elongation zones) (Masson et al. 2002). Hence, in roots, the gravity-induced lateral auxin
gradient generated across the cap has to be transmitted basipetally, from the site of per-
ception and signal transduction to the site of response in order for a gravitropic curvature
to develop (Figure 2.1A and Color Section).
In aboveground organs, the signal has only to be transmitted laterally, across the organ.
In the Arabidopsisshoot elongation zone, the epidermis, cortex, gravity-perceiving endo-
dermis, and stele are arranged radially, in successive layers, and Arabidopsisstem seg-
ments dissected from any part of the elongation zone are gravitropic (Fukaki et al. 1996).
Thus, the gravity signal perceived in the endodermal cell layer may be transmitted in an
inner-to-outer fashion (laterally), leading to a unilateral, asymmetric auxin distribution
between the lower and upper flanks that results in differential cell elongation and conse-
quent gravitropic curvature (Fukaki et al. 1998; Tasaka et al. 1999). This centrifugal as-
pect of gravitropic signaling between cell types seems general in aboveground organs.
However, we do not know whether all cells in the endodermal layer play equal roles in
gravity perception and signaling (Figure 2.1B and Color Section).
Recent physiological and molecular genetic studies have provided insights into the
mechanisms that govern gravity signal transduction in roots, hypocotyls, shoots, and ce-
real pulvini. This information, summarized in this chapter, will provide the foundation
for future research aimed at better understanding how this machinery might be modu-
lated by endogenous and environmental cues.
2.2 Gravity signal transduction in roots and aboveground organs
Roots constitute an excellent system for analysis of gravity signal transduction in plants.
The physical separation existing between the primary site of gravity perception (the root
cap columella) and the site of differential cellular elongation that drives the correspon-
ding curvature response (the elongation zones) allows for careful dissection of the mo-
lecular mechanisms involved in each phase of the process. Hence, it is not surprising that
several teams have focused their research on this system to decipher some of the molec-
ular mechanisms that govern gravity signal transduction. The gravity receptor remains
unknown. However, several potential gravity signal transducers have recently been un-
covered. They include cytoplasmic cations, inositol 1,4,5-trisphosphate (InsP 3 ), and sev-
eral proteins.
Downstream effects of activation of the gravity signal transduction pathway in the root
cap were recently uncovered. They include rapid changes in gene expression within the
root tip (Moseyko et al. 2002; Kimbrough et al. 2004); a lateral polarization of the stato-
cytes as reflected by accumulation of the PIN3 auxin efflux facilitator at the lower mem-
brane of gravistimulated statocytes (Friml et al. 2002; Harrison and Masson 2006); and
a preferential downward lateral transport of auxin across the cap, with accumulation at
the lower flank (Young et al. 1990; Rashotte et al. 2001; Boonsirichai et al. 2003).
Because the subsequent lateral auxin gradient at the cap is transported basipetally along
the root tip, it will eventually direct differential cellular elongation between upper and
lower root flanks at the elongation zones, which drives most of the gravitropic curvature
(Chapter 3; reviewed in Ishikawa and Evans 1997).