that the PIN3 protein may function in this way (Friml et al. 2002; see also Chapter 2).
PIN3 is expressed in root columella cells and PIN3 localization changes in response to
reorientation of roots relative to gravity, consistent with redistribution of IAA from the
root tip (Friml et al. 2002). This protein also cycles in a BFA-dependent manner (Friml
et al. 2002). Therefore, relocalization of PIN3 in the root cap may redirect auxin flow at
the tip of roots reoriented relative to gravity. Consistent with this role, pin3mutant roots
respond to gravitropic reorientation with a slower response than wild-type (Friml et al.
2002).
Recent work has examined the physiological significance of the cycling of PIN pro-
teins in controlling auxin transport polarity, with a specific focus on the role of this
process in changing the auxin transport polarity in gravity-responding roots. Paciorek et
al. (2005) tested the possibility that IAA controls the cycling of PIN proteins. In their ex-
perimental system, the active auxins IAA, NAA, and 2,4-D prevent the BFA-induced ac-
cumulation of PIN1 into BFA bodies, as did the yuccamutant, which has elevated en-
dogenous IAA levels (Paciorek et al. 2005). In addition, it was shown that the auxins
inhibit uptake of FM 4-64, a fluorescent dye taken up by cells during endocytosis
(Paciorek et al. 2005). To determine whether the cycling of PIN2 is affected by the local
IAA concentrations, Paciorek et al. (2005) used the endogenous gradients in IAA across
a gravity-stimulated root and found that on the lower side of the root, where IAA levels
are higher, there is less accumulation of PIN2 in BFA bodies and less endocytosis of FM
4-64. Treatment with NPA, which prevents formation of the IAA gradient, resulted in a
similar PIN2 accumulation in BFA bodies and levels of endocytosis on the two sides of
CHAPTER 3 AUXIN TRANSPORT AND THE INTEGRATION OF GRAVITROPIC GROWTH 57
Figure 3.3. A model of interactions of BFA and auxin efflux inhibitors with PIN1 cycling and auxin efflux.
A magnification of vesicular cycling in the basal portion of a polarized plant cell is shown. A.PIN1 cycles
between the endosome and plasma membrane in untreated, polarized, IAA-transporting cells. B. Interruption
of PIN1 cycling in BFA-treated cells results in PIN1 accumulation in an endomembrane compartment, which
reduces IAA transport. BFA binds to GNOM, which co-localizes with endomembrane aggregations of PIN1.
BFA washout restores PIN1 cycling and IAA efflux.
This figure has been modified from Muday et al. (2003) and reprinted with permission of Elsevier
Publishing.