Glutathione depletion, e.g. by the inhibitor of its synthesis, buthionine sulphoximine (BSO), is well known to specifically reduce primary root growth. To obtain an insight into the mechanism of this inhibition, we explored the effects of BSO on Arabidopsis root growth in more detail. BSO inhibits root growth and reduces glutathione (GSH) concentration in a concentration-dependent manner leading to a linear correlation of root growth and GSH content. Microarray analysis revealed that the effect of BSO on gene expression is similar to the effects of misregulation of auxin homeostasis. In addition, auxin-resistant mutants axr1 and axr3 are less sensitive to BSO than the wild-type plants. Indeed, exposure of Arabidopsis to BSO leads to disappearance of the auxin maximum in root tips and the expression of QC cell marker. BSO treatment results in loss of the auxin carriers, PIN1, PIN2 and PIN7, from the root tips of primary roots, but not adventitious roots. Since BSO did not abolish transcription of PIN1, and since the effect of BSO was complemented by dithiothreitol, we conclude that as yet an uncharacterised post-transcriptional redox mechanism regulates the expression of PIN proteins, and thus auxin transport, in the root tips.