Class VIII myosins are required for plasmodesmatal localization of a closterovirus Hsp70 homolog

Actomyosin-driven motility and coalescence of phase-separated viral inclusion bodies are required for efficient replication of a plant rhabdovirus

Phase separation has emerged as a fundamental principle for organizing viral and cellular membraneless organelles. Although these subcellular compartments have been recognized for decades, their biogenesis and mechanisms of regulation are poorly understood. Here, we investigate the formation of membraneless inclusion bodies (IBs) induced during the infection of a plant rhabdovirus, tomato yellow mottle-associated virus (TYMaV). We generated recombinant TYMaV encoding a fluorescently labeled IB constituent protein and employed live-cell imaging to characterize the intracellular dynamics and maturation of viral IBs in infected Nicotiana benthamiana cells. We show that TYMaV IBs are phase-separated biomolecular condensates and that viral nucleoprotein and phosphoprotein are minimally required for IB formation in vivo and in vitro. TYMaV IBs move along the microfilaments, likely through the anchoring of viral phosphoprotein to myosin XIs. Furthermore, pharmacological disruption of microfilaments or inhibition of myosin XI functions suppresses IB motility, resulting in arrested IB growth and inefficient virus replication. Our study establishes phase separation as a process driving the formation of liquid viral factories and emphasizes the role of the cytoskeletal system in regulating the dynamics of condensate maturation.

Identification of myosin genes and their expression in response to biotic (PVY, PVX, PVS, and PVA) and abiotic (Drought, Heat, Cold, and High-light) stress conditions in potato

BACKGROUND

Plant organelles are highly motile where their movement is significant for fast distribution of material around the cell, facilitation of the plant's ability to respond to abiotic and biotic signals, and for appropriate growth. Abiotic and biotic stresses are among the major factors limiting crop yields, and biological membranes are the first target of these stresses. Plants utilize adaptive mechanisms namely myosin to repair injured membranes following exposure to abiotic and biotic stresses.

OBJECTIVE

Due to the economic importance and cultivation of potato grown under abiotic and biotic stress prone areas, identification and characterization of myosin family members in potato were performed in the present research.

METHODS

To identify the myosin genes in potato, we performed genome-wide analysis of myosin genes in the S. tuberosum genome using the phytozome. All putative sequences were approved with the interproscan. Bioinformatics analysis was conducted using phylogenetic tree, gene structure, cis-regulatory elements, protein-protein interaction, and gene expression.

RESULT

The majority of the cell machinery contain actin cytoskeleton and myosins, where motility of organelles are dependent on them. Homology-based analysis was applied to determine seven myosin genes in the potato genome. The members of myosin could be categorized into two groups (XI and VIII). Some of myosin proteins were sub-cellularly located in the nucleus containing 71.5% of myosin proteins and other myosin proteins were localized in the mitochondria, plasma-membrane, and cytoplasm. Determination of co-expressed network, promoter analysis, and gene structure were also performed and gene expression pattern of each gene was surveyed. Number of introns in the gene family members varied from 1 to 39. Gene expression analysis demonstrated that StMyoXI-B and StMyoVIII-2 had the highest transcripts, induced by biotic and abiotic stresses in all three tissues of stem, root, and leaves, respectively. Overall, different cis-elements including abiotic and biotic responsive, hormonal responsive, light responsive, defense responsive elements were found in the myosin promoter sequences. Among the cis-elements, the MYB, G-box, ABRE, JA, and SA contributed the most in the plant growth and development, and in response to abiotic and biotic stress conditions.

CONCLUSION

Our results showed that myosin genes can be utilized in breeding programs and genetic engineering of plants with the aim of increasing tolerance to abiotic and biotic stresses, especially to viral stresses such as PVY, PVX, PVA, PVS, high light, drought, cold and heat.

Barley Ror1 encodes a class XI myosin required for mlo-based broad-spectrum resistance to the fungal powdery mildew pathogen

Loss-of-function alleles of plant MLO genes confer broad-spectrum resistance to powdery mildews in many eudicot and monocot species. Although barley (Hordeum vulgare) mlo mutants have been used in agriculture for more than 40 years, understanding of the molecular principles underlying this type of disease resistance remains fragmentary. Forward genetic screens in barley have revealed mutations in two Required for mlo resistance (Ror) genes that partially impair immunity conferred by mlo mutants. While Ror2 encodes a soluble N-ethylmaleimide-sensitive factor-attached protein receptor (SNARE), the identity of Ror1, located at the pericentromeric region of barley chromosome 1H, remained elusive. We report the identification of Ror1 based on combined barley genomic sequence information and transcriptomic data from ror1 mutant plants. Ror1 encodes the barley class XI myosin Myo11A (HORVU.MOREX.r3.1HG0046420). Single amino acid substitutions of this myosin, deduced from non-functional ror1 mutant alleles, map to the nucleotide-binding region and the interface between the relay-helix and the converter domain of the motor protein. Ror1 myosin accumulates transiently in the course of powdery mildew infection. Functional fluorophore-labeled Ror1 variants associate with mobile intracellular compartments that partially colocalize with peroxisomes. Single-cell expression of the Ror1 tail region causes a dominant-negative effect that phenocopies ror1 loss-of-function mutants. We define a myosin motor for the establishment of mlo-mediated resistance, suggesting that motor protein-driven intracellular transport processes are critical for extracellular immunity, possibly through the targeted transfer of antifungal and/or cell wall cargoes to pathogen contact sites.

Collisions of Cortical Microtubules with Membrane Associated Myosin VIII Tail

The distribution of myosin VIII ATM1 tail in association with the plasma membrane is often observed in coordination with that of cortical microtubules (MTs). The prevailing hypothesis is that coordination between the organization of cortical MTs and proteins in the membrane results from the inhibition of free lateral diffusion of the proteins by barriers formed by MTs. Since the positioning of myosin VIII tail in the membrane is relatively stable, we ask: can it affect the organization of MTs? Myosin VIII ATM1 tail co-localized with remorin 6.6, the position of which in the plasma membrane is also relatively stable. Overexpression of myosin VIII ATM1 tail led to a larger fraction of MTs with a lower rate of orientation dispersion. In addition, collisions between MTs and cortical structures labeled by ATM1 tail or remorin 6.6 were observed. Collisions between EB1 labeled MTs and ATM1 tail clusters led to four possible outcomes: 1—Passage of MTs through the cluster; 2—Decreased elongation rate; 3—Disengagement from the membrane followed by a change in direction; and 4—retraction. EB1 tracks became straighter in the presence of ATM1 tail. Taken together, collisions of MTs with ATM1 tail labeled structures can contribute to their coordinated organization.

Plasmodesmata-Involved Battle Against Pathogens and Potential Strategies for Strengthening Hosts

Plasmodesmata (PD) are membrane-lined pores that connect adjacent cells to mediate symplastic communication in plants. These intercellular channels enable cell-to-cell trafficking of various molecules essential for plant development and stress responses, but they can also be utilized by pathogens to facilitate their infection of hosts. Some pathogens or their effectors are able to spread through the PD by modifying their permeability. Yet plants have developed various corresponding defense mechanisms, including the regulation of PD to impede the spread of invading pathogens. In this review, we aim to illuminate the various roles of PD in the interactions between pathogens and plants during the infection process. We summarize the pathogenic infections involving PD and how the PD could be modified by pathogens or hosts. Furthermore, we propose several hypothesized and promising strategies for enhancing the disease resistance of host plants by the appropriate modulation of callose deposition and plasmodesmal permeability based on current knowledge.

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.

Genome Editing for Plasmodesmal Biology

Plasmodesmata (PD) are cytoplasmic canals that facilitate intercellular communication and molecular exchange between adjacent plant cells. PD-associated proteins are considered as one of the foremost factors in regulating PD function that is critical for plant development and stress responses. Although its potential to be used for crop engineering is enormous, our understanding of PD biology was relatively limited to model plants, demanding further studies in crop systems. Recently developed genome editing techniques such as Clustered Regularly Interspaced Short Palindromic Repeats/CRISPR associate protein (CRISPR/Cas) might confer powerful approaches to dissect the molecular function of PD components and to engineer elite crops. Here, we assess several aspects of PD functioning to underline and highlight the potential applications of CRISPR/Cas that provide new insight into PD biology and crop improvement.

Viruses Reveal the Secrets of Plasmodesmal Cell Biology

Plasmodesmata (PD) are essential for intercellular trafficking of molecules required for plant life, from small molecules like sugars and ions to macromolecules including proteins and RNA molecules that act as signals to regulate plant development and defense. As obligate intracellular pathogens, plant viruses have evolved to manipulate this communication system to facilitate the initial cell-to-cell and eventual systemic spread in their plant hosts. There has been considerable interest in how viruses manipulate the PD that connect the protoplasts of neighboring cells, and viruses have yielded invaluable tools for probing the structure and function of PD. With recent advances in biochemistry and imaging, we have gained new insights into the composition and structure of PD in the presence and absence of viruses. Here, we first discuss viral strategies for manipulating PD for their intercellular movement and examine how this has shed light on our understanding of native PD function. We then address the controversial role of the cytoskeleton in trafficking to and through PD. Finally, we address how viruses could alter PD structure and consider possible mechanisms of the phenomenon described as 'gating'. This discussion supports the significance of virus research in elucidating the properties of PD, these persistently enigmatic plant organelles.

MYH9 Aggregation Induced by Direct Interaction With PRRSV GP5 Ectodomain Facilitates Viral Internalization by Permissive Cells

Prevention and control of infection by porcine reproductive and respiratory syndrome virus (PRRSV) remains a challenge, due to our limited understanding of the PRRSV invasion mechanism. Our previous study has shown that PRRSV glycoprotein GP5 interacts with MYH9 C-terminal domain protein (PRA). Here we defined that the first ectodomain of GP5 (GP5-ecto-1) directly interacted with PRA and this interaction triggered PRA and endogenous MYH9 to form filament assembly. More importantly, MYH9 filament assembly was also formed in GP5-ecto-1-transfected MARC-145 cells. Notably, PRRSV infection of MARC-145 cells and porcine alveolar macrophages also induced endogenous MYH9 aggregation and polymerization that were required for subsequent PRRSV internalization. Moreover, overexpression of S100A4, a MYH9-specific disassembly inducer, in MARC-145 cells significantly resulted in diminished MYH9 aggregation and marked inhibition of subsequent virion internalization and infection by both PRRSV-1 and PRRSV-2 isolates. The collective results of this work reveal a novel molecular mechanism employed by MYH9 that helps PRRSV gain entry into permissive cells.

A cytorhabdovirus phosphoprotein forms mobile inclusions trafficked on the actin/ER network for viral RNA synthesis

As obligate parasites, plant viruses usually hijack host cytoskeletons for replication and movement. Rhabdoviruses are enveloped, negative-stranded RNA viruses that infect vertebrates, invertebrates, and plants, but the mechanisms of intracellular trafficking of plant rhabdovirus proteins are largely unknown. Here, we used Barley yellow striate mosaic virus (BYSMV), a plant cytorhabdovirus, as a model to investigate the effects of the actin cytoskeleton on viral intracellular movement and viral RNA synthesis in a mini-replicon (MR) system. The BYSMV P protein forms mobile inclusion bodies that are trafficked along the actin/endoplasmic reticulum network, and recruit the N and L proteins into viroplasm-like structures. Deletion analysis showed that the N terminal region (aa 43-55) and the remaining region (aa 56-295) of BYSMV P are essential for the mobility and formation of inclusions, respectively. Overexpression of myosin XI-K tails completely abolishes the trafficking activity of P bodies, and is accompanied by a significant reduction of viral MR RNA synthesis. These results suggest that BYSMV P contributes to the formation and trafficking of viroplasm-like structures along the ER/actin network driven by myosin XI-K. Thus, rhabdovirus P appears to be a dynamic hub protein for efficient recruitment of viral proteins, thereby promoting viral RNA synthesis.

Key checkpoints in the movement of plant viruses through the host

Plant viruses cannot exploit any of the membrane fusion-based routes of entry described for animal viruses. In addition, one of the distinctive structures of plant cells, the cell wall, acts as the first barrier against the invasion of pathogens. To overcome the rigidity of the cell wall, plant viruses normally take advantage of the way of life of different biological vectors. Alternatively, the physical damage caused by environmental stresses can facilitate virus entry. Once inside the cell and taking advantage of the characteristic symplastic continuity of plant cells, viruses need to remodel and/or modify the restricted pore size of the plasmodesmata (channels that connect plant cells). In a successful interaction for the virus, it can reach the vascular tissue to systematically invade the plant. The connections between the different cell types in this path are not designed to allow the passage of molecules with the complexity of viruses. During this process, viruses face different cell barriers that must be overcome to reach the distal parts of the plant. In this review, we highlight the current knowledge about how plant RNA viruses enter plant cells, move between them to reach vascular cells and overcome the different physical and cellular barriers that the phloem imposes. Finally, we update the current research on cellular organelles as key regulator checkpoints in the long-distance movement of plant viruses.

A Distinct, Non-Virion Plant Virus Movement Protein Encoded by a Crinivirus Essential for Systemic Infection

Plant-infecting viruses utilize various strategies involving multiple viral and host factors to achieve successful systemic infections of their compatible hosts. Lettuce infectious yellows virus (LIYV), genus Crinivirus, family Closteroviridae, has long, filamentous flexuous virions and causes phloem-limited infections in its plant hosts. The LIYV-encoded P26 is a distinct non-virion protein that shows no similarities to proteins in current databases: it induces plasmalemma deposits over plasmadesmata (PD) pit fields and is speculated to have roles in LIYV virion transport within infected plants. In this study, P26 was demonstrated to be a PD-localized protein, and its biological significance was tested in planta by mutagenesis analysis. An LIYV P26 knockout mutant (P26X) showed viral RNA replication and virion formation in inoculated leaves of Nicotiana benthamiana plants, but failed to give systemic infection. Confirmation by using a modified green fluorescent protein (GFP)-tagged LIYV P26X showed GFP accumulation only in infiltrated leaf tissues, while wild-type LIYV GFP readily spread systemically in the phloem. Attempts to rescue P26X by complementation in trans were negative. However a translocated LIYV P26 gene in the LIYV genome rescued systemic infection, but P26 orthologs from other criniviruses did not. Mutagenesis in planta assays showed that deletions in P26, as well as 2 of 11 specific alanine-scanning mutants, abolished the ability to systemically infect N. benthamianaIMPORTANCE Plant viruses encode specific proteins that facilitate their ability to establish multicellular/systemic infections in their host plants. Relatively little is known of the transport mechanisms for plant viruses whose infections are phloem limited, including those of the family Closteroviridae. These viruses have complex, long filamentous virions that spread through the phloem. Lettuce infectious yellows virus (LIYV) encodes a non-virion protein, P26, which forms plasmalemma deposits over plasmodesmata pit fields, and LIYV virions are consistently found attached to those deposits. Here we demonstrate that P26 is a unique movement protein required for LIYV systemic infection in plants. LIYV P26 shows no sequence similarities to other proteins, but other criniviruses encode P26 orthologs. However, these failed to complement movement of LIYV P26 mutants.

Joining the Crowd: Integrating Plant Virus Proteins into the Larger World of Pathogen Effectors

The first bacterial and viral avirulence ( avr) genes were cloned in 1984. Although virus and bacterial avr genes were physically isolated in the same year, the questions associated with their characterization after discovery were very different, and these differences had a profound influence on the narrative of host-pathogen interactions for the past 30 years. Bacterial avr proteins were subsequently shown to suppress host defenses, leading to their reclassification as effectors, whereas research on viral avr proteins centered on their role in the viral infection cycle rather than their effect on host defenses. Recent studies that focus on the multifunctional nature of plant virus proteins have shown that some virus proteins are capable of suppression of the same host defenses as bacterial effectors. This is exemplified by the P6 protein of Cauliflower mosaic virus (CaMV), a multifunctional plant virus protein that facilitates several steps in the infection, including modulation of host defenses. This review highlights the modular structure and multifunctional nature of CaMV P6 and illustrates its similarities to other, well-established pathogen effectors.

Targeting of rice grassy stunt virus pc6 protein to plasmodesmata requires the ER-to-Golgi secretory pathway and an actin-myosin VIII motility system

The nonstructural protein pc6 encoded by rice grassy stunt virus (RGSV) plays a significant role in viral cell-to-cell movement, presumably by transport through plasmodesmata (PD). We confirmed the association of pc6 with PD, and also elucidated the mechanisms of protein targeting to PD. Several inhibitor treatments showed conclusively that pc6 is targeted to PD via the ER-to-Golgi secretory system and actin filaments. In addition, VIII-1 myosin was also found to be involved in pc6 PD targeting. Deletion mutants demonstrated that C-terminal amino acid residues 209-229 (transmembrane domain 2; TM2) are essential for pc6 to move through PD.

Setting Up Shop: The Formation and Function of the Viral Factories of Cauliflower mosaic virus

Similar to cells, viruses often compartmentalize specific functions such as genome replication or particle assembly. Viral compartments may contain host organelle membranes or they may be mainly composed of viral proteins. These compartments are often termed: inclusion bodies (IBs), viroplasms or viral factories. The same virus may form more than one type of IB, each with different functions, as illustrated by the plant pararetrovirus, Cauliflower mosaic virus (CaMV). CaMV forms two distinct types of IBs in infected plant cells, those composed mainly of the viral proteins P2 (which are responsible for transmission of CaMV by insect vectors) and P6 (required for viral intra-and inter-cellular infection), respectively. P6 IBs are the major focus of this review. Much of our understanding of the formation and function of P6 IBs comes from the analyses of their major protein component, P6. Over time, the interactions and functions of P6 have been gradually elucidated. Coupled with new technologies, such as fluorescence microscopy with fluorophore-tagged viral proteins, these data complement earlier work and provide a clearer picture of P6 IB formation. As the activities and interactions of the viral proteins have gradually been determined, the functions of P6 IBs have become clearer. This review integrates the current state of knowledge on the formation and function of P6 IBs to produce a coherent model for the activities mediated by these sophisticated virus-manufacturing machines.

Plasma membrane-associated cation-binding protein 1-like protein negatively regulates intercellular movement of BaMV

To establish a successful infection, a virus needs to replicate and move cell-to-cell efficiently. We investigated whether one of the genes upregulated in Nicotiana benthamiana after Bamboo mosaic virus (BaMV) inoculation was involved in regulating virus movement. We revealed the gene to be a plasma membrane-associated cation-binding protein 1-like protein, designated NbPCaP1L. The expression of NbPCaP1L in N. benthamiana was knocked down using Tobacco rattle virus-based gene silencing and consequently the accumulation of BaMV increased significantly to that of control plants. Further analysis indicated no significant difference in the accumulation of BaMV in NbPCaP1L knockdown and control protoplasts, suggesting NbPCaP1L may affect cell-to-cell movement of BaMV. Using a viral vector expressing green fluorescent protein in the knockdown plants, the mean area of viral focus, as determined by fluorescence, was found to be larger in NbPCaP1L knockdown plants. Orange fluorescence protein (OFP)-fused NbPCaP1L, NbPCaP1L-OFP, was expressed in N. benthamiana and reduced the accumulation of BaMV to 46%. To reveal the possible interaction of viral protein with NbPCaP1L, we performed yeast two-hybrid and co-immunoprecipitation experiments. The results indicated that NbPCaP1L interacted with BaMV replicase. The results also suggested that NbPCaP1L could trap the BaMV movement RNP complex via interaction with the viral replicase in the complex and so restricted viral cell-to-cell movement.

Inspirations on Virus Replication and Cell-to-Cell Movement from Studies Examining the Cytopathology Induced by Lettuce infectious yellows virus in Plant Cells

Lettuce infectious yellows virus (LIYV) is the type member of the genus Crinivirus in the family Closteroviridae. Like many other positive-strand RNA viruses, LIYV infections induce a number of cytopathic changes in plant cells, of which the two most characteristic are: Beet yellows virus-type inclusion bodies composed of vesicles derived from cytoplasmic membranes; and conical plasmalemma deposits (PLDs) located at the plasmalemma over plasmodesmata pit fields. The former are not only found in various closterovirus infections, but similar structures are known as 'viral factories' or viroplasms in cells infected with diverse types of animal and plant viruses. These are generally sites of virus replication, virion assembly and in some cases are involved in cell-to-cell transport. By contrast, PLDs induced by the LIYV-encoded P26 non-virion protein are not involved in replication but are speculated to have roles in virus intercellular movement. These deposits often harbor LIYV virions arranged to be perpendicular to the plasma membrane over plasmodesmata, and our recent studies show that P26 is required for LIYV systemic plant infection. The functional mechanism of how LIYV P26 facilitates intercellular movement remains unclear, however, research on other plant viruses provides some insights on the possible ways of viral intercellular movement through targeting and modifying plasmodesmata via interactions between plant cellular components and viral-encoded factors. In summary, beginning with LIYV, we review the studies that have uncovered the biological determinants giving rise to these cytopathological effects and their importance in viral replication, virion assembly and intercellular movement during the plant infection by closteroviruses, and compare these findings with those for other positive-strand RNA viruses.

Similarities in intracellular transport of plant viral movement proteins BMB2 and TGB3

The cell-to-cell transport of many plant viruses through plasmodesmata requires viral movement proteins (MPs) encoded by a 'triple gene block' (TGB) and termed TGB1, TGB2 and TGB3. TGB3 is a small integral membrane protein that contains subcellular targeting signals and directs both TGB2 and the helicase domain-containing TGB1 protein to plasmodesmata-associated structures. Recently, we described a 'binary movement block' (BMB) coding for two MPs, BMB1 and BMB2. The BMB2 protein associates with endoplasmic reticulum (ER) membranes, accumulates at plasmodesmata-associated membrane bodies and directs the BMB1 helicase to these structures. TGB3 transport to cell peripheral bodies was previously shown to bypass the secretory pathway and involve a non-conventional mechanism. Here, we provide evidence that the intracellular transport of both poa semilatent virus TGB3 and hibiscus green spot virus BMB2 to plasmodesmata-associated sites can occur via lateral translocation along the ER membranes. Agrobacterium-mediated transient co-expression in Nicotiana benthamiana leaves revealed that green fluorescent protein (GFP)-fused actin-binding domains of Arabidopsis fimbrin (ABD2-GFP) and mouse talin (TAL-GFP) inhibited the subcellular targeting of TGB3 and BMB2 to plasmodesmata-associated bodies, which resulted in TGB3 and BMB2 accumulation in the cytoplasm in association with aberrant ER structures. Inhibition of COPII budding complex formation by the expression of a dominant-negative mutant of the small GTPase Sar1 had no detectable effect on BMB2 subcellular targeting, which therefore could occur without exit from the ER in COPII transport vesicles. Collectively, the presented data support the current view that plant viral MPs exploit the ER:actin network for their intracellular transport.

A model for intracellular movement of Cauliflower mosaic virus: the concept of the mobile virion factory

The genomes of many plant viruses have a coding capacity limited to <10 proteins, yet it is becoming increasingly clear that individual plant virus proteins may interact with several targets in the host for establishment of infection. As new functions are uncovered for individual viral proteins, virologists have realized that the apparent simplicity of the virus genome is an illusion that belies the true impact that plant viruses have on host physiology. In this review, we discuss our evolving understanding of the function of the P6 protein of Cauliflower mosaic virus (CaMV), a process that was initiated nearly 35 years ago when the CaMV P6 protein was first described as the 'major inclusion body protein' (IB) present in infected plants. P6 is now referred to in most articles as the transactivator (TAV)/viroplasmin protein, because the first viral function to be characterized for the Caulimovirus P6 protein beyond its role as an inclusion body protein (the viroplasmin) was its role in translational transactivation (the TAV function). This review will discuss the currently accepted functions for P6 and then present the evidence for an entirely new function for P6 in intracellular movement.

Viral Replication Protein Inhibits Cellular Cofilin Actin Depolymerization Factor to Regulate the Actin Network and Promote Viral Replicase Assembly

RNA viruses exploit host cells by co-opting host factors and lipids and escaping host antiviral responses. Previous genome-wide screens with Tomato bushy stunt virus (TBSV) in the model host yeast have identified 18 cellular genes that are part of the actin network. In this paper, we show that the p33 viral replication factor interacts with the cellular cofilin (Cof1p), which is an actin depolymerization factor. Using temperature-sensitive (ts) Cof1p or actin (Act1p) mutants at a semi-permissive temperature, we find an increased level of TBSV RNA accumulation in yeast cells and elevated in vitro activity of the tombusvirus replicase. We show that the large p33 containing replication organelle-like structures are located in the close vicinity of actin patches in yeast cells or around actin cable hubs in infected plant cells. Therefore, the actin filaments could be involved in VRC assembly and the formation of large viral replication compartments containing many individual VRCs. Moreover, we show that the actin network affects the recruitment of viral and cellular components, including oxysterol binding proteins and VAP proteins to form membrane contact sites for efficient transfer of sterols to the sites of replication. Altogether, the emerging picture is that TBSV, via direct interaction between the p33 replication protein and Cof1p, controls cofilin activities to obstruct the dynamic actin network that leads to efficient subversion of cellular factors for pro-viral functions. In summary, the discovery that TBSV interacts with cellular cofilin and blocks the severing of existing filaments and the formation of new actin filaments in infected cells opens a new window to unravel the way by which viruses could subvert/co-opt cellular proteins and lipids. By regulating the functions of cofilin and the actin network, which are central nodes in cellular pathways, viruses could gain supremacy in subversion of cellular factors for pro-viral functions.

The actin network, which is a central node in cellular pathways, is frequently targeted by various pathogens to modulate cellular responses. In this paper, the authors show that TBSV interacts with cofilin actin depolymerization factor leading to inhibition of the dynamic function of the actin network in infected cells. This allows TBSV to utilize the existing actin filaments to efficiently recruit host proteins and lipids for viral replication and to build viral replication compartments for robust viral replication. Altogether, subversion of the actin network by TBSV is a key step for the virus to gain access to cellular resources required for virus replication.

Arabidopsis Myosins XI1, XI2, and XIK Are Crucial for Gravity-Induced Bending of Inflorescence Stems

Myosins and actin filaments in the actomyosin system act in concert in regulating cell structure and dynamics and are also assumed to contribute to plant gravitropic response. To investigate the role of the actomyosin system in the inflorescence stem gravitropism, we used single and multiple mutants affecting each of the 17 Arabidopsis myosins of class VIII and XI. We show that class XI but not class VIII myosins are required for stem gravitropism. Simultaneous loss of function of myosins XI1, XI2, and XIK leads to impaired gravitropic bending that is correlated with altered growth, stiffness, and insufficient sedimentation of gravity sensing amyloplasts in stem endodermal cells. The gravitropic defect of the corresponding triple mutant xi1 xi2 xik could be rescued by stable expression of the functional XIK:YFP in the mutant background, indicating a role of class XI myosins in this process. Altogether, our results emphasize the critical contributions of myosins XI in stem gravitropism of Arabidopsis.

Cucumber Necrosis Virus Recruits Cellular Heat Shock Protein 70 Homologs at Several Stages of Infection

RNA viruses often depend on host factors for multiplication inside cells due to the constraints of their small genome size and limited coding capacity. One such factor that has been exploited by several plant and animal viruses is heat shock protein 70 (HSP70) family homologs which have been shown to play roles for different viruses in viral RNA replication, viral assembly, disassembly, and cell-to-cell movement. Using next generation sequence analysis, we reveal that several isoforms of Hsp70 and Hsc70 transcripts are induced to very high levels during cucumber necrosis virus (CNV) infection of Nicotiana benthamiana and that HSP70 proteins are also induced by at least 10-fold. We show that HSP70 family protein homologs are co-opted by CNV at several stages of infection. We have found that overexpression of Hsp70 or Hsc70 leads to enhanced CNV genomic RNA, coat protein (CP), and virion accumulation, whereas downregulation leads to a corresponding decrease. Hsc70-2 was found to increase solubility of CNV CP in vitro and to increase accumulation of CNV CP independently of viral RNA replication during coagroinfiltration in N. benthamiana. In addition, virus particle assembly into virus-like particles in CP agroinfiltrated plants was increased in the presence of Hsc70-2. HSP70 was found to increase the targeting of CNV CP to chloroplasts during infection, reinforcing the role of HSP70 in chloroplast targeting of host proteins. Hence, our findings have led to the discovery of a highly induced host factor that has been co-opted to play multiple roles during several stages of the CNV infection cycle.

IMPORTANCE

Because of the small size of its RNA genome, CNV is dependent on interaction with host cellular components to successfully complete its multiplication cycle. We have found that CNV induces HSP70 family homologs to a high level during infection, possibly as a result of the host response to the high levels of CNV proteins that accumulate during infection. Moreover, we have found that CNV co-opts HSP70 family homologs to facilitate several aspects of the infection process such as viral RNA, coat protein and virus accumulation. Chloroplast targeting of the CNV CP is also facilitated, which may aid in CNV suppression of host defense responses. Several viruses have been shown to induce HSP70 during infection and others to utilize HSP70 for specific aspects of infection such as replication, assembly, and disassembly. We speculate that HSP70 may play multiple roles in the infection processes of many viruses.

Class XI Myosins Move Specific Organelles in Pollen Tubes and Are Required for Normal Fertility and Pollen Tube Growth in Arabidopsis

Pollen tube growth is an essential aspect of plant reproduction because it is the mechanism through which nonmotile sperm cells are delivered to ovules, thus allowing fertilization to occur. A pollen tube is a single cell that only grows at the tip, and this tip growth has been shown to depend on actin filaments. It is generally assumed that myosin-driven movements along these actin filaments are required to sustain the high growth rates of pollen tubes. We tested this conjecture by examining seed set, pollen fitness, and pollen tube growth for knockout mutants of five of the six myosin XI genes expressed in pollen of Arabidopsis (Arabidopsis thaliana). Single mutants had little or no reduction in overall fertility, whereas double mutants of highly similar pollen myosins had greater defects in pollen tube growth. In particular, myo11c1 myo11c2 pollen tubes grew more slowly than wild-type pollen tubes, which resulted in reduced fitness compared with the wild type and a drastic reduction in seed set. Golgi stack and peroxisome movements were also significantly reduced, and actin filaments were less organized in myo11c1 myo11c2 pollen tubes. Interestingly, the movement of yellow fluorescent protein-RabA4d-labeled vesicles and their accumulation at pollen tube tips were not affected in the myo11c1 myo11c2 double mutant, demonstrating functional specialization among myosin isoforms. We conclude that class XI myosins are required for organelle motility, actin organization, and optimal growth of pollen tubes.

DEVELOPMENTALLY REGULATED PLASMA MEMBRANE PROTEIN of Nicotiana benthamiana contributes to potyvirus movement and transports to plasmodesmata via the early secretory pathway and the actomyosin system

The intercellular movement of plant viruses requires both viral and host proteins. Previous studies have demonstrated that the frame-shift protein P3N-PIPO (for the protein encoded by the open reading frame [ORF] containing 5'-terminus of P3 and a +2 frame-shift ORF called Pretty Interesting Potyviridae ORF and embedded in the P3) and CYLINDRICAL INCLUSION (CI) proteins were required for potyvirus cell-to-cell movement. Here, we provide genetic evidence showing that a Tobacco vein banding mosaic virus (TVBMV; genus Potyvirus) mutant carrying a truncated PIPO domain of 58 amino acid residues could move between cells and induce systemic infection in Nicotiana benthamiana plants; mutants carrying a PIPO domain of seven, 20, or 43 amino acid residues failed to move between cells and cause systemic infection in this host plant. Interestingly, the movement-defective mutants produced progeny that eliminated the previously introduced stop codons and thus restored their systemic movement ability. We also present evidence showing that a developmentally regulated plasma membrane protein of N. benthamiana (referred to as NbDREPP) interacted with both P3N-PIPO and CI of the movement-competent TVBMV. The knockdown of NbDREPP gene expression in N. benthamiana impeded the cell-to-cell movement of TVBMV. NbDREPP was shown to colocalize with TVBMV P3N-PIPO and CI at plasmodesmata (PD) and traffic to PD via the early secretory pathway and the actomyosin motility system. We also show that myosin XI-2 is specially required for transporting NbDREPP to PD. In conclusion, NbDREPP is a key host protein within the early secretory pathway and the actomyosin motility system that interacts with two movement proteins and influences virus movement.

Plasmodesmata: channels for viruses on the move

The symplastic communication network established by plasmodesmata (PD) and connected phloem provides an essential pathway for spatiotemporal intercellular signaling in plant development but is also exploited by viruses for moving their genomes between cells in order to infect plants systemically. Virus movement depends on virus-encoded movement proteins (MPs) that target PD and therefore represent important keys to the cellular mechanisms underlying the intercellular trafficking of viruses and other macromolecules. Viruses and their MPs have evolved different mechanisms for intracellular transport and interaction with PD. Some viruses move from cell to cell by interacting with cellular mechanisms that control the size exclusion limit of PD whereas other viruses alter the PD architecture through assembly of specialized transport structures within the channel. Some viruses move between cells in the form of assembled virus particles whereas other viruses may interact with nucleic acid transport mechanisms to move their genomes in a non-encapsidated form. Moreover, whereas several viruses rely on the secretory pathway to target PD, other viruses interact with the cortical endoplasmic reticulum and associated cytoskeleton to spread infection. This chapter provides an introduction into viruses and their role in studying the diverse cellular mechanisms involved in intercellular PD-mediated macromolecular trafficking.

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.

Subcellular localization of p29, a putative movement protein of pepper ringspot virus

Pepper ringspot virus (PepRSV) is a member of the genus Tobravirus. It possesses a bipartite single-strand RNA genome in a positive-sense polarity. The p29 protein is encoded by RNA 1 and is presumed to be the movement protein (MP) of this virus. In this study, the intracellular distribution of the p29 protein was analyzed by confocal microscopy. Transient expression of the PepRSV p29 protein fused to green fluorescent protein was observed as punctate spots localized next to the cell wall. This protein partially co-localized with the eCFP-tagged tobacco mosaic virus 30K MP, which is known to associate with plasmodesmata. This result suggests that the p29 protein is most probably the movement protein for PepRSV.

Analysis of the role of myosins in targeting proteins to plasmodesmata

Plasmodesmata (PD) are dynamic cell wall microchannels that facilitate the intercellular trafficking of RNA and protein macromolecules playing cell nonautonomous roles in the orchestration of plant development, growth, and plant defense. The trafficking of macromolecules and organelles within cells depends on cytoskeletal components and their associated motor proteins. Plant viruses evolved to hijack this transport system to move their infectious genomes to PD. Current efforts concentrate on dissecting the role of specific myosin motors in transporting plant or viral proteins to the channels. Here we describe a method that addresses the role of specific myosins by expression of myosin tails that cause the repression of myosin activity in a dominant-negative manner. As an example, we explain the use of myosin tails from Nicotiana benthamiana to address the role of N. benthamiana myosins in the targeting of PLASMODESMATA-LOCATED PROTEIN 1 (PDLP1) to PD.

Plasmodesmata in integrated cell signalling: insights from development and environmental signals and stresses

To survive as sedentary organisms built of immobile cells, plants require an effective intercellular communication system, both locally between neighbouring cells within each tissue and systemically across distantly located organs. Such a system enables cells to coordinate their intracellular activities and produce concerted responses to internal and external stimuli. Plasmodesmata, membrane-lined intercellular channels, are essential for direct cell-to-cell communication involving exchange of diffusible factors, including signalling and information molecules. Recent advances corroborate that plasmodesmata are not passive but rather highly dynamic channels, in that their density in the cell walls and gating activities are tightly linked to developmental and physiological processes. Moreover, it is becoming clear that specific hormonal signalling pathways play crucial roles in relaying primary cellular signals to plasmodesmata. In this review, we examine a number of studies in which plasmodesmal structure, occurrence, and/or permeability responses are found to be altered upon given cellular or environmental signals, and discuss common themes illustrating how plasmodesmal regulation is integrated into specific cellular signalling pathways.

The unique enzymatic and mechanistic properties of plant myosins

Myosins are molecular motors that move along actin-filament tracks. Plants express two main classes of myosins, myosin VIII and myosin XI. Along with their relatively conserved sequence and functions, plant myosins have acquired some unique features. Myosin VIII has the enzymatic characteristics of a tension sensor and/or a tension generator, similar to functions found in other eukaryotes. Interestingly, class XI plant myosins have gained a novel function that consists of propelling the exceptionally rapid cytoplasmic streaming. This specific class includes the fastest known translocating molecular motors, which can reach an extremely high velocity of about 60μms(-1). However, the enzymatic properties and mechanistic basis for these remarkable manifestations are not yet fully understood. Here we review recent progress in understanding the uniqueness of plant myosins, while emphasizing the unanswered questions.

Probing plasmodesmata function with biochemical inhibitors

To investigate plasmodesmata (PD) function, a useful technique is to monitor the effect on cell-to-cell transport of applying an inhibitor of a physiological process, protein, or other cell component of interest. Changes in PD transport can then be monitored in one of several ways, most commonly by measuring the cell-to-cell movement of fluorescent tracer dyes or of free fluorescent proteins. Effects on PD structure can be detected in thin sections of embedded tissue observed using an electron microscope, most commonly a Transmission Electron Microscope (TEM). This chapter outlines commonly used inhibitors, methods for treating different tissues, how to detect altered cell-to-cell transport and PD structure, and important caveats.

Myosins VIII and XI play distinct roles in reproduction and transport of tobacco mosaic virus

Viruses are obligatory parasites that depend on host cellular factors for their replication as well as for their local and systemic movement to establish infection. Although myosin motors are thought to contribute to plant virus infection, their exact roles in the specific infection steps have not been addressed. Here we investigated the replication, cell-to-cell and systemic spread of Tobacco mosaic virus (TMV) using dominant negative inhibition of myosin activity. We found that interference with the functions of three class VIII myosins and two class XI myosins significantly reduced the local and long-distance transport of the virus. We further determined that the inactivation of myosins XI-2 and XI-K affected the structure and dynamic behavior of the ER leading to aggregation of the viral movement protein (MP) and to a delay in the MP accumulation in plasmodesmata (PD). The inactivation of myosin XI-2 but not of myosin XI-K affected the localization pattern of the 126k replicase subunit and the level of TMV accumulation. The inhibition of myosins VIII-1, VIII-2 and VIII-B abolished MP localization to PD and caused its retention at the plasma membrane. These results suggest that class XI myosins contribute to the viral propagation and intracellular trafficking, whereas myosins VIII are specifically required for the MP targeting to and virus movement through the PD. Thus, TMV appears to recruit distinct myosins for different steps in the cell-to-cell spread of the infection.

Actin is required for IFT regulation in Chlamydomonas reinhardtii

Assembly of cilia and flagella requires intraflagellar transport (IFT), a highly regulated kinesin-based transport system that moves cargo from the basal body to the tip of flagella [1]. The recruitment of IFT components to basal bodies is a function of flagellar length, with increased recruitment in rapidly growing short flagella [2]. The molecular pathways regulating IFT are largely a mystery. Because actin network disruption leads to changes in ciliary length and number, actin has been proposed to have a role in ciliary assembly. However, the mechanisms involved are unknown. In Chlamydomonas reinhardtii, conventional actin is found in both the cell body and the inner dynein arm complexes within flagella [3, 4]. Previous work showed that treating Chlamydomonas cells with the actin-depolymerizing compound cytochalasin D resulted in reversible flagellar shortening [5], but how actin is related to flagellar length or assembly remains unknown. Here we utilize small-molecule inhibitors and genetic mutants to analyze the role of actin dynamics in flagellar assembly in Chlamydomonas reinhardtii. We demonstrate that actin plays a role in IFT recruitment to basal bodies during flagellar elongation and that when actin is perturbed, the normal dependence of IFT recruitment on flagellar length is lost. We also find that actin is required for sufficient entry of IFT material into flagella during assembly. These same effects are recapitulated with a myosin inhibitor, suggesting that actin may act via myosin in a pathway by which flagellar assembly is regulated by flagellar length.

Recent progress in research on cell-to-cell movement of rice viruses

To adapt to plants as hosts, plant viruses have evolutionally needed the capacity to modify the host plasmodesmata (PD) that connect adjacent cells. Plant viruses have acquired one or more genes that encode movement proteins (MPs), which facilitate the cell-to-cell movement of infectious virus entities through PD to adjacent cells. Because of the diversity in their genome organization and in their coding sequences, rice viruses may each have a distinct cell-to-cell movement strategy. The complexity of their unusual genome organizations and replication strategies has so far hampered reverse genetic research on their genome in efforts to investigate virally encoded proteins that are involved in viral movement. However, the MP of a particular virus can complement defects in cell-to-cell movement of other distantly related or even unrelated viruses. Trans-complementation experiments using a combination of a movement-defective virus and viral proteins of interest to identify MPs of several rice viruses have recently been successful. In this article, we reviewed recent research that has advanced our understanding of cell-to-cell movement of rice viruses.

Molecular characterization and subcellular localization of Arabidopsis class VIII myosin, ATM1

Land plants possess myosin classes VIII and XI. Although some information is available on the molecular properties of class XI myosins, class VIII myosins are not characterized. Here, we report the first analysis of the enzymatic properties of class VIII myosin. The motor domain of Arabidopsis class VIII myosin, ATM1 (ATM1-MD), and the motor domain plus one IQ motif (ATM1-1IQ) were expressed in a baculovirus system and characterized. ATM1-MD and ATM1-1IQ had low actin-activated Mg(2+)-ATPase activity (Vmax = 4 s(-1)), although their affinities for actin were high (Kactin = 4 μM). The actin-sliding velocities of ATM1-MD and ATM1-1IQ were 0.02 and 0.089 μm/s, respectively, from which the value for full-length ATM1 is calculated to be ∼0.2 μm/s. The results of actin co-sedimentation assay showed that the duty ratio of ATM1 was ∼90%. ADP dissociation from the actin·ATM1 complex (acto-ATM1) was extremely slow, which accounts for the low actin-sliding velocity, low actin-activated ATPase activity, and high duty ratio. The rate of ADP dissociation from acto-ATM1 was markedly biphasic with fast and slow phase rates (5.1 and 0.41 s(-1), respectively). Physiological concentrations of free Mg(2+) modulated actin-sliding velocity and actin-activated ATPase activity by changing the rate of ADP dissociation from acto-ATM1. GFP-fused full-length ATM1 expressed in Arabidopsis was localized to plasmodesmata, plastids, newly formed cell walls, and actin filaments at the cell cortex. Our results suggest that ATM1 functions as a tension sensor/generator at the cell cortex and other structures in Arabidopsis.

Processing-body movement in Arabidopsis depends on an interaction between myosins and DECAPPING PROTEIN1

Processing (P)-bodies are cytoplasmic RNA protein aggregates responsible for the storage, degradation, and quality control of translationally repressed messenger RNAs in eukaryotic cells. In mammals, P-body-related RNA and protein exchanges are actomyosin dependent, whereas P-body movement requires intact microtubules. In contrast, in plants, P-body motility is actin based. In this study, we show the direct interaction of the P-body core component DECAPPING PROTEIN1 (DCP1) with the tails of different unconventional myosins in Arabidopsis (Arabidopsis thaliana). By performing coexpression studies with AtDCP1, dominant-negative myosin fragments, as well as functional full-length myosin XI-K, the association of P-bodies and myosins was analyzed in detail. Finally, the combination of mutant analyses and characterization of P-body movement patterns showed that myosin XI-K is essential for fast and directed P-body transport. Together, our data indicate that P-body movement in plants is governed by myosin XI members through direct binding to AtDCP1 rather than through an adapter protein, as known for membrane-coated organelles. Interspecies and intraspecies interaction approaches with mammalian and yeast protein homologs suggest that this mechanism is evolutionarily conserved among eukaryotes.

Genome-wide identification, splicing, and expression analysis of the myosin gene family in maize (Zea mays)

The actin-based myosin system is essential for the organization and dynamics of the endomembrane system and transport network in plant cells. Plants harbour two unique myosin groups, class VIII and class XI, and the latter is structurally and functionally analogous to the animal and fungal class V myosin. Little is known about myosins in grass, even though grass includes several agronomically important cereal crops. Here, we identified 14 myosin genes from the genome of maize (Zea mays). The relatively larger sizes of maize myosin genes are due to their much longer introns, which are abundant in transposable elements. Phylogenetic analysis indicated that maize myosin genes could be classified into class VIII and class XI, with three and 11 members, respectively. Apart from subgroup XI-F, the remaining subgroups were duplicated at least in one analysed lineage, and the duplication events occurred more extensively in Arabidopsis than in maize. Only two pairs of maize myosins were generated from segmental duplication. Expression analysis revealed that most maize myosin genes were expressed universally, whereas a few members (XI-1, -6, and -11) showed an anther-specific pattern, and many underwent extensive alternative splicing. We also found a short transcript at the O1 locus, which conceptually encoded a headless myosin that most likely functions at the transcriptional level rather than via a dominant-negative mechanism at the translational level. Together, these data provide significant insights into the evolutionary and functional characterization of maize myosin genes that could transfer to the identification and application of homologous myosins of other grasses.

Understanding myosin functions in plants: are we there yet?

Myosins are motor proteins that drive movements along actin filaments and have long been assumed to be responsible for cytoplasmic streaming in plant cells. This conjecture is now firmly established by genetic analysis in the reference species, Arabidopsis thaliana. This work and similar approaches in the moss, Physcomitrella patens, also established that myosin-driven movements are necessary for cell growth and polarity, organelle distribution and shape, and actin organization and dynamics. Identification of a mechanistic link between intracellular movements and cell expansion has proven more challenging, not the least because of the high level of apparent genetic redundancy among myosin family members. Recent progress in the creation of functional complementation constructs and identification of interaction partners promises a way out of this dilemma.

Nucleocapsid of Tomato spotted wilt tospovirus forms mobile particles that traffic on an actin/endoplasmic reticulum network driven by myosin XI-K

A number of viral proteins from plant viruses, other than movement proteins, have been shown to traffic intracellularly along actin filaments and to be involved in viral infection. However, there has been no report that a viral capsid protein may traffic within a cell by utilizing the actin/endoplasmic reticulum (ER) network. We used Tomato spotted wilt tospovirus (TSWV) as a model virus to study the cell biological properties of a nucleocapsid (N) protein. We found that TSWV N protein was capable of forming highly motile cytoplasmic inclusions that moved along the ER and actin network. The disruption of actin filaments by latrunculin B, an actin-depolymerizing agent, almost stopped the intracellular movement of N inclusions, whereas treatment with a microtubule-depolymerizing reagent, oryzalin, did not. The over-expression of a myosin XI-K tail, functioning in a dominant-negative manner, completely halted the movement of N inclusions. Latrunculin B treatment strongly inhibited the formation of TSWV local lesions in Nicotiana tabacum cv Samsun NN and delayed systemic infection in N. benthamiana. Collectively, our findings provide the first evidence that the capsid protein of a plant virus has the novel property of intracellular trafficking. The findings add capsid protein as a new class of viral protein that traffics on the actin/ER system.

A putative Rab-GTPase activation protein from Nicotiana benthamiana is important for Bamboo mosaic virus intercellular movement

The cDNA-amplified fragment length polymorphism technique was applied to isolate the differentially expressed genes during Bamboo mosaic virus (BaMV) infection on Nicotiana benthamiana plants. One of the upregulated genes was cloned and predicted to contain a TBC domain designated as NbRabGAP1 (Rab GTPase activation protein 1). No significant difference was observed in BaMV accumulation in the NbRabGAP1-knockdown and the control protoplasts. However, BaMV accumulation was 50% and 2% in the inoculated and systemic leaves, respectively, of the knockdown plants to those of the control plants. By measuring the spreading area of BaMV infection foci in the inoculated leaves, we found that BaMV moved less efficiently in the NbRabGAP1-knockdown plants than in the control plants. Transient expression of the wild type NbRabGAP1 significantly increases BaMV accumulation in N. benthamiana. These results suggest that NbRabGAP1 with a functional Rab-GAP activity is involved in virus movement.

Contribution of host intracellular transport machineries to intercellular movement of turnip mosaic virus

The contribution of different host cell transport systems in the intercellular movement of turnip mosaic virus (TuMV) was investigated. To discriminate between primary infections and secondary infections associated with the virus intercellular movement, a gene cassette expressing GFP-HDEL was inserted adjacent to a TuMV infectious cassette expressing 6K₂:mCherry, both within the T-DNA borders of the binary vector pCambia. In this system, both gene cassettes were delivered to the same cell by a single binary vector and primary infection foci emitted green and red fluorescence while secondarily infected cells emitted only red fluorescence. Intercellular movement was measured at 72 hours post infiltration and was estimated to proceed at an average rate of one cell being infected every three hours over an observation period of 17 hours. To determine if the secretory pathway were important for TuMV intercellular movement, chemical and protein inhibitors that blocked both early and late secretory pathways were used. Treatment with Brefeldin A or Concanamycin A or expression of ARF1 or RAB-E1d dominant negative mutants, all of which inhibit pre- or post-Golgi transport, reduced intercellular movement by the virus. These treatments, however, did not inhibit virus replication in primary infected cells. Pharmacological interference assays using Tyrphostin A23 or Wortmannin showed that endocytosis was not important for TuMV intercellular movement. Lack of co-localization by endocytosed FM4-64 and Ara7 (AtRabF2b) with TuMV-induced 6K₂-tagged vesicles further supported this conclusion. Microfilament depolymerizing drugs and silencing expression of myosin XI-2 gene, but not myosin VIII genes, also inhibited TuMV intercellular movement. Expression of dominant negative myosin mutants confirmed the role played by myosin XI-2 as well as by myosin XI-K in TuMV intercellular movement. Using this dual gene cassette expression system and transport inhibitors, components of the secretory and actomyosin machinery were shown to be important for TuMV intercellular spread.

Cell-cell contact-mediated hepatitis C virus (HCV) transfer, productive infection, and replication and their requirement for HCV receptors

Hepatitis C virus (HCV) infection is believed to begin with interactions between cell-free HCV and cell receptors that include CD81, scavenger receptor B1 (SR-B1), claudin-1 (CLDN1), and occludin (OCLN). In this study, we have demonstrated that HCV spreading from infected hepatocytes to uninfected hepatocytes leads to the transfer of HCV and the formation of infection foci and is cell density dependent. This cell-cell contact-mediated (CCCM) HCV transfer occurs readily and requires all these known HCV receptors and an intact actin cytoskeleton. With a fluorescently labeled replication-competent HCV system, the CCCM transfer process was further dissected by live-cell imaging into four steps: donor cell-target cell contact, formation of viral puncta-target cell conjugation, transfer of viral puncta, and posttransfer. Importantly, the CCCM HCV transfer leads to productive infection of target cells. Taken together, these results show that CCCM HCV transfer constitutes an important and effective route for HCV infection and dissemination. These findings will aid in the development of new and novel strategies for preventing and treating HCV infection.

The secretory pathway and the actomyosin motility system are required for plasmodesmatal localization of the P7-1 of rice black-streaked dwarf virus

Rice black-streaked dwarf virus (RBSDV), a plant-infecting reovirus (genus Fijivirus), generally induces virus-containing tubules in infected cells. The nonstructural protein P7-1, encoded by the first open reading frame of segment 7, is involved in forming the structural matrix of these tubules. In experiments to investigate the subcellular localization of P7-1 in Nicotiana benthamiana epidermal cells, fluorescence of P7-1:eGFP was observed in the nucleus, cytoplasm and cell periphery, and in punctate points along the cell wall of plasmolyzed cells. Co-localization with plasmodesmata-located protein 1 showed that P7-1 formed the punctate points at plasmodesmata. Mutational analysis demonstrated that transmembrane domain 1 and adjacent residues were necessary and sufficient for P7-1 to form punctate structures at the cell wall in the plasmolyzed cells. Chemical drug and protein inhibitor treatments indicated that P7-1 utilized the ER-to-Golgi secretory pathway and the actomyosin motility system for its intracellular transport. The plasmodesmatal localization of RBSDV P7-1 is therefore dependent on the secretory pathway and the actomyosin motility system.

Grapevine leafroll-associated virus 3

Grapevine leafroll disease (GLD) is one of the most important grapevine viral diseases affecting grapevines worldwide. The impact on vine health, crop yield, and quality is difficult to assess due to a high number of variables, but significant economic losses are consistently reported over the lifespan of a vineyard if intervention strategies are not implemented. Several viruses from the family Closteroviridae are associated with GLD. However, Grapevine leafroll-associated virus 3 (GLRaV-3), the type species for the genus Ampelovirus, is regarded as the most important causative agent. Here we provide a general overview on various aspects of GLRaV-3, with an emphasis on the latest advances in the characterization of the genome. The full genome of several isolates have recently been sequenced and annotated, revealing the existence of several genetic variants. The classification of these variants, based on their genome sequence, will be discussed and a guideline is presented to facilitate future comparative studies. The characterization of sgRNAs produced during the infection cycle of GLRaV-3 has given some insight into the replication strategy and the putative functionality of the ORFs. The latest nucleotide sequence based molecular diagnostic techniques were shown to be more sensitive than conventional serological assays and although ELISA is not as sensitive it remains valuable for high-throughput screening and complementary to molecular diagnostics. The application of next-generation sequencing is proving to be a valuable tool to study the complexity of viral infection as well as plant pathogen interaction. Next-generation sequencing data can provide information regarding disease complexes, variants of viral species, and abundance of particular viruses. This information can be used to develop more accurate diagnostic assays. Reliable virus screening in support of robust grapevine certification programs remains the cornerstone of GLD management.

The closterovirus-derived gene expression and RNA interference vectors as tools for research and plant biotechnology

Important progress in understanding replication, interactions with host plants, and evolution of closteroviruses enabled engineering of several vectors for gene expression and virus-induced gene silencing. Due to the broad host range of closteroviruses, these vectors expanded vector applicability to include important woody plants such as citrus and grapevine. Furthermore, large closterovirus genomes offer genetic capacity and stability unrivaled by other plant viral vectors. These features provided immense opportunities for using closterovirus vectors for the functional genomics studies and pathogen control in economically valuable crops. This review briefly summarizes advances in closterovirus research during the last decade, explores the relationships between virus biology and vector design, and outlines the most promising directions for future application of closterovirus vectors.