Prevalence and molecular characterization of important potato viruses in the Tokat province of Turkey

BACKGROUND

It is believed that viruses affect potato yield more than any other pathogens worldwide.

METHOD AND RESULTS

We report here on a survey of the four most common potato viruses in the Tokat Province of northern Turkey. Leaf samples were collected from potato plants showing signs of viral diseases in five districts of the province. Over 400 leaf samples were tested using RT-PCR with virus-specific primers. Among the one or more viruses detected in 218 (52%) leaf samples, Potato virus Y (PVY) was the most common (47.1%), followed by potato virus S (PVS; 16.7%), potato virus X (PVX; 6.0%) and potato leaf roll virus (PLRV; 5.3%). The most common mixed infections were PVY + PVS (6.9%). A phylogenetic analysis of the gene sequences showed all Turkish PVS isolates to be clustered with the PVS^O group, two PVY isolates with the PVY^N-WI group and one isolate with the PVY^NTN group. Turkish PVX isolates are in the Type X group of the two major PVX isolate groups. The Turkish PLRV isolates were separated into two major groups depending on the results of the phylogenetic analysis, with six cases in Group 1 and one in Group 2.

CONCLUSIONS

PVY, PVX, PVS and PLRV were detected in potato production areas in Tokat. A phylogenetic comparison of the gene sequences revealed all Turkish isolates to be immigrant members of the world populations of these viruses. Our results emphasize the importance of the strict quarantine control of plant materials entering Turkey.

A GoldenBraid-Compatible Virus-Based Vector System for Transient Expression of Heterologous Proteins in Plants

We have developed a Potato virus X (PVX)-based vector system compatible with the GoldenBraid 2.0 (GB) cloning strategy to transiently express heterologous proteins or peptides in plants for biotechnological purposes. This vector system consists of three domestication vectors carrying three GB parts-the cauliflower mosaic virus (CaMV) 35S promoter with PVX upstream of the second subgenomic promoter of the PVX coat protein (PVX CP SGP), nopaline synthase (NOS) terminator with PVX downstream of the first PVX CP SGP and the gene of interest (GOI). The full-length PVX clone carrying the sequence encoding a green fluorescent protein (GFP) as GOI was incorporated into the binary GB vector in a one-step reaction of three GB parts using the four-nucleotide GB standard syntax. We investigated whether the obtained vector named GFP/pGBX enables systemic PVX infection and expression of GFP in Nicotiana benthamiana plants. We show that this GB-compatible vector system can be used for simple and efficient assembly of PVX-based expression constructs and that it meets the current need for interchange of standard biological parts used in different expression systems.

Transient expression of recombinant proteins in plants using potato virus X based vectors

Plants become a promising biofactory for the large-scale production of recombinant proteins due to low cost, scalability, and safety. Agroinfiltration of plant leaves with a plant viral vector carrying a gene of interest is a rapid and efficient method for protein production in plants. Currently this method is in use for producing a wide range of proteins for multiple applications, including vaccine antigens, antibodies, and protein nanoparticles such as virus-like particles. A number of pharmaceutical proteins produced by transient expression are currently in clinical development. Here, we describe potato virus X based vector pEff-GFP enabling fast and high-level expression of recombinant proteins in Nicotiana benthamiana plants. The pEff vector provides green fluorescent protein expression levels of up to 30% of total soluble protein (about 1mg per g of fresh leaf tissue) and was successfully applied for the production of the immunogens of potential clinical interest.

Attachment of Ultralow Amount of Engineered Plant Viral Nanoparticles to Mesenchymal Stem Cells Enhances Osteogenesis and Mineralization

Hydrogel-based materials are widely used to mimic the extracellular matrix in bone tissue engineering, although they often lack biofunctional cues. In the authors' previous work, Potato virus X (PVX), a flexible rod-shaped biocompatible plant virus nanoparticle (VNP) with 1270 coat protein subunits, is genetically modified to present functional peptides for generating a bone substitute. Here, PVX is engineered to present mineralization- and osteogenesis-associated peptides and laden in hydrogels at a concentration lower by two orders of magnitude. Its competence in mineralization is demonstrated both on 2D surfaces and in hydrogels and the superiority of enriched peptides on VNPs is verified and compared with free peptides and VNPs presenting fewer functional peptides. Alkaline phosphatase activity and Alizarin red staining of human mesenchymal stem cells increase 1.2-1.7 times when stimulate by VNPs. Engineered PVX adheres to cells, exhibiting a stimulation of biomimetic peptides in close proximity to the cells. The retention of VNPs in hydrogels is monitored and more than 80% of VNPs remain inside after several washing steps. The mechanical properties of VNP-laden hydrogels are investigated, including viscosity, gelling temperature, and compressive tangent modulus. This study demonstrates that recombinant PVX nanoparticles are excellent candidates for hydrogel nanocomposites in bone tissue engineering.

A plant-based transient expression system for the rapid production of highly immunogenic Hepatitis E virus-like particles

OBJECTIVE

Hepatitis E virus (HEV) infection is a major cause of acute hepatitis worldwide. The aim of the study is the development of plant expression system for the production of virus-like particles formed by HEV capsid and the characterization of their immunogenicity.

RESULTS

Open reading frame (ORF) 2 encodes the viral capsid protein and possesses candidate for vaccine production. In this study, we used truncated genotype 3 HEV ORF 2 consisting of aa residues 110 to 610. The recombinant protein was expressed in Nicotiana benthamiana plants using the self-replicating potato virus X-based vector pEff up to 10% of the soluble protein fraction. The yield of HEV 110-610 after purification was 150-200 µg per 1 g of green leaf biomass. The recombinant protein formed nanosized virus-like particles. The immunization of mice with plant-produced HEV 110-610 protein induced high levels of HEV-specific serum antibodies.

CONCLUSIONS

HEV ORF 2 (110-610 aa) can be used as candidate for the development of a plant-produced vaccine against Hepatitis E.

RNA silencing-related genes contribute to tolerance of infection with potato virus X and Y in a susceptible tomato plant

BACKGROUND

In plants, the RNA silencing system functions as an antiviral defense mechanism following its induction with virus-derived double-stranded RNAs. This occurs through the action of RNA silencing components, including Dicer-like (DCL) nucleases, Argonaute (AGO) proteins, and RNA-dependent RNA polymerases (RDR). Plants encode multiple AGOs, DCLs, and RDRs. The functions of these components have been mainly examined in Arabidopsis thaliana and Nicotiana benthamiana. In this study, we investigated the roles of DCL2, DCL4, AGO2, AGO3 and RDR6 in tomato responses to viral infection. For this purpose, we used transgenic tomato plants (Solanum lycopersicum cv. Moneymaker), in which the expression of these genes were suppressed by double-stranded RNA-mediated RNA silencing.

METHODS

We previously created multiple DCL (i.e., DCL2 and DCL4) (hpDCL2.4) and RDR6 (hpRDR6) knockdown transgenic tomato plants and here additionally did multiple AGO (i.e., AGO2 and AGO3) knockdown plants (hpAGO2.3), in which double-stranded RNAs cognate to these genes were expressed to induce RNA silencing to them. Potato virus X (PVX) and Y (PVY) were inoculated onto these transgenic tomato plants, and the reactions of these plants to the viruses were investigated. In addition to observation of symptoms, viral coat protein and genomic RNA were detected by western and northern blotting and reverse transcription-polymerase chain reaction (RT-PCR). Host mRNA levels were investigated by quantitative RT-PCR.

RESULTS

Following inoculation with PVX, hpDCL2.4 plants developed a more severe systemic mosaic with leaf curling compared with the other inoculated plants. Systemic necrosis was also observed in hpAGO2.3 plants. Despite the difference in the severity of symptoms, the accumulation of PVX coat protein (CP) and genomic RNA in the uninoculated upper leaves was not obviously different among hpDCL2.4, hpRDR6, and hpAGO2.3 plants and the empty vector-transformed plants. Moneymaker tomato plants were asymptomatic after infection with PVY. However, hpDCL2.4 plants inoculated with PVY developed symptoms, including leaf curling. Consistently, PVY CP was detected in the uninoculated symptomatic upper leaves of hpDCL2.4 plants through western blotting. Of note, PVY CP was rarely detected in other asymptomatic transgenic or wild-type plants. However, PVY was detected in the uninoculated upper leaves of all the inoculated plants using reverse transcription-polymerase chain reactions. These findings indicated that PVY systemically infected asymptomatic Moneymaker tomato plants at a low level (i.e., no detection of CP via western blotting).

CONCLUSION

Our results indicate that the tomato cultivar Moneymaker is susceptible to PVX and shows mild mosaic symptoms, whereas it is tolerant and asymptomatic to systemic PVY infection with a low virus titer. In contrast, in hpDCL2.4 plants, PVX-induced symptoms became more severe and PVY infection caused symptoms. These results indicate that DCL2, DCL4, or both contribute to tolerance to infection with PVX and PVY. PVY CP and genomic RNA accumulated to a greater extent in DCL2.4-knockdown plants. Hence, the contribution of these DCLs to tolerance to infection with PVY is at least partly attributed to their roles in anti-viral RNA silencing, which controls the multiplication of PVY in tomato plants. The necrotic symptoms observed in the PVX-infected hpAGO2.3 plants suggest that AGO2, AGO3 or both are also distinctly involved in tolerance to infection with PVX.

Utilizing Potato Virus X to Monitor RNA Movement

Mobility assays coupled with RNA profiling have revealed the presence of hundreds of full-length non-cell-autonomous messenger RNAs that move through the whole plant via the phloem cell system. Monitoring the movement of these RNA signals can be difficult and time consuming. Here we describe a simple, virus-based system for surveying RNA movement by replacing specific sequences within the viral RNA genome of potato virus X (PVX) that are critical for movement with other sequences that facilitate movement. PVX is a RNA virus dependent on three small proteins that facilitate cell-to-cell transport and a coat protein (CP) required for long-distance spread of PVX. Deletion of the CP blocks movement, whereas replacing the CP with phloem-mobile RNA sequences reinstates mobility. Two experimental models validating this assay system are discussed. One involves the movement of the flowering locus T RNA that regulates floral induction and the second involves movement of StBEL5, a long-distance RNA signal that regulates tuber formation in potato.

Optimized production of a biologically active Clostridium perfringens glycosyl hydrolase phage endolysin PlyCP41 in plants using virus-based systemic expression

BACKGROUND

Clostridium perfringens, a gram-positive, anaerobic, rod-shaped bacterium, is the third leading cause of human foodborne bacterial disease and a cause of necrotic enteritis in poultry. It is controlled using antibiotics, widespread use of which may lead to development of drug-resistant bacteria. Bacteriophage-encoded endolysins that degrade peptidoglycans in the bacterial cell wall are potential replacements for antibiotics. Phage endolysins have been identified that exhibit antibacterial activities against several Clostridium strains.

RESULTS

An Escherichia coli codon-optimized gene encoding the glycosyl hydrolase endolysin (PlyCP41) containing a polyhistidine tag was expressed in E. coli. In addition, The E. coli optimized endolysin gene was engineered for expression in plants (PlyCP41p) and a plant codon-optimized gene (PlyCP41pc), both containing a polyhistidine tag, were expressed in Nicotiana benthamiana plants using a potato virus X (PVX)-based transient expression vector. PlyCP41p accumulated to ~ 1% total soluble protein (100μg/gm f. wt. leaf tissue) without any obvious toxic effects on plant cells, and both the purified protein and plant sap containing the protein lysed C. perfringens strain Cp39 in a plate lysis assay. Optimal systemic expression of PlyCP41p was achieved at 2 weeks-post-infection. PlyCP41pc did not accumulate to higher levels than PlyCP41p in infected tissue.

CONCLUSION

We demonstrated that functionally active bacteriophage PlyCP41 endolysin can be produced in systemically infected plant tissue with potential for use of crude plant sap as an effective antimicrobial agent against C. perfringens.

Evaluation of Potato virus X mild mutants for cross protection against severe infection in China

BACKGROUND

Cross protection is a promising alternative to control plant viral diseases. One critical factor limiting the application of cross protection is the availability of attenuated mutants or mild strains. Potato virus X (PVX) infects many crops and induces huge economic losses to agricultural production. However, researches on the variability and mechanism of PVX virulence are scarce.

METHODS

The mutants were obtained by introducing mutations into the RNA dependent RNA polymerase (RdRp) gene of PVX via site-directed mutagenesis. Attenuated mutants were screen according to their symptoms in Nicotiana benthamiana plants. The protection efficacy against severe infection were evaluated with interval of 5, 10 and 15 days.

RESULTS

Among the 40 mutants obtained, four mutants carrying substitutions of either Glu46, Asn863, Asn968 or Glu1001 to Ala in PVX RdRp showed drastically attenuated symptom, accompanying with reduced accumulation levels of coat protein, plus- and minus-sense RNAs. When the interval between protective and challenging inoculations was 15 days, mutant E1001A (with substitution of Glu1001 to Ala in RdRp) provided complete protection against severe infection in both Nicotiana benthamiana and tomato, while E46A (Glu46 mutated to Ala) provided incomplete protection. To reduce the risk of reverse mutation, we constructed mutant dM which carries double mutations of both Glu46 and Glu1001 to Ala in RdRp. The mutant dM could provide effective protection against severe PVX infection.

CONCLUSION

Mutations of Glu46, Asn863, Asn968 or Glu1001 to Ala in PVX RdRp significantly reduced the viral symptoms. Mutants E1001A and E46A could provide effective protection against wild type PVX in both Nicotiana benthamiana and tomato. These results provide theoretical and practical bases for the control of PVX via cross protection.

The Potato Virus X TGBp2 Protein Plays Dual Functional Roles in Viral Replication and Movement

Plant viruses usually encode one or more movement proteins (MP) to accomplish their intercellular movement. A group of positive-strand RNA plant viruses requires three viral proteins (TGBp1, TGBp2, and TGBp3) that are encoded by an evolutionarily conserved genetic module of three partially overlapping open reading frames (ORFs), termed the triple gene block (TGB). However, how these three viral movement proteins function cooperatively in viral intercellular movement is still elusive. Using a novel in vivo double-stranded RNA (dsRNA) labeling system, we showed that the dsRNAs generated by potato virus X (PVX) RNA-dependent RNA polymerase (RdRp) are colocalized with viral RdRp, which are further tightly covered by "chain mail"-like TGBp2 aggregates and localizes alongside TGBp3 aggregates. We also discovered that TGBp2 interacts with the C-terminal domain of PVX RdRp, and this interaction is required for the localization of TGBp3 and itself to the RdRp/dsRNA bodies. Moreover, we reveal that the central and C-terminal hydrophilic domains of TGBp2 are required to interact with viral RdRp. Finally, we demonstrate that knockout of the entire TGBp2 or the domain involved in interacting with viral RdRp attenuates both PVX replication and movement. Collectively, these findings suggest that TGBp2 plays dual functional roles in PVX replication and intercellular movement.IMPORTANCE Many plant viruses contain three partially overlapping open reading frames (ORFs), termed the triple gene block (TGB), for intercellular movement. However, how the corresponding three proteins coordinate their functions remains obscure. In the present study, we provided multiple lines of evidence supporting the notion that PVX TGBp2 functions as the molecular adaptor bridging the interaction between the RdRp/dsRNA body and TGBp3 by forming "chain mail"-like structures in the RdRp/dsRNA body, which can also enhance viral replication. Taken together, our results provide new insights into the replication and movement of PVX and possibly also other TGB-containing plant viruses.

Application to the detection of potato virus X

This article demonstrates a new kind of a highly sensitive lateral flow immunoassay (LFIA). It is based on the enlargement of the size of gold nanoparticles (GNPs) directly on the test strip after a conventional LFIA. Particle size enlargement is accomplished through the catalytic reduction of HAuCl₄ in the presence of H₂O₂ and through the accumulation of additional gold on the surface of the GNPs. To attain maximal enhancement of the coloration of the zone in the test strip and to achieve a minimal background, the concentration of precursors, the pH value, and the incubation time were optimized. GNPs on the test strip are enlarged from 20 to 350 nm after a 1-min treatment at room temperature. The economically important and widespread phytopathogen potato virus X (PVX) was used as the target analyte. The use of the GNP enlargement method results in a 240-fold reduction in the limit of the detection of PVX, which can be as low as 17 pg·mL^-1. The total duration of the assay, including virus extraction from the potato leaves, lateral flow, and the enhancement process, is only 12 min. The diagnostic efficiency of the technique was confirmed by its application to the analysis of potato leave samples. No false positives or false negatives were found. The technique does not depend on specific features of the target analyte, and it is conceivably applicable to numerous GNP-based LFIAs for important analytes. Graphical abstract An enlargement solution (containing HAuCl₄ and H₂O₂) was dripped on the strip after common lateral flow immunoassay. Gold nanoparticles on the strip (20 nm) catalyze gold reduction and the formation of larger particles (up to 350 nm), resulting in a 240-fold lower detection limit within 1 min.

Evaluation of potential RNA-interference-target genes to control cotton mealybug, Phenacoccus solenopsis (Hemiptera: Pseudococcuidae)

RNA interference (RNAi) of vital insect genes is a potential tool for targeted pest control. However, selection of the right target genes is a challenge because the RNAi efficacy is known to vary among insect species. Cotton mealybug, Phenacoccus solenopsis, is a phloem-feeding economically important crop pest. We evaluated the RNAi of 2 vital genes, Bursicon (PsBur) and V-ATPase (PsV-ATPase) as potential targets in P. solenopsis for its control. PCR fragments of PsBur and PsV-ATPase were amplified using cDNA synthesized from the total RNA. The PCR amplicons were cloned into Potato virus X (PVX) to develop recombinant PVX for the inoculation of Nicotiana tabacum plants for bioassays with healthy P. solenopsis. Reverse-transcription-polymerase chain reaction (RT-PCR) was used to validate the expression of transgenes in the recombinant-PVX-inoculated plants (treated), and suppression of the target genes in the mealybugs exposed to them. The RT-PCR confirmed the expression of transgenes in the treated plants. Mealybug individuals on treated plants either died or showed physical deformities. Further, the population of mealybug was significantly reduced by feeding on N. tabacum expressing RNAi triggers against PsBur and PsV-ATPase. The results conclude that RNAi is activated in P. solenopsis by feeding on N. tabacum expressing RNAi triggering elements of PsBur and PsV-ATPase genes through recombinant PVX vector. Further, V-ATPase and Bursicon genes are potential targets for RNAi-mediated control of P. solenopsis.

Double-enhanced lateral flow immunoassay for potato virus X based on a combination of magnetic and gold nanoparticles

This study presents the joint use of magnetic nanoparticles (MNPs) and gold nanoparticles (GNPs) for double enhancement in a lateral flow immunoassay (LFIA). The study realizes two types of enhancement: (1) increasing the concentration of analytes in the samples using conjugates of MNPs with specific antibodies and (2) increasing the visibility of the label through MNP aggregation caused by GNPs. The proposed strategy was implemented using a LFIA for potato virus X (PVX), a significant potato pathogen. MNPs conjugated with biotinylated antibodies specific to PVX and GNPs conjugated with streptavidin were synthesized and characterized. The LFIAs with and without the proposed enhancements were compared. The double-enhanced LFIA achieved the highest sensitivity, equal to 0.25 ng mL^-1 and 32 times more sensitivity than the non-enhanced LFIA (detection limit: 8 ng mL^-1). LFIAs using one of the types of amplification (magnetic concentration without GNPs-causing aggregation or MNP aggregation without the concentration stage) showed intermediate levels of sensitivity. The double-enhanced LFIA was successfully used for PVX detection in potato leaves. The results for PVX detection in the infected plants were similar for the double-enhanced LFIA developed and the conventional LFIA based on the GNP conjugates; however, the new system provided significant coloring enhancement. This study confirmed that a simple combination of MNPs and GNPs has great potential for high-sensitivity detection and could possibly be adopted for LFIAs of other compounds.

In Planta Production of Fluorescent Filamentous Plant Virus-Based Nanoparticles

Viral nanoparticles are attractive platforms for biomedical applications and are frequently employed for optical imaging in tissue culture and preclinical animal models as fluorescent probes. Chemical modification with organic dyes remains the most common strategy to develop such fluorescent probes. Here we report a genetic engineering approach to incorporate fluorescent proteins in viral nanoparticles, which can be propagated in their plant host. The fluorescent viral nanoparticles so obtained obviate post-harvest modifications and thereby maximize yields. Our engineering approach transforms filamentous potato virus X (PVX) to display green fluorescent protein (GFP) or mCherry as N-terminal coat protein (CP) fusions at a 1:3 fusion protein to CP ratio through integration of the foot-and-mouth disease 2A sequence. The in planta propagation of recombinant GFP-PVX or mCherry-PVX thus produced in Nicotiana benthamiana can be easily documented using fluorescence imaging. Molecular farming protocols can be accordingly optimized by monitoring chimera stability over the course of the infection cycle. Moreover, we also demonstrate the utility of recombinant mCherry-PVX in optical imaging of human cancer cells and tumor tissue in preclinical mice model. Together, these features make genetically engineered fluorescent PVX particles ideally suited for molecular imaging applications.

Enhancement of lateral flow immunoassay by alkaline phosphatase: a simple and highly sensitive test for potato virus X

Alkaline phosphatase (ALP) was used as an amplification tool in lateral flow immunoassay (LFIA). Potato virus Х (PVX) was selected as a target analyte because of its high economic importance. Two conjugates of gold nanoparticles were applied, one with mouse monoclonal antibody against PVX and one with ALP-labeled antibody against mouse IgG. They were immobilized to two fiberglass membranes on the test strip for use in LFIA. After exposure to the sample, a substrate for ALP (5-bromo-4-chloro-3-indolyl phosphate/nitro blue tetrazolium) was dropped on the test strip. The insoluble dark-violet diformazan produced by ALP precipitated on the membrane and significantly increased the color intensity of the control and test zones. The limit of detection (0.3 ng mL^-1) was 27 times lower than that of conventional LFIA for both buffer and potato leaf extracts. The ALP-enhanced LFIA does not require additional preparation procedures or washing steps and may be used by nontrained persons in resource-limited conditions. The new method of enhancement is highly promising and may lead to application for routine LFIA in different areas. Graphical abstract Two gold nanoparticles (GNP) conjugates were used - the first with monoclonal antibodies (mAb) (GNP-mAb); the second - alkaline phosphatase-labeled antibody against mAb (GNP-anti-mAb-ALP). The immuno complexes are captured by the polyclonal antibodies (pAb) in the test zone. Addition of the substrate solution (BCIP/NBT) results in the accumulation of the insoluble colored product and in a significance increase in color intensity.

Engineering Potato Virus X Particles for a Covalent Protein Based Attachment of Enzymes

Plant virus nanoparticles are often used to display functional amino acids or small peptides, thus serving as building blocks in application areas as diverse as nanoelectronics, bioimaging, vaccination, drug delivery, and bone differentiation. This is most easily achieved by expressing coat protein fusions, but the assembly of the corresponding virus particles can be hampered by factors such as the fusion protein size, amino acid composition, and post-translational modifications. Size constraints can be overcome by using the Foot and mouth disease virus 2A sequence, but the compositional limitations cannot be avoided without the introduction of time-consuming chemical modifications. SpyTag/SpyCatcher technology is used in the present study to covalently attach the Trichoderma reesei endoglucanase Cel12A to Potato virus X (PVX) nanoparticles. The formation of PVX particles is confirmed by western blot, and the ability of the particles to display Cel12A is demonstrated by enzyme-linked immunosorbent assays and transmission electron microscopy. Enzymatic assays show optimal reaction conditions of 50 °C and pH 6.5, and an increased substrate conversion rate compared to free enzymes. It is concluded that PVX displaying the SpyTag can serve as new scaffold for protein display, most notably for proteins with post-translational modifications.

Virulence determines beneficial trade-offs in the response of virus-infected plants to drought via induction of salicylic acid

It has been hypothesized that plants can get beneficial trade-offs from viral infections when grown under drought conditions. However, experimental support for a positive correlation between virus-induced drought tolerance and increased host fitness is scarce. We investigated whether increased virulence exhibited by the synergistic interaction involving Potato virus X (PVX) and Plum pox virus (PPV) improves tolerance to drought and host fitness in Nicotiana benthamiana and Arabidopsis thaliana. Infection by the pair PPV/PVX and by PPV expressing the virulence protein P25 of PVX conferred an enhanced drought-tolerant phenotype compared with single infections with either PPV or PVX. Decreased transpiration rates in virus-infected plants were correlated with drought tolerance in N. benthamiana but not in Arabidopsis. Metabolite and hormonal profiles of Arabidopsis plants infected with the different viruses showed a range of changes that positively correlated with a greater impact on drought tolerance. Virus infection enhanced drought tolerance in both species by increasing salicylic acid accumulation in an abscisic acid-independent manner. Viable offspring derived from Arabidopsis plants infected with PPV increased relative to non-infected plants, when exposed to drought. By contrast, the detrimental effect caused by the more virulent viruses overcame potential benefits associated with increased drought tolerance on host fitness.

Validation of reference genes for quantifying changes in gene expression in virus-infected tobacco

To facilitate quantification of gene expression changes in virus-infected tobacco plants, eight housekeeping genes were evaluated for their stability of expression during infection by one of three systemically-infecting viruses (cucumber mosaic virus, potato virus X, potato virus Y) or a hypersensitive-response-inducing virus (tobacco mosaic virus; TMV) limited to the inoculated leaf. Five reference-gene validation programs were used to establish the order of the most stable genes for the systemically-infecting viruses as ribosomal protein L25 > β-Tubulin > Actin, and the least stable genes Ubiquitin-conjugating enzyme (UCE) < PP2A < GAPDH. For local infection by TMV, the most stable genes were EF1α > Cysteine protease > Actin, and the least stable genes were GAPDH < PP2A < UCE. Using two of the most stable and the two least stable validated reference genes, three defense responsive genes were examined to compare their relative changes in gene expression caused by each virus.

Identification of MAPKs as signal transduction components required for the cell death response during compatible infection by the synergistic pair Potato virus X-Potato virus Y

Systemic necrosis is one of the most severe symptoms caused in compatible plant-virus interactions and shares common features with the hypersensitive response (HR). Mitogen-activated protein kinase (MAPK) cascades are associated with responses to compatible and incompatible host-virus interactions. Here, we show that virus-induced gene silencing of the Nicotiana benthamiana MAPK genes salicylic acid-induced protein kinase (SIPK) and wound-induced protein kinase (WIPK), and the MAPK kinase (MAPKK) genes MEK1 and MKK1, partially compromised the HR-like response induced by the synergistic interaction of Potato virus X with Potato virus Y (PVX-PVY). Nevertheless, ameliorated cell death induced by PVX-PVY in the MAPK(K)-silenced plants did not facilitate virus accumulation in systemically infected leaves. Dual silencing of SIPK and of the oxylipin biosynthetic gene 9-Lipoxygenase showed that the latter was epistatic to SIPK in response to PVX-PVY infection. These findings demonstrate that SIPK, WIPK, MEK1 and MKK1 function as positive regulators of PVX-PVY-induced cell death.

A biodistribution study of two differently shaped plant virus nanoparticles reveals new peculiar traits

Self-assembling plant virus nanoparticles (pVNPs) have started to be explored as nanometre-sized objects for biomedical applications, such as vaccine or drug delivery and imaging. Plant VNPs may be ideal tools in terms of biocompatibility and biodegradability endowed with a wide diversity of symmetries and dimensions, easy chemical/biological engineering, and rapid production in plants. Recently, we defined that icosahedral Tomato bushy stunt virus (TBSV) and filamentous Potato virus X (PVX) are neither toxic nor teratogenic. We report here the results of an interdisciplinary study aimed to define for the first time the biodistribution of unlabelled, unpegylated, underivatized TBSV and PVX by proved detecting antibodies. These data add new insights on the in vivo behaviour of these nano-objects and demonstrate that the pVNPs under scrutiny are each intrinsically endowed with peculiar properties foreshadowing different applications in molecular medicine.

Antimicrobial Activity of Bacteriophage Endolysin Produced in Nicotiana benthamiana Plants

The increasing spread of antibiotic-resistant pathogens has raised the interest in alternative antimicrobial treatments. In our study, the functionally active gram-negative bacterium bacteriophage CP933 endolysin was produced in Nicotiana benthamiana plants by a combination of transient expression and vacuole targeting strategies, and its antimicrobial activity was investigated. Expression of the cp933 gene in E. coli led to growth inhibition and lysis of the host cells or production of trace amounts of CP933. Cytoplasmic expression of the cp933 gene in plants using Potato virus X-based transient expression vectors (pP2C2S and pGR107) resulted in death of the apical portion of experimental plants. To protect plants against the toxic effects of the CP933 protein, the cp933 coding region was fused at its Nterminus to an N-terminal signal peptide from the potato proteinase inhibitor I to direct CP933 to the delta-type vacuoles. Plants producing the CP933 fusion protein did not exhibit the severe toxic effects seen with the unfused protein and the level of expression was 0.16 mg/g of plant tissue. Antimicrobial assays revealed that, in contrast to gram-negative bacterium E. coli (BL21(DE3)), the gram-positive plant pathogenic bacterium Clavibacter michiganensis was more susceptible to the plant-produced CP933, showing 18% growth inhibition. The results of our experiments demonstrate that the combination of transient expression and protein targeting to the delta vacuoles is a promising approach to produce functionally active proteins that exhibit toxicity when expressed in plant cells.

Activation of senescence-associated Dark-inducible (DIN) genes during infection contributes to enhanced susceptibility to plant viruses

Virus infections in plants cause changes in host gene expression that are common to other environmental stresses. In this work, we found extensive overlap in the transcriptional responses between Arabidopsis thaliana plants infected with Tobacco rattle virus (TRV) and plants undergoing senescence. This is exemplified by the up-regulation during infection of several senescence-associated Dark-inducible (DIN) genes, including AtDIN1 (Senescence 1, SEN1), AtDIN6 (Asparagine synthetase 1, AtASN1) and AtDIN11. DIN1, DIN6 and DIN11 homologues were also activated in Nicotiana benthamiana in response to TRV and Potato virus X (PVX) infection. Reduced TRV levels in RNA interference (RNAi) lines targeting AtDIN11 indicate that DIN11 is an important modulator of susceptibility to TRV in Arabidopsis. Furthermore, low accumulation of TRV in Arabidopsis protoplasts from RNAi lines suggests that AtDIN11 supports virus multiplication in this species. The effect of DIN6 on virus accumulation was negligible in Arabidopsis, perhaps as a result of gene or functional redundancy. However, TRV-induced silencing of NbASN, the DIN6 homologue in N. benthamiana, compromises TRV and PVX accumulation in systemically infected leaves. Interestingly, NbASN inactivation correlates with the appearance of morphological defects in infected leaves. We found that DIN6 and DIN11 regulate virus multiplication in a step prior to the activation of plant defence responses. We hypothesize on the possible roles of DIN6 and DIN11 during virus infection.

Inoculation of Nicotiana tabacum with recombinant potato virus X induces RNA interference in the solenopsis mealybug, Phenacoccus solenopsis Tinsley (Hemiptera: Pseudococcidae)

OBJECTIVES

The chitin synthase 1 (CHS1) gene in Phenacoccus solenopsis (PsCHS1) was evaluated as a potential target of RNA interference (RNAi) by using Potato virus X (PVX) as a vector (recombinant PVX) for expressing RNAi triggering elements in Nicotiana tabacum L.

RESULTS

RT-PCR analysis confirmed the expression of PsCHS1 in N. tabacum inoculated with recombinant-PVX-PsCHS1 (treated). RT- and multiplex-PCR further showed a reduction in mRNA levels of the target gene in mealybugs feeding on treated plants. Mortality in parent adults and emerging nymphs (21 and 29%) exposed to the treated plants was significantly higher (P < 0.05) than those exposed to uninoculated (-ve control) or inoculated with non-recombinant PVX (PVX-control). The number of surviving adults and the combined number of adults and nymphs (47 and 60%) was significantly (P < 0.05) lower on the treated plants than the -ve (76%) or PVX (74%) control. The visual observations verified the physical deformities in mealybugs exposed to the treated plants.

CONCLUSION

chitin synthase 1 is a potential RNAi target in P. solenopsis and the recombinant PVX can be used as a tool to evaluate candidate RNAi triggering elements in plants.

Development of a Rapid Detection Method for Potato virus X by Reverse Transcription Loop-Mediated Isothermal Amplification

The primary step for efficient control of viral diseases is the development of simple, rapid, and sensitive virus detection. Reverse transcription loop-mediated isothermal amplification (RT-LAMP) has been used to detect viral RNA molecules because of its simplicity and high sensitivity for a number of viruses. RT-LAMP for the detection of Potato virus X (PVX) was developed and compared with conventional reverse transcription polymerase chain reaction (RT-PCR) to demonstrate its advantages over RT-PCR. RT-LAMP reactions were conducted with or without a set of loop primers since one out of six primers showed PVX specificity. Based on real-time monitoring, RT-LAMP detected PVX around 30 min, compared to 120 min for RT-PCR. By adding a fluorescent reagent during the reaction, the extra step of visualization by gel electrophoresis was not necessary. RT-LAMP was conducted using simple inexpensive instruments and a regular incubator to evaluate whether RNA could be amplified at a constant temperature instead of using an expensive thermal cycler. This study shows the potential of RT-LAMP for the diagnosis of viral diseases and PVX epidemiology because of its simplicity and rapidness compared to RT-PCR.

The 5'-proximal region of Potato virus X RNA involves the potential cap-dependent "conformational element" for encapsidation

Filamentous helical Potato virus X (PVX) can be regarded as one of the well-studied viruses. Nevertheless, some aspects of the PVX assembly remained obscure. Previously, we have shown that the presence of a cap structure at the 5' end of PVX RNA is indispensable for assembly of viral ribonucleoprotein (vRNP) particles varying in length. Here, most significantly, removal of the cap structure from previously capped PVX RNA did not affect the efficiency of decapped RNA molecules to be assembled into vRNP. This result provided evidence that the cap structure by itself does not act as a signal for initiation of vRNP assembly. These observations allowed to presume that the capping triggers some spatial changes in the 5'-proximal site of PVX RNA creating a "conformational encapsidation signal for vRNP assembly", which is capable of triggering vRNP assembly in the absence of cap structure. Apparently, during capping the 5'-proximal segment of PVX RNA acquires a unique conformation which is stable to be retained even after cap removal.

Stealth filaments: Polymer chain length and conformation affect the in vivo fate of PEGylated potato virus X

Nanoparticles hold great promise for delivering medical cargos to cancerous tissues to enhance contrast and sensitivity of imaging agents or to increase specificity and efficacy of therapeutics. A growing body of data suggests that nanoparticle shape, in combination with surface chemistry, affects their in vivo fates, with elongated filaments showing enhanced tumor targeting and tissue penetration, while promoting immune evasion. The synthesis of high aspect ratio filamentous materials at the nanoscale remains challenging using synthetic routes; therefore we turned toward nature's materials, developing and studying the filamentous structures formed by the plant virus potato virus X (PVX). We recently demonstrated that PVX shows enhanced tumor homing in various preclinical models. Like other nanoparticle systems, the proteinaceous platform is cleared from circulation and tissues by the mononuclear phagocyte system (MPS). To increase bioavailability we set out to develop PEGylated stealth filaments and evaluate the effects of PEG chain length and conformation on pharmacokinetics, biodistribution, as well as potential immune and inflammatory responses. We demonstrate that PEGylation effectively reduces immune recognition while increasing pharmacokinetic profiles. Stealth filaments show reduced interaction with cells of the MPS; the protein:polymer hybrids are cleared from the body tissues within hours to days indicating biodegradability and biocompatibility. Tissue compatibility is indicated with no apparent inflammatory signaling in vivo. Tailoring PEG chain length and conformation (brush vs. mushroom) allows tuning of the pharmacokinetics, yielding long-circulating stealth filaments for applications in nanomedicine.

In vitro and in vivo toxicity evaluation of plant virus nanocarriers

The use of biological self-assembling materials, plant virus nanoparticles in particular, appears very intriguing as it allows a great choice of symmetries and dimensions, easy chemical and biological engineering of both surface and/or internal cavity as well as safe and rapid production in plants. In this perspective, we present an initial evaluation of the safety profile of two structurally different plant viruses produced in Nicotiana benthamiana L. plants: the filamentous Potato virus X and the icosahedral Tomato bushy stunt virus. In vitro haemolysis assay was used to test the cytotoxic effects, which could arise by pVNPs interaction with cellular membranes, while early embryo assay was used to evaluate toxicity and teratogenicity in vivo. Data indicates that these structurally robust particles, still able to infect plants after incubation in serum up to 24h, have neither toxic nor teratogenic effects in vitro and in vivo. This work represents the first safety-focused characterization of pVNPs in view of their possible use as drug delivery carriers.

Potato virus X-based expression vectors are stabilized for long-term production of proteins and larger inserts

Plus-strand RNA viruses such as Potato virus X (PVX) are often used as high-yielding expression vectors in plants, because they tolerate extra transgene insertion and expression without disrupting normal virus functions. However, sequence redundancy due to promoter duplication often leads to genetic instability. Although heterologous subgenomic promoter-like sequences (SGPs) have been successfully used in Tobacco mosaic virus vectors, only homologous SGP duplications have been used in PVX vectors. We stabilized PVX-based vectors by combining heterologous SGPs from related potexviruses with an N-terminal coat protein (CP) deletion. We selected two SGPs with core sequences homologous to PVX, from Bamboo mosaic virus (BaMV) and Cassava common mosaic virus, as well as a SGP with a heterologous core sequence from Foxtail mosaic virus (FoMV). We found that only the BaMV and CsCMV SGPs were utilized by the PVX replicase. However, the transgene remained unstable, due to the presence of an additional region with strong sequence similarity at the 5' end of the cp gene. The BaMV SGP combined with an N-terminal CP deletion achieved high PVX vector stability. This new expression vector is particularly useful for long-term production of proteins and for larger inserts. The improved PVX-based vectors are suitable for the systemic expression of any gene of interest in PVX host plants. The PVX-based vector can be advantageous for the overexpression of proteins, to analyze protein functions in planta or as a system for virus-induced gene silencing.

Complete genome sequences of new divergent potato virus X isolates and discrimination between strains in a mixed infection using small RNAs sequencing approach

Potato virus X (PVX; genus Potexvirus, family Alphaflexiviridae, order Tymovirales) is one of the most widespread and intensively studied viruses of potato. However, little is known about its diversity in its likely center of radiation, the Andean region of South America. To fill this gap, the strategy of Illumina deep sequencing of small RNAs was used to obtain complete or near complete genome sequence of PVX from 5 symptomatically infected greenhouse and 3 field samples (Solanum tuberosum) from Peru. PVX sequences determined in this study were assigned into three different phylogenetic groups of isolates. Notably, a complete genome sequence of a representative of a new PVX phylogenetic lineage was obtained, which shows a high level of sequence dissimilarity to other completely sequenced isolates (∼17%). The new PVX genotype was detected in greenhouse and field samples. One of the field samples was infected with the mixture of two PVX strains, which were efficiently discriminated using small RNA sequencing approach. The study confirms the utility of small RNAs deep sequencing for successful viral strain differentiation and discovery of new viral strains and indicates a high diversity of PVX in the Andean region of South America, a pattern which may be expected also for other potato pathogens.

Molecular characterization and field survey of Iranian potato virus X isolates

Six hundred and one symptomatic potato samples were collected from nine provinces in Iran. Screening by double-antibody sandwich enzyme linked immunosorbent assay using a potato virus X (PVX) together with a few potyviruses polyclonal antibodies, produced positive reactions in 4.3 % of samples against PVX. Based on symptoms on different test plant, the isolates were divided into two groups: the first groups causing blistering and malformation of leaves and the second showed mild mosaic and vein clearing in Nicotiana glutinosa. The almost complete nucleotide sequence of two isolates as a representative of severe and a mild isolates were determined. Genomes of two PVX Iranian isolates are identical to that of the most PVX isolates comprise 6435 nucleotides in length excluding 101 nucleotide in the 5' end of the genome and shares 94.8-96.7 % identities with European and Asian, and 77-96.1 % with American isolates. Furthermore, the 3'-terminal sequences, including the coat protein coding region of other 13 Iranian isolates were determined and compared with the GenBank sequences. Phylogenetic analysis of the cp gene of 13 Iranian isolates together all those available in public databases indicated that the 13 Iranian isolates all belong to low diversity clade I.

StRemorin1.3 hampers Potato virus X TGBp1 ability to increase plasmodesmata permeability, but does not interfere with its silencing suppressor activity

The Triple Gene Block 1 (TGBp1) protein encoded by the Potato virus X is a multifunctional protein that acts as a suppressor of RNA silencing or facilitates the passage of virus from cell to cell by promoting the plasmodesmata opening. We previously showed that the membrane raft protein StRemorin1.3 is able to impair PVX infection. Here, we show that overexpressed StRemorin1.3 does not impair the silencing suppressor activity of TGBp1, but affects its ability to increase plasmodesmata permeability. A similar effect on plasmodesmata permeability was observed with other movement proteins, suggesting that REM is a general regulator of plasmodesmal size exclusion limit. These results add to our knowledge of the mechanisms underlying the StREM1.3 role in virus infection.

Virus-induced gene complementation in tomato

Virus-induced gene complementation (VIGC), a plant virus technology based on Potato virus X for transient overexpression of endogenous genes complemented tomato mutants, resulting in non-ripening fruits to ripen. This efficient "gain-of-function" approach involves no stable transformation, and reveals a fruit-specific transcriptional network that may exist among key transcription factors in modulating tomato ripening. Thus, VIGC represents a novel and feasible strategy for gene functional analysis in plants.

The role of the 5'-cap structure in viral ribonucleoproteins assembly from potato virus X coat protein and RNAs

The potato virus X (PVX) virion can be reconstituted in vitro from the virus coat protein (CP) and RNA; heterologous RNAs may be used as well. In our recent study, structure and properties of cognate and heterologous viral ribonucleoproteins (vRNPs) were demonstrated to be similar to those of native virions. The assembly was found to be initiated at the 5' terminus of an RNA and was not dependent on RNA sequence. The aim of the present study was to search for a signal or an essential structural element that directs packaging of viral genetic material into vRNPs. vRNPs were formed by incubation of the PVX CP with heterologous capped RNAs, their functional fragments lacking the cap structure, as well as the capped and uncapped transcripts corresponding to the 5'-terminal region of the genomic PVX RNA. Experimental data show that the presence of the cap structure at the 5' end of a nucleic acid is an important condition for vRNP assembly from RNA and CP. Presumably, the 5'-cap affects conformational state of the RNA region responsible for the efficient interaction with CP and creates conformational encapsidation signal for vRNP assembly.