associated molecules within the root statocytes (Boonsirichai et al. 2003). It is interesting
to note that several of the molecules suggested to function as second messengers in grav-
ity signal transduction, including Ca2+, proton flux, and phosphoinositides, have been
shown to function in the regulation of vesicular trafficking in other cell types, thus argu-
ing for a direct connection between gravity signal transduction and vesicular trafficking
(reviewed in Blancaflor and Masson 2003). However, we have not yet eliminated the pos-
sibility that gravity-induced PIN3 relocalization in the root statocytes is a consequence of
transporter activity regulation right at the plasma membrane. Indeed, changes in auxin
level have been shown to result in changes in the cycling and polar localization of PIN pro-
teins in other cell types (Blilou et al. 2005; Paciorek et al. 2005). More work is needed to
resolve this ambiguity.
The situation is even more complex if one considers potential effects of gravity signal
transduction on the polarity of vesicular trafficking and on the localization of auxin trans-
porters in shoot statocytes. Indeed, PIN3 is also an ideal candidate for molecular linkage
between gravity perception and asymmetric auxin distribution in shoots. Here, PIN3 is
localized to the plasma membrane of the inner longitudinal and bottom sides of endoder-
mal cells in vertically oriented organs (Friml et al. 2002), and no data currently exist to
support or contradict a possible intracellular relocalization of PIN3 upon gravistimula-
tion. Careful immunolocalization or GFP-PIN fusion expression studies are needed to de-
termine whether such a relocalization also occurs in shoot statocytes, and to establish
whether it involves other auxin transporters. It is essential to establish whether gravistim-
ulation regulates the activity of auxin transporters on the outer side of lower flank stato-
cytes, or directly regulates the vesicular trafficking of auxin transporters. As reviewed in
Chapter 3, the transduction pathway could also lead to differential phosphorylation of
auxin transport facilitators, or it could regulate the levels of small-molecule effectors of
auxin transporters, thereby contributing to the regulation of lateral auxin transport and
lateral gradient formation in the absence of transporter relocalization. These possibilities
will have to be investigated carefully in order to gain a better understanding of the mo-
lecular mechanisms that govern gravity signal transduction in both roots and shoots.
2.2.7 Could cytokinin also contribute to the gravitropic signal?
Although lateral gradients of auxin have long been considered the biochemical signals
that inform the tissues involved in the differential cell elongation of gravitropic signals
perceived by the statocytes, cytokinin was recently proposed as an alternative signal me-
diating early phases of gravitropic curvature in roots (Aloni et al. 2004). Careful kinemat-
ical studies of early phases of root gravitropic curvature have long puzzled researchers
(Ishikawa and Evans 1993; Baluˇska et al. 1996; Wolverton et al. 2002). These investiga-
tions have revealed a complexity of the early root-growth response to gravistimulation
that could not be explained by the simple model of auxin redistribution described above.
Instead, it revealed a first step where overall growth was inhibited at both the upper and
lower sides of gravistimulated roots, followed by a second, auxin-gradient-independent
phase where the gravitropic curvature is initiated by an enhancement in the rate of cellu-
lar elongation on the upper flank of the distal elongation zone (reviewed in Wolverton et
al. 2002).