(Katekar and Geissler 1980; Muday and Haworth 1994; Rashotte et al. 2000). Recently,
synthetic and naturally occurring inhibitors of auxin influx have been identified and
these compounds also inhibit gravitropic bending (Rahman et al. 2001a; Parry et al.
2001a).
Although the validity of the Cholodny-Went hypothesis has been debated (Trewavas
1992), molecular and genetic evidence has provided significant support to this hypothe-
sis (as reviewed in Blancaflor and Masson 2003). As discussed in Chapters 1 and 2, one
powerful test of this hypothesis has been the examination of auxin-induced gene expres-
sion across gravity-stimulated plants. Transgenic plants carrying several different auxin-
responsive promoters driving the expression of the gene encoding ß-glucuronidase
(GUS) or green fluorescent protein (GFP) have now been used to show asymmetric
auxin-induced gene expression across gravity-stimulated shoots (McClure and Guilfoyle
1989; Li et al. 1991; Li et al. 1999) or roots (Larkin et al. 1996; Luschnig et al. 1998;
Rashotte et al. 2001; Ottenschläger et al. 2003). Although GUS and GFP reporters indi-
rectly measure changes in auxin accumulation, the ability to easily observe the expres-
sion with high spatial resolution makes this a powerful approach to explore the role of
auxin in tropisms.
The asymmetric expression of the DR5::GUS reporter in Arabidopsisroots that are
vertical and roots reoriented 90 degrees relative to gravity is shown in Figure 3.2 (also
see Color Section. The inhibition of gravitropic bending and differential auxin-regulated
gene expression by IAA efflux inhibitors, as shown in Figure 3.2C (Li et al. 1991;
Rashotte et al. 2001; Paciorek et al. 2005), indicates that lateral auxin transport is re-
quired for differential gene expression. It is likely that this asymmetric gene expression
also requires a change in auxin sensitivity (Salisbury et al. 1988), perhaps through acti-
vation of transcription factors necessary for auxin-induced gene expression (as reviewed
in Leyser 2006 and discussed below).
A critical question about these expression studies is whether they support the idea that
gradients in auxin precede differential growth in response to gravity. In roots, particularly
ofArabidopsis, the spatial and temporal character of the response to gravity stimulation
is well characterized. Root gravitropic bending is narrowly constrained to the distal elon-
gation zone (DEZ) (as reviewed in Evans 1991; Wolverton et al. 2002), which in a
gravity-stimulated Arabidopsisroot is localized on the lower side to a region between 100
and 300 microns from the tip (Mullen et al. 1998). The DR5::GUS reporter detects asym-
metric auxin-induced gene expression in the same region as the differential growth in the
DEZ (Rashotte et al. 2001), as shown in Figure 3.2B. This epidermal localization is con-
sistent with previous evidence suggesting that the gravity signal in roots moves through
cells of the epidermis and/or cortex (Yang et al. 1990; Björkman and Cleland 1991). Yet,
gradients in DR5::GUS and GH3::GUS expression are detected after roots have begun to
bend (Larkin et al. 1996; Rashotte et al. 2001). In DR5rev::GFP transgenic similar asym-
metric gene expression is visible by 2 to 3 hours after gravity stimulation, and even ear-
lier asymmetries in auxin-induced gene expression in the root cap are detectable at ~1.5
hours after gravity stimulation (Ottenschläger et al. 2003; Paciorek et al. 2005), which
temporally precedes the asymmetry in the epidermal/cortical cells that is shown in Figure
3.2. This timing of this root cap asymmetry more closely parallels the initiation of grav-
itropic curvature.
frankie
(Frankie)
#1