Plant virus-specific transport function and resistance of plants to viruses

A historical overview of long-distance signalling in plants

Be it a small herb or a large tree, intra- and intercellular communication and long-distance signalling between distant organs are crucial for every aspect of plant development. The vascular system, comprising xylem and phloem, acts as a major conduit for the transmission of long-distance signals in plants. In addition to expanding our knowledge of vascular development, numerous reports in the past two decades revealed that selective populations of RNAs, proteins, and phytohormones function as mobile signals. Many of these signals were shown to regulate diverse physiological processes, such as flowering, leaf and root development, nutrient acquisition, crop yield, and biotic/abiotic stress responses. In this review, we summarize the significant discoveries made in the past 25 years, with emphasis on key mobile signalling molecules (mRNAs, proteins including RNA-binding proteins, and small RNAs) that have revolutionized our understanding of how plants integrate various intrinsic and external cues in orchestrating growth and development. Additionally, we provide detailed insights on the emerging molecular mechanisms that might control the selective trafficking and delivery of phloem-mobile RNAs to target tissues. We also highlight the cross-kingdom movement of mobile signals during plant-parasite relationships. Considering the dynamic functions of these signals, their implications in crop improvement are also discussed.

Diversity of Plant Virus Movement Proteins: What Do They Have in Common?

The modern view of the mechanism of intercellular movement of viruses is based largely on data from the study of the tobacco mosaic virus (TMV) 30-kDa movement protein (MP). The discovered properties and abilities of TMV MP, namely, (a) in vitro binding of single-stranded RNA in a non-sequence-specific manner, (b) participation in the intracellular trafficking of genomic RNA to the plasmodesmata (Pd), and (c) localization in Pd and enhancement of Pd permeability, have been used as a reference in the search and analysis of candidate proteins from other plant viruses. Nevertheless, although almost four decades have passed since the introduction of the term “movement protein” into scientific circulation, the mechanism underlying its function remains unclear. It is unclear why, despite the absence of homology, different MPs are able to functionally replace each other in trans-complementation tests. Here, we consider the complexity and contradictions of the approaches for assessment of the ability of plant viral proteins to perform their movement function. We discuss different aspects of the participation of MP and MP/vRNA complexes in intra- and intercellular transport. In addition, we summarize the essential MP properties for their functioning as “conditioners”, creating a favorable environment for viral reproduction.

Small hydrophobic viral proteins involved in intercellular movement of diverse plant virus genomes

Most plant viruses code for movement proteins (MPs) targeting plasmodesmata to enable cell-to-cell and systemic spread in infected plants. Small membrane-embedded MPs have been first identified in two viral transport gene modules, triple gene block (TGB) coding for an RNA-binding helicase TGB1 and two small hydrophobic proteins TGB2 and TGB3 and double gene block (DGB) encoding two small polypeptides representing an RNA-binding protein and a membrane protein. These findings indicated that movement gene modules composed of two or more cistrons may encode the nucleic acid-binding protein and at least one membrane-bound movement protein. The same rule was revealed for small DNA-containing plant viruses, namely, viruses belonging to genus Mastrevirus (family Geminiviridae) and the family Nanoviridae. In multi-component transport modules the nucleic acid-binding MP can be viral capsid protein(s), as in RNA-containing viruses of the families Closteroviridae and Potyviridae. However, membrane proteins are always found among MPs of these multicomponent viral transport systems. Moreover, it was found that small membrane MPs encoded by many viruses can be involved in coupling viral replication and cell-to-cell movement. Currently, the studies of evolutionary origin and functioning of small membrane MPs is regarded as an important pre-requisite for understanding of the evolution of the existing plant virus transport systems. This paper represents the first comprehensive review which describes the whole diversity of small membrane MPs and presents the current views on their role in plant virus movement.

The Tobamoviral Movement Protein: A "Conditioner" to Create a Favorable Environment for Intercellular Spread of Infection

During their evolution, viruses acquired genes encoding movement protein(s) (MPs) that mediate the intracellular transport of viral genetic material to plasmodesmata (Pd) and initiate the mechanisms leading to the increase in plasmodesmal permeability. Although the current view on the role of the viral MPs was primarily formed through studies on tobacco mosaic virus (TMV), the function of its MP has not been fully elucidated. Given the intercellular movement of MPs independent of genomic viral RNA (vRNA), this characteristic may induce favorable conditions ahead of the infection front for the accelerated movement of the vRNA (i.e. the MP plays a role as a "conditioner" of viral intercellular spread). This idea is supported by (a) the synthesis of MP from genomic vRNA early in infection, (b) the Pd opening and the MP transfer to neighboring cells without formation of the viral replication complex (VRC), and (c) the MP-mediated movement of VRCs beyond the primary infected cell. Here, we will consider findings that favor the TMV MP as a "conditioner" of enhanced intercellular virus movement. In addition, we will discuss the mechanism by which TMV MP opens Pd for extraordinary transport of macromolecules. Although there is no evidence showing direct effects of TMV MP on Pd leading to their dilatation, recent findings indicate that MPs exert their influence indirectly by modulating Pd external and structural macromolecules such as callose and Pd-associated proteins. In explaining this phenomenon, we will propose a mechanism for TMV MP functioning as a conditioner for virus movement.

Chimeric Virus Made from crTMV RNA and the Coat Protein of Potato Leafroll Virus is Targeted to the Nucleolus and Can Infect Nicotiana benthamiana Mechanically

A genetically engineered chimeric virus crTMV-CP-PLRV composed of the crucifer-infecting tobacco mosaic virus (crTMV) RNA and the potato leafroll virus (PLRV) coat protein (CP) was obtained by agroinfiltration of Nicotiana benthamiana with the binary vector pCambia-crTMV-CP_PLRV. The significant levels of the chimeric virus enabled direct visualization of crTMV-CP-PLRV in the cell and to investigate the mechanism of the pathogenesis. Localization of the crTMV-CP-PLRV in plant cells was examined by immunoblot techniques, as well as light, and transmission electron microscopy. The chimera can transfer between vascular and nonvascular tissues. The chimeric virus inoculum is capable to infect N. benthamiana mechanically. The distinguishing feature of the chimeric virus, the RNA virus with the positive genome, was found to localize in the nucleolus. We also investigated the role of the N-terminal sequence of the PLRV P3 coat protein in the cellular localization of the virus. We believe that the gene of the PLRV CP can be substituted with genes from other challenging-to-study plant pathogens to produce other useful recombinant viruses.

Maize Dwarf Mosaic Virus: From Genome to Disease Management

Maize dwarf mosaic virus (MDMV) is a serious maize pathogen, epidemic worldwide, and one of the most common virus diseases for monocotyledonous plants, causing up to 70% loss in corn yield globally since 1960. MDMV belongs to the genus Potyvirus (Potyviridae) and was first identified in 1964 in Illinois in corn and Johnsongrass. MDMV is a single stranded positive sense RNA virus and is transmitted in a non-persistent manner by several aphid species. MDMV is amongst the most important virus diseases in maize worldwide. This review will discuss its genome, transmission, symptomatology, diagnosis and management. Particular emphasis will be given to the current state of knowledge on the diagnosis and control of MDMV, due to its importance in reducing the impact of maize dwarf mosaic disease, to produce an enhanced quality and quantity of maize.

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.

Identification of a movement protein of Mirafiori lettuce big-vein ophiovirus

Mirafiori lettuce big-vein virus (MiLBVV) is a member of the genus Ophiovirus, which is a segmented negative-stranded RNA virus. In microprojectile bombardment experiments to identify a movement protein (MP) gene of ophioviruses that can trans-complement intercellular movement of an MP-deficient heterologous virus, a plasmid containing an infectious clone of a tomato mosaic virus (ToMV) derivative expressing the GFP was co-bombarded with plasmids containing one of three genes from MiLBVV RNAs 1, 2 and 4 onto Nicotiana benthamiana. Intercellular movement of the movement-defective ToMV was restored by co-expression of the 55 kDa protein gene, but not with the two other genes. Transient expression in epidermal cells of N. benthamiana and onion showed that the 55 kDa protein with GFP was localized on the plasmodesmata. The 55 kDa protein encoded in the MiLBVV RNA2 can function as an MP of the virus. This report is the first to describe an ophiovirus MP.

The movement protein encoded by gene 3 of rice transitory yellowing virus is associated with virus particles

Gene 3 in the genomes of several plant-infecting rhabdoviruses, including rice transitory yellowing virus (RTYV), has been postulated to encode a cell-to-cell movement protein (MP). Trans-complementation experiments using a movement-defective tomato mosaic virus and the P3 protein of RTYV, encoded by gene 3, facilitated intercellular transport of the mutant virus. In transient-expression experiments with the GFP-fused P3 protein in epidermal leaf cells of Nicotiana benthamiana, the P3 protein was associated with the nucleus and plasmodesmata. Immunogold-labelling studies of thin sections of RTYV-infected rice plants using an antiserum against Escherichia coli-expressed His(6)-tagged P3 protein indicated that the P3 protein was located in cell walls and on virus particles. In Western blots using antisera against E. coli-expressed P3 protein and purified RTYV, the P3 protein was detected in purified RTYV, whilst antiserum against purified RTYV reacted with the E. coli-expressed P3 protein. After immunogold labelling of crude sap from RTYV-infected rice leaves, the P3 protein, as well as the N protein, was detected on the ribonucleocapsid core that emerged from partially disrupted virus particles. These results provide evidence that the P3 protein of RTYV, which functions as a viral MP, is a viral structural protein and seems to be associated with the ribonucleocapsid core of virus particles.

Callose deposition at plasmodesmata is a critical factor in restricting the cell-to-cell movement of Soybean mosaic virus

Callose is a β-l,3-glucan with diverse roles in the viral pathogenesis of plants. It is widely believed that the deposition of callose and hypersensitive reaction (HR) are critical defence responses of host plants against viral infection. However, the sequence of these two events and their resistance mechanisms are unclear. By exploiting a point inoculation approach combined with aniline blue staining, immuno-electron microscopy and external sphincters staining with tannic acid, we systematically investigated the possible roles of callose deposition during viral infection in soybean. In the incompatible combination, callose deposition at the plasmodesmata (PD) was clearly visible at the sites of inoculation but viral RNA of coat protein (CP-RNA) was not detected by RT-PCR in the leaf above the inoculated one (the upper leaf). In the compatible combination, however, callose deposition at PD was not detected at the site of infection but the viral CP-RNA was detected by RT-PCR in the upper leaf. We also found that in the incompatible combination the fluorescence due to callose formation at the inoculation point disappeared following the injection of 2-deoxy-D-glucose (DDG, an inhibitor of callose synthesis). At same time, in the incompatible combination, necrosis was observed and the viral CP-RNA was detected by RT-PCR in the upper leaf and HR characteristics were evident at the inoculation sites. These results show that, during the defensive response of soybean to viral infection, callose deposition at PD is mainly responsible for restricting the movement of the virus between cells and it occurs prior to the HR response.

The nonstructural protein pC6 of rice grassy stunt virus trans-complements the cell-to-cell spread of a movement-defective tomato mosaic virus

The nonstructural protein pC6 encoded by rice grassy stunt virus is thought to correspond functionally to the nonstructural protein pC4 of rice stripe virus, which can support viral cell-to-cell movement. In a trans-complementation experiment with a movement-defective tomato mosaic virus, pC6 and pC4 facilitated intercellular transport of the virus. Transient expression of pC6, fused with green fluorescent protein, in epidermal cells was predominantly observed close to the cell wall as well as in a few punctate structures, presumably associated with plasmodesmata. These results suggest that pC6 has a role similar to that of pC4 in viral cell-to-cell movement.

Transfer of genetic material between the chloroplast and nucleus: how is it related to stress in plants?

BACKGROUND

The presence of chloroplast-related DNA sequences in the nuclear genome is generally regarded as a relic of the process by which genes have been transferred from the chloroplast to the nucleus. The remaining chloroplast encoded genes are not identical across the plant kingdom indicating an ongoing transfer of genes from the organelle to the nucleus.

SCOPE

This review focuses on the active processes by which the nuclear genome might be acquiring or removing DNA sequences from the chloroplast genome. Present knowledge of the contribution to the nuclear genome of DNA originating from the chloroplast will be reviewed. In particular, the possible effects of stressful environments on the transfer of genetic material between the chloroplast and nucleus will be considered. The significance of this research and suggestions for the future research directions to identify drivers, such as stress, of the nuclear incorporation of plastid sequences are discussed.

CONCLUSIONS

The transfer to the nuclear genome of most of the protein-encoding functions for chloroplast-located proteins facilitates the control of gene expression. The continual transfer of fragments, including complete functional genes, from the chloroplast to the nucleus has been observed. However, the mechanisms by which the loss of functions and physical DNA elimination from the chloroplast genome following the transfer of those functions to the nucleus remains obscure. The frequency of polymorphism across chloroplast-related DNA fragments within a species will indicate the rate at which these DNA fragments are incorporated and removed from the chromosomes.

Localization by immunogold cytochemistry of the virus-coded 30K protein in plasmodesmata of leaves infected with tobacco mosaic virus

The 30K protein of tobacco mosaic virus (TMV) was localized to the plasmodesmata of infected tobacco leaves by immunogold cytochemistry. This protein has been reported to be in the nuclear fraction of TMV-infected protoplasts, but as it has been proposed to function in cell-to-cell transport of virus, probably via the plasmodesmata, intact tissue was investigated with particular attention directed to plasmodesmata and nuclei. Thin sections were made from leaves mechanically inoculated with TMV at different times. Affinity-purified antibodies against a synthetic peptide corresponding to the C-terminal sequence of the 30K protein were used in the incubations, and parallel sections were incubated with antibodies against TMV. The 30K protein label accumulated inside the plasmodesmata, with a maximum 24 hr after inoculation. No specific label was found in the nuclei or at any other site in the cells.

Role of plant virus movement proteins

Plant viruses spread from the initially infected cells to the rest of the plant in several distinct stages. First, the virus (in the form of virions or nucleic acid protein complexes) moves intracellularly from the sites of replication to plasmodesmata (PD, plant-specific intercellular membranous channels), the virus then transverses the PD to spread intercellularly (cell-to-cell movement). Long-distance movement of virus occurs through phloem sieve tubes. The processes of plant virus movement are controlled by specific viral movement proteins (MPs). No extensive sequence similarity has been found in MPs belonging to different plant virus taxonomic groups. Moreover, different MPs were shown to use different pathways and mechanisms for virus transport. Some viral transport systems require a single MP while others require additional virus-encoded proteins to transport viral genomes. In this review, we focus on the functions and properties of different classes of MPs encoded by RNA containing plant viruses.

Plant viral movement proteins: Agents for cell-to-cell trafficking of viral genomes

Plants viruses spread throughout their hosts using a number of pathways, the most common being movement cell to cell through plasmodesmata (PD), unique intercellular organelles of the plant kingdom, and between organs by means of the vascular system. Pioneering studies on plant viruses revealed that PD allow the cell-to-cell trafficking of virally encoded proteins, termed the movement proteins (MPs). This non-cell-autonomous protein (NCAP) pathway is similarly employed by the host to traffic macromolecules. Viral MPs bind RNA/DNA in a sequence nonspecific manner to form nucleoprotein complexes (NPC). Host proteins are then involved in the delivery of MPs and NPC to the PD orifice, and a role for the cytoskeleton has been implicated. Trafficking of NCAPs through the PD structure involves three steps in which the MP: (a) interacts with a putative PD docking complex, (b) induces dilation in the PD microchannels, and (c) binds to an internal translocation system for delivery into the neighboring cytoplasm. Viral genera that use this NCAP pathway have evolved a combination of a MP and ancillary proteins that work in concert to enable the formation of a stable NPC that can compete with endogenous NCAPs for the PD trafficking machinery. Incompatible MP-host protein interactions may underlie observed tissue tropisms and restricted infection domains. These pivotal discoveries are discussed in terms of the need to develop a more comprehensive understanding of the (a) three-dimensional structure of MPs, (b) PD supramolecular complex, and (c) host proteins involved in this cell-to-cell trafficking process.

[Epitope mapping of the recombinant movement protein of the tobacco mosaic virus using monoclonal antibodies]

The movement protein (MP) of the tobacco mosaic virus (TMV) provides the intercellular transport of the viral RNA through plasmodesmata. The MP fulfills its function while interacting with host cell factors over the whole path of its intracellular movement from the subcellular site of its synthesis to the plasmodesmata of cellular walls. The MP conformation during its intracellular movement and fulfillment of the transport function still remains unknown. In this study, we describe the preparation of murine monoclonal antibodies (MAs) to TMV MP and mapping of the MP epitopes. Stable hybridoma lines that produce MAs to the partially denatured recombinant MP (MPr) were obtained. MAs were tested by immunoblotting and ELISA with the use of deletion variants of MPr. The epitopes of TMV MPr that recognize specific MAs were determined.

Tobacco plants respond to the constitutive expression of the tospovirus movement protein NS(M) with a heat-reversible sealing of plasmodesmata that impairs development

Viral infection often results in typical symptoms, the biological background of which has remained elusive. We show that constitutive expression of the NSM viral movement protein (MP) of tomato spotted wilt virus in Nicotiana tabacum is sufficient to induce severe, infection-like symptoms, including pronounced deficiencies in root and shoot development. Leaves failed to expand and were arranged in a rosette due to the absence of internode elongation. Following the sink-source transition they accumulated excessive amounts of starch and developed fusing chlorotic patches in the mesophyll, resembling virus-induced chlorotic lesions. Eventually, the leaves became entirely white and brittle. With a combination of techniques, including photosystem II quantum-yield measurements, iontophoresis of symplasmic tracers, bombardment with pPVX.GFP and double immunolabelling it was shown that these symptoms correlated with the obstruction of NSM-targeted mesophyll plasmodesmata (Pd) in source tissues by depositions of 1,3-beta-D-glucan (GLU) or callose. Temperature-shift treatments (TST; 22-->32 degrees C), known to abolish chlorotic local lesions, also abolished the chlorotic 'superlesions' of transgenic plants and rescued plant development, by restoring the transport capacity of Pd through the action of 1,3-beta-D-glucanase (GLU-h) or callase. Return of these elongated, TST-recovered plants to 22 degrees C reintroduced superlesions and arrested shoot elongation, resulting in the formation of a rosette of clustered leaves at the shoot tip. Collectively, this indicates that the symptoms of NSM plants are self-inflicted and due to a basal defence response that counteracts prolonged interference of the MP with Pd functioning. This type of defence may also play a role in the formation of symptoms during viral infection.

Triple gene block: modular design of a multifunctional machine for plant virus movement

Many plant virus genera encode a 'triple gene block' (TGB), a specialized evolutionarily conserved gene module involved in the cell-to-cell and long-distance movement of viruses. The TGB-based transport system exploits the co-ordinated action of three polypeptides to deliver viral genomes to plasmodesmata and to accomplish virus entry into neighbouring cells. Although data obtained on both the TGB and well-studied single protein transport systems clearly demonstrate that plant viruses employ host cell pathways for intra- and intercellular trafficking of genomic nucleic acids and proteins, there is no integral picture of the details of molecular events during TGB-mediated virus movement. Undoubtedly, understanding the molecular basis of the concerted action of TGB-encoded proteins in transporting viral genomes from cell to cell should provide new insights into the general principles of movement protein function. This review describes the structure, phylogeny and expression of TGB proteins, their roles in virus cell-to-cell movement and potential influence on host antiviral defences.

Characterization of mutant tobacco mosaic virus coat protein that interferes with virus cell-to-cell movement

Expression of tobacco mosaic virus (TMV) coat protein (CP) in plants confers resistance to infection by TMV and related tobamoviruses. Certain mutants of the CP (CP(T42W)) provide much greater levels of resistance than wild-type (wt) CP. In the present work, infection induced by RNA transcripts of TMV clones that contain wt CP or mutant CP(T42W) fused to the green fluorescent protein (GFP) (TMV-CP:GFP, TMV-CP(T42W):GFP) and clones harboring TMV movement protein (MP):GFP were followed in nontransgenic and transgenic tobacco BY-2 protoplasts and Nicotiana tabaccum Xanthi-nn plants that express wt CP or CP(T42W). On nontransgenic and wt CP transgenic plants, TMV-CP:GFP produced expanding, highly fluorescent disk-shaped areas. On plants expressing CP(T42W), infection by TMV-CP:GFP or TMV-MP:GFP-CP produced infection sites of smaller size that were characterized by low fluorescence, reflecting reduced levels of virus spread and reduced accumulation of both CP:GFP and MP:GFP. TMV-CP(T42W):GFP failed to produce visible infection sites on nontransgenic plants, yet produced normal infection sites on MP-transgenic plants that produce MP. TMV infection of transgenic BY-CP(T42W) protoplasts resulted in very low levels of MP accumulation, whereas on BY-CP protoplasts (containing wt CP), infection produced higher levels of MP than in nontransgenic BY-2 cells. The results suggest that wt CP has a positive effect on the production of MP, whereas the CP(T42W) has a negative effect on MP accumulation and/or function. This effect results in very high levels of resistance to TMV infection in plants containing CP(T42W). This report shows that the CP of a plant virus regulates production of the MP, and that a mutant CP interferes with MP accumulation and cell-to-cell movement of infection.

RNA-binding properties of the 63 kDa protein encoded by the triple gene block of poa semilatent hordeivirus

The 63 kDa '63K' movement protein encoded by the triple gene block of poa semilatent virus (PSLV) comprises the C-terminal NTPase/helicase domain and the N-terminal extension domain, which contains two positively charged sequence motifs, A and B. In this study, the in vitro RNA-binding properties of PSLV 63K and its mutants were analysed. Membrane-immobilized 63K and N-63K (isolated N-terminal extension domain) bound RNA at high NaCl concentrations. In contrast, C-63K (isolated NTPase/helicase domain) was able to bind RNA only at NaCl concentrations of up to 50 mM. In gel-shift assays, C-63K bound RNA to form complexes that were unable to enter an agarose gel, whereas complexes formed by N-63K could enter the gel. Full-length 63K formed both types of complexes. Visualization of the RNA-protein complexes formed by 63K, N-63K and C-63K by atomic force microscopy demonstrated that each complex had a different shape. Collectively, these data indicate that 63K has two distinct RNA-binding activities associated with the NTPase/helicase domain and the N-terminal extension domain. Mutations in either of the positively charged sequence motifs A and B had little effect on the RNA binding of the N-terminal extension domain, whereas mutations in both motifs together inhibited RNA binding. Hybrid viruses with mutations in motifs A and B were able to infect inoculated leaves of Nicotiana benthamiana plants, but were unable to move systemically to uninoculated leaves, suggesting that the RNA-binding activity of the N-terminal extension domain of PSLV 63K is associated with virus long-distance movement.

In vitro phosphorylation of the movement protein of tomato mosaic tobamovirus by a cellular kinase

The movement protein (MP) of tomato mosaic virus (ToMV) was produced in E. coli as a soluble fusion protein with glutathione S-transferase. When immobilized on glutathione affinity beads, the recombinant protein was phosphorylated in vitro by incubating with cell extracts of Nicotiana tabacum and tobacco suspension culture cells (BY-2) in the presence of [gamma-(32)P]ATP. Phosphorylation occurred even after washing the beads with a detergent-containing buffer, indicating that the recombinant MP formed a stable complex with some protein kinase(s) during incubation with the cell extract. Phosphoamino acid analysis revealed that the MP was phosphorylated on serine and threonine residues. Phosphorylation of the MP was decreased by addition of kinase inhibitors such as heparin, suramin and quercetin, which are known to be effective for casein kinase II (CK II). The phosphorylation level was not changed by other types of inhibitor. In addition, as shown for animal and plant CK II, [gamma-(32)P]GTP was efficiently used as a phosphoryl donor. Phosphorylation was not affected by amino acid replacements at serine-37 and serine-238, but was completely inhibited by deletion of the carboxy-terminal 9 amino acids, including threonine-256, serine-257, serine-261 and serine-263. These results suggest that the MP of ToMV could be phosphorylated in plant cells by a host protein kinase that is closely related to CK II.

Cell-to-cell movement of TMV RNA is temperature-dependent and corresponds to the association of movement protein with microtubules

The movement protein (MP) of tobacco mosaic virus (TMV) is essential for spread of the viral RNA genome from cell to cell. During infection, the MP associates with microtubules, and it has been proposed that the cytoskeleton transports the viral ribonucleoprotein complex from ER sites of synthesis to plasmodesmata through which infection spreads into adjacent cells. However, microtubule association of MP was observed in cells undergoing late infection rather than in cells undergoing early infection at the leading edge of expanding infection sites where virus RNA cell-to-cell spread occurs. Therefore, alternative roles for microtubules in virus infection have been proposed, including a role in MP degradation. To further investigate the role of microtubules in virus pathogenesis, we tested the efficiency of cell-to-cell spread of infection and microtubule association of the MP in response to changes in temperature. We show that the subcellular distribution of MP is temperature-dependent and that a higher efficiency of intercellular transport of virus RNA at elevated temperatures corresponds to an increased association of MP with microtubules early in infection.

Characterization of the brome mosaic virus movement protein expressed in E. coli

The biochemical and functional properties of the movement protein (MP) of brome mosaic virus (BMV) were investigated. Expression and purification of the BMV MP from Escherichia coli resulted in a pure and soluble protein preparation. Sucrose gradient centrifugation revealed that BMV MP forms oligomers consisting of two or more copies but no higher order multimers even when different ionic strengths and pHs were applied. Nitro-cellulose filter binding and gel retardation studies showed that in vitro the BMV MP preferentially bound to ss nucleic acids (RNA and DNA); the affinity to ssRNA was lower compared to BMV coat protein. The binding to ss nucleic acid was cooperative and not sequence specific and the hypothetical binding site was calculated to be around three to six nucleotides per MP monomer. The nucleic acid binding properties of the BMV MP are discussed in relation to the recent finding that this protein is also able to form tubular structures in infected protoplasts.

The geminivirus BL1 movement protein is associated with endoplasmic reticulum-derived tubules in developing phloem cells

Plant viruses encode movement proteins that are essential for systemic infection of their host but dispensable for replication and encapsidation. BL1, one of the two movement proteins encoded by the bipartite geminivirus squash leaf curl virus, was immunolocalized to unique approximately 40-nm tubules that extended up to and across the walls of procambial cells in systemically infected pumpkin leaves. These tubules were not found in procambial cells from pumpkin seedlings inoculated with BL1 mutants that are defective in movement. The tubules also specifically stained with antisera to binding protein (BiP), indicating that they were derived from the endoplasmic reticulum. Independent confirmation of this endoplasmic reticulum association was obtained by subcellular fractionation studies in which BL1 was localized to fractions that contained both endoplasmic reticulum membranes and BiP. Thus, squash leaf curl virus appears to recruit the endoplasmic reticulum as a conduit for cell-to-cell movement of the viral genome.

Accumulation kinetics of CMV RNA 3-encoded proteins and subcellular localization of the 3a protein in infected and transgenic tobacco plants

The complete nucleotide sequence of RNA 3 of a Spanish isolate of cucumber mosaic virus (CMV-24) has been determined. The encoded putative cell-to-cell movement protein (3a protein) and the coat protein are 279 and 218 amino acids long, respectively. The 3a protein was expressed in Escherichia coli using the vector pT7-7 and was used to raise an immunoserum. We have followed the time course of accumulation of the 3a protein, in parallel to that of the coat protein, and its subcellular localization as a function of time after CMV-24 infection on tobacco plants. The maximum accumulation level of the 3a protein was reached at early stages of infection, being detected in the cytosolic and the cell wall fractions. At later stages of infection, a decline in accumulation levels of the 3a protein was observed, and the protein was essentially associated with the cell wall fractions. These data were corroborated by immunocytochemistry performed in both infected and 3a-expressing transgenic tobacco plants.

The immobilized movement proteins of two tobamoviruses form stable ribonucleoprotein complexes with full-length viral genomic RNA

The movement proteins of two tobamoviruses (tobacco mosaic virus, TMV, common strain U1 and cruciferous TMV strain) containing amino-terminal hexahistidine affinity tags were overexpressed in Escherichia coli and purified by metal chelate affinity chromatography. Purified recombinant proteins were immobilized to a Ni(2+)-chelate adsorbent and their ability to interact with full-length genomic TMV RNA was tested. Here we report that binding of viral RNA to hexahistidine fusion movement proteins results in the formation of stable ribonucleoprotein complexes.

Movement of virus and photoassimilate in the phloem: a comparative analysis

Recent progress in the study of short-distance (cell-to-cell) movement of plant virus, facilitated by 'movement proteins', has led to a resurgence of interest in long-distance virus transport in the phloem. Relatively little is known about phloem-specific barriers to virus movement or about the form in which virus enters, travels within and exists this tissue. Progress in understanding virus and photoassimilate transport is limited by a paucity of information on the substructure and properties of plasmodesmata at specific interfaces. The direction of virus movement, once it has entered the phloem, can be understood by following photoassimilate translocation, a complex and dynamic process influenced by plant growth, development and vascular topology.

Molecular basis for virus disease resistance in plants

Classical studies of virus disease resistance in plants have provided the basis for recent molecular studies of resistance. Three common approaches to the study of resistance have been used. In one approach, nucleotide and/or amino acid sequences of virus strains that overcome disease resistance genes in the host are compared with sequences of strains that do not induce disease in these hosts. In the second approach, resistance/susceptibility of protoplasts is compared with the response of intact plants from which they are derived, to develop hypotheses regarding whether resistance acts at the level of the individual cell or by inhibiting cell-to-cell movement. In the third approach, the mechanism of virus cell-to-cell movement has been studied to clarify one of the basic steps in pathogenesis and to determine the mechanism of disease resistance for certain virus-host interactions.

Coat protein gene duplication in a filamentous RNA virus of plants

Computer-assisted analysis revealed a striking sequence similarity between the putative 24-kDa protein (p24) encoded by open reading frame (ORF) 5 of beet yellows closterovirus and the coat protein of this virus encoded by the adjacent ORF6. Both of these proteins are closely related to the homologous proteins of another closterovirus, citrus tristeza virus. It is hypothesized that the genes for coat protein and its diverged tandem copy have evolved by duplication. Phylogenetic analysis using various methods for tree generation suggested that the duplication was already present in the genome of the common ancestor of the two closteroviruses. The genes for p24 and coat protein of beet yellows closterovirus were cloned, transcribed, and translated in vitro yielding products of the expected size. It was shown that p24 is translated starting from the first of the two alternative AUG codons located near the 5' terminus of ORF5. The presence of a single protein species in beet yellows closterovirus virions and the near identity of the amino acid composition of this protein with the composition of the ORF6 but not the ORF5 product indicated that p24 is not a major virion component. Most of the amino acids that are conserved in the coat proteins of filamentous viruses of plants are retained also in p24. These observations suggest that p24 may share some structural and functional features with the coat protein but probably fulfills a distinct function in virus reproduction.

HSP70-related 65 kDa protein of beet yellows closterovirus is a microtubule-binding protein

Beet yellows virus (BYV) genome encodes a 65 kDa protein homologous to the HSP70 family of cellular heat-shock proteins (Agranovsky, A.A., Boyko, V.P., Karasev, A.V., Koonin, E.V. and Dolja, V.V. (1991) J. Mol. Biol. 217, 603-610). The respective gene was cloned and expressed in vitro yielding a product of the expected size (p65). This product was found to bind to the purified microtubules with a binding constant of 4 x 10(-7) M. The binding of p65 was stimulated if ATP presented in the translation mixture was hydrolyzed by apyrase. Removal of the short C-terminal domains of alpha- and beta-tubulin by subtilisin digestion abolished the binding, demonstrating its specificity. The possible role of p65 association with microtubules in the movement of virus within and/or between plant cells is proposed.

Reinvestigation of intracellular localization of the 30K protein in tobacco protoplasts infected with tobacco mosaic virus RNA

It has been shown that the 30K protein of tobacco mosaic virus (TMV) is responsible for the cell-to-cell movement function of the virus. It is still obscure how the protein is involved in this function at the molecular level. We formerly found that the 30K protein is localized to the plasmodesmata of TMV-infected plants. We also reported that the 30K protein was detected in a nuclei-rich fraction of TMV-infected protoplasts after biochemical fractionation. To clarify the inconsistency, the 30K protein was immunocytologically localized in TMV-infected protoplasts using a newly prepared antibody against the 30K protein. On some sections, the 30K protein was found near the nucleus but not in or on the nucleus. At later stages of infection a novel electron-transparent structure was detected in the cytoplasm where the 30K proteins were localized. This structure might reflect an intermediate form between its synthesis in the cytoplasm and its targeting to the plasmodesmata in whole plants.

Effects of terminal deletion mutations on function of the movement protein of tobacco mosaic virus

A series of carboxy- and amino-terminal deletion mutations in the movement protein (MP) gene of tobacco mosaic virus (TMV) were ligated into a cloned TMV cDNA deleted for the endogenous MP gene. RNA transcripts were produced in vitro from clones carrying the various mutated MP genes. The effect of the deletion mutations on local and systemic movements of the infection was evaluated. Deletion of 9 or 33 amino acids from the carboxy terminus of the movement protein did not effect cell-to-cell movement as reflected by local lesion formation on Nicotiana tabacum cv. Xanthi NN plants. Deletion of 55 amino acids resulted in impaired MP that supported the formation of local lesions of 1 mm in diameter compared to lesions of 3-5 mm caused by the wild-type MP. Deletion of 74 amino acids (or more) from the carboxy terminus resulted in a protein that could not support virus movement. Modified viruses that contained repeated sequences in the 3' region of the MP gene lost the repeated sequences during replication and reverted to the wild type. This was evidenced by the size of the MP produced and by sequence analysis of reverse-transcribed PCR-amplified products, following infection by the modified virus. MP deleted for as few as 3 amino acids at the amino terminus could not support virus movement thus indicating that the amino-terminal domain is critical for MP activity.

How do plant virus nucleic acids move through intercellular connections?

In addition to their function in transport of water, ions, small metabolites, and growth factors in normal plant tissue, the plasmodesmata presumably serve as routes for cell-to-cell movement of plant viruses in infected tissue. Virus cell-to-cell spread through plasmodesmata is an active process mediated by specialized virus encoded movement proteins; however, the mechanism by which these proteins operate is not clear. We incorporate recent information on the biochemical properties of plant virus movement proteins and their interaction with plasmodesmata in a model for transport of nucleic acids through plasmodesmatal channels. We propose that only single stranded (ss) nucleic acids can be transported efficiently through plasmodesmata, and that movement proteins function as molecular chaperones for ss nucleic acids to form unfolded movement protein-ss nucleic acid complexes. These complexes are targeted to plasmodesmata. Plasmodesmatal permeability is then increased following interaction with movement protein and the entire movement complex or its nucleic acid component is translocated across the plasmodesmatal channel.

Tobacco rattle virus RNA-1 29K gene product potentiates viral movement and also affects symptom induction in tobacco

In order to investigate the function of the 29K protein of tobacco rattle virus (TRV), we introduced different mutations in the 29K protein gene and analyzed the biological properties of the subsequent transcripts in tobacco plants. Although none of the mutant RNAs was able to accumulate to a detectable level, the defects in the 29K protein could be complemented by coinoculation with wild-type TRV or tobacco mosaic virus (TMV). Complementation was also achieved in transgenic plants expressing the homologous TMV 30K protein which is involved in cell-to-cell movement, but without inducing distinctive symptoms. Transcripts of chimeric TRV clones containing duplicate genes for the 29K protein initiated infections with formation of necrotic lesions and the progeny retained only one copy of the gene. These experiments demonstrate that the 29K protein is not required for viral RNA replication and, because the TRV transcripts do not encode the coat protein, that the 29K and 30K proteins act on nonencapsidated RNA. In addition to potentiating viral movement, the TRV 29K protein may also play a role in symptom induction on tobacco.

Gene I, a potential cell-to-cell movement locus of cauliflower mosaic virus, encodes an RNA-binding protein

Cauliflower mosaic virus (CaMV) is a double-stranded DNA (dsDNA) pararetrovirus capable of cell-to-cell movement presumably through intercellular connections, the plasmodesmata, of the infected plant. This movement is likely mediated by a specific viral protein encoded by the gene I locus. Here we report that the purified gene I protein binds RNA and single-stranded DNA (ssDNA) but not dsDNA regardless of nucleotide sequence specificity. The binding is highly cooperative, and the affinity of the gene I protein for RNA is 10-fold higher than for ssDNA. CaMV replicates by reverse transcription of a 358 RNA that is homologous to the entire genome. We propose that the 35S RNA may be involved in cell-to-cell movement of CaMV as an intermediate that is transported through plasmodesmata as an RNA-gene I protein complex.

In vivo complementation of infectious transcripts from mutant tobacco mosaic virus cDNAs in transgenic plants

A full-length cDNA clone of the U1 (common) strain of tobacco mosaic virus (TMV) was constructed, and highly infectious transcripts were produced in vitro using bacteriophage T7 RNA polymerase. Frameshift mutations designed to cause premature termination of translation were introduced into either the 30-kDa movement protein (MP) gene or the coat protein (CP) gene. The MP-frameshift mutant was unable to locally or systemically infect inoculated tobacco plants. However, inoculation of transgenic tobacco plants that expressed a wild-type TMV MP gene resulted in both local and systemic viral infection. The CP-frameshift mutant, although unable to move systemically in nontransformed tobacco, exhibited systemic movement in transgenic plants that expressed a wild-type TMV CP gene. Transgenic tobacco plants that expressed the appropriate wild-type TMV gene were thus able to complement, in trans, mutant viruses lacking a functional MP or CP gene.

Transfer of the movement protein gene between two tobamoviruses: influence on local lesion development

The effects of transfer of the movement gene between the tobamoviruses tobacco mosaic virus (TMV) and tobacco mild green mosaic virus (TMGMV) were studied. The movement protein (MP) gene of TMGMV was cloned into an infectious cDNA of TMV to build the recombinant virus V23. V23, like TMV and TMGMV, caused systemic infection in Nicotiana tabacum Xanthi. In N. sylvestris V23 and TMV spread systemically although TMGMV produces necrotic local lesions on this host. V23 and TMV cause systemic infection on tomato plants while TMGMV does not infect tomato. In Xanthi nc plants, V23 produced necrotic local lesions similar in size to those produced by TMGMV. On the other hand in transgenic Xanthi nc tobacco plants that express a gene encoding the MP of TMV the necrotic lesions produced by V23 and TMGMV were similar in size to those produced by TMV. These results indicate that the size of necrotic lesions produced by TMGMV and TMV on Xanthi nc plants is influenced by the MP gene.

Systemic movement of an RNA plant virus determined by a point substitution in a 5' leader sequence

The ability of viruses to move through infected plants is an important determinant of host range and pathogenicity. We have investigated the genetic basis for the inability of the Type strain of barley stripe mosaic hordeivirus to undergo long-range systemic movement in the tobacco Nicotiana benthamiana. We show that, in this model system, a short open reading frame in the 5' leader of the smallest viral genomic RNA prevents long-range vascular movement. As predicted by the ribosome scanning model, the leader open reading frame decreases the efficiency with which the 5'-proximal gene is translated in vitro. Thus, systemic pathogenicity in this system may be determined by the efficiency of translation of a viral gene in vivo and is not determined by the primary sequence of the encoded protein.

Infection of tomato by the tomato yellow leaf curl virus: susceptibility to infection, symptom development, and accumulation of viral DNA

Symptom development in tomato plants following whitefly-mediated inoculation with tomato yellow leaf curl virus (TYLCV) was related to the occurrence of viral DNA using a specific DNA probe. Although disease symptoms were first observed 15 days post-inoculation, viral DNA could be detected 7 days earlier. TYLCV-DNA concentrations reached an optimum 4 days before symptoms appeared. The highest concentrations of TYLCV-DNA were found in rapidly growing tissues (shoot apex, young leaves, roots) and in the stems; the lowest concentrations were found in the older leaves and cotyledons. Plants were also inoculated on specific sites. Young leaves and apices were the best targets for virus inoculation. In these tissues, the viral DNA replicated at the site of inoculation and was transported first to the roots, then to the shoot apex and to the neighboring leaves and the flowers. Inoculation through the oldest leaves was inefficient.

Long-distance movement and viral assembly of tobacco mosaic virus mutants

Spreading of tobacco mosaic virus in infected plants is of two modes: cell-to-cell movement (to adjacent cells) and long-distance movement (to distant parts of the plant). Viral coat protein has been suggested to be involved in long-distance movement. To analyze the function of coat protein in the movement, we used mutants with modifications in the coat protein gene or in the assembly origin on the genomic RNA. A mutant which has the coding region for the C-terminal 5 amino acids of the protein deleted and mutants with 1 amino acid inserted after residue 101 or 152 of the protein retained both the abilities of long-distance movement and assembly into virus particles. Other mutants in the coat protein gene eliminated the two abilities. A mutant with modifications in the assembly origin displayed greatly reduced abilities of both the movement and assembly. These results suggest that both the coat protein with its ability to assemble into virus particles and the assembly origin are involved in long-distance movement, and that virus particles may play a pivotal role in the movement.

Regeneration of a functional RNA virus genome by recombination between deletion mutants and requirement for cowpea chlorotic mottle virus 3a and coat genes for systemic infection

RNAs 1 and 2 of the tripartite cowpea chlorotic mottle virus (CCMV) genome are sufficient for RNA replication in protoplasts, whereas systemic infection of cowpea plants additionally requires RNA3, which encodes the 3a noncapsid protein and coat protein. By using biologically active CCMV cDNA clones, we find that deletions in either RNA3 gene block systemic infection. Thus, though some plant RNA viruses are able to spread systemically without encapsidation, both the coat and 3a genes are required for systemic infection of cowpeas by CCMV. When plants were coinoculated with CCMV RNAs 1 and 2 and both the 3a and coat deletion mutants of RNA3, 30-60% rapidly developed systemic infection. Progeny RNA recovered from systemically infected leaves in such infections contained neither of the starting deletion mutants but rather a single full-length RNA3 component with both genes intact. Nucleotide substitutions introduced into the coat protein deletion mutant as an artificial marker were recovered in the full-length progeny RNA, confirming its recombinant nature. Intermolecular RNA recombination in planta can, therefore, rescue a complete infectious genome from coinoculated mutants independently disabled for systemic spread. These results have implications for the repair of defective genomes produced by frequent natural replication errors, the possible emergence of newly adapted RNA viruses upon coinfection of new hosts, and further studies of RNA virus recombination.

The P30 movement protein of tobacco mosaic virus is a single-strand nucleic acid binding protein

The P30 protein of tobacco mosaic virus (TMV) is required for cell to cell movement of viral RNA, which presumably occurs through plant intercellular connections, the plasmodesmata. The mechanism by which P30 mediates transfer of TMV RNA molecules through plasmodesmata channels is unknown. We have identified P30 as an RNA and single-stranded (ss) DNA binding protein. Binding of purified P30 to ss nucleic acids is strong, highly cooperative, and sequence nonspecific with a minimal binding site of 4-7 nucleotides per P30 monomer. In-frame deletions across P30 were used to localize the ss nucleic acid binding domain to within amino acid residues 65-86 of the protein. We propose that binding of P30 to TMV RNA creates an unfolded protein-RNA complex that functions as an intermediate in virus cell to cell movement through plasmodesmata.

Movement protein of tobacco mosaic virus modifies plasmodesmatal size exclusion limit

The function of the 30-kilodalton movement protein (MP) of tobacco mosaic virus is to facilitate cell-to-cell movement of viral progeny in an infected plant. A novel method for delivering non-plasmalemma-permeable fluorescent probes to the cytosol of spongy mesophyll cells of tobacco leaves was used to study plasmodesmatal size exclusion limits in transgenic plants that express the MP gene. Movement of fluorescein isothiocyanate-labeled dextran (F-dextran) with an average molecular mass of 9400 daltons and an approximate Stokes radius of 2.4 nanometers was detected between cells of the transgenic plants, whereas the size exclusion limit for the control plants was 700 to 800 daltons. No evidence of F-dextran metabolism in the leaves of the transgenic plants was found. Thus, the tobacco mosaic virus movement protein has a direct effect on a plasmodesmatal function.