SignificanceWithin plants, controlled gradients of the hormone auxin are essential for development. These gradients are achieved through intracellular polar positioning of auxin transporter proteins, such as PIN-FORMED proteins (PINs), at the plasma membrane, thus guiding the direction of auxin transport. The establishment and maintenance of PIN polarity is controlled within each cell by complicated trafficking pathways of the endomembrane system. In the model plantArabidopsis, it has long been known that endocytosis and recycling trafficking routes play roles in PIN polarity. In this study, we reveal the role of a secretory route in this process. Our evidence shows that this secretory trafficking route selectively controls basal (rootward) but not apical (shootward) PIN polarity.