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
In view of their predominant sessile lifestyle, plants need to be able to adapt to changes in their environment. Environmental signals such as light and gravity modulate plant growth and... Show moreIn view of their predominant sessile lifestyle, plants need to be able to adapt to changes in their environment. Environmental signals such as light and gravity modulate plant growth and architecture by redirecting polar cell-to-cell transport of auxin, thus causing changes in the distribution of this plant hormone. The PIN auxin efflux carriers are key drivers of auxin transport that determine the direction of auxin flow through their asymmetric subcellular distribution. An important component in PIN polarity establishment is the plant protein kinase PINOID (PID). PID instructs apical (shoot meristem facing) PIN polarity by phosphorylating the central hydrophylic loop of PIN proteins (PINHL). In this thesis we investigated modulation of PID activity by the calcium binding proteins TCH3 and PBP1, and by the protein kinase PDK1. All three proteins were found to regulate both the enzymatic activity and the sub-cellular localisation of PID in response to calcium and phospholipids, respectively, and as such they are likely to be involved in translating environmental signals into PIN polarity changes. In addition, we show that PID and its close homologs act both redundantly and differentially in orienting plant development by instructing the subcellular distribution of PINs. Show less
