The phytohormone auxin triggers transcriptional reprogramming through a well-characterized perception machinery in the nucleus. By contrast, mechanisms that underlie fast effects of auxin, such as the regulation of ion fluxes, rapid phosphorylation of proteins or auxin feedback on its transport, remain unclear1-3. Whether auxin-binding protein 1 (ABP1) is an auxin receptor has been a source of debate for decades1,4. Here we show that a fraction of Arabidopsis thaliana ABP1 is secreted and binds auxin specifically at an acidic pH that is typical of the apoplast. ABP1 and its plasma-membrane-localized partner, transmembrane kinase 1 (TMK1), are required for the auxin-induced ultrafast global phospho-response and for downstream processes that include the activation of H+-ATPase and accelerated cytoplasmic streaming. abp1 and tmk mutants cannot establish auxin-transporting channels and show defective auxin-induced vasculature formation and regeneration. An ABP1(M2X) variant that lacks the capacity to bind auxin is unable to complement these defects in abp1 mutants. These data indicate that ABP1 is the auxin receptor for TMK1-based cell-surface signalling, which mediates the global phospho-response and auxin canalization.

• MeSH
• Arabidopsis * genetics   metabolism   MeSH
• Phosphorylation MeSH
• Hydrogen-Ion Concentration MeSH
• Indoleacetic Acids * metabolism   MeSH
• Mutation MeSH
• Protein Serine-Threonine Kinases * genetics   metabolism   MeSH
• Arabidopsis Proteins * genetics   metabolism   MeSH
• Proton-Translocating ATPases metabolism   MeSH
• Cytoplasmic Streaming MeSH
• Plant Growth Regulators metabolism   MeSH
• Publication type
• Journal Article MeSH
• Names of Substances
• AT1G66150 protein, Arabidopsis MeSH Browser
• auxin-binding protein 1 MeSH Browser
• Indoleacetic Acids * MeSH
• Protein Serine-Threonine Kinases * MeSH
• Arabidopsis Proteins * MeSH
• Proton-Translocating ATPases MeSH
• Plant Growth Regulators MeSH

Spontaneously arising channels that transport the phytohormone auxin provide positional cues for self-organizing aspects of plant development such as flexible vasculature regeneration or its patterning during leaf venation. The auxin canalization hypothesis proposes a feedback between auxin signaling and transport as the underlying mechanism, but molecular players await discovery. We identified part of the machinery that routes auxin transport. The auxin-regulated receptor CAMEL (Canalization-related Auxin-regulated Malectin-type RLK) together with CANAR (Canalization-related Receptor-like kinase) interact with and phosphorylate PIN auxin transporters. camel and canar mutants are impaired in PIN1 subcellular trafficking and auxin-mediated PIN polarization, which macroscopically manifests as defects in leaf venation and vasculature regeneration after wounding. The CAMEL-CANAR receptor complex is part of the auxin feedback that coordinates polarization of individual cells during auxin canalization.

• MeSH
• Arabidopsis enzymology   genetics   MeSH
• Biological Transport MeSH
• Indoleacetic Acids metabolism   MeSH
• Protein Interaction Mapping MeSH
• Membrane Transport Proteins metabolism   MeSH
• Protein Kinases genetics   metabolism   MeSH
• Arabidopsis Proteins metabolism   MeSH
• Transcription Factors metabolism   MeSH
• Publication type
• Journal Article MeSH
• Research Support, Non-U.S. Gov't MeSH
• Names of Substances
• Indoleacetic Acids MeSH
• Membrane Transport Proteins MeSH
• PIN1 protein, Arabidopsis MeSH Browser
• Protein Kinases MeSH
• Arabidopsis Proteins MeSH
• Transcription Factors MeSH
• WRKY23 protein, Arabidopsis MeSH Browser

Directional transport of the phytohormone auxin is a versatile, plant-specific mechanism regulating many aspects of plant development. The recently identified plant hormones, strigolactones (SLs), are implicated in many plant traits; among others, they modify the phenotypic output of PIN-FORMED (PIN) auxin transporters for fine-tuning of growth and developmental responses. Here, we show in pea and Arabidopsis that SLs target processes dependent on the canalization of auxin flow, which involves auxin feedback on PIN subcellular distribution. D14 receptor- and MAX2 F-box-mediated SL signaling inhibits the formation of auxin-conducting channels after wounding or from artificial auxin sources, during vasculature de novo formation and regeneration. At the cellular level, SLs interfere with auxin effects on PIN polar targeting, constitutive PIN trafficking as well as clathrin-mediated endocytosis. Our results identify a non-transcriptional mechanism of SL action, uncoupling auxin feedback on PIN polarity and trafficking, thereby regulating vascular tissue formation and regeneration.

• MeSH
• Arabidopsis genetics   metabolism   MeSH
• Heterocyclic Compounds, 3-Ring metabolism   MeSH
• Pisum sativum genetics   metabolism   MeSH
• Indoleacetic Acids metabolism   MeSH
• Lactones metabolism   MeSH
• Arabidopsis Proteins genetics   metabolism   MeSH
• Gene Expression Regulation, Plant genetics   physiology   MeSH
• Plant Growth Regulators metabolism   MeSH
• Publication type
• Journal Article MeSH
• Research Support, Non-U.S. Gov't MeSH
• Names of Substances
• GR24 strigolactone MeSH Browser
• Heterocyclic Compounds, 3-Ring MeSH
• Indoleacetic Acids MeSH
• Lactones MeSH
• Arabidopsis Proteins MeSH
• Plant Growth Regulators MeSH

Plant survival depends on vascular tissues, which originate in a self-organizing manner as strands of cells co-directionally transporting the plant hormone auxin. The latter phenomenon (also known as auxin canalization) is classically hypothesized to be regulated by auxin itself via the effect of this hormone on the polarity of its own intercellular transport. Correlative observations supported this concept, but molecular insights remain limited. In the current study, we established an experimental system based on the model Arabidopsis thaliana, which exhibits auxin transport channels and formation of vasculature strands in response to local auxin application. Our methodology permits the genetic analysis of auxin canalization under controllable experimental conditions. By utilizing this opportunity, we confirmed the dependence of auxin canalization on a PIN-dependent auxin transport and nuclear, TIR1/AFB-mediated auxin signaling. We also show that leaf venation and auxin-mediated PIN repolarization in the root require TIR1/AFB signaling. Further studies based on this experimental system are likely to yield better understanding of the mechanisms underlying auxin transport polarization in other developmental contexts.

• Keywords
• Arabidopsis thaliana, PIN1, TIR1/AFB, auxin, auxin canalization, cell polarity,
• MeSH
• Arabidopsis * genetics   metabolism   MeSH
• F-Box Proteins * genetics   MeSH
• Indoleacetic Acids MeSH
• Arabidopsis Proteins * genetics   metabolism   MeSH
• Receptors, Cell Surface genetics   metabolism   MeSH
• Gene Expression Regulation, Plant MeSH
• Plant Growth Regulators MeSH
• Signal Transduction MeSH
• Publication type
• Journal Article MeSH
• Research Support, Non-U.S. Gov't MeSH
• Names of Substances
• F-Box Proteins * MeSH
• Indoleacetic Acids MeSH
• Arabidopsis Proteins * MeSH
• Receptors, Cell Surface MeSH
• Plant Growth Regulators MeSH
• TIR1 protein, Arabidopsis MeSH Browser

Cell polarity, manifested by the localization of proteins to distinct polar plasma membrane domains, is a key prerequisite of multicellular life. In plants, PIN auxin transporters are prominent polarity markers crucial for a plethora of developmental processes. Cell polarity mechanisms in plants are distinct from other eukaryotes and still largely elusive. In particular, how the cell polarities are propagated and maintained following cell division remains unknown. Plant cytokinesis is orchestrated by the cell plate-a transient centrifugally growing endomembrane compartment ultimately forming the cross wall1. Trafficking of polar membrane proteins is typically redirected to the cell plate, and these will consequently have opposite polarity in at least one of the daughter cells2-5. Here, we provide mechanistic insights into post-cytokinetic re-establishment of cell polarity as manifested by the apical, polar localization of PIN2. We show that the apical domain is defined in a cell-intrinsic manner and that re-establishment of PIN2 localization to this domain requires de novo protein secretion and endocytosis, but not basal-to-apical transcytosis. Furthermore, we identify a PINOID-related kinase WAG1, which phosphorylates PIN2 in vitro6 and is transcriptionally upregulated specifically in dividing cells, as a crucial regulator of post-cytokinetic PIN2 polarity re-establishment.

• MeSH
• Arabidopsis cytology   genetics   physiology   MeSH
• Cell Membrane metabolism   MeSH
• Cell Division * MeSH
• Cytokinesis MeSH
• Endocytosis MeSH
• Phenotype MeSH
• Phosphorylation MeSH
• Plant Roots cytology   genetics   physiology   MeSH
• Cell Polarity * MeSH
• Arabidopsis Proteins genetics   metabolism   MeSH
• Recombinant Fusion Proteins MeSH
• Genes, Reporter MeSH
• Protein Transport MeSH
• Publication type
• Journal Article MeSH
• Research Support, Non-U.S. Gov't MeSH
• Names of Substances
• PIN2 protein, Arabidopsis MeSH Browser
• Arabidopsis Proteins MeSH
• Recombinant Fusion Proteins MeSH

Intercellular distribution of the plant hormone auxin largely depends on the polar subcellular distribution of the plasma membrane PIN-FORMED (PIN) auxin transporters. PIN polarity switches in response to different developmental and environmental signals have been shown to redirect auxin fluxes mediating certain developmental responses. PIN phosphorylation at different sites and by different kinases is crucial for PIN function. Here we investigate the role of PIN phosphorylation during gravitropic response. Loss- and gain-of-function mutants in PINOID and related kinases but not in D6PK kinase as well as mutations mimicking constitutive dephosphorylated or phosphorylated status of two clusters of predicted phosphorylation sites partially disrupted PIN3 phosphorylation and caused defects in gravitropic bending in roots and hypocotyls. In particular, they impacted PIN3 polarity rearrangements in response to gravity and during feed-back regulation by auxin itself. Thus PIN phosphorylation, besides regulating transport activity and apical-basal targeting, is also important for the rapid polarity switches in response to environmental and endogenous signals.

• MeSH
• Arabidopsis drug effects   physiology   MeSH
• Phosphorylation MeSH
• Gravitropism * MeSH
• Plant Roots drug effects   physiology   MeSH
• Indoleacetic Acids pharmacology   MeSH
• Gravity Sensing MeSH
• Cell Polarity * MeSH
• Arabidopsis Proteins genetics   metabolism   MeSH
• Plant Growth Regulators pharmacology   MeSH
• Amino Acid Sequence MeSH
• Publication type
• Journal Article MeSH
• Research Support, Non-U.S. Gov't MeSH
• Names of Substances
• Indoleacetic Acids MeSH
• PIN3 protein, Arabidopsis MeSH Browser
• Arabidopsis Proteins MeSH
• Plant Growth Regulators MeSH

Auxin is unique among plant hormones due to its directional transport that is mediated by the polarly distributed PIN auxin transporters at the plasma membrane. The canalization hypothesis proposes that the auxin feedback on its polar flow is a crucial, plant-specific mechanism mediating multiple self-organizing developmental processes. Here, we used the auxin effect on the PIN polar localization in Arabidopsis thaliana roots as a proxy for the auxin feedback on the PIN polarity during canalization. We performed microarray experiments to find regulators of this process that act downstream of auxin. We identified genes that were transcriptionally regulated by auxin in an AXR3/IAA17- and ARF7/ARF19-dependent manner. Besides the known components of the PIN polarity, such as PID and PIP5K kinases, a number of potential new regulators were detected, among which the WRKY23 transcription factor, which was characterized in more detail. Gain- and loss-of-function mutants confirmed a role for WRKY23 in mediating the auxin effect on the PIN polarity. Accordingly, processes requiring auxin-mediated PIN polarity rearrangements, such as vascular tissue development during leaf venation, showed a higher WRKY23 expression and required the WRKY23 activity. Our results provide initial insights into the auxin transcriptional network acting upstream of PIN polarization and, potentially, canalization-mediated plant development.

• MeSH
• Arabidopsis genetics   growth & development   MeSH
• Plants, Genetically Modified MeSH
• Gene Regulatory Networks * drug effects   MeSH
• Plant Roots drug effects   genetics   growth & development   metabolism   MeSH
• Indoleacetic Acids metabolism   pharmacology   MeSH
• Membrane Transport Proteins genetics   metabolism   MeSH
• Microarray Analysis MeSH
• Cell Polarity * genetics   MeSH
• Arabidopsis Proteins genetics   metabolism   physiology   MeSH
• Gene Expression Regulation, Plant drug effects   MeSH
• Gene Expression Profiling MeSH
• Transcription Factors physiology   MeSH
• Feedback, Physiological drug effects   MeSH
• Publication type
• Journal Article MeSH
• Research Support, Non-U.S. Gov't MeSH
• Names of Substances
• Indoleacetic Acids MeSH
• Membrane Transport Proteins MeSH
• PIN1 protein, Arabidopsis MeSH Browser
• Arabidopsis Proteins MeSH
• Transcription Factors MeSH
• WRKY23 protein, Arabidopsis MeSH Browser