High-Affinity Auxin Transport by the AUX1 Influx Carrier Protein

Abstract

In plants, auxin is a key regulator of development and is unique among plant hormones in that its function requires polarized transport between neighboring cells to form concentration gradients across various plant tissues. Although putative auxin-influx and -efflux transporters have been identified by using molecular genetic approaches, a detailed functional understanding for many of these transporters remains undetermined. Here we present the functional characterization of the auxin-influx carrier AUX1. Upon expression of AUX1 in Xenopus oocytes, saturable, pH-dependent uptake of 3H-IAA was measured. Mutations in AUX1 that abrogate physiological responses to IAA in planta resulted in loss or reduction of 3H-IAA uptake in AUX1-expressing oocytes. AUX1-mediated uptake of 3H-IAA was reduced by the IAA analogs 2,4-D and 1-NOA, but not by other auxin analogs. The measured Km for AUX1-mediated uptake of 3H-IAA was at concentrations at which physiological responses are observed for exogenously added IAA and 2,4-D. This is the first report demonstrating detailed functional characteristics of a plant auxin-influx transporter. This biochemical characterization provides new insights and a novel tool for studying auxin entry into cells and its pivotal roles in plant growth and development.

Alkoxy-auxins Are Selective Inhibitors of Auxin Transport Mediated by PIN, ABCB, and AUX1 Transporters

“…3F). AUX1 importer recognizes the synthetic auxin 2,4-D and natural auxin IAA, but not NAA as a substrate (13,14). This substrate profile of AUX1 is consistent with the inhibitory profile of AUX1 by alkoxy-IAA and -NAA (6a-7b).…”

“…The artificial auxin 2,4-D is incorporated into cells by the AUX1 importer and inhibits primary root growth. The auxin influx inhibitor 1-napthyloxy acetic acid restores 2,4-D-inhibited primary root growth by reducing the uptake of 2,4-D by the AUX1 importer (13,23). In an analogous experiment, Bz-IAA, but not Bz-NAA rescued 2,4-D inhibited root growth (supplemental Fig.…”

“…The relative activities of inhibitors in this class are also consistent with observed specificities of the AUX1, PIN, and ABCB auxin transporters. For instance, AUX1 exhibits a preference for IAA and 2,4-D over NAA (13,23), and Bz-IAA (7a) inhibits AUX1 transport, but Bz-NAA (7b) does not. Considering the structural analogy to auxin, alkoxy-auxins would be recognized at the transport site of auxin transporters and therefore, the selective inhibition of alkoxy-NAA was observed on PIN and ABCB rather than AUX1.…”

Spatiotemporal asymmetric auxin distribution: a means to coordinate plant development

“…Third, auxin influx inhibitors, 1-NOA and CHPAA, phenocopy the root phenotype of aux1 [51]. Recently, experiments that included the heterologous expression of AUX1 in Xenopus oocytes directly demonstrated that AUX1 functions as a high-affinity, specific auxin uptake carrier [57]. AUX1 is expressed in various tissues, including protophloem cells, root columella cells, the lateral root cap and the shoot apical meristem [55,58].…”

Auxin: at the root of nodule development?

“…It can also enter into cells by auxin importers of the amino acid permease families AUX1 (Auxin resistant 1), LAX (like-AUX1) and PGP4, a member of the MDR/PGP (Multidrug resistance/P-glycoprotein) families (Terasaka et al 2005;Yang et al 2006). This involves proton symport for AUX1 and ATP-driven uptake for PGPs (Fig.…”