Actin and myosin regulate cytoplasm stiffness in plant cells: a study using optical tweezers

The actin motor protein OsMYA1 associates with OsExo70H1 and contributes to rice secretory defense by modulating OsSyp121 distribution

Magnaporthe oryzae (M. oryzae) is a devastating hemibiotrophic pathogen. Its biotrophic invasive hyphae (IH) are enclosed in the extrainvasive hyphal membrane produced by plant cells, thus generating a front line of the battlefield between the pathogen and the host plants. In plants, defense-related complexes such as proteins, callose-rich materials and vesicles, are directionally secreted to this interface to confer defense responses, but the underlying molecular mechanism is poorly understood. In this study, we found that a Myosin gene, Myosin A1 (OsMYA1), contributed to rice defense. The OsMYA1 knockout mutant exhibited decreased resistance to M. oryzae infection. OsMYA1 localizes to the actin cytoskeleton and surrounds the IH of M. oryzae. OsMYA1 interacts with an exocyst subunit, OsExo70H1, and regulates its accumulation at the plasma membrane (PM) and pathogen-plant interface. Furthermore, OsExo70H1 interacted with the rice syntaxin of the plants121 protein (OsSyp121), and the distribution of OsSyp121 to the PM or the pathogen-plant interface was disrupted in both the OsMYA1 and OsExo70H1 mutants. Overall, these results not only reveal a new function of OsMYA1 in rice blast resistance, but also uncover a molecular mechanism by which plants regulate defense against M. oryzae by OsMYA1-initiated vesicle secretory pathway, which originates from the actin cytoskeleton to the PM.

Using Optical Tweezers Combined with Total Internal Reflection Microscopy to Study Interactions Between the ER and Golgi in Plant Cells

Optical tweezers have been used to trap and micro-manipulate several biological specimens ranging from DNA, macromolecules, organelles, to single-celled organisms. Using a combination of the refraction and scattering of laser light from a focused laser beam, refractile objects are physically captured and can be moved within the surrounding media. The technique is routinely used to determine biophysical properties such as the forces exerted by motor proteins. Here, we describe how optical tweezers combined with total internal reflection fluorescence microscopy (TIRF) can be used to assess physical interactions between organelles, more specifically the ER and Golgi bodies in plant cells.

At the Nexus between Cytoskeleton and Vacuole: How Plant Cytoskeletons Govern the Dynamics of Large Vacuoles

Large vacuoles are a predominant cell organelle throughout the plant body. They maximally account for over 90% of cell volume and generate turgor pressure that acts as a driving force of cell growth, which is essential for plant development. The plant vacuole also acts as a reservoir for sequestering waste products and apoptotic enzymes, thereby enabling plants to rapidly respond to fluctuating environments. Vacuoles undergo dynamic transformation through repeated enlargement, fusion, fragmentation, invagination, and constriction, eventually resulting in the typical 3-dimensional complex structure in each cell type. Previous studies have indicated that such dynamic transformations of plant vacuoles are governed by the plant cytoskeletons, which consist of F-actin and microtubules. However, the molecular mechanism of cytoskeleton-mediated vacuolar modifications remains largely unclear. Here we first review the behavior of cytoskeletons and vacuoles during plant development and in response to environmental stresses, and then introduce candidates that potentially play pivotal roles in the vacuole-cytoskeleton nexus. Finally, we discuss factors hampering the advances in this research field and their possible solutions using the currently available cutting-edge technologies.

Optical Trapping in Plant Cells

Optical tweezers allow for noninvasive manipulation of subcellular compartments to study their physical interactions and attachments. By measuring (delay of) displacements, (semi)quantitative force measurements within a living cell can be performed. In this chapter, we provide practical tips for setting up such experiments paying special attention to the technical considerations for integrating optical tweezers into a confocal microscope. Next, we describe experimental approaches we have taken to trap intracellular structures in plant cells.

Lessons from optical tweezers: quantifying organelle interactions, dynamics and modelling subcellular events

Optical tweezers enable users to physically trap organelles and move them laterally within the plant cell. Recent advances have highlighted physical interactions between functionally related organelle pairs, such as ER-Golgi and peroxisome-chloroplast, and have shown how organelle positioning affects plant growth. Quantification of these processes has provided insight into the force components which ultimately drive organelle movement and positioning in plant cells. Application of optical tweezers has therefore revolutionised our understanding of plant organelle dynamics.

Molecular Mechanisms of T Cells Activation by Dendritic Cells in Autoimmune Diseases

The interaction between T cell and dendritic cells (DCs) that leads to T cell activation affects the progression of the immune response including autoimmune diseases. Antigen presentation on immune cell surface, formation of an immunological synapse (IS), and specific identification of complex by T cells including two activating signals are necessary steps that lead to T cell activation. The formation of stimulatory IS involves the inclusion of costimulatory molecules, such as ICAM-1/LFA-1 and CD28/B7-1, and so on. Some fusion proteins and monoclonal antibodies targeting costimulatory molecules have been developed and approved to treat autoimmune diseases, including rheumatoid arthritis (RA), systemic lupus erythematosus (SLE), multiple sclerosis (MS), type I diabetes (T1D), inflammatory bowel disease (IBD), and psoriasis. These biological agents, including CTLA-4- and LFA-3-Ig, anti-CD3 monoclonal antibody, could prevent the successful engagement of DCs by T cell with significant efficacy and safety profile. In this article, we reviewed the molecular mechanisms of T cell activation during the interaction between T cells and DCs, and summarized some biological agents that target costimulatory molecules involved in the regulation of T cell activation.

Using Optical Tweezers Combined with Total Internal Reflection Microscopy to Study Interactions Between the ER and Golgi in Plant Cells

Optical tweezers have been used to trap and micromanipulate several biological specimens ranging from DNA, macromolecules, organelles to single celled organisms. Using a combination of the refraction and scattering of laser light from a focused laser beam, refractile objects are physically captured and can be moved within the surrounding media. The technique is routinely used to determine biophysical properties such as the forces exerted by motor proteins. Here, we describe how optical tweezers combined with total internal reflection fluorescence (TIRF) microscopy can be used to assess physical interactions between organelles, more specifically the ER and Golgi bodies in plant cells.

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.

Structure and components of the globular and filamentous viroplasms induced by Rice black-streaked dwarf virus

Viroplasms of members of the family Reoviridae are considered to be viral factories for genome replication and virion assembly. Globular and filamentous phenotypes have different components and probably have different functions. We used transmission electron microscopy and electron tomography to examine the structure and components of the two viroplasm phenotypes induced by Rice black-streaked dwarf virus (RBSDV). Immuno-gold labeling was used to localize each of the 13 RBSDV encoded proteins as well as double-stranded RNA, host cytoskeleton actin-11 and α-tubulin. Ten of the RBSDV proteins were localized in one or both types of viroplasm. P5-1, P6 and P9-1 were localized on both viroplasm phenotypes but P5-1 was preferentially associated with filaments and P9-1 with the matrix. Structural analysis by electron tomography showed that osmiophilic granules 6-8nm in diameter served as the fundamental unit for constructing both of the viroplasm phenotypes but were more densely packed in the filamentous phenotype.

REGULATOR OF BULB BIOGENESIS1 (RBB1) Is Involved in Vacuole Bulb Formation in Arabidopsis

Vacuoles are dynamic compartments with constant fluctuations and transient structures such as trans-vacuolar strands and bulbs. Bulbs are highly dynamic spherical structures inside vacuoles that are formed by multiple layers of membranes and are continuous with the main tonoplast. We recently carried out a screen for mutants with abnormal trafficking to the vacuole or aberrant vacuole morphology. We characterized regulator of bulb biogenesis1-1 (rbb1-1), a mutant in Arabidopsis that contains increased numbers of bulbs when compared to the parental control. rbb1-1 mutants also contain fewer transvacuolar strands than the parental control, and we propose the hypothesis that the formation of transvacuolar strands and bulbs is functionally related. We propose that the bulbs may function transiently to accommodate membranes and proteins when transvacuolar strands fail to elongate. We show that RBB1 corresponds to a very large protein of unknown function that is specific to plants, is present in the cytosol, and may associate with cellular membranes. RBB1 is involved in the regulation of vacuole morphology and may be involved in the establishment or stability of trans-vacuolar strands and bulbs.

Mechanical role of actin dynamics in the rheology of the Golgi complex and in Golgi-associated trafficking events

BACKGROUND

In vitro studies have shown that physical parameters, such as membrane curvature, tension, and composition, influence the budding and fission of transport intermediates. Endocytosis in living cells also appears to be regulated by the mechanical load experienced by the plasma membrane. In contrast, how these parameters affect intracellular membrane trafficking in living cells is not known. To address this question, we investigate here the impact of a mechanical stress on the organization of the Golgi complex and on the formation of transport intermediates from the Golgi complex.

RESULTS

Using confocal microscopy, we visualize the deformation of Rab6-positive Golgi membranes applied by an internalized microsphere trapped in optical tweezers and simultaneously measure the corresponding forces. Our results show that the force necessary to deform Golgi membranes drops when actin dynamics is altered and correlates with myosin II activity. We also show that the applied stress has a long-range effect on Golgi membranes, perturbs the dynamics of Golgi-associated actin, and induces a sharp decrease in the formation of Rab6-positive vesicles from the Golgi complex as well as tubulation of Golgi membranes.

CONCLUSIONS

We suggest that acto-myosin contractility strongly contributes to the local rigidity of the Golgi complex and regulates the mechanics of the Golgi complex to control intracellular membrane trafficking.

Root hairs

Roots hairs are cylindrical extensions of root epidermal cells that are important for acquisition of nutrients, microbe interactions, and plant anchorage. The molecular mechanisms involved in the specification, differentiation, and physiology of root hairs in Arabidopsis are reviewed here. Root hair specification in Arabidopsis is determined by position-dependent signaling and molecular feedback loops causing differential accumulation of a WD-bHLH-Myb transcriptional complex. The initiation of root hairs is dependent on the RHD6 bHLH gene family and auxin to define the site of outgrowth. Root hair elongation relies on polarized cell expansion at the growing tip, which involves multiple integrated processes including cell secretion, endomembrane trafficking, cytoskeletal organization, and cell wall modifications. The study of root hair biology in Arabidopsis has provided a model cell type for insights into many aspects of plant development and cell biology.

Optical trapping in plant cells

Optical tweezers allow noninvasive manipulation of subcellular compartments to study their physical interactions and attachments. By measuring (delay of) displacements, (semi-)quantitative force measurements within a living cell can be performed. In this chapter, we provide practical tips for setting up such experiments paying special attention to the technical considerations for integrating optical tweezers into a confocal microscope. Next, we describe some working protocols to trap intracellular structures in plant cells.

The actin cytoskeleton in root hairs: all is fine at the tip

Filamentous actin forms characteristic bundles in plant cells that facilitate cytoplasmic streaming. In contrast, networks of actin exhibiting fast turnover are found especially near sites of rapid cell expansion. These networks may serve various functions including delivering and retaining vesicles while preventing penetration of organelles into the area where cell growth occurs thereby allowing fast turnover of vesicles to and from the plasma membrane. Root hairs elongate by polarized growth at their tips and the local accumulation of fine F-actin near the tip has provided valuable insight into the organization of these networks. Here we will sequentially focus on the role of the actin cytoskeleton in root hair tip growth and on how activities of different actin binding proteins in the apical part of growing root hairs contribute to build the fine F-actin configuration that correlates with tip growth.

Fluorescent protein-based technologies: shedding new light on the plant endomembrane system

Without doubt, GFP and spectral derivatives have revolutionized the way biologists approach their journey toward the discovery of how plant cells function. It is fascinating that in its early days GFP was used merely for localization studies, but as time progressed researchers successfully explored new avenues to push the power of GFP technology to reach new and exciting research frontiers. This has had a profound impact on the way we can now study complex and dynamic systems such as plant endomembranes. Here we briefly describe some of the approaches where GFP has revolutionized in vivo studies of protein distribution and dynamics and focus on two emerging approaches for the application of GFP technology in plant endomembranes, namely optical tweezers and forward genetics approaches, which are based either on the light or on genetic manipulation of secretory organelles to gain insights on the factors that control their activities and integrity.

Arabidopsis VILLIN2 and VILLIN3 are required for the generation of thick actin filament bundles and for directional organ growth

In plant cells, actin filament bundles serve as tracks for myosin-dependent organelle movement and play a role in the organization of the cytoplasm. Although virtually all plant cells contain actin filament bundles, the role of the different actin-bundling proteins remains largely unknown. In this study, we investigated the role of the actin-bundling protein villin in Arabidopsis (Arabidopsis thaliana). We used Arabidopsis T-DNA insertion lines to generate a double mutant in which VILLIN2 (VLN2) and VLN3 transcripts are truncated. Leaves, stems, siliques, and roots of vln2 vln3 double mutant plants are twisted, which is caused by local differences in cell length. Microscopy analysis of the actin cytoskeleton showed that in these double mutant plants, thin actin filament bundles are more abundant while thick actin filament bundles are virtually absent. In contrast to full-length VLN3, truncated VLN3 lacking the headpiece region does not rescue the phenotype of the vln2 vln3 double mutant. Our results show that villin is involved in the generation of thick actin filament bundles in several cell types and suggest that these bundles are involved in the regulation of coordinated cell expansion.

The relative effect of citral on mitotic microtubules in wheat roots and BY2 cells

The plant volatile monoterpene citral is a highly active compound with suggested allelopathic traits. Seed germination and seedling development are inhibited in the presence of citral, and it disrupts microtubules in both plant and animal cells in interphase. We addressed the following additional questions: can citral interfere with cell division; what is the relative effect of citral on mitotic microtubules compared to interphase cortical microtubules; what is its effect on newly formed cell plates; and how does it affect the association of microtubules with γ-tubulin? In wheat seedlings, citral led to inhibition of root elongation, curvature of newly formed cell walls and deformation of microtubule arrays. Citral's effect on microtubules was both dose- and time-dependent, with mitotic microtubules appearing to be more sensitive to citral than cortical microtubules. Association of γ-tubulin with microtubules was more sensitive to citral than were the microtubules themselves. To reveal the role of disrupted mitotic microtubules in dictating aberrations in cell plates in the presence of citral, we used tobacco BY2 cells expressing GFP-Tua6. Citral disrupted mitotic microtubules, inhibited the cell cycle and increased the frequency of asymmetric cell plates in these cells. The time scale of citral's effect in BY2 cells suggested a direct influence on cell plates during their formation. Taken together, we suggest that at lower concentrations, citral interferes with cell division by disrupting mitotic microtubules and cell plates, and at higher concentrations it inhibits cell elongation by disrupting cortical microtubules.

Myosin XIK is a major player in cytoplasm dynamics and is regulated by two amino acids in its tail

It has recently been found that among the 17 Arabidopsis myosins, six (XIC, XIE, XIK, XI-I, MYA1, and MYA2) have a major role in the motility of Golgi bodies and mitochondria in Nicotiana benthamiana and Nicotiana tabacum. Here, the same dominant negative tail fragments were also found to arrest the movement of Gogi bodies when transiently expressed in Arabidopsis plants. However, when a Golgi marker was transiently expressed in plants knocked out in these myosins, its movement was dramatically inhibited only in the xik mutant. In addition, a tail fragment of myosin XIK could inhibit the movement of several post-Golgi organelles, such as the trans-Golgi network, pre-vacuolar compartment, and endosomes, as well as total cytoplasmic streaming, suggesting that myosin XIK is a major player in cytoplasm kinetics. However, no co-localization of myosin tails with the arrested organelles was observed. Several deletion truncations of the myosin XIK tail were generated to corroborate function with localization. All deletion mutants possessing an inhibitory effect on organelle movement exhibited a diffuse cytoplasmic distribution. Point mutations in the tail of myosin XIK revealed that Arg1368 and Arg1443 are essential for its activity. These residues correspond to Lys1706 and Lys1779 from mouse myosin Va, which mediate the inhibitory head-tail interaction in this myosin. Therefore, such an interaction might underlie the dominant negative effect of truncated plant myosin tails and explain the mislocalization with target organelles.

High expression of Lifeact in Arabidopsis thaliana reduces dynamic reorganization of actin filaments but does not affect plant development

Lifeact is a novel probe that labels actin filaments in a wide range of organisms. We compared the localization and reorganization of Lifeact:Venus-labeled actin filaments in Arabidopsis root hairs and root epidermal cells of lines that express different levels of Lifeact: Venus with that of actin filaments labeled with GFP:FABD2, a commonly used probe in plants. Unlike GFP:FABD2, Lifeact:Venus labeled the highly dynamic fine F-actin in the subapical region of tip-growing root hairs. Lifeact:Venus expression at varying levels was not observed to affect plant development. However, at expression levels comparable to those of GFP:FABD2 in a well-characterized marker line, Lifeact:Venus reduced reorganization rates of bundles of actin filaments in root epidermal cells. Reorganization rates of cytoplasmic strands, which reflect the reorganization of the actin cytoskeleton, were also reduced in these lines. Moreover, in the same line, Lifeact:Venus-decorated actin filaments were more resistant to depolymerization by latrunculin B than those in an equivalent GFP:FABD2-expressing line. In lines where Lifeact: Venus is expressed at lower levels, these effects are less prominent or even absent. We conclude that Lifeact: Venus reduces remodeling of the actin cytoskeleton in Arabidopsis in a concentration-dependent manner. Since this reduction occurs at expression levels that do not cause defects in plant development, selection of normally growing plants is not sufficient to determine optimal Lifeact expression levels. When correct expression levels of Lifeact have been determined, it is a valuable probe that labels dynamic populations of actin filaments such as fine F-actin, better than FABD2 does.

Fungal lectin of Peltigera canina induces chemotropism of compatible Nostoc cells by constriction-relaxation pulses of cyanobiont cytoskeleton

A glycosylated arginase acting as a fungal lectin from Peltigera canina is able to produce recruitment of cyanobiont Nostoc cells and their adhesion to the hyphal surface. This implies that the cyanobiont would develop organelles to motility towards the chemoattractant. However when visualized by transmission electron microscopy, Nostoc cells recently isolated from P. canina thallus do not reveal any motile, superficial organelles, although their surface was covered by small spindles and serrated layer related to gliding. The use of S-(3,4-dichlorobenzyl)isothiourea, blebbistatin, phalloidin and latrunculin A provide circumstantial evidence that actin microfilaments rather than MreB, the actin-like protein from prokaryota, and, probably, an ATPase which develops contractile function similar to that of myosin II, are involved in cell motility. These experimental facts, the absence of superficial elements (fimbriae, pili or flagellum) related to cell movement, and the appearance of sunken cells during of after movement verified by scanning electron microscopy, support the hypothesis that the motility of lichen cyanobionts could be achieved by contraction-relaxation episodes of the cytoskeleton induced by fungal lectin act as a chemoattractant.

Golgi body motility in the plant cell cortex correlates with actin cytoskeleton organization

The actin cytoskeleton is involved in the transport and positioning of Golgi bodies, but the actin-based processes that determine the positioning and motility behavior of Golgi bodies are not well understood. In this work, we have studied the relationship between Golgi body motility behavior and actin organization in intercalary growing root epidermal cells during different developmental stages. We show that in these cells two distinct actin configurations are present, depending on the developmental stage. In small cells of the early root elongation zone, fine filamentous actin (F-actin) occupies the whole cell, including the cortex. In larger cells in the late elongation zone that have almost completed cell elongation, actin filament bundles are interspersed with areas containing this fine F-actin and areas without F-actin. Golgi bodies in areas with the fine F-actin exhibit a non-directional, wiggling type of motility. Golgi bodies in areas containing actin filament bundles move up to 7 μm s⁻¹. Since the motility of Golgi bodies changes when they enter an area with a different actin configuration, we conclude that the type of movement depends on the actin organization and not on the individual organelle. Our results show that the positioning of Golgi bodies depends on the local actin organization.

Recent advances in understanding plant myosin function: life in the fast lane

Plant myosins are required for organelle movement, and a role in actin organization has recently come to light. Myosin mutants display several gross morphological phenotypes, the most severe being dwarfism and reduced fecundity, and there is a correlation between reduced organelle movement and morphological defects. This review aims to discuss recent findings in plants relating to the role of myosins in actin dynamics, development, and organelle movement, more specifically the endoplasmic reticulum (ER). One overarching theme is that there still appear to be more questions than answers relating to plant myosin function and regulation.

Optical tweezers for the micromanipulation of plant cytoplasm and organelles

Laser trapping of micron-sized particles can be achieved utilizing the radiation pressure generated by a focused infrared laser beam. Thus, it is theoretically possible to trap and manipulate organelles within the cytoplasm and remodel the architecture of the cytoplasm and membrane systems. Here we describe recent progress, using this under utilized technology, in the manipulation of cytoplasmic strands and organelles in plant cells.

Motoring around the plant cell: insights from plant myosins

Organelle movement in plants cells is extremely dynamic. Movement is driven by the acto-myosin system. Higher plant myosins fall into two classes: classes XI and VIII. Localization studies have highlighted that myosins are present throughout the cytosol, label motile puncta and decorate the nuclear envelope and plasma membrane. Functional studies through expression of dominant-negative myosin variants, RNAi (RNA interference) and T-DNA insertional analysis have shown that class XI myosins are required for organelle movement. Intriguingly, organelle movement is also linked to Arabidopsis growth and development. The present review tackles current findings relating to plant organelle movement and the role of myosins.

Organelle biogenesis and positioning in plants

The biogenesis and positioning of organelles involves complex interacting processes and precise control. Progress in our understanding is being made rapidly as advances in analysing the nuclear and organellar genome and proteome combine with developments in live-cell microscopy and manipulation at the subcellular level. This paper introduces the collected papers resulting from Organelle Biogenesis and Positioning in Plants, the 2009 Biochemical Society Annual Symposium. Including papers on the nuclear envelope and all major organelles, it considers current knowledge and progress towards unifying themes that will elucidate the mechanisms by which cells generate the correct complement of organelles and adapt and change it in response to environmental and developmental signals.

Probing cytoplasmic organization and the actin cytoskeleton of plant cells with optical tweezers

In interphase plant cells, the actin cytoskeleton is essential for intracellular transport and organization. To fully understand how the actin cytoskeleton functions as the structural basis for cytoplasmic organization, both molecular and physical aspects of the actin organization have to be considered. In the present review, we discuss literature that gives an insight into how cytoplasmic organization is achieved and in which actin-binding proteins have been identified that play a role in this process. We discuss how physical properties of the actin cytoskeleton in the cytoplasm of live plant cells, such as deformability and elasticity, can be probed by using optical tweezers. This technique allows non-invasive manipulation of cytoplasmic organization. Optical tweezers, integrated in a confocal microscope, can be used to manipulate cytoplasmic organization while studying actin dynamics. By combining this with mutant studies and drug applications, insight can be obtained about how the physical properties of the actin cytoskeleton, and thus the cytoplasmic organization, are influenced by different cellular processes.