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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]

Gravitropism is directional growth of a plant organ in response to gravity. A downward-growing root shows a positive response; an upward-growing shoot shows a negative one. Reorienting a growing plant can therefore produce curvature as growth becomes unequal on opposite sides. The response concerns the direction of growth, rather than passive bending under the organ's weight.

In Arabidopsis thaliana, auxin links perception near the root tip to growth farther back. Ranjan Swarup and collaborators used tissue-targeted expression of the influx carrier AUX1 to map where transport was needed. Their 2005 experiments implicated the lateral root cap and elongating plant epidermis. Separately, blocking auxin responsiveness in expanding plant epidermis cells abolished the gravity response. The comparison distinguishes delivery of a signal from the capacity of receiving cells to respond to it.^[ref]

A horizontal root provides a useful observation: record the tip position over time and inspect curvature through the root elongation zone, rather than expecting the entire existing root to rotate as a rigid rod. Holding the light direction fixed helps distinguish a gravity response from phototropism. Transport, cellular growth and final tip direction are related measurements, but they are not interchangeable.

Ranjan Swarup is a researcher in plant development, particularly auxin transport and root gravitropism. In a 2001 study, Ranjan Swarup and collaborators investigated where the auxin influx carrier AUX1 operates in Arabidopsis thaliana root tissues. Functional epitope tag versions of AUX1 restored the gravity response of an AUX1 mutant, allowing the team to localize a carrier that still performed its biological function.^[ref]

A chemical rescue provides an illuminating example. At the same tested concentration, the membrane-permeable auxin analogue naphthaleneacetic acid (1-NAA) restored the mutant root gravity response and outer-tissue reporter gene expression, whereas 2,4-D did not. Combining the rescue with carrier localization linked the visible bending response to the delivery of an auxin signal to growing tissues. The paper proposed distinct inward-to-tip and outward-from-tip transport roles for AUX1, showing how the location of a membrane transport protein helps explain growth of a whole organ.^[ref]