The phosphoinositide signature guides the final step of plant cytokinesis.

Plant cytokinesis, which fundamentally differs from that in animals, requires the outward expansion of a plasma membrane precursor named the cell plate. How the transition from a cell plate to a plasma membrane occurs remains poorly understood. Here, we report that the acquisition of plasma membrane identity occurs through lateral patterning of the phosphatidylinositol 4,5-bisphosphate PI(4,5)P2 at the newly formed cell plate membrane. There, the phosphoinositide phosphatase SAC9 emerges as a key regulator, colocalizing with and regulating the function of the microtubule-associated protein MAP65-3 at the cell plate leading zone. In sac9-3 mutant, the polar distribution of PI(4,5)P2 at the cell plate is altered, leading to ectopic recruitment of the cytokinesis apparatus and formation of an additional cell plate insertion site. We propose that at the cell plate, SAC9 drives the depletion of PI(4,5)P2, which acts as a polar cue to spatially separate cell plate expansion from the acquisition of plasma membrane identity during final step of cytokinesis.

Maintaining asymmetry in cell division.

Promoting asymmetric division through microtubule dynamics establishes cell fate.

Connecting the plant cytoskeleton to the cell surface via the phosphoinositides.

Plants have developed fine-tuned cellular mechanisms to respond to a variety of intracellular and extracellular signals. These responses often necessitate the rearrangement of the plant cytoskeleton to modulate cell shape and/or to guide vesicle trafficking. At the cell periphery, both actin filaments and microtubules associate with the plasma membrane that acts as an integrator of the intrinsic and extrinsic environments. At this membrane, acidic phospholipids such as phosphatidic acid, and phosphoinositides contribute to the selection of peripheral proteins and thereby regulate the organization and dynamic of the actin and microtubules. After recognition of the importance of phosphatidic acid on cytoskeleton dynamics and rearrangement, it became apparent that the other lipids might play a specific role in shaping the cytoskeleton. This review focuses on the emerging role of the phosphatidylinositol 4,5-bisphosphate for the regulation of the peripherical cytoskeleton during cellular processes such as cytokinesis, polar growth, biotic and abiotic responses.

The Arabidopsis SAC9 enzyme is enriched in a cortical population of early endosomes and restricts PI(4,5)P2 at the plasma membrane.

Membrane lipids, and especially phosphoinositides, are differentially enriched within the eukaryotic endomembrane system. This generates a landmark code by modulating the properties of each membrane. Phosphatidylinositol 4,5-bisphosphate [PI(4,5)P2] specifically accumulates at the plasma membrane in yeast, animal, and plant cells, where it regulates a wide range of cellular processes including endocytic trafficking. However, the functional consequences of mispatterning PI(4,5)P2 in plants are unknown. Here, we functionally characterized the putative phosphoinositide phosphatase SUPPRESSOR OF ACTIN9 (SAC9) in Arabidopsis thaliana (Arabidopsis). We found that SAC9 depletion led to the ectopic localization of PI(4,5)P2 on cortical intracellular compartments, which depends on PI4P and PI(4,5)P2 production at the plasma membrane. SAC9 localizes to a subpopulation of trans-Golgi Network/early endosomes that are enriched in a region close to the cell cortex and that are coated with clathrin. Furthermore, it interacts and colocalizes with Src Homology 3 Domain Protein 2 (SH3P2), a protein involved in endocytic trafficking. In the absence of SAC9, SH3P2 localization is altered and the clathrin-mediated endocytosis rate is reduced. Together, our results highlight the importance of restricting PI(4,5)P2 at the plasma membrane and illustrate that one of the consequences of PI(4,5)P2 misspatterning in plants is to impact the endocytic trafficking.

Tools for studying the cytoskeleton during plant cell division.

The plant cytoskeleton regulates fundamental biological processes, including cell division. How to experimentally perturb the cytoskeleton is a key question if one wants to understand the role of both actin filaments (AFs) and microtubules (MTs) in a given biological process. While a myriad of mutants are available, knock-out in cytoskeleton regulators, when nonlethal, often produce little or no phenotypic perturbation because such regulators are often part of a large family, leading to functional redundancy. In this review, alternative techniques to modify the plant cytoskeleton during plant cell division are outlined. The different pharmacological and genetic approaches already developed in cell culture, transient assays, or in whole organisms are presented. Perspectives on the use of optogenetics to perturb the plant cytoskeleton are also discussed.

Experimental manipulation of phosphoinositide lipids: from cells to organisms.

Phosphoinositides (PIs) have critical roles in various cellular, physiological, developmental, pathological, and infectious processes. They are signaling phospholipids that can affect every aspect of membrane biology, including protein function (e.g., recruitment and activity), membrane physicochemical properties (e.g., curvature, surface charges, and packing), and the generation of secondary messengers. PIs act at precise locations within the cell in a dose-dependent manner, and their local concentration can vary drastically during signaling and trafficking. Thus, techniques able to manipulate PI amounts acutely and with subcellular accuracy are paramount to understanding the role of these lipids in vivo. Here, we review these methods and emphasize approaches recently developed to perturb PI levels in multicellular organisms.

Dynamic apico-basal enrichment of the F-actin during cytokinesis in Arabidopsis cells embedded in their tissues.

Cell division is a tightly regulated mechanism, notably in tissues where malfunctions can lead to tumour formation or developmental defects. This is particularly true in land plants, where cells cannot relocate and therefore cytokinesis determines tissue topology. In plants, cell division is executed in radically different manners than in animals, with the appearance of new structures and the disappearance of ancestral mechanisms. Whilst F-actin and microtubules closely co-exist, recent studies mainly focused on the involvement of microtubules in this key process. Here, we used a root tracking system to image the spatio-temporal dynamics of both F-actin reporters and cell division markers in dividing cells embedded in their tissues. In addition to the F-actin accumulation at the phragmoplast, we observed and quantified a dynamic apico-basal enrichment of F-actin from the prophase/metaphase transition until the end of the cytokinesis.

A nanodomain-anchored scaffolding complex is required for the function and localization of phosphatidylinositol 4-kinase alpha in plants.

Phosphoinositides are low-abundant lipids that participate in the acquisition of membrane identity through their spatiotemporal enrichment in specific compartments. Phosphatidylinositol 4-phosphate (PI4P) accumulates at the plant plasma membrane driving its high electrostatic potential, and thereby facilitating interactions with polybasic regions of proteins. PI4Kα1 has been suggested to produce PI4P at the plasma membrane, but how it is recruited to this compartment is unknown. Here, we pin-point the mechanism that tethers Arabidopsis thaliana phosphatidylinositol 4-kinase alpha1 (PI4Kα1) to the plasma membrane via a nanodomain-anchored scaffolding complex. We established that PI4Kα1 is part of a complex composed of proteins from the NO-POLLEN-GERMINATION, EFR3-OF-PLANTS, and HYCCIN-CONTAINING families. Comprehensive knockout and knockdown strategies revealed that subunits of the PI4Kα1 complex are essential for pollen, embryonic, and post-embryonic development. We further found that the PI4Kα1 complex is immobilized in plasma membrane nanodomains. Using synthetic mis-targeting strategies, we demonstrate that a combination of lipid anchoring and scaffolding localizes PI4Kα1 to the plasma membrane, which is essential for its function. Together, this work opens perspectives on the mechanisms and function of plasma membrane nanopatterning by lipid kinases.

Imaging the living plant cell: From probes to quantification.

At the center of cell biology is our ability to image the cell and its various components, either in isolation or within an organism. Given its importance, biological imaging has emerged as a field of its own, which is inherently highly interdisciplinary. Indeed, biologists rely on physicists and engineers to build new microscopes and imaging techniques, chemists to develop better imaging probes, and mathematicians and computer scientists for image analysis and quantification. Live imaging collectively involves all the techniques aimed at imaging live samples. It is a rapidly evolving field, with countless new techniques, probes, and dyes being continuously developed. Some of these new methods or reagents are readily amenable to image plant samples, while others are not and require specific modifications for the plant field. Here, we review some recent advances in live imaging of plant cells. In particular, we discuss the solutions that plant biologists use to live image membrane-bound organelles, cytoskeleton components, hormones, and the mechanical properties of cells or tissues. We not only consider the imaging techniques per se, but also how the construction of new fluorescent probes and analysis pipelines are driving the field of plant cell biology.

Methods to Visualize the Actin Cytoskeleton During Plant Cell Division.

Cell division in plants consists of separating the mother cell in two daughter cells by the centrifugal growth of a new wall. This process involves the reorganization of the structural elements of the cell, namely the microtubules and actin cytoskeleton which allow the coordination, the orientation, and the progression of mitosis. In addition to its implication in those plant-specific structures, the actin cytoskeleton, in close association with the plasma membrane, exhibits specific patterning at the cortex of the dividing cells, and might act as a signaling component. This review proposes an overview of the techniques available to visualize the actin cytoskeleton in fixed tissues or living cells during division, including electron, fluorescent, and super-resolution microscopy techniques.

Arabidopsis ADR1 helper NLR immune receptors localize and function at the plasma membrane in a phospholipid dependent manner.

Activation of nucleotide-binding leucine-rich repeat receptors (NLRs) results in immunity and a localized cell death. NLR cell death activity requires oligomerization and in some cases plasma membrane (PM) localization. The exact mechanisms underlying PM localization of NLRs lacking predicted transmembrane domains or recognizable lipidation motifs remain elusive. We used confocal microscopy, genetically encoded molecular tools and protein-lipid overlay assays to determine whether PM localization of members of the Arabidopsis HeLo-/RPW8-like domain 'helper' NLR (RNL) family is mediated by the interaction with negatively charged phospholipids of the PM. Our results show that PM localization and stability of some RNLs and one CC-type NLR (CNL) depend on the direct interaction with PM phospholipids. Depletion of phosphatidylinositol-4-phosphate from the PM led to a mis-localization of the analysed NLRs and consequently inhibited their cell death activity. We further demonstrate homo- and hetero-association of members of the RNL family. Our results provide new insights into the molecular mechanism of NLR localization and defines an important role of phospholipids for CNL and RNL PM localization and consequently, for their function. We propose that RNLs interact with anionic PM phospholipids and that RNL-mediated cell death and immune responses happen at the PM.

Inducible depletion of PI(4,5)P2 by the synthetic iDePP system in Arabidopsis.

Phosphatidylinositol 4,5-bisphosphate (PI(4,5)P2) is a low-abundance membrane lipid essential for plasma membrane function1,2. In plants, mutations in phosphatidylinositol 4-phosphate (PI4P) 5-kinases (PIP5K) suggest that PI(4,5)P2 production is involved in development, immunity and reproduction3-5. However, phospholipid synthesis is highly intricate6. It is thus likely that steady-state depletion of PI(4,5)P2 triggers confounding indirect effects. Furthermore, inducible tools available in plants allow PI(4,5)P2 to increase7-9 but not decrease, and no PIP5K inhibitors are available. Here, we introduce iDePP (inducible depletion of PI(4,5)P2 in plants), a system for the inducible and tunable depletion of PI(4,5)P2 in plants in less than three hours. Using this strategy, we confirm that PI(4,5)P2 is critical for various aspects of plant development, including root growth, root-hair elongation and organ initiation. We show that PI(4,5)P2 is required to recruit various endocytic proteins, including AP2-µ, to the plasma membrane, and thus to regulate clathrin-mediated endocytosis. Finally, we find that inducible PI(4,5)P2 perturbation impacts the dynamics of the actin cytoskeleton as well as microtubule anisotropy. Together, we propose that iDePP is a simple and efficient genetic tool to test the importance of PI(4,5)P2 in given cellular or developmental responses, and also to evaluate the importance of this lipid in protein localization.

Assessing Extrinsic Membrane Protein Dependency to PI4P Using a Plasma Membrane to Endosome Relocalization Transient Assay in Nicotiana benthamiana.

Phosphoinositides are key players from which the various membranes of the cells acquire their identity. The relative accumulation of these low-abundant anionic phospholipids in the cytosolic leaflet of the plasma membrane and of various organelles generates a landmark code, responsible for the selective recruitment of extrinsic proteins at given membranes. One of the key players in the protein/lipid interaction at the plasma membrane in plant cells, is phosphatidylinositol 4-phosphate (PI4P), which patterns the recruitment of effector proteins from the plasma membrane to organelles along the endocytic pathway. Here we describe a fast assay to assess the requirement of PI4P for membrane localization of extrinsic membrane proteins in vivo. This system relies on perturbing PI4P distribution in plant cells via the action of a PI4P phosphatase that depletes the pool of PI4P at a given membrane. This system efficiently decreases PI4P levels, and can therefore be easily used to assess requirement of PI4P (and electrostatics) for the targeting of extrinsic membrane proteins to the plasma membrane or endosomes. Ultimately, this system could also be extended to test the phosphatase activity in planta of enzymes putatively involved in PI4P metabolism.

Anionic Lipids: A Pipeline Connecting Key Players of Plant Cell Division.

How cells position their division plane is a critical component of cell division. Indeed, it defines whether the two daughter cells divide symmetrically (with equal volumes) or not, and as such is critical for cell differentiation and lineage specification across eukaryotes. However, oriented cell divisions are of special significance for organisms with cell walls, such as plants, because their cells are embedded and cannot relocate. Correctly positioning the division plane is therefore of prevailing importance in plants, as it controls not only the occurrence of asymmetric cell division, but also tissue morphogenesis and organ integrity. While cytokinesis is executed in radically different manners in animals and plants, they both rely on the dynamic interplay between the cytoskeleton and membrane trafficking to precisely deliver molecular components to the future site of cell division. Recent research has shown that strict regulation of the levels and distribution of anionic lipids, which are minor components of the cell membrane's lipids, is required for successful cytokinesis in non-plant organisms. This review focused on the recent evidence pointing to whether such signaling lipids have roles in plant cell division.

A Combinatorial Lipid Code Shapes the Electrostatic Landscape of Plant Endomembranes.

Membrane surface charge is critical for the transient, yet specific recruitment of proteins with polybasic regions to certain organelles. In eukaryotes, the plasma membrane (PM) is the most electronegative compartment of the cell, which specifies its identity. As such, membrane electrostatics is a central parameter in signaling, intracellular trafficking, and polarity. Here, we explore which are the lipids that control membrane electrostatics using plants as a model. We show that phosphatidylinositol-4-phosphate (PI4P), phosphatidic acidic (PA), and phosphatidylserine (PS) are separately required to generate the electrostatic signature of the plant PM. In addition, we reveal the existence of an electrostatic territory that is organized as a gradient along the endocytic pathway and is controlled by PS/PI4P combination. Altogether, we propose that combinatorial lipid composition of the cytosolic leaflet of organelles not only defines the electrostatic territory but also distinguishes different functional compartments within this territory by specifying their varying surface charges.

Automated Tracking of Root for Confocal Time-lapse Imaging of Cellular Processes.

Here we describe a protocol that enables to automatically perform time-lapse imaging of growing root tips for several hours. Plants roots expressing fluorescent proteins or stained with dyes are imaged while they grow using automatic movement of the microscope stage that compensates for root growth and allows to follow a given region of the root over time. The protocol makes possible the image acquisition of multiple growing root tips, therefore increasing the number of recorded mitotic events in a given experiment. The protocol also allows the visualization of more than one fluorescent protein or dye simultaneously, using multiple channel acquisition. We particularly focus on imaging of cytokinesis in Arabidopsis root tip meristem, but this protocol is also suitable to follow root hair growth, pollen tube growth, and other regions of root over time, in various plant species. It may as well be amendable to automatically track non-plant structures with an apical growth.

A PtdIns(4)P-driven electrostatic field controls cell membrane identity and signalling in plants.

Many signalling proteins permanently or transiently localize to specific organelles. It is well established that certain lipids act as biochemical landmarks to specify compartment identity. However, they also influence membrane biophysical properties, which emerge as important features in specifying cellular territories. Such parameters include the membrane inner surface potential, which varies according to the lipid composition of each organelle. Here, we found that the plant plasma membrane (PM) and the cell plate of dividing cells have a unique electrostatic signature controlled by phosphatidylinositol-4-phosphate (PtdIns(4)P). Our results further reveal that, contrarily to other eukaryotes, PtdIns(4)P massively accumulates at the PM, establishing it as a critical hallmark of this membrane in plants. Membrane surface charges control the PM localization and function of the polar auxin transport regulator PINOID as well as proteins from the BRI1 KINASE INHIBITOR1 (BKI1)/MEMBRANE ASSOCIATED KINASE REGULATOR (MAKR) family, which are involved in brassinosteroid and receptor-like kinase signalling. We anticipate that this PtdIns(4)P-driven physical membrane property will control the localization and function of many proteins involved in development, reproduction, immunity and nutrition.

The Arabidopsis microtubule-associated protein MAP65-3 supports infection by filamentous biotrophic pathogens by down-regulating salicylic acid-dependent defenses.

The oomycete Hyaloperonospora arabidopsidis and the ascomycete Erysiphe cruciferarum are obligate biotrophic pathogens causing downy mildew and powdery mildew, respectively, on Arabidopsis. Upon infection, the filamentous pathogens induce the formation of intracellular bulbous structures called haustoria, which are required for the biotrophic lifestyle. We previously showed that the microtubule-associated protein AtMAP65-3 plays a critical role in organizing cytoskeleton microtubule arrays during mitosis and cytokinesis. This renders the protein essential for the development of giant cells, which are the feeding sites induced by root knot nematodes. Here, we show that AtMAP65-3 expression is also induced in leaves upon infection by the downy mildew oomycete and the powdery mildew fungus. Loss of AtMAP65-3 function in the map65-3 mutant dramatically reduced infection by both pathogens, predominantly at the stages of leaf penetration. Whole-transcriptome analysis showed an over-represented, constitutive activation of genes involved in salicylic acid (SA) biosynthesis, signaling, and defense execution in map65-3, whereas jasmonic acid (JA)-mediated signaling was down-regulated. Preventing SA synthesis and accumulation in map65-3 rescued plant susceptibility to pathogens, but not the developmental phenotype caused by cytoskeleton defaults. AtMAP65-3 thus has a dual role. It positively regulates cytokinesis, thus plant growth and development, and negatively interferes with plant defense against filamentous biotrophs. Our data suggest that downy mildew and powdery mildew stimulate AtMAP65-3 expression to down-regulate SA signaling for infection.

In Vivo Imaging of Microtubule Organization in Dividing Giant Cell.

Mitosis which is a major step during plant development can also be observed in physiopathological conditions. During the compatible interaction between the root-knot nematode Meloidogyne incognita and its host Arabidopsis, the pathogen induce through repeated divisions without complete cytokinesis the formation of hypertrophied and multinucleate feeding cells, named giant cells. Due to the presence of hypertrophied plant cell material surrounding the giant cells, classical live cell imaging gave therefore very poor resolution. Here, we describe a protocol which allows the in vivo observation of the mitotic apparatus in developing giant cells using confocal imaging of vibrosliced tissues. This approach can also be used to visualize in vivo other cellular processes occurring in different steps of giant cells.