ER network dynamics are differentially controlled by myosins XI-K, XI-C, XI-E, XI-I, XI-1, and XI-2

The VAMP-associated protein VAP27-1 plays a crucial role in plant resistance to ER stress by modulating ER-PM contact architecture in Arabidopsis

The endoplasmic reticulum (ER) and the plasma membrane (PM) form ER-PM contact sites (EPCSs) that allow the ER and PM to exchange materials and information. Stress-induced disruption of protein folding triggers ER stress, and the cell initiates the unfolded protein response (UPR) to resist the stress. However, whether EPCSs play a role in ER stress in plants remains unclear. VESICLE-ASSOCIATED MEMBRANE PROTEIN (VAMP)-ASSOCIATED PROTEIN 27-1 (VAP27-1) functions in EPCS tethering and is encoded by a family of 10 genes (VAP27-1-10) in Arabidopsis thaliana. Here, we used CRISPR-Cas9-mediated genome editing to obtain a homozygous vap27-1 vap27-3 vap27-4 (vap27-1/3/4) triple mutant lacking three of the key VAP27 family members in Arabidopsis. The vap27-1/3/4 mutant exhibits defects in ER-PM connectivity and EPCS architecture, as well as excessive UPR signaling. We further showed that relocation of VAP27-1 to the PM mediates specific VAP27-1-related EPCS remodeling and expansion under ER stress. Moreover, the spatiotemporal dynamics of VAP27-1 at the PM increase ER-PM connectivity and enhance Arabidopsis resistance to ER stress. In addition, we revealed an important role for intracellular calcium homeostasis in the regulation of UPR signaling. Taken together, these results broaden our understanding of the molecular and cellular mechanisms of ER stress and UPR signaling in plants, providing additional clues for improving plant broad-spectrum resistance to different stresses.

Dancing with the Stars: Using Image Analysis to Study the Choreography of the Endoplasmic Reticulum and Its Partners and of Movement Within Its Tubules

In this chapter, approaches to the image analysis of the choreography of the plant endoplasmic reticulum (ER) labeled with fluorescent fusion proteins ("stars," if you wish) are presented. The approaches include the analyses of those parts of the ER that are attached through membrane contact sites to moving or non-moving partners (other "stars"). Image analysis is also used to understand the nature of the tubular polygonal network, the hallmark of this organelle, and how the polygons change over time due to tubule sliding or motion. Furthermore, the remodeling polygons of the ER interact with regions of fundamentally different topologies, the ER cisternae, and image analysis can be used to separate the tubules from the cisternae. ER cisternae, like polygons and tubules, can be motile or stationary. To study which parts are attached to non-moving partners, such as domains of the ER that form membrane contact sites with the plasma membrane/cell wall, an image analysis approach called persistency mapping has been used. To study the domains of the ER that move rapidly and stream through the cell, image analysis of optic flow has been used. However, optic flow approaches confuse the movement of the ER itself with the movement of proteins within the ER. As an overall measure of ER dynamics, optic flow approaches are of value, but their limitation as to what exactly is "flowing" needs to be specified. Finally, there are important imaging approaches that directly address the movement of fluorescent proteins within the ER lumen or in the membrane of the ER. Of these, fluorescence recovery after photobleaching (FRAP), inverse FRAP (iFRAP), and single particle tracking approaches are described.

Manipulation of the Cellular Membrane-Cytoskeleton Network for RNA Virus Replication and Movement in Plants

Viruses infect all cellular life forms and cause various diseases and significant economic losses worldwide. The majority of viruses are positive-sense RNA viruses. A common feature of infection by diverse RNA viruses is to induce the formation of altered membrane structures in infected host cells. Indeed, upon entry into host cells, plant-infecting RNA viruses target preferred organelles of the cellular endomembrane system and remodel organellar membranes to form organelle-like structures for virus genome replication, termed as the viral replication organelle (VRO) or the viral replication complex (VRC). Different viruses may recruit different host factors for membrane modifications. These membrane-enclosed virus-induced replication factories provide an optimum, protective microenvironment to concentrate viral and host components for robust viral replication. Although different viruses prefer specific organelles to build VROs, at least some of them have the ability to exploit alternative organellar membranes for replication. Besides being responsible for viral replication, VROs of some viruses can be mobile to reach plasmodesmata (PD) via the endomembrane system, as well as the cytoskeleton machinery. Viral movement protein (MP) and/or MP-associated viral movement complexes also exploit the endomembrane-cytoskeleton network for trafficking to PD where progeny viruses pass through the cell-wall barrier to enter neighboring cells.

The sieve-element endoplasmic reticulum: A focal point of phytoplasma-host plant interaction?

The rough endoplasmic reticulum (r-ER) is of paramount importance for adaptive responses to biotic stresses due to an increased demand for de novo synthesis of immunity-related proteins and signaling components. In nucleate cells, disturbance of r-ER integrity and functionality leads to the "unfolded protein response" (UPR), which is an important component of innate plant immune signalling. In contrast to an abundance of reports on r-ER responses to biotic challenges, sieve-element endoplasmic reticulum (SE-ER) responses to phytoplasma infection have not been investigated. We found that morphological SE-ER changes, associated with phytoplasma infection, are accompanied by differential expression of genes encoding proteins involved in shaping and anchoring the reticulum. Phytoplasma infection also triggers an increased release of bZIP signals from the (SE-ER)/r-ER and consequent differential expression of UPR-related genes. The modified expression patterns seem to reflect a trade-off between survival of host cells, needed for the phytoplasmic biotrophic lifestyle, and phytoplasmas. Specialized plasmodesmata between sieve element and companion cell may provide a corridor for transfer of phytoplasma effectors inducing UPR-related gene expression in companion cells.

intER-ACTINg: the structure and dynamics of ER and actin are interlinked

The actin cytoskeleton is the driver of gross ER remodelling and the movement and positioning of other membrane-bound organelles such as Golgi bodies. Rapid ER membrane remodelling is a feature of most plant cells and is important for normal cellular processes, including targeted secretion, immunity and signalling. Modifications to the actin cytoskeleton, through pharmacological agents such as Latrunculin B and phalloidin, or disruption of normal myosin function also affect ER structure and/or dynamics. Here, we investigate the impact of changes in the actin cytoskeleton on structure and dynamics on the ER as well as in return the impact of modified ER structure on the architecture of the actin cytoskeleton. By expressing actin markers that affect actin dynamics, or expressing of ER-shaping proteins that influence ER architecture, we found that the structure of ER-actin networks is closely inter-related; affecting one component is likely to have a direct effect on the other. Therefore, our results indicate that a complicated regulatory machinery and cross-talk between these two structures must exist in plants to co-ordinate the function of ER-actin network during multiple subcellular processes. In addition, when considering organelle structure and dynamics, the choice of actin marker is essential in preventing off-target organelle structure and dynamics modifications. This article is protected by copyright. All rights reserved.

Myosin XI-B is involved in the transport of vesicles and organelles in pollen tubes of Arabidopsis thaliana

The movement of organelles and vesicles in pollen tubes depends on F-actin. However, the molecular mechanism through which plant myosin XI drives the movement of organelles is still controversial, and the relationship between myosin XI and vesicle movement in pollen tubes is also unclear. In this study, we found that the siliques of the myosin xi-b/e mutant were obviously shorter than those of the wild-type (WT) and that the seed set of the mutant was severely deficient. The pollen tube growth of myosin xi-b/e was significantly inhibited both in vitro and in vivo. Fluorescence recovery after photobleaching showed that the velocity of vesicle movement in the pollen tube tip of the myosin xi-b/e mutant was lower than that of the WT. It was also found that peroxisome movement was significantly inhibited in the pollen tubes of the myosin xi-b/e mutant, while the velocities of the Golgi stack and mitochondrial movement decreased relatively less in the pollen tubes of the mutant. The endoplasmic reticulum streaming in the pollen tube shanks was not significantly different between the WT and the myosin xi-b/e mutant. In addition, we found that myosin XI-B-GFP colocalized obviously with vesicles and peroxisomes in the pollen tubes of Arabidopsis. Taken together, these results indicate that myosin XI-B may bind mainly to vesicles and peroxisomes, and drive their movement in pollen tubes. These results also suggest that the mechanism by which myosin XI drives organelle movement in plant cells may be evolutionarily conserved compared with other eukaryotic cells.

Arabidopsis thaliana myosin XIK is recruited to the Golgi through interaction with a MyoB receptor

Plant cell organelles are highly mobile and their positioning play key roles in plant growth, development and responses to changing environmental conditions. Movement is acto-myosin dependent. Despite controlling the dynamics of several organelles, myosin and myosin receptors identified so far in Arabidopsis thaliana generally do not localise to the organelles whose movement they control, raising the issue of how specificity is determined. Here we show that a MyoB myosin receptor, MRF7, specifically localises to the Golgi membrane and affects its movement. Myosin XI-K was identified as a putative MRF7 interactor through mass spectrometry analysis. Co-expression of MRF7 and XI-K tail triggers the relocation of XI-K to the Golgi, linking a MyoB/myosin complex to a specific organelle in Arabidopsis. FRET-FLIM confirmed the in vivo interaction between MRF7 and XI-K tail on the Golgi and in the cytosol, suggesting that myosin/myosin receptor complexes perhaps cycle on and off organelle membranes. This work supports a traditional mechanism for organelle movement where myosins bind to receptors and adaptors on the organelle membranes, allowing them to actively move on the actin cytoskeleton, rather than passively in the recently proposed cytoplasmic streaming model.

Perico et al. use co-expression analysis and a FRET-FLIM approach to show that the Arabidopsis MyoB myosin receptor, MRF7, triggers the relocation of Myosin XI-K to the Golgi. As such, this study provides evidence for plant myosin recruitment and control of organelle movement.

Plant cytoskeletons and the endoplasmic reticulum network organization

Plant endoplasmic reticulum (ER) remodelling is likely to be important for its function in targeted protein secretion, organelle interaction and signal exchange. It has been known for decades that the structure and movement of the ER network is mainly regulated by the actin cytoskeleton through actin motor proteins and membrane-cytoskeleton adaptors. Recent discoveries also revealed alternative pathways that influence ER movement, through a microtubule-based machinery. Therefore, plants utilize both cytoskeletal components to drive ER dynamics, a process that is likely to be dependent on the cell type and the developmental stages. On the other hand, the ER membrane also has a direct effect towards the organization of the cytoskeletal network and disrupting the tethering factors at the ER-PM interface also rearranges the cytoskeletal structure. However, the influence of the ER network on the cytoskeleton organization has not been studied. In this review, we will provide an overview of the ER-cytoskeleton network in plants, and discuss the most recent discoveries in the field.

IntEResting structures: formation and applications of organized smooth endoplasmic reticulum in plant cells

The endoplasmic reticulum (ER) is an organelle with remarkable plasticity, capable of rapidly changing its structure to accommodate different functions based on intra- and extracellular cues. One of the ER structures observed in plants is known as "organized smooth endoplasmic reticulum" (OSER), consisting of symmetrically stacked ER membrane arrays. In plants, these structures were first described in certain specialized tissues, e.g. the sieve elements of the phloem, and more recently in transgenic plants overexpressing ER membrane resident proteins. To date, much of the investigation of OSER focused on yeast and animal cells but research into plant OSER has started to grow. In this update, we give a succinct overview of research into the OSER phenomenon in plant cells with case studies highlighting both native and synthetic occurrences of OSER. We also assess the primary driving forces that trigger the formation of OSER, collating evidence from the literature to compare two competing theories for the origin of OSER: that OSER formation is initiated by oligomerizing protein accumulation in the ER membrane or that OSER is the result of ER membrane proliferation. This has long been a source of controversy in the field and here we suggest a way to integrate arguments from both sides into a single unifying theory. Finally, we discuss the potential biotechnological uses of OSER as a tool for the nascent plant synthetic biology field with possible applications as a synthetic microdomain for metabolic engineering and as an extensive membrane surface for synthetic chemistry or protein accumulation.

Maintaining the structural and functional homeostasis of the plant endoplasmic reticulum

The endoplasmic reticulum (ER) is a ubiquitous organelle that is vital to the life of eukaryotic cells. It synthesizes essential lipids and proteins and initiates the glycosylation of intracellular and surface proteins. As such, the ER is necessary for cell growth and communication with the external environment. The ER is also a highly dynamic organelle, whose structure is continuously remodeled through an interaction with the cytoskeleton and the action of specialized ER shapers. Recent and significant advances in ER studies have brought to light conserved and unique features underlying the structure and function of this organelle in plant cells. In this review, exciting developments in the understanding of the mechanisms for plant ER structural and functional homeostasis, particularly those that underpin ER network architecture and ER degradation, are presented and discussed.

Actin and an unconventional myosin motor, TgMyoF, control the organization and dynamics of the endomembrane network in Toxoplasma gondii

Toxoplasma gondii is an obligate intracellular parasite that relies on three distinct secretory organelles, the micronemes, rhoptries, and dense granules, for parasite survival and disease pathogenesis. Secretory proteins destined for these organelles are synthesized in the endoplasmic reticulum (ER) and sequentially trafficked through a highly polarized endomembrane network that consists of the Golgi and multiple post-Golgi compartments. Currently, little is known about how the parasite cytoskeleton controls the positioning of the organelles in this pathway, or how vesicular cargo is trafficked between organelles. Here we show that F-actin and an unconventional myosin motor, TgMyoF, control the dynamics and organization of the organelles in the secretory pathway, specifically ER tubule movement, apical positioning of the Golgi and post-Golgi compartments, apical positioning of the rhoptries, and finally, the directed transport of Rab6-positive and Rop1-positive vesicles. Thus, this study identifies TgMyoF and actin as the key cytoskeletal components that organize the endomembrane system in T. gondii.

Endomembrane trafficking is a vital cellular process in all eukaryotic cells. In most cases the molecular motors myosin, kinesin, and dynein transport cargo including vesicles, organelles and transcripts along actin and microtubule filaments in a manner analogous to a train moving on its tracks. For the unicellular eukaryote Toxoplasma gondii, the accurate trafficking of proteins through the endomembrane system is vital for parasite survival and pathogenicity. However, the mechanisms of cargo transport in this parasite are poorly understood. In this study, we fluorescently labeled multiple endomembrane organelles and imaged their movements using live cell microscopy. We demonstrate that filamentous actin and an unconventional myosin motor named TgMyoF control both the positioning of organelles in this pathway and the movement of transport vesicles throughout the parasite cytosol. This data provides new insight into the mechanisms of cargo transport in this important pathogen and expands our understanding of the biological roles of actin in the intracellular phase of the parasite’s growth cycle.

The plant endoplasmic reticulum: an organized chaos of tubules and sheets with multiple functions

The endoplasmic reticulum is a fascinating organelle at the core of the secretory pathway. It is responsible for the synthesis of one third of the cellular proteome and, in plant cells, it produces receptors and transporters of hormones as well as the proteins responsible for the biosynthesis of critical components of a cellulosic cell wall. The endoplasmic reticulum structure resembles a spider-web network of interconnected tubules and cisternae that pervades the cell. The study of the dynamics and interaction of this organelles with other cellular structures such as the plasma membrane, the Golgi apparatus and the cytoskeleton, have been permitted by the implementation of fluorescent protein and advanced confocal imaging. In this review, we report on the findings that contributed towards the understanding of the endoplasmic reticulum morphology and function with the aid of fluorescent proteins, focusing on the contributions provided by pioneering work from the lab of the late Professor Chris Hawes.

Light microscopy of the endoplasmic reticulum-membrane contact sites in plants

The existence of membrane contact sites (MCS) has been reported in different systems in the past decade, and their importance has been recognised by the cell biology community. Amongst all endomembrane structures, the endoplasmic reticulum (ER) plays vital roles in organising the organelle interaction network with the plasma membrane (PM), Golgi bodies, mitochondria, plastids, endosomes and autophagosomes. A number of methods have been used to study the establishment and functions of these interactions, among them, light microscopy appears to be one of the most effective approaches. Here, we present an overview of the discovery of ER-PM contact sites, and highlight the latest developments in light microscopical-based techniques that can be used for their study.

Defining the dance: quantification and classification of endoplasmic reticulum dynamics

The availability of quantification methods for subcellular organelle dynamic analysis has increased rapidly over the last 20 years. The application of these techniques to contiguous subcellular structures that exhibit dynamic remodelling over a range of scales and orientations is challenging, as quantification of 'movement' rarely corresponds to traditional, qualitative classifications of types of organelle movement. The plant endoplasmic reticulum represents a particular challenge for dynamic quantification as it itself is an entirely contiguous organelle that is in a constant state of flux and gross remodelling, controlled by the actinomyosin cytoskeleton.

In vivo interactions between myosin XI, vesicles and filamentous actin are fast and transient in Physcomitrella patens

The actin cytoskeleton and active membrane trafficking machinery are essential for polarized cell growth. To understand the interactions between myosin XI, vesicles and actin filaments in vivo, we performed fluorescence recovery after photobleaching and showed that the dynamics of myosin XIa at the tip of the spreading earthmoss Physcomitrella patens caulonemal cells are actin-dependent and that 50% of myosin XI is bound to vesicles. To obtain single-particle information, we used variable-angle epifluorescence microscopy in protoplasts to demonstrate that protein myosin XIa and VAMP72-labeled vesicles localize in time and space over periods lasting only a few seconds. By tracking data with Hidden Markov modeling, we showed that myosin XIa and VAMP72-labeled vesicles exhibit short runs of actin-dependent directed transport. We also found that the interaction of myosin XI with vesicles is short-lived. Together, this vesicle-bound fraction, fast off-rate and short average distance traveled seem be crucial for the dynamic oscillations observed at the tip, and might be vital for regulation and recycling of the exocytosis machinery, while simultaneously promoting vesicle focusing and vesicle secretion at the tip, necessary for cell wall expansion.

Plant organelle dynamics: cytoskeletal control and membrane contact sites

Contents Summary 381 I. Introduction 381 II. Basic movement characteristics 382 III. Actin and associated motors, myosins, play a primary role in plant organelle movement and positioning 382 IV. Mechanisms of myosin recruitment: a tightly regulated system? 384 V. Microtubules, associated motors and interplay with actin 386 VI. Role of organelle interactions: tales of tethers 387 VII. Summary model to describe organelle movement in higher plants 390 VIII. Why is organelle movement important? 390 IX. Conclusions and future perspectives 391 Acknowledgements 391 References 391 SUMMARY: Organelle movement and positioning are correlated with plant growth and development. Movement characteristics are seemingly erratic yet respond to external stimuli including pathogens and light. Given these clear correlations, we still do not understand the specific roles that movement plays in these processes. There are few exceptions including organelle inheritance during cell division and photorelocation of chloroplasts to prevent photodamage. The molecular and biophysical components that drive movement can be broken down into cytoskeletal components, motor proteins and tethers, which allow organelles to physically interact with one another. Our understanding of these components and concepts has exploded over the past decade, with recent technological advances allowing an even more in-depth profiling. Here, we provide an overview of the cytoskeletal and tethering components and discuss the mechanisms behind organelle movement in higher plants.

Bacterial expression, purification and biophysical characterization of the smallest plant reticulon isoform, RTNLB13

Reticulons are a large family of integral membrane proteins that are ubiquitous in eukaryotes and play a key role in functional remodelling of the endoplasmic reticulum membrane. The reticulon family is especially large in plants, with the Arabidopsis thaliana genome containing twenty-one isoforms. Reticulons vary in length but all contain a conserved C-terminal reticulon homology domain (RHD) that associates with membranes. An understanding of the structure and membrane interactions of RHDs is key to unlocking their mechanism of function, however no three-dimensional structure has been solved. We believe that this is, in part, due to difficulties in obtaining reticulon proteins in yields sufficient for structural study. To address this, we report here the first bacterial overexpression, purification, and biophysical investigation of a reticulon protein from plants, the RTNLB13 protein from A. thaliana. RTNLB13 is the smallest plant reticulon and is made up of a single RHD. We used circular dichroism, SDS-PAGE and analytical ultracentrifugation to reveal that RTNLB13 is 45% α-helical in a number of detergent environments, monomeric at low concentrations, and capable of self-association at higher concentrations. We used solution-state NMR to screen the effect of detergent type on the fold of isotopically-enriched RTNLB13, and found that ∼60% of the expected protein peaks were broadened due to slow dynamics. This broadening points toward a large network of protein-membrane interactions throughout the sequence. We have interpreted our results in light of current literature and suggest a preliminary description of RTNLB13 structure and topology.

Kinesins and Myosins: Molecular Motors that Coordinate Cellular Functions in Plants

Kinesins and myosins are motor proteins that can move actively along microtubules and actin filaments, respectively. Plants have evolved a unique set of motors that function as regulators and organizers of the cytoskeleton and as drivers of long-distance transport of various cellular components. Recent progress has established the full complement of motors encoded in plant genomes and has revealed valuable insights into the cellular functions of many kinesin and myosin isoforms. Interestingly, several of the motors were found to functionally connect the two cytoskeletal systems and thereby to coordinate their activities. In this review, we discuss the available genetic, cell biological, and biochemical data for each of the plant kinesin and myosin families from the context of their subcellular mechanism of action as well as their physiological function in the whole plant. We particularly emphasize work that illustrates mechanisms by which kinesins and myosins coordinate the activities of the cytoskeletal system.

Modeling the Geometry and Dynamics of the Endoplasmic Reticulum Network

The endoplasmic reticulum (ER) is an intricate network that pervades the entire cortex of plant cells and its geometric shape undergoes drastic changes. This paper proposes a mathematical model to reconstruct geometric network dynamics by combining the node movements within the network and topological changes engendered by these nodes. The network topology in the model is determined by a modified optimization procedure from the work (Lemarchand, et al. 2014) which minimizes the total length taking into account both degree and angle constraints, beyond the conditions of connectedness and planarity. A novel feature for solving our optimization problem is the use of "lifted" angle constraints, which allows one to considerably reduce the solution runtimes. Using this optimization technique and a Langevin approach for the branching node movement, the simulated network dynamics represent the ER network dynamics observed under latrunculin B treated condition and recaptures features such as the appearance/disappearance of loops within the ER under the native condition. The proposed modeling approach allows quantitative comparison of networks between the model and experimental data based on topological changes induced by node dynamics. An increased temporal resolution of experimental data will allow a more detailed comparison of network dynamics using this modeling approach.

Dancing with the Stars: Using Image Analysis to Study the Choreography of the Endoplasmic Reticulum and Its Partners and of Movement Within Its Tubules

In this chapter, approaches to the image analysis of the choreography of the plant endoplasmic reticulum (ER) labeled with fluorescent fusion proteins ("stars," if you wish) are presented. The approaches include the analyses of those parts of the ER that are attached through membrane contact sites to moving or nonmoving partners (other "stars"). Image analysis is also used to understand the nature of the tubular polygonal network, the hallmark of this organelle, and how the polygons change over time due to tubule sliding or motion. Furthermore, the remodeling polygons of the ER interact with regions of fundamentally different topology, the ER cisternae, and image analysis can be used to separate the tubules from the cisternae. ER cisternae, like polygons and tubules, can be motile or stationary. To study which parts are attached to nonmoving partners, such as domains of the ER that form membrane contact sites with the plasma membrane/cell wall, an image analysis approach called persistency mapping has been used. To study the domains of the ER that are moving rapidly and streaming through the cell, the image analysis of optic flow has been used. However, optic flow approaches confuse the movement of the ER itself with the movement of proteins within the ER. As an overall measure of ER dynamics, optic flow approaches are of value, but their limitation as to what exactly is "flowing" needs to be specified. Finally, there are important imaging approaches that directly address the movement of fluorescent proteins within the ER lumen or in the membrane of the ER. Of these, fluorescence recovery after photobleaching (FRAP), inverse FRAP (iFRAP), and single particle tracking approaches are described.

Update on Myosin Motors: Molecular Mechanisms and Physiological Functions

Recent progress has revealed aspects of the molecular mechanisms that allow myosin motors to carry outtheir physiological functions.

Emerging roles of cortical microtubule-membrane interactions

Plant cortical microtubules have crucial roles in cell wall development. Cortical microtubules are tightly anchored to the plasma membrane in a highly ordered array, which directs the deposition of cellulose microfibrils by guiding the movement of the cellulose synthase complex. Cortical microtubules also interact with several endomembrane systems to regulate cell wall development and other cellular events. Recent studies have identified new factors that mediate interactions between cortical microtubules and endomembrane systems including the plasma membrane, endosome, exocytic vesicles, and endoplasmic reticulum. These studies revealed that cortical microtubule-membrane interactions are highly dynamic, with specialized roles in developmental and environmental signaling pathways. A recent reconstructive study identified a novel function of the cortical microtubule-plasma membrane interaction, which acts as a lateral fence that defines plasma membrane domains. This review summarizes recent advances in our understanding of the mechanisms and functions of cortical microtubule-membrane interactions.

From shaping organelles to signalling platforms: the emerging functions of plant ER-PM contact sites

The plant endoplasmic reticulum (ER) defines the biosynthetic site of lipids and proteins destined for secretion, but also contains important signal transduction and homeostasis components that regulate multiple hormonal and developmental responses. To achieve its various functions, the ER has a unique architecture, both reticulated and highly plastic, that facilitates the spatial-temporal segregation of biochemical reactions and the establishment of inter-organelle communication networks. At the cell cortex, the cortical ER (cER) anchors to and functionally couples with the PM through largely static structures known as ER-PM contact sites (EPCS). These spatially confined microdomains are emerging as critical regulators of the geometry of the cER network, and as highly specialized signalling hubs. In this review, we share recent insights into how EPCS regulate cER remodelling, and discuss the proposed roles for plant EPCS components in the integration of environmental and developmental signals at the cER-PM interface.

Connecting membranes to the actin cytoskeleton

In plants, the actin cytoskeleton plays a major role in organelle movement, cargo transport, maintaining cell polarity and controlling the morphogenesis of endomembrane systems. All of these events require a direct connection between membrane structures and the cytoskeleton. Our knowledge in this field has been greatly advanced by a few recent discoveries including the identification of the plant specific NETWORKED family of proteins, which can mediate such linkages. Other proteins that are known to regulate actin nucleation and polymerization are also likely to be involved, but many key questions still remain unanswered. In this paper, we will focus on recent research on the interfaces between the actin cytoskeleton and membranes of the endoplasmic reticulum, the vacuole and autophagosomes.

Modeling Endoplasmic Reticulum Network Maintenance in a Plant Cell

The endoplasmic reticulum (ER) in plant cells forms a highly dynamic network of complex geometry. ER network morphology and dynamics are influenced by a number of biophysical processes, including filament/tubule tension, viscous forces, Brownian diffusion, and interactions with many other organelles and cytoskeletal elements. Previous studies have indicated that ER networks can be thought of as constrained minimal-length networks acted on by a variety of forces that perturb and/or remodel the network. Here, we study two specific biophysical processes involved in remodeling. One is the dynamic relaxation process involving a combination of tubule tension and viscous forces. The other is the rapid creation of cross-connection tubules by direct or indirect interactions with cytoskeletal elements. These processes are able to remodel the ER network: the first reduces network length and complexity whereas the second increases both. Using live cell imaging of ER network dynamics in tobacco leaf epidermal cells, we examine these processes on ER network dynamics. Away from regions of cytoplasmic streaming, we suggest that the dynamic network structure is a balance between the two processes, and we build an integrative model of the two processes for network remodeling. This model produces quantitatively similar ER networks to those observed in experiments. We use the model to explore the effect of parameter variation on statistical properties of the ER network.

Plasmolysis-deplasmolysis causes changes in endoplasmic reticulum form, movement, flow, and cytoskeletal association

Plasmolysis of hypocotyl cells of transgenic Arabidopsis thaliana and Nicotiana benthamiana diminishes the dynamics of the remodeling of the endoplasmic reticulum (ER) in the central protoplast, namely that withdrawn from the cell wall, and more persistent cisternae are formed, yet little change in the actin network in the protoplast occurs. Also, protein flow within the ER network in the protoplast, as detected with fluorescence recovery after photobleaching (FRAP), is not affected by plasmolysis. After plasmolysis, another network of strictly tubular ER remains attached to the plasma membrane-wall interface and is contained within the Hechtian strands and reticulum. FRAP studies indicate that protein flow within these ER tubules diminishes. Actin is largely absent from the Hechtian reticulum and the ER becomes primarily associated with altered, branched microtubules. The smaller volume of the central protoplast is accompanied by decreased movement rates of tubules, cisternae, and spheroid organelles, but this reduced movement is not readily reversed by the increase in volume that accompanies deplasmolysis.

Plant Endoplasmic Reticulum-Plasma Membrane Contact Sites

The endoplasmic reticulum (ER) acts as a superhighway with multiple sideroads that connects the different membrane compartments including the ER to the plasma membrane (PM). ER-PM contact sites (EPCSs) are a common feature in eukaryotic organisms, but have not been studied well in plants owing to the lack of molecular markers and to the difficulty in resolving the EPCS structure using conventional microscopy. Recently, however, plant protein complexes required for linking the ER and PM have been identified. This is a further step towards understanding the structure and function of plant EPCSs. We highlight some recent studies in this field and suggest several hypotheses that relate to the possible function of EPCSs in plants.

NETWORKED 3B: a novel protein in the actin cytoskeleton-endoplasmic reticulum interaction

In plants movement of the endoplasmic reticulum (ER) is dependent on the actin cytoskeleton. However little is known about proteins that link the ER membrane and the actin cytoskeleton. Here we identified a novel protein, NETWORKED 3B (NET3B), which is associated with the ER and actin cytoskeleton in vivo. NET3B belongs to a superfamily of plant specific actin binding proteins, the NETWORKED family. NET3B associates with the actin cytoskeleton in vivo through an N-terminal NET actin binding (NAB) domain, which has been well-characterized in other members of the NET family. A three amino acid insertion, Val-Glu-Asp, in the NAB domain of NET3B appears to lower its ability to localize to the actin cytoskeleton compared with NET1A, the founding member of the NET family. The C-terminal domain of NET3B links the protein to the ER. Overexpression of NET3B enhanced the association between the ER and the actin cytoskeleton, and the extent of this association was dependent on the amount of NET3B available. Another effect of NET3B overexpression was a reduction in ER membrane diffusion. In conclusion, our results revealed that NET3B modulates ER and actin cytoskeleton interactions in higher plants.

Plant ER geometry and dynamics: biophysical and cytoskeletal control during growth and biotic response

The endoplasmic reticulum (ER) is an intricate and dynamic network of membrane tubules and cisternae. In plant cells, the ER 'web' pervades the cortex and endoplasm and is continuous with adjacent cells as it passes through plasmodesmata. It is therefore the largest membranous organelle in plant cells. It performs essential functions including protein and lipid synthesis, and its morphology and movement are linked to cellular function. An emerging trend is that organelles can no longer be seen as discrete membrane-bound compartments, since they can physically interact and 'communicate' with one another. The ER may form a connecting central role in this process. This review tackles our current understanding and quantification of ER dynamics and how these change under a variety of biotic and developmental cues.

Phosphorylation of the C Terminus of RHD3 Has a Critical Role in Homotypic ER Membrane Fusion in Arabidopsis

The endoplasmic reticulum (ER) consists of dynamically changing tubules and cisternae. In animals and yeast, homotypic ER membrane fusion is mediated by fusogens (atlastin and Sey1p, respectively) that are membrane-associated dynamin-like GTPases. In Arabidopsis (Arabidopsis thaliana), another dynamin-like GTPase, ROOT HAIR DEFECTIVE3 (RHD3), has been proposed as an ER membrane fusogen, but direct evidence is lacking. Here, we show that RHD3 has an ER membrane fusion activity that is enhanced by phosphorylation of its C terminus. The ER network was RHD3-dependently reconstituted from the cytosol and microsome fraction of tobacco (Nicotiana tabacum) cultured cells by exogenously adding GTP, ATP, and F-actin. We next established an in vitro assay system of ER tubule formation with Arabidopsis ER vesicles, in which addition of GTP caused ER sac formation from the ER vesicles. Subsequent application of a shearing force to this system triggered the formation of tubules from the ER sacs in an RHD-dependent manner. Unexpectedly, in the absence of a shearing force, Ser/Thr kinase treatment triggered RHD3-dependent tubule formation. Mass spectrometry showed that RHD3 was phosphorylated at multiple Ser and Thr residues in the C terminus. An antibody against the RHD3 C-terminal peptide abolished kinase-triggered tubule formation. When the Ser cluster was deleted or when the Ser residues were replaced with Ala residues, kinase treatment had no effect on tubule formation. Kinase treatment induced the oligomerization of RHD3. Neither phosphorylation-dependent modulation of membrane fusion nor oligomerization has been reported for atlastin or Sey1p. Taken together, we propose that phosphorylation-stimulated oligomerization of RHD3 enhances ER membrane fusion to form the ER network.

Protein Bodies in Leaves Exchange Contents through the Endoplasmic Reticulum

Protein bodies (PBs) are organelles found in seeds whose main function is the storage of proteins that are used during germination for sustaining growth. PBs can also be induced to form in leaves when foreign proteins are produced at high levels in the endoplasmic reticulum (ER) and when fused to one of three tags: Zera®, elastin-like polypeptides (ELP), or hydrophobin-I (HFBI). In this study, we investigate the differences between ELP, HFBI and Zera PB formation, packing, and communication. Our results confirm the ER origin of all three fusion-tag-induced PBs. We show that secretory pathway proteins can be sequestered into all types of PBs but with different patterns, and that different fusion tags can target a specific protein to different PBs. Zera PBs are mobile and dependent on actomyosin motility similar to ELP and HFBI PBs. We show in vivo trafficking of proteins between PBs using GFP photoconversion. We also show that protein trafficking between ELP or HFBI PBs is faster and proteins travel further when compared to Zera PBs. Our results indicate that fusion-tag-induced PBs do not represent terminally stored cytosolic organelles, but that they form in, and remain part of the ER, and dynamically communicate with each other via the ER. We hypothesize that the previously documented PB mobility along the actin cytoskeleton is associated with ER movement rather than independent streaming of detached organelles.

Functions of plant-specific myosin XI: from intracellular motility to plant postures

The plant-specific protein motor class myosin XI is known to function in rapid bulk flow of the cytoplasm (cytoplasmic streaming) and in organellar movements. Recent studies unveiled a wide range of physiological functions of myosin XI motors, from intracellular motility to organ movements. Arabidopsis thaliana has 13 members of myosin XI class. In vegetative organs, myosins XIk, XI1, and XI2 primarily contribute to dynamics and spatial configurations of endoplasmic reticulum that develops a tubular network in the cell periphery and thick strand-like structures in the inner cell regions. Myosin XI-i forms a nucleocytoplasmic linker and is responsible for nuclear movement and shape. In addition to these intracellular functions, myosin XIf together with myosin XIk is involved in the fundamental nature of plants; the actin-myosin XI cytoskeleton regulates organ straightening to adjust plant posture.

The endoplasmic reticulum: a dynamic and well-connected organelle

The endoplasmic reticulum forms the first compartment in a series of organelles which comprise the secretory pathway. It takes the form of an extremely dynamic and pleomorphic membrane-bounded network of tubules and cisternae which have numerous different cellular functions. In this review, we discuss the nature of endoplasmic reticulum structure and dynamics, its relationship with closely associated organelles, and its possible function as a highway for the distribution and delivery of a diverse range of structures from metabolic complexes to viral particles.

Nucleocapsid protein from fig mosaic virus forms cytoplasmic agglomerates that are hauled by endoplasmic reticulum streaming

Although many studies have demonstrated intracellular movement of viral proteins or viral replication complexes, little is known about the mechanisms of their motility. In this study, we analyzed the localization and motility of the nucleocapsid protein (NP) of Fig mosaic virus (FMV), a negative-strand RNA virus belonging to the recently established genus Emaravirus. Electron microscopy of FMV-infected cells using immunogold labeling showed that NPs formed cytoplasmic agglomerates that were predominantly enveloped by the endoplasmic reticulum (ER) membrane, while nonenveloped NP agglomerates also localized along the ER. Likewise, transiently expressed NPs formed agglomerates, designated NP bodies (NBs), in close proximity to the ER, as was the case in FMV-infected cells. Subcellular fractionation and electron microscopic analyses of NP-expressing cells revealed that NBs localized in the cytoplasm. Furthermore, we found that NBs moved rapidly with the streaming of the ER in an actomyosin-dependent manner. Brefeldin A treatment at a high concentration to disturb the ER network configuration induced aberrant accumulation of NBs in the perinuclear region, indicating that the ER network configuration is related to NB localization. Dominant negative inhibition of the class XI myosins, XI-1, XI-2, and XI-K, affected both ER streaming and NB movement in a similar pattern. Taken together, these results showed that NBs localize in the cytoplasm but in close proximity to the ER membrane to form enveloped particles and that this causes passive movements of cytoplasmic NBs by ER streaming.

IMPORTANCE

Intracellular trafficking is a primary and essential step for the cell-to-cell movement of viruses. To date, many studies have demonstrated the rapid intracellular movement of viral factors but have failed to provide evidence for the mechanism or biological significance of this motility. Here, we observed that agglomerates of nucleocapsid protein (NP) moved rapidly throughout the cell, and we performed live imaging and ultrastructural analysis to identify the mechanism of motility. We provide evidence that cytoplasmic protein agglomerates were passively dragged by actomyosin-mediated streaming of the endoplasmic reticulum (ER) in plant cells. In virus-infected cells, NP agglomerates were surrounded by the ER membranes, indicating that NP agglomerates form the basis of enveloped virus particles in close proximity to the ER. Our work provides a sophisticated model of macromolecular trafficking in plant cells and improves our understanding of the formation of enveloped particles of negative-strand RNA viruses.

Interactions between plant endomembrane systems and the actin cytoskeleton

Membrane trafficking, organelle movement, and morphogenesis in plant cells are mainly controlled by the actin cytoskeleton. Not all proteins that regulate the cytoskeleton and membrane dynamics in animal systems have functional homologs in plants, especially for those proteins that form the bridge between the cytoskeleton and membrane; the membrane-actin adaptors. Their nature and function is only just beginning to be elucidated and this field has been greatly enhanced by the recent identification of the NETWORKED (NET) proteins, which act as membrane-actin adaptors. In this review, we will summarize the role of the actin cytoskeleton and its regulatory proteins in their interaction with endomembrane compartments and where they potentially act as platforms for cell signaling and the coordination of other subcellular events.

Organelle trafficking, the cytoskeleton, and pollen tube growth

The pollen tube is fundamental for the reproduction of seed plants. Characteristically, it grows relatively quickly and uni-directionally ("polarized growth") to extend the male gametophyte to reach the female gametophyte. The pollen tube forms a channel through which the sperm cells move so that they can reach their targets in the ovule. To grow quickly and directionally, the pollen tube requires an intense movement of organelles and vesicles that allows the cell's contents to be distributed to sustain the growth rate. While the various organelles distribute more or less uniformly within the pollen tube, Golgi-released secretory vesicles accumulate massively at the pollen tube apex, that is, the growing region. This intense movement of organelles and vesicles is dependent on the dynamics of the cytoskeleton, which reorganizes differentially in response to external signals and coordinates membrane trafficking with the growth rate of pollen tubes.

Role of plant myosins in motile organelles: is a direct interaction required?

Plant organelles are highly motile, with speed values of 3-7 µm/s in cells of land plants and about 20-60 µm/s in characean algal cells. This movement is believed to be important for rapid distribution of materials around the cell, for the plant's ability to respond to environmental biotic and abiotic signals and for proper growth. The main machinery that propels motility of organelles within plant cells is based on the actin cytoskeleton and its motor proteins the myosins. Most plants express multiple members of two main classes: myosin VIII and myosin XI. While myosin VIII has been characterized as a slow motor protein, myosins from class XI were found to be the fastest motor proteins known in all kingdoms. Paradoxically, while it was found that myosins from class XI regulate most organelle movement, it is not quite clear how or even if these motor proteins attach to the organelles whose movement they regulate.

Formins: linking cytoskeleton and endomembranes in plant cells

The cytoskeleton plays a central part in spatial organization of the plant cytoplasm, including the endomebrane system. However, the mechanisms involved are so far only partially understood. Formins (FH2 proteins), a family of evolutionarily conserved proteins sharing the FH2 domain whose dimer can nucleate actin, mediate the co-ordination between actin and microtubule cytoskeletons in multiple eukaryotic lineages including plants. Moreover, some plant formins contain transmembrane domains and participate in anchoring cytoskeletal structures to the plasmalemma, and possibly to other membranes. Direct or indirect membrane association is well documented even for some fungal and metazoan formins lacking membrane insertion motifs, and FH2 proteins have been shown to associate with endomembranes and modulate their dynamics in both fungi and metazoans. Here we summarize the available evidence suggesting that formins participate in membrane trafficking and endomembrane, especially ER, organization also in plants. We propose that, despite some methodological pitfalls inherent to in vivo studies based on (over)expression of truncated and/or tagged proteins, formins are beginning to emerge as candidates for the so far somewhat elusive link between the plant cytoskeleton and the endomembrane system.

ER - the key to the highway

The endoplasmic reticulum (ER) is the key organelle at the start of the secretory pathway and the list of its functions is continually growing. The ER organization as a tubular/cisternal network at the cortex of plant cells has recently been shown to be governed by the membrane tubulation proteins of the reticulon family working alongside plant atlastin homologues, members of the RHD3 group of proteins. Such a network has intimate connections with other organelles such as peroxisomes via peroxules, chloroplasts, Golgi bodies and at the cell cortex to the plasma membrane with cytoskeleton at so called 'anchor/contact sites'. The ER network is by no means static displaying a range of different movements and acting as a subcellular highway supports the motility of organelles such as peroxisomes, mitochondria and Golgi bodies plus the transport of macromolecules such as viral movement proteins, nucleocapsid proteins and RNA. Here we highlight recent and exciting discoveries on the maintenance of the ER structure and its role on movement and biology of other organelles.

Structure and dynamics of ER: minimal networks and biophysical constraints

The endoplasmic reticulum (ER) in live cells is a highly mobile network whose structure dynamically changes on a number of timescales. The role of such drastic changes in any system is unclear, although there are correlations with ER function. A better understanding of the fundamental biophysical constraints on the system will allow biologists to determine the effects of molecular factors on ER dynamics. Previous studies have identified potential static elements that the ER may remodel around. Here, we use these structural elements to assess biophysical principles behind the network dynamics. By analyzing imaging data of tobacco leaf epidermal cells under two different conditions, i.e., native state (control) and latrunculin B (treated), we show that the geometric structure and dynamics of ER networks can be understood in terms of minimal networks. Our results show that the ER network is well modeled as a locally minimal-length network between the static elements that potentially anchor the ER to the cell cortex over longer timescales; this network is perturbed by a mixture of random and deterministic forces. The network need not have globally minimum length; we observe cases where the local topology may change dynamically between different Euclidean Steiner network topologies. The networks in the treated cells are easier to quantify, because they are less dynamic (the treatment suppresses actin dynamics), but the same general features are found in control cells. Using a Langevin approach, we model the dynamics of the nonpersistent nodes and use this to show that the images can be used to estimate both local viscoelastic behavior of the cytoplasm and filament tension in the ER network. This means we can explain several aspects of the ER geometry in terms of biophysical principles.