Stitching Organelles: Organization and Function of Specialized Membrane Contact Sites in Plants

Talk to your neighbors in an emergency: Stromule-mediated chloroplast-nucleus communication in plant immunity

Hypersensitive response-programmed cell death (HR-PCD) is a response mounted by plants to defend themselves against pathogens. Communication between the chloroplast and the nucleus is critical for the progression of HR-PCD. Tubular protrusions of chloroplasts, known as stromules, are tightly associated with the HR-PCD progression. There is emerging evidence that signaling molecules originating from chloroplasts are transferred to the nucleus through stromules. The translocation of signaling molecules from the chloroplast to the nucleus might trigger defense responses, including transcriptional reprogramming. In this review, we discuss the possible functions of stromules in the rapid transfer of signaling molecules in the chloroplast-nucleus communication.

Plastid Transient and Stable Interactions with Other Cell Compartments

Plastids are organelles delineated by two envelopes playing important roles in different cellular processes such as energy production or lipid biosynthesis. To regulate their biogenesis and their function, plastids have to communicate with other cellular compartments. This communication can be mediated by metabolites, signaling molecules, and by the establishment of direct contacts between the plastid envelope and other organelles such as the endoplasmic reticulum, mitochondria, peroxisomes, plasma membrane, and the nucleus. These interactions are highly dynamic and respond to different biotic and abiotic stresses. However, the mechanisms involved in the formation of plastid-organelle contact sites and their functions are still far from being understood. In this chapter, we summarize our current knowledge about plastid contact sites and their role in the regulation of plastid biogenesis and function.

The plasma membrane - cell wall nexus in plant cells: focus on the Hechtian structure

Across all kingdoms of life, cells secrete an extracellular polymer mesh that in turn feeds back onto them. This entails physical connections between the plasma membrane and the polymer mesh. In plant cells, one connection stands out: the Hechtian strand which, during plasmolysis, reflects the existence of a physical link between the plasma membrane of the retracting protoplast and the cell wall. The Hechtian strand is part of a larger structure, which we call the Hechtian structure, that comprises the Hechtian strand, the Hechtian reticulum and the Hechtian attachment sites. Although it has been observed for more than 100 years, its molecular composition and biological functions remain ill-described. A comprehensive characterization of the Hechtian structure is a critical step towards understanding this plasma membrane-cell wall connection and its relevance in cell signaling. This short review intends to highlight the main features of the Hechtian structure, in order to provide a clear framework for future research in this under-explored and promising field.

Fine Structure of Plasmodesmata-Associated Membrane Bodies Formed by Viral Movement Protein

Cell-to-cell transport of plant viruses through plasmodesmata (PD) requires viral movement proteins (MPs) often associated with cell membranes. The genome of the Hibiscus green spot virus encodes two MPs, BMB1 and BMB2, which enable virus cell-to-cell transport. BMB2 is known to localize to PD-associated membrane bodies (PAMBs), which are derived from the endoplasmic reticulum (ER) structures, and to direct BMB1 to PAMBs. This paper reports the fine structure of PAMBs. Immunogold labeling confirms the previously observed localization of BMB1 and BMB2 to PAMBs. EM tomography data show that the ER-derived structures in PAMBs are mostly cisterns interconnected by numerous intermembrane contacts that likely stabilize PAMBs. These contacts predominantly involve the rims of the cisterns rather than their flat surfaces. Using FRET-FLIM (Förster resonance energy transfer between fluorophores detected by fluorescence-lifetime imaging microscopy) and chemical cross-linking, BMB2 is shown to self-interact and form high-molecular-weight complexes. As BMB2 has been shown to have an affinity for highly curved membranes at cisternal rims, the interaction of BMB2 molecules located at rims of adjacent cisterns is suggested to be involved in the formation of intermembrane contacts in PAMBs.

Mind the gap: Methods to study membrane contact sites

Organelles are dynamic entities whose functions are essential for the optimum functioning of cells. It is now known that the juxtaposition of organellar membranes is essential for the exchange of metabolites and their communication. These functional apposition sites are termed membrane contact sites. Dynamic membrane contact sites between various sub-cellular structures such as mitochondria, endoplasmic reticulum, peroxisomes, Golgi apparatus, lysosomes, lipid droplets, plasma membrane, endosomes, etc. have been reported in various model systems. The burgeoning area of research on membrane contact sites has witnessed several manuscripts in recent years that identified the contact sites and components involved. Several methods have been developed to identify, measure and analyze the membrane contact sites. In this manuscript, we aim to discuss important methods developed to date that are used to study membrane contact sites.

Compartment-specific Ca2+ imaging in the green alga Chlamydomonas reinhardtii reveals high light-induced chloroplast Ca2+ signatures

To investigate the role of intracellular Ca2+ signaling in the perception and response mechanisms to light in unicellular microalgae, the genetically encoded ratiometric Ca2+ indicator Yellow Cameleon (YC3.6) was expressed in the model organism for green algae Chlamydomonas reinhardtii, targeted to cytosol, chloroplast, and mitochondria. Through in vivo single-cell confocal microscopy imaging, light-induced Ca2+ signaling was investigated in different conditions and different genotypes, including the photoreceptors mutants phot and acry. A genetically encoded H2 O2 sensor was also adopted to investigate the possible role of H2 O2 formation in light-dependent Ca2+ signaling. Light-dependent Ca2+ response was observed in Chlamydomonas reinhardtii cells only in the chloroplast as an organelle-autonomous response, influenced by light intensity and photosynthetic electron transport. The absence of blue and red-light photoreceptor aCRY strongly reduced the light-dependent chloroplast Ca2+ response, while the absence of the blue photoreceptor PHOT had no significant effects. A correlation between high light-induced chloroplast H2 O2 gradients and Ca2+ transients was drawn, supported by H2 O2 -induced chloroplast Ca2+ transients in the dark. In conclusion, different triggers are involved in the light-induced chloroplast Ca2+ signaling as saturation of the photosynthetic electron transport, H2 O2 formation, and aCRY-dependent light perception.

Peroxisomes undergo morphological changes in a light-dependent manner with proximity to the nucleus

The size and shape of organelles can influence the rate of biochemical reactions in cells. Previous studies have suggested that organelle morphology changes due to intra- and extracellular environmental responses, affecting the metabolic efficiency of and signal transduction emanating from neighboring organelles. In this study, we tested the possibility that intracellularly distributed organelles exhibit a heterogeneous response to intra- and extracellular environments. We detected a high correlation between peroxisome morphology and distance to the nucleus in light-exposed cells. Moreover, the proximity area between chloroplasts and peroxisomes varied with distance to the nucleus. These results indicate that peroxisome morphology varies with proximity to the nucleus, suggesting the presence of a nucleus-peroxisome signal transduction cascade mediated by chloroplasts.

Biological roles of plant synaptotagmins

Plant synaptotagmins (SYTs) are resident proteins of the endoplasmic reticulum (ER). They are characterized by an N-terminal transmembrane region and C2 domains at the C-terminus, which tether the ER to the plasma membrane (PM). In addition to their tethering role, SYTs contain a lipid-harboring SMP domain, essential for shuttling lipids between the ER and the PM. There is now abundant literature on Arabidopsis SYT1, the best-characterized family member, which link it to biotic and abiotic responses as well as to ER morphology. Here, we review the current knowledge of SYT members, focusing on their role in stress, and discuss how these roles can be related to their tethering and lipid transport functions. Finally, we contextualize this information about SYTs with their homologs, the yeast tricalbins and the mammalian extended synaptotagmins.

Reconstitution of membrane contact by unilamellar vesicles

Eukaryotic cells compartmentalize diverse biochemical functions within organelles defined by intracellular membranes. Recent focus has intensified on studying the interactions among organelles and the role of membrane contacts in maintaining cellular balance. While analyzing these contacts mainly involves fluorescence and electron microscopy, as well as biochemical cell fractionation, understanding their mechanisms and responses to genetic and environmental changes remains challenging. Here we describe an approach employing in vitro reconstitution of membrane contacts using unilamellar vesicles. This technique offers insights into contact mechanisms when combined with established methods like fluorescence imaging and mass spectrometry, potentially deepening our understanding of membrane contacts and organelle networks.

A tell tail sign: a conserved C-terminal tail-anchor domain targets a subset of pathogen effectors to the plant endoplasmic reticulum

The endoplasmic reticulum (ER) is the entry point to the secretory pathway and, as such, is critical for adaptive responses to biotic stress, when the demand for de novo synthesis of immunity-related proteins and signalling components increases significantly. Successful phytopathogens have evolved an arsenal of small effector proteins which collectively reconfigure multiple host components and signalling pathways to promote virulence; a small, but important, subset of which are targeted to the endomembrane system including the ER. We identified and validated a conserved C-terminal tail-anchor motif in a set of pathogen effectors known to localize to the ER from the oomycetes Hyaloperonospora arabidopsidis and Plasmopara halstedii (downy mildew of Arabidopsis and sunflower, respectively) and used this protein topology to develop a bioinformatic pipeline to identify putative ER-localized effectors within the effectorome of the related oomycete, Phytophthora infestans, the causal agent of potato late blight. Many of the identified P. infestans tail-anchor effectors converged on ER-localized NAC transcription factors, indicating that this family is a critical host target for multiple pathogens.

Keep in contact: multiple roles of endoplasmic reticulum-membrane contact sites and the organelle interaction network in plants

Functional regulation and structural maintenance of the different organelles in plants contribute directly to plant development, reproduction and stress responses. To ensure these activities take place effectively, cells have evolved an interconnected network amongst various subcellular compartments, regulating rapid signal transduction and the exchange of biomaterial. Many proteins that regulate membrane connections have recently been identified in plants, and this is the first step in elucidating both the mechanism and function of these connections. Amongst all organelles, the endoplasmic reticulum is the key structure, which likely links most of the different subcellular compartments through membrane contact sites (MCS) and the ER-PM contact sites (EPCS) have been the most intensely studied in plants. However, the molecular composition and function of plant MCS are being found to be different from other eukaryotic systems. In this article, we will summarise the most recent advances in this field and discuss the mechanism and biological relevance of these essential links in plants.

Three-dimensional nanoscale analysis of light-dependent organelle changes in Arabidopsis mesophyll cells

Different organelles function coordinately in numerous intracellular processes. Photorespiration incidental to photosynthetic carbon fixation is organized across three subcellular compartments: chloroplasts, peroxisomes, and mitochondria. Under light conditions, these three organelles often form a ternary organellar complex in close proximity, suggesting a connection with metabolism during photorespiration. However, due to the heterogeneity of intercellular organelle localization and morphology, organelles' responses to changes in the external environment remain poorly understood. Here, we used array tomography by field emission scanning electron microscopy to image organelles inside the whole plant cell at nanometer resolution, generating a three-dimensional (3D) spatial map of the light-dependent positioning of chloroplasts, peroxisomes, nuclei, and vacuoles. Our results show, in light-treated cells, the volume of peroxisomes increased, and mitochondria were simplified. In addition, the population of free organelles decreased, and the ternary complex centered on chloroplasts increased. Moreover, our results emphasized the expansion of the proximity area rather than the increase in the number of proximity sites interorganelles. All of these phenomena were quantified for the first time on the basis of nanoscale spatial maps. In summary, we provide the first 3D reconstruction of Arabidopsis mesophyll cells, together with nanoscale quantified organelle morphology and their positioning via proximity areas, and then evidence of their light-dependent changes.

Distinct Mechanisms of Endomembrane Reorganization Determine Dissimilar Transport Pathways in Plant RNA Viruses

Plant viruses exploit the endomembrane system of infected cells for their replication and cell-to-cell transport. The replication of viral RNA genomes occurs in the cytoplasm in association with reorganized endomembrane compartments induced by virus-encoded proteins and is coupled with the virus intercellular transport via plasmodesmata that connect neighboring cells in plant tissues. The transport of virus genomes to and through plasmodesmata requires virus-encoded movement proteins (MPs). Distantly related plant viruses encode different MP sets, or virus transport systems, which vary in the number of MPs and their properties, suggesting their functional differences. Here, we discuss two distinct virus transport pathways based on either the modification of the endoplasmic reticulum tubules or the formation of motile vesicles detached from the endoplasmic reticulum and targeted to endosomes. The viruses with the movement proteins encoded by the triple gene block exemplify the first, and the potyviral system is the example of the second type. These transport systems use unrelated mechanisms of endomembrane reorganization. We emphasize that the mode of virus interaction with cell endomembranes determines the mechanism of plant virus cell-to-cell transport.

Intertwined Roles of Reactive Oxygen Species and Salicylic Acid Signaling Are Crucial for the Plant Response to Biotic Stress

One of the earliest hallmarks of plant immune response is production of reactive oxygen species (ROS) in different subcellular compartments, which regulate plant immunity. A suitable equilibrium, which is crucial to prevent ROS overaccumulation leading to oxidative stress, is maintained by salicylic acid (SA), a chief regulator of ROS. However, ROS not only act downstream of SA signaling, but are also proposed to be a central component of a self-amplifying loop that regulates SA signaling as well as the interaction balance between different phytohormones. The exact role of this crosstalk, the position where SA interferes with ROS signaling and ROS interferes with SA signaling and the outcome of this regulation, depend on the origin of ROS but also on the pathosystem. The precise spatiotemporal regulation of organelle-specific ROS and SA levels determine the effectiveness of pathogen arrest and is therefore crucial for a successful immune response. However, the regulatory interplay behind still remains poorly understood, as up until now, the role of organelle-specific ROS and SA in hypersensitive response (HR)-conferred resistance has mostly been studied by altering the level of a single component. In order to address these aspects, a sophisticated combination of research methods for monitoring the spatiotemporal dynamics of key players and transcriptional activity in plants is needed and will most probably consist of biosensors and precision transcriptomics.

A glossary of plant cell structures: Current insights and future questions

In this glossary of plant cell structures, we asked experts to summarize a present-day view of plant organelles and structures, including a discussion of outstanding questions. In the following short reviews, the authors discuss the complexities of the plant cell endomembrane system, exciting connections between organelles, novel insights into peroxisome structure and function, dynamics of mitochondria, and the mysteries that need to be unlocked from the plant cell wall. These discussions are focused through a lens of new microscopy techniques. Advanced imaging has uncovered unexpected shapes, dynamics, and intricate membrane formations. With a continued focus in the next decade, these imaging modalities coupled with functional studies are sure to begin to unravel mysteries of the plant cell.

Trophic state alters the mechanism whereby energetic coupling between photosynthesis and respiration occurs in Euglena gracilis

The coupling between mitochondrial respiration and photosynthesis plays an important role in the energetic physiology of green plants and some secondary-red photosynthetic eukaryotes (diatoms), allowing an efficient CO₂ assimilation and optimal growth. Using the flagellate Euglena gracilis, we first tested if photosynthesis-respiration coupling occurs in this species harbouring secondary green plastids (i.e. originated from an endosymbiosis between a green alga and a phagotrophic euglenozoan). Second, we tested how the trophic state (mixotrophy and photoautotrophy) of the cell alters the mechanisms involved in the photosynthesis-respiration coupling. Energetic coupling between photosynthesis and respiration was determined by testing the effect of respiratory inhibitors on photosynthesis, and measuring the simultaneous variation of photosynthesis and respiration rates as a function of temperature (i.e. thermal response curves). The mechanism involved in the photosynthesis-respiration coupling was assessed by combining proteomics, biophysical and cytological analyses. Our work shows that there is photosynthesis-respiration coupling and membrane contacts between mitochondria and chloroplasts in E. gracilis. However, whereas in mixotrophy adjustment of the chloroplast ATP/NADPH ratio drives the interaction, in photoautotrophy the coupling is conditioned by CO₂ limitation and photorespiration. This indicates that maintenance of photosynthesis-respiration coupling, through plastic metabolic responses, is key to E. gracilis functioning under changing environmental conditions.

The structure and flexibility analysis of the Arabidopsis synaptotagmin 1 reveal the basis of its regulation at membrane contact sites

Non-vesicular lipid transfer at ER and plasma membrane (PM) contact sites (CS) is crucial for the maintenance of membrane lipid homeostasis. Extended synaptotagmins (E-Syts) play a central role in this process as they act as molecular tethers of ER and PM and as lipid transfer proteins between these organelles. E-Syts are proteins constitutively anchored to the ER through an N-terminal hydrophobic segment and bind the PM via a variable number of C-terminal C2 domains. Synaptotagmins (SYTs) are the plant orthologous of E-Syts and regulate the ER-PM communication in response to abiotic stress. Combining different structural and biochemical techniques, we demonstrate that the binding of SYT1 to lipids occurs through a Ca2+-dependent lipid-binding site and by a site for phosphorylated forms of phosphatidylinositol, thus integrating two different molecular signals in response to stress. In addition, we show that SYT1 displays three highly flexible hinge points that provide conformational freedom to facilitate lipid extraction, protein loading, and subsequent transfer between PM and ER.

NbSOBIR1 Partitions Into Plasma Membrane Microdomains and Binds ER-Localized NbRLP1

The receptor-like kinase Suppressor of BIR1 (SOBIR1) binds various receptor-like proteins (RLPs) that perceive microbe-associated molecular patterns (MAMPs) at the plasma membrane, which is thought to activate plant pattern-triggered immunity (PTI) against pathogen invasion. Despite its potentially crucial role, how SOBIR1 transmits immune signaling to ultimately elicit PTI remains largely unresolved. Herein, we report that a Nicotiana benthamiana gene NbRLP1, like NbSOBIR1, was highly induced upon Phytophthora parasitica infection. Intriguingly, NbRLP1 is characterized as a receptor-like protein localizing to the endoplasmic reticulum (ER) membrane rather than the plasma membrane. Using bimolecular fluorescence complementation and affinity purification assays, we established that NbRLP1 is likely to associate with NbSOBIR1 through the contact between the ER and plasma membrane. We further found that NbSOBIR1 at the plasma membrane partitions into mobile microdomains that undergo frequent lateral movement and internalization. Remarkably, the dynamics of NbSOBIR1 microdomain is coupled to the remodeling of the cortical ER network. When NbSOBIR1 microdomains were induced by the P. parasitica MAMP ParA1, tobacco cells overexpressing NbRLP1 accelerated NbSOBIR1 internalization. Overexpressing NbRLP1 in tobacco further exaggerated the ParA1-induced necrosis. Together, these findings have prompted us to propose that ER and the ER-localized NbRLP1 may play a role in transmitting plant immune signals by regulating NbSOBIR1 internalization.

Synaptotagmins at the endoplasmic reticulum-plasma membrane contact sites maintain diacylglycerol homeostasis during abiotic stress

Endoplasmic reticulum-plasma membrane contact sites (ER-PM CS) play fundamental roles in all eukaryotic cells. Arabidopsis thaliana mutants lacking the ER-PM protein tether synaptotagmin1 (SYT1) exhibit decreased PM integrity under multiple abiotic stresses, such as freezing, high salt, osmotic stress, and mechanical damage. Here, we show that, together with SYT1, the stress-induced SYT3 is an ER-PM tether that also functions in maintaining PM integrity. The ER-PM CS localization of SYT1 and SYT3 is dependent on PM phosphatidylinositol-4-phosphate and is regulated by abiotic stress. Lipidomic analysis revealed that cold stress increased the accumulation of diacylglycerol at the PM in a syt1/3 double mutant relative to wild-type while the levels of most glycerolipid species remain unchanged. In addition, the SYT1-green fluorescent protein fusion preferentially binds diacylglycerol in vivo with little affinity for polar glycerolipids. Our work uncovers a SYT-dependent mechanism of stress adaptation counteracting the detrimental accumulation of diacylglycerol at the PM produced during episodes of abiotic stress.

DeepLearnMOR: a deep-learning framework for fluorescence image-based classification of organelle morphology

The proper biogenesis, morphogenesis, and dynamics of subcellular organelles are essential to their metabolic functions. Conventional techniques for identifying, classifying, and quantifying abnormalities in organelle morphology are largely manual and time-consuming, and require specific expertise. Deep learning has the potential to revolutionize image-based screens by greatly improving their scope, speed, and efficiency. Here, we used transfer learning and a convolutional neural network (CNN) to analyze over 47,000 confocal microscopy images from Arabidopsis wild-type and mutant plants with abnormal division of one of three essential energy organelles: chloroplasts, mitochondria, or peroxisomes. We have built a deep-learning framework, DeepLearnMOR (Deep Learning of the Morphology of Organelles), which can rapidly classify image categories and identify abnormalities in organelle morphology with over 97% accuracy. Feature visualization analysis identified important features used by the CNN to predict morphological abnormalities, and visual clues helped to better understand the decision-making process, thereby validating the reliability and interpretability of the neural network. This framework establishes a foundation for future larger-scale research with broader scopes and greater data set diversity and heterogeneity.

Live monitoring of plant redox and energy physiology with genetically encoded biosensors

Genetically encoded biosensors pave the way for understanding plant redox dynamics and energy metabolism on cellular and subcellular levels.

Chloroplast-derived photo-oxidative stress causes changes in H2O2 and EGSH in other subcellular compartments

Metabolic fluctuations in chloroplasts and mitochondria can trigger retrograde signals to modify nuclear gene expression. Mobile signals likely to be involved are reactive oxygen species (ROS), which can operate protein redox switches by oxidation of specific cysteine residues. Redox buffers, such as the highly reduced glutathione pool, serve as reservoirs of reducing power for several ROS-scavenging and ROS-induced damage repair pathways. Formation of glutathione disulfide and a shift of the glutathione redox potential (EGSH) toward less negative values is considered as hallmark of several stress conditions. Here we used the herbicide methyl viologen (MV) to generate ROS locally in chloroplasts of intact Arabidopsis (Arabidopsis thaliana) seedlings and recorded dynamic changes in EGSH and H2O2 levels with the genetically encoded biosensors Grx1-roGFP2 (for EGSH) and roGFP2-Orp1 (for H2O2) targeted to chloroplasts, the cytosol, or mitochondria. Treatment of seedlings with MV caused rapid oxidation in chloroplasts and, subsequently, in the cytosol and mitochondria. MV-induced oxidation was significantly boosted by illumination with actinic light, and largely abolished by inhibitors of photosynthetic electron transport. MV also induced autonomous oxidation in the mitochondrial matrix in an electron transport chain activity-dependent manner that was milder than the oxidation triggered in chloroplasts by the combination of MV and light. In vivo redox biosensing resolves the spatiotemporal dynamics of compartmental responses to local ROS generation and provides a basis for understanding how compartment-specific redox dynamics might operate in retrograde signaling and stress acclimation in plants.

Geometry and cellular function of organelle membrane interfaces

A vast majority of cellular processes take root at the surface of biological membranes. By providing a two-dimensional platform with limited diffusion, membranes are, by nature, perfect devices to concentrate signaling and metabolic components. As such, membranes often act as "key processors" of cellular information. Biological membranes are highly dynamic and deformable and can be shaped into curved, tubular, or flat conformations, resulting in differentiated biophysical properties. At membrane contact sites, membranes from adjacent organelles come together into a unique 3D configuration, forming functionally distinct microdomains, which facilitate spatially regulated functions, such as organelle communication. Here, we describe the diversity of geometries of contact site-forming membranes in different eukaryotic organisms and explore the emerging notion that their shape, 3D architecture, and remodeling jointly define their cellular activity. The review also provides selected examples highlighting changes in membrane contact site architecture acting as rapid and local responses to cellular perturbations, and summarizes our current understanding of how those structural changes confer functional specificity to those cellular territories.

Attracted to membranes: lipid-binding domains in plants

Membranes are essential for cells and organelles to function. As membranes are impermeable to most polar and charged molecules, they provide electrochemical energy to transport molecules across and create compartmentalized microenvironments for specific enzymatic and cellular processes. Membranes are also responsible for guided transport of cargoes between organelles and during endo- and exocytosis. In addition, membranes play key roles in cell signaling by hosting receptors and signal transducers and as substrates and products of lipid second messengers. Anionic lipids and their specific interaction with target proteins play an essential role in these processes, which are facilitated by specific lipid-binding domains. Protein crystallography, lipid-binding studies, subcellular localization analyses, and computer modeling have greatly advanced our knowledge over the years of how these domains achieve precision binding and what their function is in signaling and membrane trafficking, as well as in plant development and stress acclimation.

Review: Membrane tethers control plasmodesmal function and formation

Cell-to-cell communication is crucial in coordinating diverse biological processes in multicellular organisms. In plants, communication between adjacent cells occurs via nanotubular passages called plasmodesmata (PD). The PD passage is composed of an appressed endoplasmic reticulum (ER) internally, and plasma membrane (PM) externally, that traverses the cell wall, and associates with the actin-cytoskeleton. The coordination of the ER, PM and cytoskeleton plays a potential role in maintaining the architecture and conductivity of PD. Many data suggest that PD-associated proteins can serve as tethers that connect these structures in a functional PD, to regulate cell-to-cell communication. In this review, we summarize the organization and regulation of PD activity via tethering proteins, and discuss the importance of PD-mediated cell-to-cell communication in plant development and defense against environmental stress.

Plastid Genomes of Flowering Plants: Essential Principles

The plastid genome (plastome ) has proved a valuable source of data for evaluating evolutionary relationships among angiosperms. Through basic and applied approaches, plastid transformation technology offers the potential to understand and improve plant productivity, providing food, fiber, energy, and medicines to meet the needs of a burgeoning global population. The growing genomic resources available to both phylogenetic and biotechnological investigations is allowing novel insights and expanding the scope of plastome research to encompass new species. In this chapter, we present an overview of some of the seminal and contemporary research that has contributed to our current understanding of plastome evolution and attempt to highlight the relationship between evolutionary mechanisms and the tools of plastid genetic engineering.

Reticulon-like properties of a plant virus-encoded movement protein

Plant viruses encode movement proteins (MPs) that ensure the transport of viral genomes through plasmodesmata (PD) and use cell endomembranes, mostly the endoplasmic reticulum (ER), for delivery of viral genomes to PD and formation of PD-anchored virus replication compartments. Here, we demonstrate that the Hibiscus green spot virus BMB2 MP, an integral ER protein, induces constrictions of ER tubules, decreases the mobility of ER luminal content, and exhibits an affinity to highly curved membranes. These properties are similar to those described for reticulons, cellular proteins that induce membrane curvature to shape the ER tubules. Similar to reticulons, BMB2 adopts a W-like topology within the ER membrane. BMB2 targets PD and increases their size exclusion limit, and these BMB2 activities correlate with the ability to induce constrictions of ER tubules. We propose that the induction of ER constrictions contributes to the BMB2-dependent increase in PD permeability and formation of the PD-associated replication compartments, therefore facilitating the virus intercellular spread. Furthermore, we show that the ER tubule constrictions also occur in cells expressing TGB2, one of the three MPs of Potato virus X (PVX), and in PVX-infected cells, suggesting that reticulon-like MPs are employed by diverse RNA viruses.

Plant endomembranes and cytoskeleton: moving targets in immunity

Pathogens attack plant cells to divert resources toward pathogen proliferation. To resist pathogens, plant cells rely on multilayered signaling pathways that hinge upon the secretory pathway for the synthesis and trafficking of pathogen sensors and defense molecules. In recent years, significant strides have been made in the understanding of the functional relationship between pathogen response and membrane traffic. Here we discuss how the plant cytoskeleton and endomembranes are targeted by pathogen effectors and highlight an emerging role of membrane contact sites in biotic stress responses.

A central role of the endoplasmic reticulum in the cell emerges from its functional contact sites with multiple organelles

Early eukaryotic cells emerged from the compartmentalization of metabolic processes into specific organelles through the development of an endomembrane system (ES), a precursor of the endoplasmic reticulum (ER), which was essential for their survival. Recently, substantial evidence emerged on how organelles communicate among themselves and with the plasma membrane (PM) through contact sites (CSs). From these studies, the ER-the largest single structure in eukaryotic cells-emerges as a central player communicating with all organelles to coordinate cell functions and respond to external stimuli to maintain cellular homeostasis. Herein we review the functional insights into the ER-CSs with other organelles in a physiological perspective. We hypothesize that, in addition to the primitive role by the ES in the appearance of proto-eukaryotes, its successor-the ER-emerges as the key coordinator of inter-organelle/PM communication. The ER thus appears to be the 'maestro' driving eukaryotic cell evolution by incorporating new functions/organelles, while remaining the real coordinator overarching cellular functions and orchestrating them with the external milieu.

Plasma membrane aquaporins interact with the endoplasmic reticulum resident VAP27 proteins at ER-PM contact sites and endocytic structures

Plasma membrane (PM) intrinsic proteins (PIPs) are aquaporins facilitating the diffusion of water and small solutes. The functional importance of the PM organisation of PIPs in the interaction with other cellular structures is not completely understood. We performed a pull-down assay using maize (Zea mays) suspension cells expressing YFP-ZmPIP2;5 and validated the protein interactions by yeast split-ubiquitin and bimolecular fluorescence complementation assays. We expressed interacting proteins tagged with fluorescent proteins in Nicotiana benthamiana leaves and performed water transport assays in oocytes. Finally, a phylogenetic analysis was conducted. The PM-located ZmPIP2;5 physically interacts with the endoplasmic reticulum (ER) resident ZmVAP27-1. This interaction requires the ZmVAP27-1 cytoplasmic major sperm domain. ZmPIP2;5 and ZmVAP27-1 localise in close vicinity in ER-PM contact sites (EPCSs) and endocytic structures upon exposure to salt stress conditions. This interaction enhances PM water permeability in oocytes. Similarly, the Arabidopsis ZmVAP27-1 paralogue, AtVAP27-1, interacts with the AtPIP2;7 aquaporin. Together, these data indicate that the PIP2-VAP27 interaction in EPCSs is evolutionarily conserved, and suggest that VAP27 might stabilise the aquaporins and guide their endocytosis in response to salt stress.

Optical Forces: From Fundamental to Biological Applications

Optical forces, generally arising from changes of field gradients or linear momentum carried by photons, form the basis for optical trapping and manipulation. Advances in optical forces help to reveal the nature of light-matter interactions, giving answers to a wide range of questions and solving problems across various disciplines, and are still yielding new insights in many exciting sciences, particularly in the fields of biological technology, material applications, and quantum sciences. This review focuses on recent advances in optical forces, ranging from fundamentals to applications for biological exploration. First, the basics of different types of optical forces with new light-matter interaction mechanisms and near-field techniques for optical force generation beyond the diffraction limit with nanometer accuracy are described. Optical forces for biological applications from in vitro to in vivo are then reviewed. Applications from individual manipulation to multiple assembly into functional biophotonic probes and soft-matter superstructures are discussed. At the end future directions for application of optical forces for biological exploration are provided.

Sticking With It: ER-PM Membrane Contact Sites as a Coordinating Nexus for Regulating Lipids and Proteins at the Cell Cortex

Membrane contact sites between the cortical endoplasmic reticulum (ER) and the plasma membrane (PM) provide a direct conduit for small molecule transfer and signaling between the two largest membranes of the cell. Contact is established through ER integral membrane proteins that physically tether the two membranes together, though the general mechanism is remarkably non-specific given the diversity of different tethering proteins. Primary tethers including VAMP-associated proteins (VAPs), Anoctamin/TMEM16/Ist2p homologs, and extended synaptotagmins (E-Syts), are largely conserved in most eukaryotes and are both necessary and sufficient for establishing ER-PM association. In addition, other species-specific ER-PM tether proteins impart unique functional attributes to both membranes at the cell cortex. This review distils recent functional and structural findings about conserved and species-specific tethers that form ER-PM contact sites, with an emphasis on their roles in the coordinate regulation of lipid metabolism, cellular structure, and responses to membrane stress.

Rare earth elements induce cytoskeleton-dependent and PI4P-associated rearrangement of SYT1/SYT5 endoplasmic reticulum-plasma membrane contact site complexes in Arabidopsis

In plant cells, environmental stressors promote changes in connectivity between the cortical endoplasmic reticulum (ER) and the plasma membrane (PM). Although this process is tightly regulated in space and time, the molecular signals and structural components mediating these changes in interorganelle communication are only starting to be characterized. In this report, we confirm the presence of a putative tethering complex containing the synaptotagmins 1 and 5 (SYT1 and SYT5) and the Ca2+- and lipid-binding protein 1 (CLB1/SYT7). This complex is enriched at ER-PM contact sites (EPCSs), has slow responses to changes in extracellular Ca2+, and displays severe cytoskeleton-dependent rearrangements in response to the trivalent lanthanum (La3+) and gadolinium (Gd3+) rare earth elements (REEs). Although REEs are generally used as non-selective cation channel blockers at the PM, here we show that the slow internalization of REEs into the cytosol underlies the activation of the Ca2+/calmodulin intracellular signaling, the accumulation of phosphatidylinositol-4-phosphate (PI4P) at the PM, and the cytoskeleton-dependent rearrangement of the SYT1/SYT5 EPCS complexes. We propose that the observed EPCS rearrangements act as a slow adaptive response to sustained stress conditions, and that this process involves the accumulation of stress-specific phosphoinositide species at the PM.

The pigment binding behaviour of water-soluble chlorophyll protein (WSCP)

Water-soluble chlorophyll proteins (WSCPs) are homotetrameric proteins that bind four chlorophyll (Chl) molecules in identical binding sites, which makes WSCPs a good model to study protein-pigment interactions. In a previous study, we described preferential binding of Chl a or Chl b in various WSCP versions. Chl b binding is preferred when a hydrogen bond can be formed between the C7 formyl of the chlorin macrocycle and the protein, whereas Chl a is preferred when Chl b binding is sterically unfavorable. Here, we determined the binding affinities and kinetics of various WSCP versions not only for Chl a/b, but also for chlorophyllide (Chlide) a/b and pheophytin (Pheo) a/b. Altered KD values are responsible for the Chl a/b selectivity in WSCP whereas differences in the reaction kinetics are neglectable in explaining different Chl a/b preferences. WSCP binds both Chlide and Pheo with a lower affinity than Chl, which indicates the importance of the phytol chain and the central Mg2+ ion as interaction sites between WSCP and pigment. Pheophorbide (Pheoide), lacking both the phytol chain and the central Mg2+ ion, can only be bound as Pheoide b to a WSCP that has a higher affinity for Chl b than Chl a, which underlines the impact of the C7 formyl-protein interaction. Moreover, WSCP was able to bind protochlorophyllide and Mg-protoporphyrin IX, which suggests that neither the size of the π electron system of the macrocycle nor the presence of a fifth ring at the macrocycle notably affect the binding to WSCP. WSCP also binds heme to form a tetrameric complex, suggesting that heme is bound in the Chl-binding site.

It Started With a Kiss: Monitoring Organelle Interactions and Identifying Membrane Contact Site Components in Plants

Organelle movement and interaction are dynamic processes. Interpreting the functional role and mechanistic detail of interactions at membrane contact sites requires careful quantification of parameters such as duration, frequency, proximity, and surface area of contact, and identification of molecular components. We provide an overview of current methods used to quantify organelle interactions in plants and other organisms and propose novel applications of existing technologies to tackle this emerging topic in plant cell biology.

Structural and functional relationships between plasmodesmata and plant endoplasmic reticulum-plasma membrane contact sites consisting of three synaptotagmins

Synaptotagmin 1 (SYT1) has been recognised as a tethering factor of plant endoplasmic reticulum (ER)-plasma membrane (PM) contact sites (EPCSs) and partially localises to around plasmodesmata (PD). However, other components of EPCSs associated with SYT1 and functional links between the EPCSs and PD have not been identified. We explored interactors of SYT1 by immunoprecipitation and mass analysis. The dynamics, morphology and spatial arrangement of the ER in Arabidopsis mutants lacking the EPCS components were investigated using confocal microscopy and electron microscopy. PD permeability of EPCS mutants was assessed using a virus movement protein and free green fluorescent protein (GFP) as indicators. We identified two additional components of the EPCSs, SYT5 and SYT7, that interact with SYT1. The mutants of the three SYTs were defective in the anchoring of the ER to the PM. The ER near the PD entrance appeared to be weakly squeezed in the triple mutant compared with the wild-type. The triple mutant suppressed cell-to-cell movement of the virus movement protein, but not GFP diffusion. We revealed major additional components of EPCS associated with SYT1 and suggested that the EPCSs arranged around the PD squeeze the ER to regulate active transport via PD.

Stress signalling dynamics of the mitochondrial electron transport chain and oxidative phosphorylation system in higher plants

BACKGROUND

Mitochondria play a diversity of physiological and metabolic roles under conditions of abiotic or biotic stress. They may be directly subjected to physico-chemical constraints, and they are also involved in integrative responses to environmental stresses through their central position in cell nutrition, respiration, energy balance and biosyntheses. In plant cells, mitochondria present various biochemical peculiarities, such as cyanide-insensitive alternative respiration, and, besides integration with ubiquitous eukaryotic compartments, their functioning must be coupled with plastid functioning. Moreover, given the sessile lifestyle of plants, their relative lack of protective barriers and present threats of climate change, the plant cell is an attractive model to understand the mechanisms of stress/organelle/cell integration in the context of environmental stress responses.

SCOPE

The involvement of mitochondria in this integration entails a complex network of signalling, which has not been fully elucidated, because of the great diversity of mitochondrial constituents (metabolites, reactive molecular species and structural and regulatory biomolecules) that are linked to stress signalling pathways. The present review analyses the complexity of stress signalling connexions that are related to the mitochondrial electron transport chain and oxidative phosphorylation system, and how they can be involved in stress perception and transduction, signal amplification or cell stress response modulation.

CONCLUSIONS

Plant mitochondria are endowed with a diversity of multi-directional hubs of stress signalling that lead to regulatory loops and regulatory rheostats, whose functioning can amplify and diversify some signals or, conversely, dampen and reduce other signals. Involvement in a wide range of abiotic and biotic responses also implies that mitochondrial stress signalling could result in synergistic or conflicting outcomes during acclimation to multiple and complex stresses, such as those arising from climate change.

Microstructure of floral nectaries in Robinia viscosa var. hartwigii (Papilionoideae, Fabaceae)-a valuable but little-known melliferous plant

Floral nectaries are important components of floral architecture and significant taxonomic traits facilitating assessment of relationships between taxa and can contribute substantially to studies on the ecology and evolution of a particular genus. Knowledge of nectary structure and functioning allows better understanding of the mutualistic interactions between the pollinator and the plant. Robinia viscosa var. hartwigii (Hartweg's locust), planted in many European countries as an ornamental plant and used for recovery of degraded areas and urban arborisation, is a valuable melliferous species often visited by honeybees and bumblebees. The aim of this study was to investigate the microstructure of the floral nectaries of R. viscosa var. hartwigii with the use of light, fluorescence, scanning, and transmission electron microscopes. The photosynthetic nectaries were located on the inner surface of the cup-like receptacle. The components of pre-nectar were synthesised in the chloroplasts of the glandular parenchyma and transported via the conducting elements of the phloem. Nectar was released through modified nectarostomata. Nectar secretion presumably proceeded in the eccrine mode, whereas nectar transport represented the symplastic and apoplastic types. The cuticle on the nectary epidermis surface contained lipids, essentials oils, and flavonoids, while proteins and flavonoids were present in the glandular parenchyma cells. Idioblasts containing phenolic compounds, tannins, and polysaccharides were observed between the glandular parenchyma cells. The location of the nectaries and the mode of nectar production in the flowers of the Hartweg's locust follow the common location and structure pattern characteristic for the nectaries in some members of the subfamily Papilionoideae and can be a significant taxonomic trait for the genus Robinia and the tribe Robinieae.

Remorin interacting with PCaP1 impairs Turnip mosaic virus intercellular movement but is antagonised by VPg

Group 1 Remorins (REMs) are extensively involved in virus trafficking through plasmodesmata (PD). However, their roles in Potyvirus cell-to-cell movement are not known. The plasma membrane (PM)-associated Ca²⁺ binding protein 1 (PCaP1) interacts with the P3N-PIPO of Turnip mosaic virus (TuMV) and is required for TuMV cell-to-cell movement, but the underlying mechanism remains elusive. The mutant plants with overexpression or knockout of REM1.2 were used to investigate its role in TuMV cell-to-cell movement. Arabidopsis thaliana complementary mutants of pcap1 were used to investigate the role of PCaP1 in TuMV cell-to-cell movement. Yeast-two-hybrid, bimolecular fluorescence complementation, co-immunoprecipitation and RT-qPCR assays were employed to investigate the underlying molecular mechanism. The results show that TuMV-P3N-PIPO recruits PCaP1 to PD and the actin filament-severing activity of PCaP1 is required for TuMV intercellular movement. REM1.2 negatively regulates the cell-to-cell movement of TuMV via competition with PCaP1 for binding actin filaments. As a counteractive response, TuMV mediates REM1.2 degradation via both 26S ubiquitin-proteasome and autophagy pathways through the interaction of VPg with REM1.2 to establish systemic infection in Arabidopsis. This work unveils the actin cytoskeleton and PM nanodomain-associated molecular events underlying the cell-to-cell movement of potyviruses.

On the move: redox-dependent protein relocation in plants

Compartmentation of proteins and processes is a defining feature of eukaryotic cells. The growth and development of organisms is critically dependent on the accurate sorting of proteins within cells. The mechanisms by which cytosol-synthesized proteins are delivered to the membranes and membrane compartments have been extensively characterized. However, the protein complement of any given compartment is not precisely fixed and some proteins can move between compartments in response to metabolic or environmental triggers. The mechanisms and processes that mediate such relocation events are largely uncharacterized. Many proteins can in addition perform multiple functions, catalysing alternative reactions or performing structural, non-enzymatic functions. These alternative functions can be equally important functions in each cellular compartment. Such proteins are generally not dual-targeted proteins in the classic sense of having targeting sequences that direct de novo synthesized proteins to specific cellular locations. We propose that redox post-translational modifications (PTMs) can control the compartmentation of many such proteins, including antioxidant and/or redox-associated enzymes.

The functional universe of membrane contact sites

Organelles compartmentalize eukaryotic cells, enhancing their ability to respond to environmental and developmental changes. One way in which organelles communicate and integrate their activities is by forming close contacts, often called 'membrane contact sites' (MCSs). Interest in MCSs has grown dramatically in the past decade as it is has become clear that they are ubiquitous and have a much broader range of critical roles in cells than was initially thought. Indeed, functions for MCSs in intracellular signalling (particularly calcium signalling, reactive oxygen species signalling and lipid signalling), autophagy, lipid metabolism, membrane dynamics, cellular stress responses and organelle trafficking and biogenesis have now been reported.

Tricalbin-Mediated Contact Sites Control ER Curvature to Maintain Plasma Membrane Integrity

Membrane contact sites (MCS) between the endoplasmic reticulum (ER) and the plasma membrane (PM) play fundamental roles in all eukaryotic cells. ER-PM MCS are particularly abundant in Saccharomyces cerevisiae, where approximately half of the PM surface is covered by cortical ER (cER). Several proteins, including Ist2, Scs2/22, and Tcb1/2/3 are implicated in cER formation, but the specific roles of these molecules are poorly understood. Here, we use cryo-electron tomography to show that ER-PM tethers are key determinants of cER morphology. Notably, Tcb proteins (tricalbins) form peaks of extreme curvature on the cER membrane facing the PM. Combined modeling and functional assays suggest that Tcb-mediated cER peaks facilitate the transport of lipids between the cER and the PM, which is necessary to maintain PM integrity under heat stress. ER peaks were also present at other MCS, implying that membrane curvature enforcement may be a widespread mechanism to regulate MCS function.

Plant AtEH/Pan1 proteins drive autophagosome formation at ER-PM contact sites with actin and endocytic machinery

The Arabidopsis EH proteins (AtEH1/Pan1 and AtEH2/Pan1) are components of the endocytic TPLATE complex (TPC) which is essential for endocytosis. Both proteins are homologues of the yeast ARP2/3 complex activator, Pan1p. Here, we show that these proteins are also involved in actin cytoskeleton regulated autophagy. Both AtEH/Pan1 proteins localise to the plasma membrane and autophagosomes. Upon induction of autophagy, AtEH/Pan1 proteins recruit TPC and AP-2 subunits, clathrin, actin and ARP2/3 proteins to autophagosomes. Increased expression of AtEH/Pan1 proteins boosts autophagosome formation, suggesting independent and redundant pathways for actin-mediated autophagy in plants. Moreover, AtEHs/Pan1-regulated autophagosomes associate with ER-PM contact sites (EPCS) where AtEH1/Pan1 interacts with VAP27-1. Knock-down expression of either AtEH1/Pan1 or VAP27-1 makes plants more susceptible to nutrient depleted conditions, indicating that the autophagy pathway is perturbed. In conclusion, we identify the existence of an autophagy-dependent pathway in plants to degrade endocytic components, starting at the EPCS through the interaction among AtEH/Pan1, actin cytoskeleton and the EPCS resident protein VAP27-1.

Arabidopsis EH/Pan1 proteins are part of the TPLATE complex (TPC) that is required for endocytosis in plants. Here, the authors show AtEH/Pan1 proteins also act in actin-mediated autophagy, by interacting with VAP27-1 at ER-PM contact sites and recruiting TPLATE and AP-2 complex subunits, clathrin and ARP2/3/ proteins to autophagosomes.

Multiple downy mildew effectors target the stress-related NAC transcription factor LsNAC069 in lettuce

To cause disease in lettuce, the biotrophic oomycete Bremia lactucae secretes potential RxLR effector proteins. Here we report the discovery of an effector-target hub consisting of four B. lactucae effectors and one lettuce protein target by a yeast-two-hybrid (Y2H) screening. Interaction of the lettuce tail-anchored NAC transcription factor, LsNAC069, with B. lactucae effectors does not require the N-terminal NAC domain but depends on the C-terminal region including the transmembrane domain. Furthermore, in Y2H experiments, B. lactucae effectors interact with Arabidopsis and potato tail-anchored NACs, suggesting that they are conserved effector targets. Transient expression of RxLR effector proteins BLR05 and BLR09 and their target LsNAC069 in planta revealed a predominant localization to the endoplasmic reticulum. Phytophthora capsici culture filtrate and polyethylene glycol treatment induced relocalization to the nucleus of a stabilized LsNAC069 protein, lacking the NAC-domain (LsNAC069ΔNAC ). Relocalization was significantly reduced in the presence of the Ser/Cys-protease inhibitor TPCK indicating proteolytic cleavage of LsNAC069 allows for relocalization. Co-expression of effectors with LsNAC069ΔNAC reduced its nuclear accumulation. Surprisingly, LsNAC069 silenced lettuce lines had decreased LsNAC069 transcript levels but did not show significantly altered susceptibility to B. lactucae. In contrast, LsNAC069 silencing increased resistance to Pseudomonas cichorii bacteria and reduced wilting effects under moderate drought stress, indicating a broad role of LsNAC069 in abiotic and biotic stress responses.

Dynamic metabolic solutions to the sessile life style of plants

Plants are sessile organisms. To compensate for not being able to escape when challenged by unfavorable growth conditions, pests or herbivores, plants have perfected their metabolic plasticity by having developed the capacity for on demand dynamic biosynthesis and storage of a plethora of phytochemicals.

Covering: up to 2018

Plants are sessile organisms. To compensate for not being able to escape when challenged by unfavorable growth conditions, pests or herbivores, plants have perfected their metabolic plasticity by having developed the capacity for on demand synthesis of a plethora of phytochemicals to specifically respond to the challenges arising during plant ontogeny. Key steps in the biosynthesis of phytochemicals are catalyzed by membrane-bound cytochrome P450 enzymes which in plants constitute a superfamily. In planta, the P450s may be organized in dynamic enzyme clusters (metabolons) and the genes encoding the P450s and other enzymes in a specific pathway may be clustered. Metabolon formation facilitates transfer of substrates between sequential enzymes and therefore enables the plant to channel the flux of general metabolites towards biosynthesis of specific phytochemicals. In the plant cell, compartmentalization of the operation of specific biosynthetic pathways in specialized plastids serves to avoid undesired metabolic cross-talk and offers distinct storage sites for molar concentrations of specific phytochemicals. Liquid–liquid phase separation may lead to formation of dense biomolecular condensates within the cytoplasm or vacuole allowing swift activation of the stored phytochemicals as required upon pest or herbivore attack. The molecular grid behind plant plasticity offers an endless reservoir of functional modules, which may be utilized as a synthetic biology tool-box for engineering of novel biological systems based on rational design principles. In this review, we highlight some of the concepts used by plants to coordinate biosynthesis and storage of phytochemicals.

Re-evaluation of physical interaction between plant peroxisomes and other organelles using live-cell imaging techniques

The dynamic behavior of organelles is essential for plant survival under various environmental conditions. Plant organelles, with various functions, migrate along actin filaments and contact other types of organelles, leading to physical interactions at a specific site called the membrane contact site. Recent studies have revealed the importance of physical interactions in maintaining efficient metabolite flow between organelles. In this review, we first summarize peroxisome function under different environmental conditions and growth stages to understand organelle interactions. We then discuss current knowledge regarding the interactions between peroxisome and other organelles, i.e., the oil bodies, chloroplast, and mitochondria from the perspective of metabolic and physiological regulation, with reference to various organelle interactions and techniques for estimating organelle interactions occurring in plant cells.

Manipulating Endoplasmic Reticulum-Plasma Membrane Tethering in Plants Through Fluorescent Protein Complementation

The bimolecular fluorescence complementation (BiFC) assay has been widely used to examine interactions between integral and peripheral proteins within putative plasma membrane (PM) microdomains. In the course of using BiFC assays to examine the co-localization of plasma membrane (PM) targeted receptor-like kinases (RLKs), such as FLS2, with PM micro-domain proteins such as remorins, we unexpectedly observed heterogeneous distribution patterns of fluorescence on the PM of Nicotiana benthamiana leaf cortical cells. These patterns appeared to co-localize with the endoplasmic reticulum (ER) and with ER-PM contact sites, and closely resembled patterns caused by over-expression of the ER-PM tether protein Synaptotagmin1 (SYT1). Using domain swap experiments with SYT1, we inferred that non-specific dimerization between FLS2-VenusN and VenusC-StRem1.3 could create artificial ER-PM tether proteins analogous to SYT1. The same patterns of ER-PM tethering were produced when a representative set of integral membrane proteins were partnered in BiFC complexes with PM-targeted peripheral membrane proteins, including PtdIns(4)P-binding proteins. We inferred that spontaneous formation of mature fluorescent proteins caused the BiFC complexes to trap the integral membrane proteins in the ER during delivery to the PM, producing a PM-ER tether. This phenomenon could be a useful tool to deliberately manipulate ER-PM tethering or to test protein membrane localization. However, this study also highlights the risk of using the BiFC assay to study membrane protein interactions in plants, due to the possibility of alterations in cellular structures and membrane organization, or misinterpretation of protein-protein interactions. A number of published studies using this approach may therefore need to be revisited.

Coming together to define membrane contact sites

Close proximities between organelles have been described for decades. However, only recently a specific field dealing with organelle communication at membrane contact sites has gained wide acceptance, attracting scientists from multiple areas of cell biology. The diversity of approaches warrants a unified vocabulary for the field. Such definitions would facilitate laying the foundations of this field, streamlining communication and resolving semantic controversies. This opinion, written by a panel of experts in the field, aims to provide this burgeoning area with guidelines for the experimental definition and analysis of contact sites. It also includes suggestions on how to operationally and tractably measure and analyze them with the hope of ultimately facilitating knowledge production and dissemination within and outside the field of contact-site research.

Ethylene and hydrogen peroxide regulate formation of a sterol-enriched domain essential for wall labyrinth assembly in transfer cells

Transfer cells (TCs) facilitate high rates of nutrient transport into, and within, the plant body. Their transport function is conferred by polarized wall ingrowth papillae, deposited upon a specialized uniform wall layer, that form a scaffold supporting an amplified area of plasma membrane enriched in nutrient transporters. We explored the question of whether lipid-enriched domains of the TC plasma membrane could serve as organizational platforms for proteins regulating the construction of the intricate TC wall labyrinth using developing Vicia faba cotyledons. When these cotyledons are placed in culture, their adaxial epidermal cells trans-differentiate to a TC phenotype regulated by auxin, ethylene, extracellular hydrogen peroxide (apoH2O2), and cytosolic Ca2+ ([Ca2+]cyt) arranged in series. Staining cultured cotyledons with the sterol-specific dye, Filipin III, detected a polarized sterol-enriched domain in the plasma membrane of their trans-differentiating epidermal transfer cells (ETCs). Ethylene activated sterol biosynthesis while extracellular apoH2O2 directed sterol-enriched vesicles to fuse with the outer periclinal region of the ETC plasma membrane. The sterol-enriched domain was essential for generating the [Ca2+]cyt signal and orchestrating construction of both the uniform wall layer and wall ingrowth papillae. A model is presented outlining how the sterol-enriched plasma membrane domain forms and functions to regulate wall labyrinth assembly.