Polar cell-to-cell transport of plant hormone auxin mediated by plasma membrane (PM)-localized PIN-FORMED (PIN) auxin efflux carriers generates auxin gradients that provide positional information... Show morePolar cell-to-cell transport of plant hormone auxin mediated by plasma membrane (PM)-localized PIN-FORMED (PIN) auxin efflux carriers generates auxin gradients that provide positional information for various plant developmental processes. The apical-basal polar localization of the PIN proteins that determines the direction of auxin flow is directed by reversible phosphorylation of the PIN hydrophilic loop (PINHL). Here, we identified three evolutionarily conserved TPRXS(N/S) motifs within the PINHL, and proved that the central serine residues located in three motifs were phosphorylated by the PINOID (PID) serine/threonine kinase protein and its closely related AGC3 kinases WAG1 and WAG2. Loss-of-phosphorylation PIN1 or PIN2 protein (serine to alanine mutation) induced auxin-related plant defects in inflorescence development or root gravity-response, respectively, correlating with their apical-to-basal polarity changing in their expressing plant tissues. Furthermore, phosphorylation at the conserved serine residues is important for PIN protein PM localization, as loss-of-phosphorylation PIN1 protein exhibited internalized signals in plant embryos and enhanced vacuolar accumulation in dark-incubated Arabidopsis protoplast cells. Our data indicate that phosphorylation of our identified conserved serine residues in the PIN1HL by AGC3 kinases is required for proper PIN polar localization, and is thus essential for generating the differential auxin distribution that directs plant development. Show less
