Polar auxin transport--old questions and new concepts?

Polar auxin transport controls multiple aspects of plant development including differential growth, embryo and root patterning and vascular tissue differentiation. Identification of proteins involved in this process and availability of new tools enabling 'visualization' of auxin and auxin routes in planta largely contributed to the significant progress that has recently been made. New data support classical concepts, but several recent findings are likely to challenge our view on the mechanism of auxin transport. The aim of this review is to provide a comprehensive overview of the polar auxin transport field. It starts with classical models resulting from physiological studies, describes the genetic contributions and discusses the molecular basis of auxin influx and efflux. Finally, selected questions are presented in the context of developmental biology, integrating available data from different fields.