AUX1 is a plant membrane protein that promotes uptake of the auxin indole-3-acetic acid into cells. It is an influx carrier: its role concerns entry across a cell membrane, whereas the distribution of auxin through a tissue also depends on export, cellular geometry and the placement of transporters. These distinctions connect a molecular uptake mechanism to polar auxin transport without treating one carrier as the whole tissue-level pathway.[ref]
High-affinity auxin transport by the AUX1 influx carrier protein by Yaodong Yang and others. Abstract: functional identification of AUX1-mediated auxin influx.
Direct transport evidence came from expressing AUX1 in Xenopus oocytes and measuring uptake of tritium-labeled indole-3-acetic acid. Uptake was saturable and pH-dependent. Saturation matters: increasing substrate cannot increase a finite population of carrier-mediated transport events without bound. Dependence on pH connects uptake to the chemical environment across the biological membrane, rather than to AUX1 abundance alone.[ref]
High-affinity auxin transport by the AUX1 influx carrier protein by Yaodong Yang and others. Oocyte expression and radiolabeled IAA uptake assay.
In Arabidopsis thaliana roots, Ranjan Swarup and collaborators localized AUX1 and compared transport-related mutant phenotypes using different auxin compounds. Their results supported distinct routes for hormone delivery and redistribution near the root apex. The combination is instructive: localization asks where a carrier operates; heterologous expression tests what it can transport. Neither measurement alone completely specifies the resulting auxin pattern in an intact root.[ref]
Localization of the auxin permease AUX1 suggests two functionally distinct hormone transport pathways operate in the Arabidopsis root apex by Ranjan Swarup and others. Genes & Development 15 (2001), 2648–2653; localization and Figure 3 mutant/auxin comparisons.
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