Abstract
The phytohormone auxin plays a major role in embryonic and postembryonic plant development. The temporal and spatial distribution of auxin largely depends on the subcellular polar localization of members of the PIN-FORMED (PIN) auxin efflux carrier family. The Ser/Thr protein kinase PINOID (PID) catalyzes PIN phosphorylation and crucially contributes to the regulation of apical-basal PIN polarity. The GTP exchange factor on ADP-ribosylation factors (ARF-GEF), GNOM preferentially mediates PIN recycling at the basal side of the cell. Interference with GNOM activity leads to dynamic PIN transcytosis between different sides of the cell. Our genetic, pharmacological, and cell biological approaches illustrate that PID and GNOM influence PIN polarity and plant development in an antagonistic manner and that the PID-dependent PIN phosphorylation results in GNOM-independent polar PIN targeting. The data suggest that PID and the protein phosphatase 2A not only regulate the static PIN polarity, but also act antagonistically on the rate of GNOM-dependent polar PIN transcytosis. We propose a model that includes PID-dependent PIN phosphorylation at the plasma membrane and the subsequent sorting of PIN proteins to a GNOM-independent pathway for polarity alterations during developmental processes, such as lateral root formation and leaf vasculature development.