Genetic analysis of the Arabidopsis TIR1/AFB auxin receptors reveals both overlapping and specialized functions

© 2020, Prigge et al. The TIR1/AFB auxin co-receptors mediate diverse responses to the plant hormone auxin. The Arabidopsis genome encodes six TIR1/AFB proteins representing three of the four clades that were established prior to angiosperm radiation. To determine the role of these proteins in plant development we performed an extensive genetic analysis involving the generation and characterization of all possible multiply-mutant lines. We find that loss of all six TIR1/AFB proteins results in early embryo defects and eventually seed abortion, and yet a single wild-type allele of TIR1 or AFB2 is sufficient to support growth throughout development. Our analysis reveals extensive functional overlap between even the most distantly related TIR1/AFB genes except for AFB1. Surprisingly, AFB1 has a specialized function in rapid auxin-dependent inhibition of root growth and early phase of root gravitropism. This activity may be related to a difference in subcellular localization compared to the other members of the family

Auxin-regulated reversible inhibition of TMK1 signaling by MAKR2 modulates the dynamics of root gravitropism

Plants are able to orient their growth according to gravity, which ultimately controls both shoot and root architecture.1 Gravitropism is a dynamic process whereby gravistimulation induces the asymmetric distribution of the plant hormone auxin, leading to asymmetric growth, organ bending, and subsequent reset of auxin distribution back to the original pre-gravistimulation situation.1, 2, 3 Differential auxin accumulation during the gravitropic response depends on the activity of polarly localized PIN-FORMED (PIN) auxin-efflux carriers.1, 2, 3, 4 In particular, the timing of this dynamic response is regulated by PIN2,5,6 but the underlying molecular mechanisms are poorly understood. Here, we show that MEMBRANE ASSOCIATED KINASE REGULATOR2 (MAKR2) controls the pace of the root gravitropic response. We found that MAKR2 is required for the PIN2 asymmetry during gravitropism by acting as a negative regulator of the cell-surface signaling mediated by the receptor-like kinase TRANSMEMBRANE KINASE1 (TMK1).2,7, 8, 9, 10 Furthermore, we show that the MAKR2 inhibitory effect on TMK1 signaling is antagonized by auxin itself, which triggers rapid MAKR2 membrane dissociation in a TMK1-dependent manner. Our findings suggest that the timing of the root gravitropic response is orchestrated by the reversible inhibition of the TMK1 signaling pathway at the cell surface

HY5 and phytochrome activity modulate shoot to root coordination during thermomorphogenesis.

This is the author accepted manuscript. The final version is available from The Company of Biologists via the DOI in this record Temperature is one of the most impactful environmental factors to which plants adjust their growth and development. While the regulation of temperature signaling has been extensively investigated for the aerial part of plants, much less is known and understood about how roots sense and modulate their growth in response to fluctuating temperatures. Here we found that shoot and root growth responses to high ambient temperature are coordinated during early seedling development. A shoot signaling module that includes HY5, the phytochromes and the PIFs exerts a central function in coupling these growth responses and maintain auxin levels in the root. In addition to the HY5/PIF-dependent shoot module, a regulatory axis composed of auxin biosynthesis and auxin perception factors controls root responses to high ambient temperature. Together, our findings show that shoot and root developmental responses to temperature are tightly coupled during thermomorphogenesis and suggest that roots integrate energy signals with local hormonal inputs.National Institute of General Medical Sciences of the National Institutes of Healt

The receptor kinase SRF3 coordinates iron- level and flagellin dependent defense and growth responses in plants

Iron is critical for host–pathogen interactions. While pathogens seek to scavenge iron to spread, the host aims at decreasing iron availability to reduce pathogen virulence. Thus, iron sensing and homeostasis are of particular importance to prevent host infection and part of nutritional immunity. While the link between iron homeostasis and immunity pathways is well established in plants, how iron levels are sensed and integrated with immune response pathways remains unknown. Here we report a receptor kinase SRF3, with a role in coordinating root growth, iron homeostasis and immunity pathways via regulation of callose synthases. These processes are modulated by iron levels and rely on SRF3 extracellular and kinase domains which tune its accumulation and partitioning at the cell surface. Mimicking bacterial elicitation with the flagellin peptide flg22 phenocopies SRF3 regulation upon low iron levels and subsequent SRF3-dependent responses. We propose that SRF3 is part of nutritional immunity responses involved in sensing external iron levels

Localisation et fonction des lipides anioniques dans l'organisation cellulaire et le développement des plantes

The « electrostatic territory» is part of the eukaryotic membrane organization and is defined by the enrichment of negatively charged phospholipids at the membrane cytosolic face. This feature is involved in the membrane recruitment of cytosolic proteins, which contain positively charged motifs and/or domains. In this work, we used Arabidopsis thaliana as a model and explored the existence of an electrostatic territory in plant cells. We found that the plasma membrane is the most anionic intracellular membrane (Simon, Platre et al., 2016 Nature Plants). This electrostatic field is maintained by lipid cooperation between, phosphatidic acid, phosphatidylserine and phosphatidylinositol-4-phosphate. The cell surface unique feature is involved in the regulation of hormonal signalling such as auxin and brassinosteroids pathways. We found that phosphatidylserine tunes the spatiotemporal dynamics of small GTPases from the Rho family. During auxin response, PS is required to cluster Rho into specialized membrane domains. We show that nanocluster formation is required for Rho-mediated auxin signaling including the regulation of endocytosis, cytoskeleton organization, morphogenesis and the root gravitropic response.Les cellules eucaryotes possèdent un territoire membranaire dit « électrostatique » qui est définit par la présence de phospholipides négativement chargés sur la face cytosolique des membranes. Cette propriété permet le recrutement de protéine cytosolique contenant des motifs/domaines positivement chargés au niveau des membranes via des interactions électrostatiques. Nous nous sommes demandés si le territoire électrostatique est présent chez les cellules végétales et quel est son organisation ? Quels sont le(s) lipide(s) anionique(s) impliqués dans son maintien ? Et quel est son (ces) rôle(s) dans la signalisation et le développement des plantes ? Premièrement, nous avons mis en avant que la membrane plasmique est le compartiment intracellulaire le plus électronégativement chargé (Simon, Platre et al., 2016 Nature Plants). Ce champ électrostatique est gouverné par trois lipides anioniques différents, l’acide phosphatidique, la phosphatidylserine et le phosphatidylinositol-4-phosphate. Nous avons montré que cette propriété unique de la membrane plasmique permet de réguler des voies de signalisation hormonale, tel que celle de l’auxine et des brassinostéroïdes. Notamment, la phosphatidylserine régule la dynamique spatiotemporelle des petites GTPases de la famille Rho. En réponse à l’auxine, ce lipide permet de regrouper les protéines Rho dans des domaines membranaires. La formation de ces domaines est requise pour l’activité de ces protéines permettant de contrôler l’endocytose, la dynamique du cytosquelette mais également régule la morphogenèse cellulaire ainsi que la réponse gravitropique de la racine

Localization and function of anionic lipids in cell organization and plant development

Les cellules eucaryotes possèdent un territoire membranaire dit « électrostatique » qui est définit par la présence de phospholipides négativement chargés sur la face cytosolique des membranes. Cette propriété permet le recrutement de protéine cytosolique contenant des motifs/domaines positivement chargés au niveau des membranes via des interactions électrostatiques. Nous nous sommes demandés si le territoire électrostatique est présent chez les cellules végétales et quel est son organisation ? Quels sont le(s) lipide(s) anionique(s) impliqués dans son maintien ? Et quel est son (ces) rôle(s) dans la signalisation et le développement des plantes ? Premièrement, nous avons mis en avant que la membrane plasmique est le compartiment intracellulaire le plus électronégativement chargé (Simon, Platre et al., 2016 Nature Plants). Ce champ électrostatique est gouverné par trois lipides anioniques différents, l’acide phosphatidique, la phosphatidylserine et le phosphatidylinositol-4-phosphate. Nous avons montré que cette propriété unique de la membrane plasmique permet de réguler des voies de signalisation hormonale, tel que celle de l’auxine et des brassinostéroïdes. Notamment, la phosphatidylserine régule la dynamique spatiotemporelle des petites GTPases de la famille Rho. En réponse à l’auxine, ce lipide permet de regrouper les protéines Rho dans des domaines membranaires. La formation de ces domaines est requise pour l’activité de ces protéines permettant de contrôler l’endocytose, la dynamique du cytosquelette mais également régule la morphogenèse cellulaire ainsi que la réponse gravitropique de la racine.The « electrostatic territory» is part of the eukaryotic membrane organization and is defined by the enrichment of negatively charged phospholipids at the membrane cytosolic face. This feature is involved in the membrane recruitment of cytosolic proteins, which contain positively charged motifs and/or domains. In this work, we used Arabidopsis thaliana as a model and explored the existence of an electrostatic territory in plant cells. We found that the plasma membrane is the most anionic intracellular membrane (Simon, Platre et al., 2016 Nature Plants). This electrostatic field is maintained by lipid cooperation between, phosphatidic acid, phosphatidylserine and phosphatidylinositol-4-phosphate. The cell surface unique feature is involved in the regulation of hormonal signalling such as auxin and brassinosteroids pathways. We found that phosphatidylserine tunes the spatiotemporal dynamics of small GTPases from the Rho family. During auxin response, PS is required to cluster Rho into specialized membrane domains. We show that nanocluster formation is required for Rho-mediated auxin signaling including the regulation of endocytosis, cytoskeleton organization, morphogenesis and the root gravitropic response

The root response to gravity: from the macro to the nanoscale

International audiencePlants are sessile organisms which adapt to their everchanging environment. The root is buried is the soil and continuously explores its surroundings. Indeed, while growing downwards to anchor the plants in the ground, it has to avoid obstacles and seek for nutrients and water. This seeking mechanism depends on the root perception of gravity. Through differential growth, the root is able to align according to the gravity vector. The growth is regulated at the cellular level by an increase of the plant hormone auxin, which activates the small Rho Guanine triphosphatase (Rho GTPase) of plant 6 (ROP6) at the plasma membrane to inhibit endocytosis and trigger cytoskeleton reorganization. Through a collaborative work, four French laboratories addressed the question of ROP6 membrane dynamics upon gravistimulation. Based on cellular biology, biochemistry and super resolution imaging approaches, they discovered that ROP6 is organized into nanoclusters at the plasma membrane of plant cells in response to auxin. The stabilization of ROP6 in these nanoclusters is required for signaling and thus the regulation of gravitropic bending. The formation of these nanoclusters is dependent upon the membrane lipid phosphatidylserine, which directly interact with ROP6. Using a genetic toolkit, the authors uncovered that phosphatidylserine is rate limiting for the ROP6-dependent nanocluster formation, which in turn tunes the cellular read outs. This work, not only explain the fine mechanism of the root response to gravity from the developmental level to the nanoscale but also provide a valuable insight towards the understanding of small GTPase signaling in eukaryotic system

Exogenous treatment of Arabidopsis seedlings with lyso-phospholipids for the inducible complementation of lipid mutants

International audienceLipids are major components of membranes with pleiotropic roles and interconnected metabolism, so experimentally addressing the primary function of individual lipid species in vivo can be difficult. Genetic approaches are particularly challenging to interpret due to compensatory mechanisms and indirect effects. Here, we describe a fast inducible approach to complement the phenotypes of Arabidopsis lipid mutants through exogenous treatment with the depleted lipid, followed by live confocal imaging to observe genetically encoded lipid sensors in wild-type and mutant root tissues

Anionic lipids and the maintenance of membrane electrostatics in eukaryotes

A wide range of signaling processes occurs at the cell surface through the reversible association of proteins from the cytosol to the plasma membrane. Some low abundant lipids are enriched at the membrane of specific compartments and thereby contribute to the identity of cell organelles by acting as biochemical landmarks. Lipids also influence membrane biophysical properties, which emerge as an important feature in specifying cellular territories. Such parameters are crucial for signal transduction and include lipid packing, membrane curvature and electrostatics. In particular, membrane electrostatics specifies the identity of the plasma membrane inner leaflet. Membrane surface charges are carried by anionic phospholipids, however the exact nature of the lipid(s) that powers the plasma membrane electrostatic field varies among eukaryotes and has been hotly debated during the last decade. Herein, we discuss the role of anionic lipids in setting up plasma membrane electrostatics and we compare similarities and differences that were found in different eukaryotic cells