They also constitute an outstanding list of markers that can be used to better define the
pathways involved in gravity signal transduction, as illustrated by the expression analy-
sis of fast gravity-responsive genes in adk1-1, arg1-2,andarl2-1mutant root tips. It is
likely that a combination of reverse genetics and marker-gene-expression analysis will
yield important insights into the molecular mechanisms that govern gravity signal trans-
duction in plants.
2.2.6 Relocalization of auxin transport facilitators or activity regulation?
As reviewed in Chapter 3, activation of the gravity signal transduction pathway in roots
results in a relocalization of the PIN3 auxin efflux facilitator to the bottom membrane of
the statocytes (Friml et al. 2002). PIN3, a member of the PIN family of transmembrane
auxin efflux facilitators, is expressed in the statocytes of both roots (columella cells) and
shoots (endodermal cells). It localizes to the plasma membrane and to vesicles that cycle
between plasma membrane and endosome. Mutations in the PIN3gene result in altered
root and hypocotyl gravitropism, supporting a role for this protein in gravity signal trans-
duction (Friml et al. 2002). In statocytes of vertical roots, PIN3 is positioned symmetri-
cally at the plasma membrane. It rapidly relocalizes laterally, to the bottom membrane,
upon gravity stimulation (Friml et al. 2002). PIN3 relocalization initiates within 2 min of
gravistimulation, therefore preceding establishment of a lateral auxin gradient across the
root tip. As discussed above, PIN3 relocalization requires the presence of fully functional
ARG1, ARL2,andADK1(Harrison and Masson, unpublished data; Young et al. 2006),
suggesting that it functions downstream of these proteins in the gravity signal transduc-
tion pathway in roots.
Because at least ARG1(and probably ARL2)has been associated with the vesicular traf-
ficking pathway (see above), it appears reasonable to postulate that gravity signal trans-
duction modulates the vesicular trafficking of PIN3 and probably of other membrane-
CHAPTER 2 SIGNAL TRANSDUCTION IN GRAVITROPISM 37
Figure 2.2. Linear (A) and bifurcated (B) models of gravity signal transduction in the root statocytes.