AUX1 promotes lateral root formation by facilitating indole-3-acetic acid distribution between sink and source tissues in the arabidopsis seedling

Arabidopsis root architecture is regulated by shoot-derived signals such as nitrate and auxin. We report that mutations in the putative auxin influx carrier AUX1 modify root architecture as a result of the disruption in hormone transport be- tween indole-3-acetic acid (IAA) source and sink tissues. Gas chromatography–selected reaction monitoring–mass spectrometry measurements revealed that the aux1 mutant exhibited altered IAA distribution in young leaf and root tis- sues, the major IAA source and sink organs, respectively, in the developing seedling. Expression studies using the auxin-inducible reporter IAA2 :: uidA revealed that AUX1 facilitates IAA loading into the leaf vascular transport system. AUX1 also facilitates IAA unloading in the primary root apex and developing lateral root primordium. Exogenous appli- cation of the synthetic auxin 1-naphthylacetic acid is able to rescue the aux1 lateral root phenotype, implying that root auxin levels are suboptimal for lateral root primordium initiation in the mutant