plex formation (as reviewed in Geisler and Murphy 2006). The altered localization of
PIN1 in the mdr1/pgp19mutant is consistent with a protein complex (Noh et al. 2003)
that is altered in the absence of MDR/PGP proteins. The possibility of cooperativity be-
tween PIN1 and PGP1 and MDR1/PGP19 was examined in pgp1/pgp19double mutants
that overexpressed PIN1 under the control of an estradiol response promoter (Petrasek et
al. 2006). PIN1 overexpression in both wild-type and pgp1/pgp19mutants was sufficient
to induce agravitropic root growth in both cases, suggesting that PIN1 action did not re-
quire PGP1 and PGP19 protein (Petrasek et al. 2006). The recent demonstration that the
potassium carrier, TRH1, can also mediate IAA efflux in vivoandin vitrofurther sug-
gests the complexity of proteins that mediate IAA efflux (Vicente-Agullo et al. 2004). A
complete understanding of the complex assembly of auxin efflux carriers awaits further
experimentation.
The specificity of IAA transport machinery for naturally occurring auxins other than
IAA has also been examined. IBA transport has been examined in several species
(Ludwig-Muller 2000; Rashotte et al. 2003). In Arabidopsis, plants that have either the
aux1orpin2/eir1/agr1 mutation exhibit wild-type levels of IBA transport (Rashotte et al.
2003). Similarly, IBA transport is not affected by auxin efflux inhibitors such as NPA
(Rashotte et al. 2003). The Arabidopsis rib1mutant has altered IBA, but not IAA, trans-
port, suggesting that the RIB1 protein may participate in IBA specific transport (Poupart
et al. 2005). Similarly, in rice roots, the arm2mutant has altered IBA uptake but wild-
type levels of IAA uptake (Chhun et al. 2005). These results suggest that there may be
additional as-yet unknown mechanisms that mediate transport of other auxins.
3.5 IAA influx carriers and their role in gravitropism
For many years the existence of an IAA influx carrier was questioned based on the
chemiosmotic model of IAA transport (as reviewed in Goldsmith 1977). This model
posits that because of the low pH of the extracellular space and a pKafor IAA of 4.8,
some extracellular IAA should be protonated and the hydrophobicity of uncharged IAA
should allow it to passively enter plant cells (as reviewed in Goldsmith 1977). Yet, the ma-
jority of IAA will be at pH above the pKa, so carrier-mediated uptake of the IAA anion
would increase IAA accumulation. The demonstrations that IAA uptake was saturable in
suspension cells (Rubery and Sheldrake 1974) and that there is substrate-specific uptake
of auxins, with IAA and 2,4-D but not 1-NAA, moving into plant cells by carrier-
mediated uptake (Delbarre et al. 1996) further supported the concept of protein-mediated
IAA uptake. Our understanding of auxin influx has been extensively improved through
the molecular and functional characterization of AUX1’s activity as an IAA influx pro-
tein (as reviewed by Parry et al. 2001b; Blakeslee et al. 2005) and identification of com-
pounds which function as auxin influx inhibitors (Imhoff et al. 2000; Rahman et al.
2001a).
The best-characterized protein with a role in auxin influx is AUX1. The aux1mutant
ofArabidopsiswas identified in a screen for seedlings with altered elongation in the pres-
ence of 2,4-D (Maher and Martindale 1980). The aux1mutant has an agravitropic root
phenotype, as well as alterations in other auxin transport-dependent processes such as