to the /ß hydrolyase super-family and the protein localizes to the ER membrane, consis-
tent with the accumulation of AUX1 protein in the ER of the axr4mutant (Dharmasiri et
al. 2006). One additional study suggested that brefeldin A (BFA), a molecule that blocks
vesicle transport (described in detail below), may prevent the localization of AUX1 to the
appropriate membrane in protophloem cells (Grebe et al. 2002).
Identification of IAA influx inhibitors has also enhanced our understanding of the
process of IAA influx. Imhoff et al. (2000) screened a large number of aryl and ary-
loxyalkylcarboxylic acids for their ability to block IAA influx in suspension-cultured
tobacco cells. Two compounds, 1-napthoxyacetic acid (1-NOA) and 3-chloro-4-
hydroxyphenylacetic acid were found to inhibit auxin influx at micromolar concentra-
tions (Imhoff et al. 2000) and to inhibit polar IAA transport in plants (Parry et al. 2001a).
These influx inhibitors phenocopy the differential auxin resistance as well as agravitropic
phenotypes of aux1,with 1-NOA having a specific effect on IAA influx (Parry et al.
2001a; Rahman et al. 2002). The ability of 1-NOA to specifically inhibit AUX1-mediated
IAA influx was confirmed when AUX1 was expressed in Xenopusoocytes, as described
above (Yang et al. 2006).
A naturally occurring plant secondary metabolite, chromosaponin I (CSI), reduces
IAA influx and phenocopies the agravitropic root growth of the aux1mutant (Rahman et
al. 2001a). In most alleles of aux1,where the mutation lies in the central domain or in the
N-terminal domain (to date, 13 alleles tested), CSI either completely inhibited the gravi-
tropic response in weak alleles or did not have any effect on the already agravitropic root
growth in strong alleles (Swarup et al. 2004). Interestingly, CSI had a very different ef-
fect in aux1-7, which carries a mutation in the C-terminal domain. CSI rescued the gravi-
tropic response and auxin uptake defects in aux1-7, indicating that CSI may directly in-
teract with AUX1 protein (Rahman et al. 2001a) via the C-terminal domain (Swarup et
al. 2004). The restoration of gravitropic response by CSI in an engineered transgenic line
expressing HA-aux1-7in a null allele (aux1-22) background confirmed this direct inter-
action (Swarup et al. 2004).
The finding that chromosaponin acts as a naturally occurring influx inhibitor parallels
the identification of flavonoids as regulators of auxin efflux, described below. The iden-
tification of these two molecules suggests that regulation of auxin movements by endoge-
nous small molecules may be an important and general mechanism to control auxin trans-
port and dependent physiological processes.
3.6 Regulation of IAA efflux protein location and activity during gravity response
For a better understanding of changes in auxin transport, two important mechanisms need
to be clarified. First, the initial establishment of polarity of auxin transport, and second,
how this polarity is changed in response to gravity stimulation. The presence of multiple
PINandMDR/PGPandLAXgenes, with distinct expression patterns and subcellular lo-
calizations, suggests that changes in synthesis and localization of IAA transporters are
adequate to mediate these changes (as reviewed in Benjamins et al. 2005; Blakeslee et al.
2005). An interesting theme in this regulation is the ability of IAA to positively regulate
the efflux of auxin through regulation of synthesis, targeting, and proteolytic degradation