The plant hormone auxin or indole-3-acetic acid (IAA) controls many aspects of plant growth and development through dynamic polar cell-to-cell transport-generated maxima and minima.The polarity of auxin transport is determined by the asymmetric subcellular distribution of the PIN1-type auxin efflux carriers.Reversible phosphorylation of these PIN proteins in their large central hydrophilic loop through the antagonistic action of PINOID (PID) and related AGC kinases and PP2A phosphatases is sufficient to direct their proper localization during embryogenesis, tropic growth, and the initiation of flowers and floral organs.The phosphorylation status of PIN proteins affects their endosomal trafficking, as phosphorylated PIN proteins are sorted via ADP-RIBOSYLATION FACTOR-GDP/GTP EXCHANGE FACTOR GNOM-independent routes.