigger a cascade of events
that would ultimately lead to the lateral polarization of statocytes, as discussed above
(Sievers et al. 1984; Sievers et al. 1989; Pickard and Ding 1993; Volkmann and Baluˇska
1999; Yoder et al. 2001).
As discussed in Chapter 5, a number of pharmacological studies support a role for
Ca2+in gravity signal transduction. Agents that inactivate mechano-sensitive ion chan-
nels (i.e., Gd3+or La3+) alter the function of Ca2+regulatory proteins (calmodulin and
Ca2+-ATPases), and Ca2+chelators all inhibit gravitropism (reviewed in Sinclair and
Trewavas 1997; Fasano et al. 2002).
If Ca2+-selective, mechano-sensitive ion channels contribute to gravity signal trans-
duction in the statocytes, one should be able to detect changes in cytosolic Ca2+levels
upon gravistimulation. In an attempt to detect such Ca2+responses, Plieth and Trewavas
(2002) generated transgenic Arabidopsis thalianaseedlings that constitutively express
apoaequorin. This protein interacts with coelenterazine (an exogenously added com-
pound that freely diffuses through membranes) to generate a cytoplasmic aequorin com-
plex whose luminescence intensity is proportional to the levels of Ca2+within the cyto-
plasm (Knight et al. 1991). They then analyzed the intensity of light emitted by groups of
seedlings in response to gravistimulation. Under these conditions, gravistimulation pro-
moted biphasic and transient peaks of aequorin luminescence, reflective of transient
peaks in cytosolic Ca2+levels. The first peak lasted a few seconds and was reminiscent
of responses to mechano-stimulation. The second peak lasted several minutes, a duration
that was related to the intensity of the stimulus (angle of plant reorientation within the
gravity field). It was inhibited by treatments that alter polar auxin transport. Clinorotation
(low-speed rotation within the gravity field to avoid constant stimulation), on the other
hand, promoted a sustained rise in luminescence. It was concluded that a change in cy-
tosolic Ca2+levels follows gravistimulation and may contribute to gravity signal trans-
duction in Arabidopsisseedlings (Plieth and Trewavas 2002).