the root (Paciorek et al. 2005). Taken together, these results suggest a mechanism by
which IAA may reinforce its own changing polarity during root gravitropism.
The pharmacological approach using BFA, described above, provides strong evidence
for the cycling of PIN proteins as a mechanism to change auxin transport polarity during
gravity response, and has uncovered some intriguing potential regulatory mechanisms.
Yet several aspects of these studies need to be carefully considered. First, the dose of BFA
needed to cause PIN1 accumulation in BFA bodies (Geldner et al. 2001) is much higher
than that needed to reduce root growth, gravity response, and lateral root formation
(Geldner et al. 2001; Geldner et al. 2004) and to alter vesicular movements (Niu et al.
2005; Parton et al. 2003). Second, these BFA bodies are not observed in untreated cells
and, although they contain endosomal markers (Geldner et al. 2003), the biological sig-
nificance of these structures is not yet clear. Aside from changes in endocytosis of FM 4-
64, which correlate with changes in BFA sensitivity to BFA body formation (Paciorek et
al. 2005), there has been no evidence of regulated endosomal cycling of PIN proteins in
the absence of BFA. Finally, although BFA has been shown to alter IAA transport in in-
florescence tissues (Geldner et al. 2003) and cultured plant cells (Morris and Robinson
1998), the effect of BFA on IAA transport in the tissues in which BFA affects PIN cy-
cling and growth and development have not been reported. These points highlight the im-
portance of additional studies to understand the intriguing idea of endosomal cycling as
a regulatory mechanism to control IAA transport polarity.
3.6.2 Regulation of IAA efflux by synthesis and degradation of efflux carriers
Changes in abundance of IAA efflux proteins may also enhance the effects of PIN pro-
tein cycling to amplify gradients in IAA. Although PIN protein cycling has been shown
to occur in the absence of protein synthesis (Geldner et al. 2001), other experiments have
also shown that there are transcriptional controls of efflux carriers that accompany
change in auxin transport (Peer et al. 2004; Vieten et al. 2005). In particular, the expres-
sion of the PIN1-6genes has been examined and shown to be controlled by changing
auxin levels as a result of application of auxins and auxin efflux inhibitors (Peer et al.
2004; Vieten et al. 2005). These results are consistent with transcriptional controls of ef-
flux carriers that may alter the capacity of plant tissues to transport auxin, and with an
additional level of feedback of auxin on its own transport. Induced synthesis of IAA
transport proteins in cells on the lower side of roots in response to gravistimulation could
then serve to enhance transport on the lower side of horizontal roots, whereas decreased
expression and enhanced turnover of auxin transporters in cells of the upper side could
reduce auxin transport on this upper side. A schematic diagram of auxin transport
changes at the root tip is shown in Figure 3.2E, F.
Just as enhanced synthesis of efflux carriers can increase IAA transport to the lower
side of a gravity-stimulated root, proteolytic degradation of existing carriers that mediate
IAA transport to the upper side of horizontal roots would also facilitate formation of
asymmetries in IAA concentrations (Sieberer et al. 2000; Abas et al. 2006). Differential
proteolysis of PIN2 protein across gravity-stimulated roots has been shown for endoge-
nous PIN2 and EIR1/PIN2 fusions to GUS and GFP (Sieberer et al. 2000; Abas et al.
2006). This proteolytic cleavage is prevented by proteosome inhibitors (Abas et al. 2006).