The importance of the plant growth regulator auxin for plant growth has long been recognized, even before the discovery of its chemical structures in the early 20th century. Physiological studies in the decades since have demonstrated that auxin is unidirectionally transported in plants, a process dubbed polar auxin transport. It is the polar auxin transport process that generates a local auxin concentration gradient and regulates a broad array of physiological and developmental processes. The discoveries of auxin transport carrier proteins that mediate auxin influx into and efflux out of...