Plants employ specialised carriers to mobilise auxins to target tissues.
Localisation of the auxin permease AUX1 in the Arabidopsis root apex has revealed a novel phloem-based transport pathway. AUX1, asymmetrically localised to the plasma membrane of root protophloem cells, is proposed to promote the acropetal, post-phloem movement of auxin to the root apex. MS analysis demonstrates that auxin accumulation in aux1 mutant root apices is impaired, consistent with an AUX1 phloem unloading function. AUX1 localisation to columella and lateral root cap tissues of the Arabidopsis root apex reveals that the IAA permease regulates a second auxin transport pathway. Expression studies using an auxin-regulated reporter demonstrate that AUX1 regulates root gravitropism by facilitating basipetal auxin transport to distal elongation zone tissues.