pressed in roots and is localized asymmetrically in the plasma membrane, consistent with
a role in mediating basipetal IAA transport (Müller et al. 1998).
A recent report suggests that PIN function is redundant, using an eir1/pin2mutant line
transformed with a PIN1::GFP variant that has localization consistent with PIN2 but op-
posite to PIN1 (Wisniewska et al. 2006). This construct restored gravitropic bending and
lateral IAA transport to eir1/pin2, whereas wild-type PIN1::GFP did not, indicating that
the polar localization of PIN proteins is sufficient to direct IAA movement with appro-
priate polarity (Wisniewska et al. 2006). Finally, the overexpression of PIN7 in tobacco
cell cultures significantly enhanced IAA, 1-NAA, and 2,4-D efflux, but not the trypto-
phan movement (Petrasek et al. 2006), conclusively demonstrating that PIN proteins can
mediate IAA efflux.
A second family of proteins has also been suggested to participate in IAA efflux by
both genetic and biochemical approaches. The multidrug resistance/P-glycoprotein
(MDR/PGP)gene family encodes proteins with sequence similarity to genes in the ATP
Binding Cassette (ABC) transporter superfamily, which transport a variety of molecules
in addition to the cytotoxic compounds for which they were initially isolated (Geisler and
Murphy 2006). AtMDR1/PGP19, AtPGP1, and AtPGP2 proteins have been shown to
bind to an NPA affinity column and expression of AtMDR1 in yeast increases NPA bind-
ing activity (Noh et al. 2001), consistent with the possibility that these proteins are the
target for IAA efflux inhibitors. The mdr1/pgp19mutant has reduced basipetal IAA
transport in inflorescence (Noh et al. 2001) and roots (Geisler et al. 2005), and has phe-
notypes consistent with altered auxin transport (Noh et al. 2001; Lin and Wang 2005;
Geisler et al. 2005), including enhanced gravitropic responses in both inflorescences
(Noh et al. 2003) and roots (Lin and Wang 2005). The pgp1mutant has a weak pheno-
type that enhances the mdr1/pgp19mutant phenotype (Noh et al. 2001; Noh et al. 2003;
Lin and Wang 2005), suggesting partially redundant functions for these two proteins. The
pgp4/mdr4mutant also has reduced root basipetal IAA transport and gravitropic response
(Terasaka et al. 2005).
The ability of PGP1 and PGP4 to mediate IAA and NAA efflux in heterologous sys-
tems (Geisler et al. 2005; Terasaka et al. 2005) further links these proteins to the process
of IAA transport (Blakeslee et al. 2005). Surprisingly, current evidence supports a role of
PGP4 in control of IAA influx, not efflux (Terasaka et al. 2005). Additionally, PGP19/
MDR1 has also been shown to mediate IAA efflux from plant tissue culture (Petrasek et
al. 2006), although it has been difficult to demonstrate its function in heterologous sys-
tems (Geisler and Murphy 2006). PGP1, PGP4, and MDR1/PGP19 all have membrane
localizations that are asymmetrically distributed across auxin transporting cells, suggest-
ing that their localization could convey directional control of auxin transport (Geisler et
al. 2005; Terasaka et al. 2005; Geisler and Murphy 2006). Additionally, AtPGP1 and 19
have been shown to participate in protein complexes with TWISTED DWARF1, a unique
plasma membrane-anchored, immunophilin-like protein, which is required for maximal
auxin transport and appropriate plant development (Geisler et al. 2003).
An important question is whether PIN and MDR proteins interact in vivoto modulate
IAA movements or whether they represent two distinct IAA transport pathways. The ex-
pression patterns of PGP1 and PGP19/MDR1 overlap with AtPIN1, whereas the PGP1
and PGP4 patterns overlap with AtPIN2 in other tissues, consistent with possible com-
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