Inhibition of auxin movement from the shoot into the root inhibits lateralroot development in arabidopsis

In roots two distinct polar movements of auxin have been reported that may control different developmental and growth events. To test the hypothesis t hat auxin derived from the shoot and transported toward the root controls l ateral root development, the two polarities of auxin transport were uncoupl ed in Arabidopsis. Local application of the auxin-transport inhibitor napht hylphthalamic acid (NPA) at the root-shoot junction decreased the number an d density of lateral roots and reduced the free indoleacetic acid (IAA) lev els in the root and [H-3]IAA transport into the root. Application of NPA to the basal half of or at several positions along the root only reduced late ral root density in regions that were in contact with NPA or in regions api cal to the site of application. Lateral root development was restored by ap plication of IAA apical to NPA application. Lateral root development in Ara bidopsis roots was also inhibited by excision of the shoot or dark growth a nd this inhibition was reversible by IAA. Together, these results are consi stent with auxin transport from the shoot into the root controlling lateral root development.