root branching and root hair formation (Maher and Martindale 1980; Marchant et al.
2002; Rahman et al. 2002). Consistent with a role in mediating IAA influx, roots of aux1
show a selective resistance to the auxins whose uptake appears to be carrier-mediated,
IAA and 2,4-D, but not to the membrane-diffusible auxin, 1-NAA (Delbarre et al. 1996;
Marchant et al. 1999; Yamamoto and Yamamoto 1998). The uptake and transport of IAA
are also reduced in aux1mutants (Rahman et al. 2001a; Rashotte et al. 2003).
Molecular cloning of AUX1 supported the idea that this protein mediates IAA trans-
port as the gene is similar in sequence to the ATF (amino acid transporter family) of pro-
teins (Bennett et al. 1996; Young et al. 1999; Ortiz-Lopez et al. 2000). In the Arabidopsis
genome, three other genes showed a high degree of sequence similarity to AUX1 and are
termed the LAX(Like AUX1) gene family (Parry et al. 2001b). However, the function of
the other family members has not yet been reported. AU X 1encodes a membrane protein
of 48 KD composed of 11 transmembrane (TM)-spanning domains, with a cytoplasmic
facing N-terminal domain (Swarup et al. 2004). The functional characterization of aux1
alleles revealed that the central region of AUX1 appears particularly important for pro-
tein function as nine of the missense mutations cluster between TM VI and VII. On
the other hand, studies of the sole conditional allele aux1-7mutant suggest that the C-
terminal region of AUX1 may perform a regulatory function (Rahman et al. 2001a;
Swarup et al. 2004). Finally, a recent report provided direct evidence that AUX1 indeed
functions as an auxin influx career (Yang et al. 2006). Expression of wild-type AUX1
protein increased auxin uptake in Xenopusoocytes. In contrast, expression of three inde-
pendent point mutants of AU X 1, which abrogate the AUX1 function in planta, did not me-
diate auxin influx in oocytes. The substrate specificity of AUX1 in heterologous expres-
sion system is similar to the auxin specificity in plants, showing movement of IAA and
2,4-D, but not NAA and IBA (Yang et al. 2006).
The tissue-specific expression pattern of AU X 1provides insight into its developmen-
tal role in planta. In the lateral root cap cells, AUX1 is localized without polarity, whereas
in epidermal cells it is mainly axial, localized at both upper and lower sides (Marchant et
al. 1999; Swarup et al. 2005). This expression pattern has been proposed to facilitate the
basipetal transport of auxin between the gravity-sensing columella cells and the gravity-
responsive cells of the distal elongation zone, but suggests that AUX1 does not specify
the basipetal polarity (Swarup et al. 2005). Consistent with this model, aux1mutations
disrupt basipetal auxin transport and lead to agravitropic growth (Rashotte et al. 2003;
Swarup et al. 2005). This idea is further confirmed by the restoration of gravitropic re-
sponse of aux1by tissue-specific expression of AUX1 (Swarup et al. 2005) in plants
transformed with constructs expressed in cells of the lateral root cap and the epidermal
cells of the elongation zone (Swarup et al. 2005).
Collectively, these results suggest that localization of AUX1 is important for its role in
plant development, so understanding the mechanisms that specify AUX1 localization is
necessary. AXR4 functions as a specific regulator of AUX1 localization in epidermal and
protophloem cells (Dharmasiri et al. 2006), but not for other membrane proteins such as
PIN1, PIN2, or PM H+-ATPase, whose localization remain unaltered in the axr4mutant
(Dharmasiri et al. 2006). These findings are consistent with the similar aux1andaxr4
mutant phenotypes, which include agravitropic and auxin-insensitive growth (Dharmasiri
et al. 2006). The AXR4gene encodes a transmembrane protein with sequence similarity
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