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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]

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]

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]

Yaodong Yang is the first author of a 2006 study directly characterizing the auxin influx carrier AUX1. The team expressed this plant membrane transport protein in Xenopus oocytes and measured entry of radioactively labeled indole-3-acetic acid. Uptake was saturable and depended on pH, providing functional evidence about transport rather than only the protein's location.^[ref]

The mutant comparison connected molecular and organismal observations. Changes in AUX1 that disrupted auxin responses in plants also reduced uptake in the experimental oocytes. Testing the same altered protein in a different cellular setting helped distinguish its transport activity from the many downstream effects of auxin on plant development. Saturation also indicates a limited transport capacity under the assay conditions, rather than an uptake rate that increases without bound.^[ref]