Replication and trafficking of a plant virus are coupled at the entrances of plasmodesmata

Plant viruses use movement proteins (MPs) to modify intercellular pores called plasmodesmata (PD) to cross the plant cell wall. Many viruses encode a conserved set of three MPs, known as the triple gene block (TGB), typified by Potato virus X (PVX). In this paper, using live-cell imaging of viral RNA (vRNA) and virus-encoded proteins, we show that the TGB proteins have distinct functions during movement. TGB2 and TGB3 established endoplasmic reticulum-derived membranous caps at PD orifices. These caps harbored the PVX replicase and nonencapsidated vRNA and represented PD-anchored viral replication sites. TGB1 mediated insertion of the viral coat protein into PD, probably by its interaction with the 5' end of nascent virions, and was recruited to PD by the TGB2/3 complex. We propose a new model of plant virus movement, which we term coreplicational insertion, in which MPs function to compartmentalize replication complexes at PD for localized RNA synthesis and directional trafficking of the virus between cells.

Fig mosaic emaravirus p4 protein is involved in cell-to-cell movement

Fig mosaic virus (FMV), a member of the newly formed genus Emaravirus, is a segmented negative-strand RNA virus. Each of the six genomic FMV segments contains a single ORF: that of RNA4 encodes the protein p4. FMV-p4 is presumed to be the movement protein (MP) of the virus; however, direct experimental evidence for this is lacking. We assessed the intercellular distribution of FMV-p4 in plant cells by confocal laser scanning microscopy and we found that FMV-p4 was localized to plasmodesmata and to the plasma membrane accompanied by tubule-like structures. A series of experiments designed to examine the movement functions revealed that FMV-p4 has the capacity to complement viral cell-to-cell movement, prompt GFP diffusion between cells, and spread by itself to neighbouring cells. Altogether, our findings demonstrated that FMV-p4 shares several properties with other viral MPs and plays an important role in cell-to-cell movement.

Systemic transport of Alfalfa mosaic virus can be mediated by the movement proteins of several viruses assigned to five genera of the 30K family

We previously showed that the movement protein (MP) gene of Alfalfa mosaic virus (AMV) is functionally exchangeable for the cell-to-cell transport of the corresponding genes of Tobacco mosaic virus (TMV), Brome mosaic virus, Prunus necrotic ringspot virus, Cucumber mosaic virus and Cowpea mosaic virus. We have analysed the capacity of the heterologous MPs to systemically transport the corresponding chimeric AMV genome. All MPs were competent in systemic transport but required the fusion at their C terminus of the coat protein-interacting C-terminal 44 aa (A44) of the AMV MP. Except for the TMV MP, the presence of the hybrid virus in upper leaves correlated with the capacity to move locally. These results suggest that all the MPs assigned to the 30K superfamily should be exchangeable not only for local virus movement but also for systemic transport when the A44 fragment is present.

[Effect of BYDV-MP nuclear localization signal on the movement of PVX]

Abstract:By using PVX derived vector pGR107, the effect of BYDV-MP nuclear localization signal on the movement of PVX was studied. BYDV-MP was cloned into pGR107 using GFP as an indicator. BYDV-MP was then shown to induce the systemic infection and exacerbate the symptom of PVX through infecting Nicotiana benthamiana. When the PVX gene encoding 25kD protein, which functioned as a systematic movemnet protein,was deleted and the above experiment was repeated, the result showed that BYDV-MP could compensate the systemic movement of PVX. A serial mutants with substitutions on the fifth, sixth and seventh amino acids of BYDV-MP nuclear localization signal was further constructed. It was found that the mutants at the fifth, sixth amino acids in BYDV-MP nuclear localization signal could only delay or weaken systemic movement of PVX whereas the mutant at seventh amino acid could entirely inhibit systemic movement of PVX.

A plant virus movement protein regulates the Gcn2p kinase in budding yeast

Virus life cycle heavily depends on their ability to command the host machinery in order to translate their genomes. Animal viruses have been shown to interfere with host translation machinery by expressing viral proteins that either maintain or inhibit eIF2α function by phosphorylation. However, this interference mechanism has not been described for any plant virus yet. Prunnus necrotic ringspot virus (PNRSV) is a serious pathogen of cultivated stone fruit trees. The movement protein (MP) of PNRSV is necessary for the cell-to-cell movement of the virus. By using a yeast-based approach we have found that over-expression of the PNRSV MP caused a severe growth defect in yeast cells. cDNA microarrays analysis carried out to characterise at the molecular level the growth interference phenotype reported the induction of genes related to amino acid deprivation suggesting that expression of MP activates the GCN pathway in yeast cells. Accordingly, PNRSV MP triggered activation of the Gcn2p kinase, as judged by increased eIF2α phosphorylation. Activation of Gcn2p by MP expression required a functional Tor1p kinase, since rapamycin treatment alleviated the yeast cell growth defect and blocked eIF2α phosphorylation triggered by MP expression. Overall, these findings uncover a previously uncharacterised function for PNRSV MP viral protein, and point out at Tor1p and Gcn2p kinases as candidate susceptibility factors for plant viral infections.

Movement protein Pns6 of rice dwarf phytoreovirus has both ATPase and RNA binding activities

Cell-to-cell movement is essential for plant viruses to systemically infect host plants. Plant viruses encode movement proteins (MP) to facilitate such movement. Unlike the well-characterized MPs of DNA viruses and single-stranded RNA (ssRNA) viruses, knowledge of the functional mechanisms of MPs encoded by double-stranded RNA (dsRNA) viruses is very limited. In particular, many studied MPs of DNA and ssRNA viruses bind non-specifically ssRNAs, leading to models in which ribonucleoprotein complexes (RNPs) move from cell to cell. Thus, it will be of special interest to determine whether MPs of dsRNA viruses interact with genomic dsRNAs or their derivative sRNAs. To this end, we studied the biochemical functions of MP Pns6 of Rice dwarf phytoreovirus (RDV), a member of Phytoreovirus that contains a 12-segmented dsRNA genome. We report here that Pns6 binds both dsRNAs and ssRNAs. Intriguingly, Pns6 exhibits non-sequence specificity for dsRNA but shows preference for ssRNA sequences derived from the conserved genomic 5'- and 3'-terminal consensus sequences of RDV. Furthermore, Pns6 exhibits magnesium-dependent ATPase activities. Mutagenesis identified the RNA binding and ATPase activity sites of Pns6 at the N- and C-termini, respectively. Our results uncovered the novel property of a viral MP in differentially recognizing dsRNA and ssRNA and establish a biochemical basis to enable further studies on the mechanisms of dsRNA viral MP functions.

Identification a coat protein region of cucumber mosaic virus (CMV) essential for long-distance movement in cucumber

To characterise the long-distance movement determinant of cucumoviral coat proteins (CPs), five mutants were engineered into the CMV CP bearing the corresponding tomato aspermy virus (TAV) loops exposed on the surface of the virion. Both viruses can move long-distance in Nicotiana clevelandii, but only CMV can move long-distance in cucumber. Investigation of the CMV chimeras identified three amino acids of the βB-βC loop that were essential for the CMV long-distance movement in cucumber. Introducing these mutations into the TAV CP was not sufficient for long-distance movement, indicating that this is not the sole region causing long-distance movement deficiency.

Plasmodesmata during development: re-examination of the importance of primary, secondary, and branched plasmodesmata structure versus function

Plasmodesmata (PD) structure and function vary temporally and spatially during all stages of plant development. PD that originate during, or post, cell division are designated as primary or secondary according to classical terminology. PD structure may be simple, twinned, or branched. Studies of PD during leaf, root, and embryo development have lead to the generalization that cells in less mature tissues contain predominantly simple PD. New quantitative analyses reveal that twinned and branched PD also occur in immature tissues. New data also highlight the versatility of viral movement proteins as tags for labeling PD in immature tissues as well as PD in mature tissues. A summary of the formation and function of primary, secondary, and branched PD during leaf, trichome, embryo, apical meristem, vascular cambium, and root development underscores the remarkable and indispensible plant-specific intercellular communication system that is mediated by PD.

Intracellular transport of viruses and their components: utilizing the cytoskeleton and membrane highways

Plant viruses are obligate organisms that require host components for movement within and between cells. A mechanistic understanding of virus movement will allow the identification of new methods to control virus systemic spread and serve as a model system for understanding host macromolecule intra- and intercellular transport. Recent studies have moved beyond the identification of virus proteins involved in virus movement and their effect on plasmodesmal size exclusion limits to the analysis of their interactions with host components to allow movement within and between cells. It is clear that individual virus proteins and replication complexes associate with and, in some cases, traffic along the host cytoskeleton and membranes. Here, we review these recent findings, highlighting the diverse associations observed between these components and their trafficking capacity. Plant viruses operate individually, sometimes within virus species, to utilize unique interactions between their proteins or complexes and individual host cytoskeletal or membrane elements over time or space for their movement. However, there is not sufficient information for any plant virus to create a complete model of its intracellular movement; thus, more research is needed to achieve that goal.

Mutational analysis of the putative pipo of soybean mosaic virus suggests disruption of PIPO protein impedes movement

The presence of a small open reading frame embedded in the P3 cistron of potyvirus turnip mosaic virus, termed "pipo," was recently discovered. We have now studied the putative pipo of soybean mosaic virus (SMV). Introduction of single, or multiple, stop codon mutations at different locations within pipo, without substitution in polyprotein amino acids, did not abolish replication, but restricted the virus to small cluster of cells within the inoculated leaves. Furthermore, extensive mutagenesis of the conserved GA(6) motif at the 5' end of pipo also generated two out of five mutants that remained restricted to small foci of infected cells within the inoculated leaves. Long-distance movement function of the movement-defective PIPO-mutants was not restored following co-inoculation with competent SMV strains. Taken together, the data suggest that the putative pipo of SMV is essential for the virus movement; however, knock out of its expression does not abolish replication.

Identification of domains of the Tomato spotted wilt virus NSm protein involved in tubule formation, movement and symptomatology

Deletion and alanine-substitution mutants of the Tomato spotted wilt virus NSm protein were generated to identify domains involved in tubule formation, movement and symptomatology using a heterologous Tobacco mosaic virus expression system. Two regions of NSm, G(19)-S(159) and G(209)-V(283), were required for both tubule formation in protoplasts and cell-to-cell movement in plants, indicating a correlation between these activities. Three amino acid groups, D(154), EYKK(205-208) and EEEEE(284-288) were linked with long-distance movement in Nicotiana benthamiana. EEEEE(284-288) was essential for NSm-mediated long-distance movement, whereas D(154) was essential for tubule formation and cell-to-cell movement; indicating separate genetic controls for cell-to-cell and long-distance movement. The region I(57)-N(100) was identified as the determinant of foliar necrosis in Nicotiana benthamiana, and mutagenesis of HH(93-94) greatly reduced necrosis. These findings are likely applicable to other tospovirus species, especially those within the 'New World' group as NSm sequences are highly conserved.

Cymbidium mosaic potexvirus isolate-dependent host movement systems reveal two movement control determinants and the coat protein is the dominant

Little is known about how plant viruses of a single species exhibit different movement behavior in different host species. Two Cymbidium mosaic potexvirus (CymMV) isolates, M1 and M2, were studied. Both can infect Phalaenopsis orchids, but only M1 can systemically infect Nicotiana benthamiana plants. Protoplast inoculation and whole-mount in situ hybridization revealed that both isolates can replicate in N. benthamiana; however, M2 was restricted to the initially infected cells. Genome shuffling between M1 and M2 revealed that two control modes are involved in CymMV host dependent movement. The M1 coat protein (CP) plays a dominant role in controlling CymMV movement between cells, because all chimeric CymMV viruses containing the M1 CP systemically infected N. benthamiana plants. Without the M1 CP, one chimeric virus containing the combination of the M1 triple gene block proteins (TGBps), the M2 5' RNA (1-4333), and the M2 CP effectively moved in N. benthamiana plants. Further complementation analysis revealed that M1 TGBp1 and TGBp3 are co-required to complement the movement of the chimeric viruses in N. benthamiana. The amino acids within the CP, TGBp1 and TGBp3 which are required or important for CymMV M2 movement in N. benthamiana plants were mapped. The required amino acids within the CP map to the predicted RNA binding domain. RNA-protein binding assays revealed that M1 CP has higher RNA binding affinity than does M2 CP. Yeast two-hybrid assays to detect all possible interactions of M1 TGBps and CP, and only TGBp1 and CP self-interactions were observed.

Inhibition of long-distance movement of RNA silencing signals in Nicotiana benthamiana by Apple chlorotic leaf spot virus 50 kDa movement protein

Apple chlorotic leaf spot virus 50 kDa movement protein (P50) acts as a suppressor of systemic silencing in Nicotiana benthamiana. Here, we investigate the mode of action of P50 suppressor. An agroinfiltration assay in GFP-expressing N. benthamiana line16c (GFP-plant) showed that P50 could not prevent the short-distance spread of silencing. In grafting experiments, the systemic silencing was inhibited in GFP-plants (scion) grafted on P50-expressing N. benthamiana (P50-plant; rootstock) when GFP silencing was induced in rootstock. In double-grafted plants, GFP-plant (scion)/P50-plant (interstock)/GFP-plant (rootstock), the systemic silencing in scion was inhibited when GFP silencing was induced in rootstock. Analysis of P50 deletion mutants indicated that the N-terminal region (amino acids 1-284) is important for its suppressor activity. In gel mobility shift assay, P50 lacks binding ability with siRNAs. These results indicated that P50 has a unique suppressor activity that specifically inhibits the long-distance movement of silencing signals.

Genetic and histological studies on the delayed systemic movement of Tobacco Mosaic Virus in Arabidopsis thaliana

BACKGROUND

Viral infections and their spread throughout a plant require numerous interactions between the host and the virus. While new functions of viral proteins involved in these processes have been revealed, current knowledge of host factors involved in the spread of a viral infection is still insufficient. In Arabidopsis thaliana, different ecotypes present varying susceptibilities to Tobacco mosaic virus strain U1 (TMV-U1). The rate of TMV-U1 systemic movement is delayed in ecotype Col-0 when compared with other 13 ecotypes.We followed viral movement through vascular tissue in Col-0 plants by electronic microscopy studies. In addition, the delay in systemic movement of TMV-U1 was genetically studied.

RESULTS

TMV-U1 reaches apical leaves only after 18 days post rosette inoculation (dpi) in Col-0, whereas it is detected at 9 dpi in the Uk-4 ecotype. Genetic crosses between Col-0 and Uk-4 ecotypes, followed by analysis of viral movement in F1 and F2 populations, revealed that this delayed movement correlates with a recessive, monogenic and nuclear locus. The use of selected polymorphic markers showed that this locus, denoted DSTM1 (Delayed Systemic Tobamovirus Movement 1), is positioned on the large arm of chromosome II. Electron microscopy studies following the virion's route in stems of Col-0 infected plants showed the presence of curved structures, instead of the typical rigid rods of TMV-U1. This was not observed in the case of TMV-U1 infection in Uk-4, where the observed virions have the typical rigid rod morphology.

CONCLUSION

The presence of defectively assembled virions observed by electron microscopy in vascular tissue of Col-0 infected plants correlates with a recessive delayed systemic movement trait of TMV-U1 in this ecotype.

Movement profiles: a tool for quantitative analysis of cell-to-cell movement of plant viral movement proteins

Movement proteins (MPs) are virally encoded factors that mediate transport of viral nucleic acid between plant cells. Many MPs are able to move between cells themselves. This feature serves as the basis for evaluation of the transport activity of individual MPs. MPs are transiently expressed as a fusion to autofluorescent proteins such as green fluorescent protein (GFP) in individual epidermal cells of leaves by biolistic delivery. Expressing cells can be directly monitored for subcellular localization and cell-to-cell movement of the MP:GFP fusion protein into neighboring cells by confocal scanning microscopy. During the time frame of transient expression, numerous cells are evaluated at several time points, and the accumulated data are depicted in a graph termed "movement profile." Thus, a movement profile will provide information on the correlation between subcellular localization of the MP in the expressing cell and the efficiency of cell-to-cell transport, the time course and efficiency of targeting of the MP to plasmodesmata, and the translocation efficiency of the MP into neighboring cells.

Infection of soybean by cucumber mosaic virus as determined by viral movement protein

To characterize the host range determinant of the soybean strain of Cucumber mosaic virus (CMV) we analyzed a series of pseudorecombinants and chimeric viruses between infectious transcripts from two soybean strains (CMV-SC and CMV-SD) and an ordinary strain (CMV-Y). CMV-Y could not infect soybeans, even locally. Systemic infection of the two soybean-adapted soybean isolates on soybean plants mapped to RNA3. Chimeric RNA3s from between CMV-SC and CMV-Y, and chimeric RNA3s from between CMV-SC and CMV-SD, were made and inoculated onto wild soybean Iwate and soybean cv. Tsurunoko. The 3a region determined the viral systemic movement in the plants. In the wild soybean ecotype Hyougo, cell-to-cell movement of two different CMV soybean strains, one of which infects systemically while the other does not, in the inoculated leaves were almost the same, suggesting that the resistance of soybean operates at the level of long-distance movement. Our results clearly suggest that movement protein is a host determinant of CMV soybean strains.

The movement protein BC1 promotes redirection of the nuclear shuttle protein BV1 of Abutilon mosaic geminivirus to the plasma membrane in fission yeast

In order to monitor their interaction and cellular localisation, the movement protein (MP; syn. BC1) and the nuclear shuttle protein (NSP; syn. BV1) of the geminivirus Abutilon mosaic virus (AbMV) were ectopically expressed in Schizosaccharomyces pombe cells, either alone or together under the control of an inducible promoter. For highest resolution, electron microscopy using freeze-fracture immunolabelling served to detect these proteins in situ. As expected from previous in planta and yeast experiments, NSP accumulated within the nuclei, whereas MP was targetted to the protoplasmic face of plasma membranes when expressed alone. Upon coexpression, NSP was localised at the plasma membranes, where it was strongly attached. These results support a model in which NSP transports viral DNA to the cell periphery to facilitate cell-to-cell movement of viral DNA within plants. In contrast to AbMV MP, no plant-specific protein seems to be necessary for the translocation of NSP to the plasma membrane.

Apple chlorotic leaf spot virus 50 kDa movement protein acts as a suppressor of systemic silencing without interfering with local silencing in Nicotiana benthamiana

Apple chlorotic leaf spot virus (ACLSV) is the type species of the genus Trichovirus and its single-stranded, plus-sense RNA genome encodes a 216 kDa protein (P216) involved in replication, a 50 kDa movement protein (P50) and a 21 kDa coat protein (CP). In this study, it was investigated whether these proteins might have RNA silencing-suppressor activities by Agrobacterium-mediated transient assay in the green fluorescent protein-expressing Nicotiana benthamiana line 16c. The results indicated that none of these proteins could suppress local silencing in infiltrated leaves. However, systemic silencing in upper leaves induced by both single- and double-stranded RNA could be suppressed by P50, but not by a frame-shift mutant of P50, P216 or CP. Moreover, when P50 was expressed separately from where silencing signals were generated in a leaf, systemic silencing in upper leaves was inhibited. Collectively, our data indicate that P50 acts as a suppressor of systemic silencing without interfering with local silencing, probably by inhibiting the movement of silencing signals.