Arabidopsis Blue Light Receptor Phototropin 1 Undergoes Blue Light-Induced Activation in Membrane Microdomains

Recruitment of HAB1 and SnRK2.2 by C2-domain protein CAR1 in plasma membrane ABA signaling

Plasma membrane (PM)-associated abscisic acid (ABA) signal transduction is an important component of ABA signaling. The C2-domain ABA-related (CAR) proteins have been reported to play a crucial role in recruiting ABA receptor PYR1/PYL/RCAR (PYLs) to the PM. However, the molecular details of the involvement of CAR proteins in membrane-delimited ABA signal transduction remain unclear. For instance, where this response process takes place and whether any additional members besides PYL are taking part in this signaling process. Here, the GUS-tagged materials for all Arabidopsis CAR members were used to comprehensively visualize the extensive expression patterns of the CAR family genes. Based on the representativeness of CAR1 in response to ABA, we determined to use it as a target to study the function of CAR proteins in PM-associated ABA signaling. Single-particle tracking showed that ABA affected the spatiotemporal dynamics of CAR1. The presence of ABA prolonged the dwell time of CAR1 on the membrane and showed faster lateral mobility. Surprisingly, we verified that CAR1 could directly recruit hypersensitive to ABA1 (HAB1) and SNF1-related protein kinase 2.2 (SnRK2.2) to the PM at both the bulk and single-molecule levels. Furthermore, PM localization of CAR1 was demonstrated to be related to membrane microdomains. Collectively, our study revealed that CARs recruited the three main components of ABA signaling to the PM to respond positively to ABA. This study deepens our understanding of ABA signal transduction.

Membrane microdomains: Structural and signaling platforms for establishing membrane polarity

Cell polarity results from the asymmetric distribution of cellular structures, molecules, and functions. Polarity is a fundamental cellular trait that can determine the orientation of cell division, the formation of particular cell shapes, and ultimately the development of a multicellular body. To maintain the distinct asymmetric distribution of proteins and lipids in cellular membranes, plant cells have developed complex trafficking and regulatory mechanisms. Major advances have been made in our understanding of how membrane microdomains influence the asymmetric distribution of proteins and lipids. In this review, we first give an overview of cell polarity. Next, we discuss current knowledge concerning membrane microdomains and their roles as structural and signaling platforms to establish and maintain membrane polarity, with a special focus on the asymmetric distribution of proteins and lipids, and advanced microscopy techniques to observe and characterize membrane microdomains. Finally, we review recent advances regarding membrane trafficking in cell polarity establishment and how the balance between exocytosis and endocytosis affects membrane polarity.

Exploring the Role of the Plant Actin Cytoskeleton: From Signaling to Cellular Functions

The plant actin cytoskeleton is characterized by the basic properties of dynamic array, which plays a central role in numerous conserved processes that are required for diverse cellular functions. Here, we focus on how actins and actin-related proteins (ARPs), which represent two classical branches of a greatly diverse superfamily of ATPases, are involved in fundamental functions underlying signal regulation of plant growth and development. Moreover, we review the structure, assembly dynamics, and biological functions of filamentous actin (F-actin) from a molecular perspective. The various accessory proteins known as actin-binding proteins (ABPs) partner with F-actin to finely tune actin dynamics, often in response to various cell signaling pathways. Our understanding of the significance of the actin cytoskeleton in vital cellular activities has been furthered by comparison of conserved functions of actin filaments across different species combined with advanced microscopic techniques and experimental methods. We discuss the current model of the plant actin cytoskeleton, followed by examples of the signaling mechanisms under the supervision of F-actin related to cell morphogenesis, polar growth, and cytoplasmic streaming. Determination of the theoretical basis of how the cytoskeleton works is important in itself and is beneficial to future applications aimed at improving crop biomass and production efficiency.

Multiscale microscopy to decipher plant cell structure and dynamics

New imaging methodologies with high contrast and molecular specificity allow researchers to analyze dynamic processes in plant cells at multiple scales, from single protein and RNA molecules to organelles and cells, to whole organs and tissues. These techniques produce informative images and quantitative data on molecular dynamics to address questions that cannot be answered by conventional biochemical assays. Here, we review selected microscopy techniques, focusing on their basic principles and applications in plant science, discussing the pros and cons of each technique, and introducing methods for quantitative analysis. This review thus provides guidance for plant scientists in selecting the most appropriate techniques to decipher structures and dynamic processes at different levels, from protein dynamics to morphogenesis.

Spatial proteomics of vesicular trafficking: coupling mass spectrometry and imaging approaches in membrane biology

In plants, membrane compartmentalization requires vesicle trafficking for communication among distinct organelles. Membrane proteins involved in vesicle trafficking are highly dynamic and can respond rapidly to changes in the environment and to cellular signals. Capturing their localization and dynamics is thus essential for understanding the mechanisms underlying vesicular trafficking pathways. Quantitative mass spectrometry and imaging approaches allow a system-wide dissection of the vesicular proteome, the characterization of ligand-receptor pairs and the determination of secretory, endocytic, recycling and vacuolar trafficking pathways. In this review, we highlight major proteomics and imaging methods employed to determine the location, distribution and abundance of proteins within given trafficking routes. We focus in particular on methodologies for the elucidation of vesicle protein dynamics and interactions and their connections to downstream signalling outputs. Finally, we assess their biological applications in exploring different cellular and subcellular processes.

Molecular insights into the phototropin control of chloroplast movements

Chloroplast movements are controlled by ultraviolet/blue light through phototropins. In Arabidopsis thaliana, chloroplast accumulation at low light intensities and chloroplast avoidance at high light intensities are observed. These responses are controlled by two homologous photoreceptors, the phototropins phot1 and phot2. Whereas chloroplast accumulation is triggered by both phototropins in a partially redundant manner, sustained chloroplast avoidance is elicited only by phot2. Phot1 is able to trigger only a small, transient chloroplast avoidance, followed by the accumulation phase. The source of this functional difference is not fully understood at either the photoreceptor or the signalling pathway levels. In this article, we review current understanding of phototropin functioning and try to dissect the differences that result in signalling to elicit two distinct chloroplast responses. First, we focus on phototropin structure and photochemical and biochemical activity. Next, we analyse phototropin expression and localization patterns. We also summarize known photoreceptor systems controlling chloroplast movements. Finally, we focus on the role of environmental stimuli in controlling phototropin activity. All these aspects impact the signalling to trigger chloroplast movements and raise outstanding questions about the mechanism involved.

Light Intensity- and Spectrum-Dependent Redox Regulation of Plant Metabolism

Both light intensity and spectrum (280–800 nm) affect photosynthesis and, consequently, the formation of reactive oxygen species (ROS) during photosynthetic electron transport. ROS, together with antioxidants, determine the redox environment in tissues and cells, which in turn has a major role in the adjustment of metabolism to changes in environmental conditions. This process is very important since there are great spatial (latitude, altitude) and temporal (daily, seasonal) changes in light conditions which are accompanied by fluctuations in temperature, water supply, and biotic stresses. The blue and red spectral regimens are decisive in the regulation of metabolism because of the absorption maximums of chlorophylls and the sensitivity of photoreceptors. Based on recent publications, photoreceptor-controlled transcription factors such as ELONGATED HYPOCOTYL5 (HY5) and changes in the cellular redox environment may have a major role in the coordinated fine-tuning of metabolic processes during changes in light conditions. This review gives an overview of the current knowledge of the light-associated redox control of basic metabolic pathways (carbon, nitrogen, amino acid, sulphur, lipid, and nucleic acid metabolism), secondary metabolism (terpenoids, flavonoids, and alkaloids), and related molecular mechanisms. Light condition-related reprogramming of metabolism is the basis for proper growth and development of plants; therefore, its better understanding can contribute to more efficient crop production in the future.

Probing membrane protein interactions and signaling molecule homeostasis in plants by Förster resonance energy transfer analysis

Membrane proteins have key functions in signal transduction, transport, and metabolism. Therefore, deciphering the interactions between membrane proteins provides crucial information on signal transduction and the spatiotemporal organization of protein complexes. However, detecting the interactions and behaviors of membrane proteins in their native environments remains difficult. Förster resonance energy transfer (FRET) is a powerful tool for quantifying the dynamic interactions and assembly of membrane proteins without disrupting their local environment, supplying nanometer-scale spatial information and nanosecond-scale temporal information. In this review, we briefly introduce the basic principles of FRET and assess the current state of progress in the development of new FRET techniques (such as FRET-FLIM, homo-FRET, and smFRET) for the analysis of plant membrane proteins. We also describe the various FRET-based biosensors used to quantify the homeostasis of signaling molecules and the active state of kinases. Furthermore, we summarize recent applications of these advanced FRET sensors in probing membrane protein interactions, stoichiometry, and protein clustering, which have shed light on the complex biological functions of membrane proteins in living plant cells.

Membrane nanodomains and transport functions in plant

Far from a homogeneous environment, biological membranes are highly structured with lipids and proteins segregating in domains of different sizes and dwell times. In addition, membranes are highly dynamics especially in response to environmental stimuli. Understanding the impact of the nanoscale organization of membranes on cellular functions is an outstanding question. Plant channels and transporters are tightly regulated to ensure proper cell nutrition and signaling. Increasing evidence indicates that channel and transporter nano-organization within membranes plays an important role in these regulation mechanisms. Here, we review recent advances in the field of ion, water, but also hormone transport in plants, focusing on protein organization within plasma membrane nanodomains and its cellular and physiological impacts.

Plant Sterol Clustering Correlates with Membrane Microdomains as Revealed by Optical and Computational Microscopy

Local inhomogeneities in lipid composition play a crucial role in the regulation of signal transduction and membrane traffic. This is particularly the case for plant plasma membrane, which is enriched in specific lipids, such as free and conjugated forms of phytosterols and typical phytosphingolipids. Nevertheless, most evidence for microdomains in cells remains indirect, and the nature of membrane inhomogeneities has been difficult to characterize. We used a new push–pull pyrene probe and fluorescence lifetime imaging microscopy (FLIM) combined with all-atom multiscale molecular dynamics simulations to provide a detailed view on the interaction between phospholipids and phytosterol and the effect of modulating cellular phytosterols on membrane-associated microdomains and phase separation formation. Our understanding of the organization principles of biomembranes is limited mainly by the challenge to measure distributions and interactions of lipids and proteins within the complex environment of living cells. Comparing phospholipids/phytosterol compositions typical of liquid-disordered (Ld) and liquid-ordered (Lo) domains, we furthermore show that phytosterols play crucial roles in membrane homeostasis. The simulation work highlights how state-of-the-art modeling alleviates some of the prior concerns and how unrefuted discoveries can be made through a computational microscope. Altogether, our results support the role of phytosterols in the lateral structuring of the PM of plant cells and suggest that they are key compounds for the formation of plant PM microdomains and the lipid-ordered phase.

Cross-talk between clathrin-dependent post-Golgi trafficking and clathrin-mediated endocytosis in Arabidopsis root cells

Coupling of post-Golgi and endocytic membrane transport ensures that the flow of materials to/from the plasma membrane (PM) is properly balanced. The mechanisms underlying the coordinated trafficking of PM proteins in plants, however, are not well understood. In plant cells, clathrin and its adaptor protein complexes, AP-2 and the TPLATE complex (TPC) at the PM, and AP-1 at the trans-Golgi network/early endosome (TGN/EE), function in clathrin-mediated endocytosis (CME) and post-Golgi trafficking. Here, we utilized mutants with defects in clathrin-dependent post-Golgi trafficking and CME, in combination with other cytological and pharmacological approaches, to further investigate the machinery behind the coordination of protein delivery and recycling to/from the TGN/EE and PM in Arabidopsis (Arabidopsis thaliana) root cells. In mutants with defective AP-2-/TPC-dependent CME, we determined that clathrin and AP-1 recruitment to the TGN/EE as well as exocytosis are significantly impaired. Likewise, defects in AP-1-dependent post-Golgi trafficking and pharmacological inhibition of exocytosis resulted in the reduced association of clathrin and AP-2/TPC subunits with the PM and a reduction in the internalization of cargoes via CME. Together, these results suggest that post-Golgi trafficking and CME are coupled via modulation of clathrin and adaptor protein complex recruitment to the TGN/EE and PM.

Single-Molecule Imaging in Living Plant Cells: A Methodological Review

Single-molecule imaging is emerging as a revolutionary approach to studying fundamental questions in plants. However, compared with its use in animals, the application of single-molecule imaging in plants is still underexplored. Here, we review the applications, advantages, and challenges of single-molecule fluorescence imaging in plant systems from the perspective of methodology. Firstly, we provide a general overview of single-molecule imaging methods and their principles. Next, we summarize the unprecedented quantitative details that can be obtained using single-molecule techniques compared to bulk assays. Finally, we discuss the main problems encountered at this stage and provide possible solutions.

In vivo single-particle tracking of the aquaporin AtPIP2;1 in stomata reveals cell type-specific dynamics

Aquaporins such as the plasma membrane intrinsic proteins (PIPs) allow water to move through cell membranes and are vital for stomatal movement in plants. Despite their importance, the dynamic changes in aquaporins during water efflux and influx have not been directly observed in real time in vivo. Here, to determine which factors regulate these changes during the bidirectional translocation of water, we examined aquaporin dynamics during the stomatal immune response to the bacterial flagellin-derived peptide flg22. The Arabidopsis (Arabidopsis thaliana) aquaporin mutant pip2;1 showed defects in the flg22-induced stomatal response. Variable-angle total internal reflection fluorescence microscopy revealed that the movement dynamics and dwell times of AQ6]GFP-AtPIP2;1 in guard cells and subsidiary cells exhibited cell type-specific dependencies on flg22. The cytoskeleton, rather than the cell wall, was the major factor regulating AtPIP2;1 dynamics, although both the cytoskeleton and cell wall might form bounded domains that restrict the diffusion of AtPIP2;1 in guard cells and subsidiary cells. Finally, our analysis revealed the different roles of cortical actin and microtubules in regulating AtPIP2;1 dynamics in guard cells, as well as subsidiary cells, under various conditions. Our observations shed light on the heterogeneous mechanisms that regulate membrane protein dynamics in plants in response to pathogens.

Anaesthetics and plants: from sensory systems to cognition-based adaptive behaviour

Plants are not only sensitive to exogenous anaesthetics, but they also produce multitudes of endogenous substances, especially when stressed, that often have anaesthetic and anelgesic properties when applied to both humans and animals. Moreover, plants rely on neurotransmitters and their receptors for cell-cell communication and integration in a similar fashion to the use of neural systems in animals and humans. Plants also use their plant-specific sensory systems and neurotransmitter-based communication, including long-distance action potentials, to manage stress via cognition-like plant-specific behaviour and adaptation.

An improved and efficient method of Agrobacterium syringe infiltration for transient transformation and its application in the elucidation of gene function in poplar

BACKGROUND

Forest trees have important economic and ecological value. As a model tree, poplar has played a significant role in elucidating the molecular mechanisms underlying tree biology. However, a lack of mutant libraries and time-consuming stable genetic transformation processes severely limit progress into the functional characterization of poplar genes. A convenient and fast transient transformation method is therefore needed to enhance progress on functional genomics in poplar.

METHODS

A total of 11 poplar clones were screened for amenability to syringe infiltration. Syringe infiltration was performed on the lower side of the leaves of young soil-grown plants. Transient expression was evaluated by visualizing the reporters β-glucuronidase (GUS) and green fluorescent protein (GFP). The experimental parameters of the syringe agroinfiltration were optimized based on the expression levels of the reporter luciferase (LUC). Stably transformed plants were regenerated from transiently transformed leaf explants through callus-induced organogenesis. The functions of Populus genes in secondary cell wall-thickening were characterized by visualizing lignin deposition therein after staining with basic fuchsin.

RESULTS

We greatly improved the transient transformation efficiency of syringe Agrobacterium infiltration in poplar through screening for a suitable poplar clone from a variety of clones and optimizing the syringe infiltration procedure. The selected poplar clone, Populus davidiana × P. bolleana, is amenable to Agrobacterium syringe infiltration, as indicated by the easy diffusion of the bacterial suspension inside the leaf tissues. Using this technique, we localized a variety of poplar proteins in specific intracellular organelles and illustrated the protein-protein and protein-DNA interactions. The transiently transformed leaves could be used to generate stably transformed plants with high efficiency through callus induction and differentiation processes. Furthermore, transdifferentiation of the protoxylem-like vessel element and ectopic secondary wall thickening were induced in the agroinfiltrated leaves via the transient overexpression of genes associated with secondary wall formation.

CONCLUSIONS

The application of P. davidiana × P. bolleana in Agrobacterium syringe infiltration provides a foundation for the rapid and high-throughput functional characterization of Populus genes in intact poplar plants, including those involved in wood formation, and provides an effective alternative to Populus stable genetic transformation.

Lighting the way: Recent insights into the structure and regulation of phototropin blue light receptors

The phototropins (phots) are light-activated kinases that are critical for plant physiology and the many diverse optogenetic tools that they have inspired. Phototropins combine two blue-light-sensing Light-Oxygen-Voltage (LOV) domains (LOV1 and LOV2) and a C-terminal serine/threonine kinase domain, using the LOV domains to control the catalytic activity of the kinase. While much is known about the structure and photochemistry of the light-perceiving LOV domains, particularly in how activation of the LOV2 domain triggers the unfolding of alpha helices that communicate the light signal to the kinase domain, many questions about phot structure and mechanism remain. Recent studies have made progress addressing these questions by utilizing small-angle X-ray scattering (SAXS) and other biophysical approaches to study multidomain phots from Chlamydomonas and Arabidopsis, leading to models where the domains have an extended linear arrangement, with the regulatory LOV2 domain contacting the kinase domain N-lobe. We discuss this and other advances that have improved structural and mechanistic understanding of phot regulation in this review, along with the challenges that will have to be overcome to obtain high-resolution structural information on these exciting photoreceptors. Such information will be essential to advancing fundamental understanding of plant physiology while enabling engineering efforts at both the whole plant and molecular levels.

Salicylic acid regulates PIN2 auxin transporter hyperclustering and root gravitropic growth via Remorin-dependent lipid nanodomain organisation in Arabidopsis thaliana

To adapt to the diverse array of biotic and abiotic cues, plants have evolved sophisticated mechanisms to sense changes in environmental conditions and modulate their growth. Growth-promoting hormones and defence signalling fine tune plant development antagonistically. During host-pathogen interactions, this defence-growth trade-off is mediated by the counteractive effects of the defence hormone salicylic acid (SA) and the growth hormone auxin. Here we revealed an underlying mechanism of SA regulating auxin signalling by constraining the plasma membrane dynamics of PIN2 auxin efflux transporter in Arabidopsis thaliana roots. The lateral diffusion of PIN2 proteins is constrained by SA signalling, during which PIN2 proteins are condensed into hyperclusters depending on REM1.2-mediated nanodomain compartmentalisation. Furthermore, membrane nanodomain compartmentalisation by SA or Remorin (REM) assembly significantly suppressed clathrin-mediated endocytosis. Consequently, SA-induced heterogeneous surface condensation disrupted asymmetric auxin distribution and the resultant gravitropic response. Our results demonstrated a defence-growth trade-off mechanism by which SA signalling crosstalked with auxin transport by concentrating membrane-resident PIN2 into heterogeneous compartments.

Stem phototropism toward blue and ultraviolet light

Positive phototropism is the process through which plants orient their organs toward a directional light source. While the blue light receptors phototropins (phot) play a major role in phototropism toward blue (B) and ultraviolet (UV) radiation, recent research showed that the UVB light receptor UVR8 also triggers phototropism toward UVB. In addition, new details of the molecular mechanisms underlying the activity of these receptors and interaction with other environmental signals have emerged in the past years. In this review, we summarize the current knowledge about hypocotyledoneous and inflorescence stem growth reorientation toward B and UVB, with a focus on the molecular mechanisms.

The cold-induced switch in direction of chloroplast relocation occurs independently of changes in endogenous phototropin levels

When exposed to fluctuating light intensity, chloroplasts move towards weak light (accumulation response), and away from strong light (avoidance response). In addition, cold treatment (5°C) induces the avoidance response even under weak-light conditions (cold-avoidance response). These three responses are mediated by the phototropin (phot), which is a blue-light photoreceptor and has also been reported to act as a thermosensory protein that perceives temperature variation. Our previous report indicated that cold-induced changes in phot biochemical activity initiate the cold-avoidance response. In this study, we further explored the induction mechanism of the cold-avoidance response in the liverwort Marchantia polymorpha and examined the relationship between changes in the amount of phot and the induction of the cold-avoidance response. The switch between the accumulation and avoidance responses occurs at a so-called ‘transitional’ light intensity. Our physiological experiments revealed that a cold-mediated decrease in the transitional light intensity leads to the induction of the cold-avoidance response. While artificial overexpression of phot decreased the transitional light intensity as much as cold treatment did, the amount of endogenous phot was not increased by cold treatment in wild-type M. polymorpha. Taken together, these findings show that the cold-avoidance response is initiated by a cold-mediated reduction of the transitional light intensity, independent of the amount of endogenous phot. This study provides a clue to understanding the mechanism underlying the switch in direction of chloroplast relocation in response to light and temperature.

Quantitative Proteomic Analysis of Alligator Weed Leaves Reveals That Cationic Peroxidase 1 Plays Vital Roles in the Potassium Deficiency Stress Response

Alligator weed is reported to have a strong ability to adapt to potassium deficiency (LK) stress. Leaves are the primary organs responsible for photosynthesis of plants. However, quantitative proteomic changes in alligator weed leaves in response to LK stress are largely unknown. In this study, we investigated the physiological and proteomic changes in leaves of alligator weed under LK stress. We found that chloroplast and mesophyll cell contents in palisade tissue increased, and that the total chlorophyll content, superoxide dismutase (SOD) activity and net photosynthetic rate (PN) increased after 15 day of LK treatment, but the soluble protein content decreased. Quantitative proteomic analysis suggested that a total of 119 proteins were differentially abundant proteins (DAPs). KEGG analysis suggested that most represented DAPs were associated with secondary metabolism, the stress response, photosynthesis, protein synthesis, and degradation pathway. The proteomic results were verified using parallel reaction monitoring mass spectrometry (PRM–MS) analysis and quantitative real-time PCR (qRT-PCR)assays. Additional research suggested that overexpression of cationic peroxidase 1 of alligator weed (ApCPX1) in tobacco increased LK tolerance. The seed germination rate, peroxidase (POD) activity, and K⁺ content increased, and the hydrogen peroxide (H₂O₂) content decreased in the three transgenic tobacco lines after LK stress. The number of root hairs of the transgenic line was significantly higher than that of WT, and net K efflux rates were severely decreased in the transgenic line under LK stress. These results confirmed that ApCPX1 played positive roles in low-K⁺ signal sensing. These results provide valuable information on the adaptive mechanisms in leaves of alligator weed under LK stress and will help identify vital functional genes to apply to the molecular breeding of LK-tolerant plants in the future.

The continuing arc toward phototropic enlightenment

Phototropism represents a simple physiological mechanism-differential growth across the growing organ of a plant-to respond to gradients of light and maximize photosynthetic light capture (in aerial tissues) and water/nutrient acquisition (in roots). The phototropin blue light receptors, phot1 and phot2, have been identified as the essential sensors for phototropism. Additionally, several downstream signal/response components have been identified, including the phot-interacting proteins NON-PHOTOTROPIC HYPOCOTYL 3 (NPH3) and PHYTOCHROME SUBSTRATE 4 (PKS4). While the structural and photochemical properties of the phots are quite well understood, much less is known about how the phots signal through downstream regulators. Recent advances have, however, provided some intriguing clues. It appears that inactive receptor phot1 is found dispersed in a monomeric form at the plasma membrane in darkness. Upon light absorption dimerizes and clusters in sterol-rich microdomains where it is signal active. Additional studies showed that the phot-regulated phosphorylation status of both NPH3 and PKS4 is linked to phototropic responsiveness. While PKS4 can function as both a positive (in low light) and a negative (in high light) regulator of phototropism, NPH3 appears to function solely as a key positive regulator. Ultimately, it is the subcellular localization of NPH3 that appears crucial, an aspect regulated by its phosphorylation status. While phot1 activation promotes dephosphorylation of NPH3 and its movement from the plasma membrane to cytoplasmic foci, phot2 appears to modulate relocalization back to the plasma membrane. Together these findings are beginning to illuminate the complex biochemical and cellular events, involved in adaptively modifying phototropic responsiveness under a wide varying range of light conditions.

Dynamics and Endocytosis of Flot1 in Arabidopsis Require CPI1 Function

Membrane microdomains are nano-scale domains (10–200 nm) enriched in sterols and sphingolipids. They have many important biological functions, including vesicle transport, endocytosis, and pathogen invasion. A previous study reported that the membrane microdomain-associated protein Flotillin1 (Flot1) was involved in plant development in Arabidopsis thaliana; however, whether sterols affect the plant immunity conveyed by Flot1 is unknown. Here, we showed that the root length in sterol-deficient cyclopropylsterol isomerase 1 (cpi1-1) mutants expressing Flot1 was significantly shorter than in control seedlings. The cotyledon epidermal cells in cpi1-1 mutants expressing Flot1 were smaller than in controls. Moreover, variable-angle total internal reflection fluorescence microscopy (VA-TIRFM) and single-particle tracking (SPT) analysis demonstrated that the long-distance Flot1-GFP movement was decreased significantly in cpi1-1 mutants compared with the control seedlings. Meanwhile, the value of the diffusion coefficient Ĝ was dramatically decreased in cpi1-1 mutants after flagelin22 (flg22) treatment compared with the control seedlings, indicating that sterols affect the lateral mobility of Flot1-GFP within the plasma membrane. Importantly, using confocal microscopy, we determined that the endocytosis of Flot1-GFP was decreased in cpi1-1 mutants, which was confirmed by fluorescence cross spectroscopy (FCS) analysis. Hence, these results demonstrate that sterol composition plays a critical role in the plant defense responses of Flot1.

Cell wall contributes to the stability of plasma membrane nanodomain organization of Arabidopsis thaliana FLOTILLIN2 and HYPERSENSITIVE INDUCED REACTION1 proteins

Current models of plasma membrane (PM) postulate its organization in various nano- and micro-domains with distinct protein and lipid composition. While metazoan PM nanodomains usually display high lateral mobility, the dynamics of plant nanodomains is often highly spatially restricted. Here we have focused on the determination of the PM distribution in nanodomains for Arabidopsis thaliana flotillin (AtFLOT) and hypersensitive induced reaction proteins (AtHIR), previously shown to be involved in response to extracellular stimuli. Using in vivo laser scanning and spinning disc confocal microscopy in Arabidopsis thaliana we present here their nanodomain localization in various epidermal cell types. Fluorescence recovery after photobleaching (FRAP) and kymographic analysis revealed that PM-associated AtFLOTs contain significantly higher immobile fraction than AtHIRs. In addition, much lower immobile fractions have been found in tonoplast pool of AtHIR3. Although members of both groups of proteins were spatially restricted in their PM distribution by corrals co-aligning with microtubules (MTs), pharmacological treatments showed no or very low role of actin and microtubular cytoskeleton for clustering of AtFLOT and AtHIR into nanodomains. Finally, pharmacological alteration of cell wall (CW) synthesis and structure resulted in changes in lateral mobility of AtFLOT2 and AtHIR1. Accordingly, partial enzymatic CW removal increased the overall dynamics as well as individual nanodomain mobility of these two proteins. Such structural links to CW could play an important role in their correct positioning during PM communication with extracellular environment.

Organization and dynamics of functional plant membrane microdomains

Plasma membranes are heterogeneous and laterally compartmentalized into distinct microdomains. These membrane microdomains consist of special lipids and proteins and are thought to act as signaling platforms. In plants, membrane microdomains have been detected by super-resolution microscopy, and there is evidence that they play roles in several biological processes. Here, we review current knowledge about the lipid and protein components of membrane microdomains. Furthermore, we summarize the dynamics of membrane microdomains in response to different stimuli. We also explore the biological functions associated with membrane microdomains as signal integration hubs. Finally, we outline challenges and questions for further studies.

The RALF1-FERONIA interaction modulates endocytosis to mediate control of root growth in Arabidopsis

The interaction between the receptor-like kinase (RLK) FERONIA (FER) and the secreted peptide Rapid Alkalinization Factor 1 (RALF1) is vital for development and stress responses in Arabidopsis. Ligand-induced membrane dynamics affect the function of several RLKs, but the effects of the RALF1-FER interaction on the dynamics of FER and the ensuing effects on its functionality are poorly understood. Here, we show that RALF1 modulated the dynamics and partitioning of FER-GFP at the plasma membrane (PM). Moreover, FER was internalized by both clathrin-mediated endocytosis (CME) and clathrin-independent endocytosis (CIE) under steady state conditions. After RALF1 treatment, FER-GFP internalization was primarily enhanced via the CME pathway, raising FER-GFP levels in the vacuole. RALF1 treatment also modulated trafficking of other PM proteins such as PIN2-GFP and BRI1-GFP, increasing their vacuolar levels by enhancing their internalization. Importantly, blocking CME attenuated RALF1-mediated root growth inhibition independently of RALF1-induced early signaling, suggesting that the RALF1 can also exert its effects via the CME pathway. These findings reveal that the RALF1-FER interaction modulates plant growth and development and this may also involve endocytosis of PM proteins.

Single-Particle Tracking for the Quantification of Membrane Protein Dynamics in Living Plant Cells

The plasma membrane is a sophisticated, organized, and highly heterogeneous structure that compartmentalizes cellular processes. To decipher the biological processes involving membrane proteins, it is necessary to analyze their spatiotemporal dynamics. However, it is difficult to directly assess the dynamics and interactions of biomolecules in living cells using traditional biochemical methods. Single-particle tracking (SPT) methods for imaging and tracking single particles conjugated with fluorescent probes offer an ideal approach to acquire valuable and complementary information about dynamic intracellular processes. SPT can be used to quantitatively monitor the diverse motions of individual particles in living cells. SPT also provides super-spatiotemporal resolution that allows early-stage or rapid response information to be obtained for a better understanding of molecular basis of associated signal transduction processes. More importantly, SPT can be used to detect the motion paths of individual biomolecules in vivo and in situ, thus unveiling the dynamic behavior of the biomolecules that support developmental processes in living cells. In this review, we give an overview of SPT methods, from image acquisition to the detection of single particles, as well as tracking and data analysis. We also discuss recent applications of SPT methods in the field of plant biology to reveal the complex biological functions of membrane proteins.

Sterols regulate endocytic pathways during flg22-induced defense responses in Arabidopsis

The plant transmembrane receptor kinase FLAGELLIN SENSING 2 (FLS2) is crucial for innate immunity. Although previous studies have reported FLS2-mediated signal transduction and endocytosis via the clathrin-mediated pathway, whether additional endocytic pathways affect FLS2-mediated defense responses remains unclear. Here, we show that the Arabidopsis thaliana sterol-deficient mutant steroid methyltransferase 1 displays defects in immune responses induced by the flagellin-derived peptide flg22. Variable-angle total internal reflection fluorescence microscopy (VA-TIRFM) coupled with single-particle tracking showed that the spatiotemporal dynamics of FLS2-GFP changed on a millisecond time scale and that the FLS2-GFP dwell time at the plasma membrane increased in cells treated with a sterol-extracting reagent when compared with untreated counterparts. We further demonstrate that flg22-induced FLS2 clustering and endocytosis involves the sterol-associated endocytic pathway, which is distinct from the clathrin-mediated pathway. Moreover, flg22 enhanced the colocalization of FLS2-GFP with the membrane microdomain marker Flot 1-mCherry and FLS2 endocytosis via the sterol-associated pathway. This indicates that plants may respond to pathogen attacks by regulating two different endocytic pathways. Taken together, our results suggest the key role of sterol homeostasis in flg22-induced plant defense responses.

Divide and Rule: Plant Plasma Membrane Organization

Since the publication of the fluid mosaic as a relevant model for biological membranes, accumulating evidence has revealed the outstanding complexity of the composition and organization of the plant plasma membrane (PM). Powerful new methodologies have uncovered the remarkable multiscale and multicomponent heterogeneity of PM subcompartmentalization, and this is emerging as a general trait with different features and properties. It is now evident that the dynamics of such a complex organization are intrinsically related to signaling pathways that regulate key physiological processes. Listing and linking recent progress in precisely qualifying these heterogeneities will help to draw an integrated picture of the plant PM. Understanding the key principles governing such a complex dynamic organization will contribute to deciphering the crucial role of the PM in cell physiology.