of IAA efflux proteins. Efflux carrier activity and/or synthesis may be regulated by phos-
phorylation and by regulatory molecules, such as flavonoids (as reviewed by Muday and
DeLong 2001; Benjamins et al. 2005). Finally, increasing evidence suggests that an in-
terplay between hormonal signaling pathways may regulate gravity response, with inter-
actions between auxin and ethylene signaling being the best developed.
3.6.1 Mechanisms that may control localization of IAA efflux carriers
The polar localization of PIN proteins has been suggested to be mediated by dynamic cy-
cling of these proteins between internal compartments and the plasma membrane
(Geldner et al. 2001; Geldner et al. 2003; as reviewed in Murphy et al. 2005). A recent
report has used live imaging of PIN1::GFP in the Arabidopsisinflorescence meristem to
identify rapid changes in protein localization that modulate auxin transport directionality
and that are linked to floral meristem initiation (Heisler et al. 2005). Pharmacological ap-
proaches also support the idea that PIN protein localization is dynamic. The drugs mon-
ensin and brefeldin A (BFA), which are inhibitors of vesicle movements, were shown to
reduce auxin transport though alterations in auxin efflux in the absence of protein syn-
thesis (Wilkinson and Morris 1994; Morris and Robinson 1998; Delbarre et al. 1998).
The asymmetric plasma membrane localization of PIN1 was altered by treatment with
BFA, which led to accumulation of PIN1 in internal compartments termed BFA bodies
(Geldner et al. 2001; Geldner et al. 2003). These BFA bodies contain endosomal markers
(Geldner et al. 2003) and PIN1 protein accumulation in these structures is fully reversible
upon removal of BFA (Geldner et al. 2001), suggesting that dynamic cycling of PIN1 be-
tween endosomes and the plasma membrane could control the localization of this and
other auxin transport proteins, as shown in Figure 3.3. BFA treatment has now been
shown to cause accumulation of PIN2, PIN3, and PIN4 in BFA bodies (Paciorek et al.
2005), suggesting that multiple IAA efflux proteins use similar mechanisms to reach their
appropriate localization on the plasma membrane.
Animal cells possess BFA-sensitive ARF-GEFs (ADP ribosylation factor-guanine nu-
cleotide exchange factors) that direct vesicle movements through several pathways
(Donaldson and Jackson 2000). The Arabidopsis gnommutant, which has a defect in a
gene encoding an ARF-GEF, exhibits altered PIN1 localization in developing embryos
(Steinmann et al. 1999). GNOM was later shown to be the target for BFA in PIN cycling,
as the GNOM transgenic plants with a mutated BFA binding site (GNOMM–L-myc)
showed resistance to BFA-regulated, auxin-mediated developmental processes such as
root gravitropic bending and lateral root formation and to accumulation of PIN1-GFP in
BFA bodies (Geldner et al. 2003). Together, these results indicate that GNOM is a target
in BFA inhibition of PIN1-dependent auxin transport, and suggest a mechanism for dif-
ferential localization of IAA efflux carriers. Accordingly, mutations in SCARFACE
(SFC), a gene recently shown to encode an ARF-GAP (GTPase activating protein) that
enhances cleavage of ARF bound GTP to GDP, thereby negatively regulating ARF activ-
ity (Randazzo and Hirsch 2004), result in altered BFA-dependent PIN1::GFP cycling and
defects in auxin transport and dependent physiological processes (Sieburth et al. 2006).
An important question is whether cycling of a PIN protein may allow changes in auxin
transport polarity that is needed to mediate gravitropism. Experimental evidence suggests