Auxin transport inhibitors block PIN1 cycling and vesicle traficking

Polar transport of the phytohormone auxin mediates various processes in pla nt growth and development, such as apical dominance, tropisms, vascular pat terning and axis formation(1,2). This view is based largely on the effects of polar auxin transport inhibitors. These compounds disrupt auxin efflux f rom the cell but their mode of action is unknown(3). It is thought that pol ar auxin flux is caused by the asymmetric distribution of efflux carriers a cting at the plasma membrane(4). The polar localization of efflux carrier c andidate PIN1 supports this model(4). Here we show that the seemingly stati c localization of PIN1 results from rapid actin-dependent cycling between t he plasma membrane and endosomal compartments. Auxin transport inhibitors b lock PIN1 cycling and inhibit trafficking of membrane proteins that are unr elated to auxin transport. Our data suggest that PIN1 cycling is of central importance for auxin transport and that auxin transport inhibitors affect efflux by generally interfering with membrane-trafficking processes. In sup port of our conclusion, the vesicle-trafficking inhibitor brefeldin A mimic s physiological effects of auxin transport inhibitors.