Regulation of Polar Auxin Transport by Protein Phosphorylation

Protein phosphorylation is one of the most important posttranslational mechanisms regulating polar auxin transport (PAT). The best known example is the phosphorylation carried out by the protein kinase PINOID (PID), which determines PIN polar targeting at the plasma membrane. PID belongs to the family of plant AGC kinases, which play prominent roles in the regulation of PAT and in the cross talk between PAT and the light-signaling pathways. Moreover, kinases belonging to other families, such as protein kinase CK2 or MAP kinases, are now emerging as important regulators of PIN recycling and/or other aspects of PAT, and the phosphoinositide biosynthetic pathway appears to play a significant role on PIN trafficking. On the other hand, identification of protein phosphatases that may reverse the effects of protein kinases is far behind, although rapid progress is being made, providing new insights about the role of protein phosphorylation on PAT regulation.