Genetic analysis of the cell-to-cell movement of beet yellows closterovirus

Tomato chlorosis virus CPm protein is a pathogenicity determinant and suppresses host local RNA silencing induced by single-stranded RNA

Introduction

Tomato chlorosis virus (ToCV) is a typical member of the genus Crinivirus, which severely threatens Solanaceae crops worldwide. The CPm protein encoded by ToCV has been reported to be associated with virus transmission by vectors and is involved in RNA silencing suppression, while the mechanisms remain ambiguous.

Methods

Here, ToCV CPm was ectopically expressed by a Potato virus X (PVX) vector and infiltrated into Nicotiana benthamiana wild-type and GFP-transgenic16c plants.

Results

The phylogenetic analysis showed that the CPm proteins encoded by criniviruses were distinctly divergent in amino acid sequences and predicted conserved domains, and the ToCV CPm protein possesses a conserved domain homologous to the TIGR02569 family protein, which does not occur in other criniviruses. Ectopic expression of ToCV CPm using a PVX vector resulted in severe mosaic symptoms followed by a hypersensitive-like response in N. benthamiana. Furthermore, agroinfiltration assays in N. benthamiana wilt type or GFP-transgenic 16c indicated that ToCV CPm protein effectively suppressed local RNA silencing induced by single-stranded but not double-stranded RNA, which probably resulted from the activity of binding double-stranded but not single-stranded RNA by ToCV CPm protein.

Conclusion

Taken together, the results of this study suggest that the ToCV CPm protein possesses the dual activities of pathogenicity and RNA silencing, which might inhibit host post-transcriptional gene silencing (PTGS)-mediated resistance and is pivotal in the primary process of ToCV infecting hosts.

Identification of Interactions between Proteins Encoded by Grapevine Leafroll-Associated Virus 3

The roles of proteins encoded by members of the genus Ampelovirus, family Closteroviridae are largely inferred by sequence homology or analogy to similarly located ORFs in related viruses. This study employed yeast two-hybrid and bimolecular fluorescence complementation assays to investigate interactions between proteins of grapevine leafroll-associated virus 3 (GLRaV-3). The p5 movement protein, HSP70 homolog, coat protein, and p20B of GLRaV-3 were all found to self-interact, however, the mechanism by which p5 interacts remains unknown due to the absence of a cysteine residue crucial for the dimerisation of the closterovirus homolog of this protein. Although HSP70h forms part of the virion head of closteroviruses, in GLRaV-3, it interacts with the coat protein that makes up the body of the virion. Silencing suppressor p20B has been shown to interact with HSP70h, as well as the major coat protein and the minor coat protein. The results of this study suggest that the virion assembly of a member of the genus Ampelovirus occurs in a similar but not identical manner to those of other genera in the family Closteroviridae. Identification of interactions of p20B with virus structural proteins provides an avenue for future research to explore the mechanisms behind the suppression of host silencing and suggests possible involvement in other aspects of the viral replication cycle.

Citrus Tristeza Virus: From Pathogen to Panacea

Citrus tristeza virus (CTV) is the most destructive viral pathogen of citrus. During the past century, CTV induced grave epidemics in citrus-growing areas worldwide that have resulted in a loss of more than 100 million trees. At present, the virus continues to threaten citrus production in many different countries. Research on CTV is accompanied by distinctive challenges stemming from the large size of its RNA genome, the narrow host range limited to slow-growing Citrus species and relatives, and the complexity of CTV populations. Despite these hurdles, remarkable progress has been made in understanding the CTV-host interactions and in converting the virus into a tool for crop protection and improvement. This review focuses on recent advances that have shed light on the mechanisms underlying CTV infection. Understanding these mechanisms is pivotal for the development of means to control CTV diseases and, ultimately, turn this virus into an ally.

Expected final online publication date for the Annual Review of Virology, Volume 9 is September 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Occurrence of Grapevine Leafroll-Associated Virus-3 (GLRaV-3), Complete Nucleotide Sequence and Cultivar Susceptibility to a GLRaV-3 Isolate from Shaanxi Province of China

Grapevine (Vitis spp.) is globally one of the most economically important fruit crops. China is the largest grapevine-growing country of the world and Shaanxi province is one of the major grapevine-growing provinces in the country. A survey of GLRaV-3 found it widespread, with 57–100% infection frequencies, in both wine and table grapevine cultivars of three grapevine-growing regions of Shaanxi province. The virus infection frequencies varied with cultivars and regions. In order to obtain the full genomic length of a new GLRaV-3 isolate, GLRaV-3-Sau (accession number MK988555), was sequenced. This isolate has a genome of 18026 nucleotides, and 14 open reading frames (ORFs). The full-genome of the isolate GLRaV-3-Sau shared 85.88% nucleotide identity to GLRaV-3-LN, another isolate found in China. Coat protein (CP) genes of GLRaV-3 isolates were identical (99%) to the Vitis vinifera isolate (accession number HQ185608.1) from the USA. Immunohistochemistry for virus localization found that distribution patterns were similar in red-berried cultivar ‘Cabernet Sauvignon’ and white-berried cultivar ‘Chardonnay’, and GLRaV-3 is restricted in phloem tissue of vascular bundles. Virus transmission by micrografting found virus transmission efficiency was higher in ‘Chardonnay’ and ‘Thompson Seedless’ than in ‘Hunan-1’, indicating that ‘Hunan-1’ was less sensitive to GLRaV-3. As far as we know, these are the most comprehensive comparisons on the genome and CP genes of GLRaV-3 worldwide and the first to have found that the grapevine ‘Hunan-1’ is less susceptible to GLRaV-3.

Virus Genome-Based Reporter for Analyzing Viral Movement Proteins and Plasmodesmata Permeability

Plant virus movement proteins (MPs) mediate cell-to-cell movement of the virus genome through plasmodesmata (PD). MPs target PD to increase their size exclusion limit (SEL), and this MP function is essential for virus intercellular trafficking. In this chapter, we describe the use of a Potato virus X genome-derived reporter for agroinfiltration-based identification of virus genome-encoded MPs and analysis of the ability of individual viral MPs or plant proteins to increase the PD SEL.

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

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

The p23 of Citrus Tristeza Virus Interacts with Host FKBP-Type Peptidyl-Prolylcis-Trans Isomerase 17-2 and Is Involved in the Intracellular Movement of the Viral Coat Protein

Citrus tristeza virus is a member of the genus Closterovirus in the family Closteroviridae. The p23 of citrus tristeza virus (CTV) is a multifunctional protein and RNA silencing suppressor. In this study, we identified a p23 interacting partner, FK506-binding protein (FKBP) 17-2, from Citrus aurantifolia (CaFKBP17-2), a susceptible host, and Nicotiana benthamiana (NbFKBP17-2), an experimental host for CTV. The interaction of p23 with CaFKBP17-2 and NbFKBP17-2 were individually confirmed by yeast two-hybrid (Y2H) and bimolecular fluorescence complementation (BiFC) assays. Subcellular localization tests showed that the viral p23 translocated FKBP17-2 from chloroplasts to the plasmodesmata of epidermal cells of N. benthamiana leaves. The knocked-down expression level of NbFKBP17-2 mRNA resulted in a decreased CTV titer in N. benthamiana plants. Further, BiFC and Y2H assays showed that NbFKBP17-2 also interacted with the coat protein (CP) of CTV, and the complexes of CP/NbFKBP17-2 rapidly moved in the cytoplasm. Moreover, p23 guided the CP/NbFKBP17-2 complexes to move along the cell wall. To the best of our knowledge, this is the first report of viral proteins interacting with FKBP17-2 encoded by plants. Our results provide insights for further revealing the mechanism of the CTV CP protein movement.

Structure and Expression of Large (+)RNA Genomes of Viruses of Higher Eukaryotes

Viral positive-sense RNA genomes evolve rapidly due to the high mutation rates during replication and RNA recombination, which allowing the viruses to acquire and modify genes for their adaptation. The size of RNA genome is limited by several factors, including low fidelity of RNA polymerases and packaging constraints. However, the 12-kb size limit is exceeded in the two groups of eukaryotic (+)RNA viruses – animal nidoviruses and plant closteroviruses. These virus groups have several traits in common. Their genomes contain 5′-proximal genes that are expressed via ribosomal frameshifting and encode one or two papain-like protease domains, membrane-binding domain(s), methyltransferase, RNA helicase, and RNA polymerase. In addition, some nidoviruses (i.e., coronaviruses) contain replication-associated domains, such as proofreading exonuclease, putative primase, nucleotidyltransferase, and endonuclease. In both nidoviruses and closteroviruses, the 3′-terminal part of the genome contains genes for structural and accessory proteins expressed via a nested set of coterminal subgenomic RNAs. Coronaviruses and closteroviruses have evolved to form flexuous helically symmetrical nucleocapsids as a mean to resolve packaging constraints. Since phylogenetic reconstructions of the RNA polymerase domains indicate only a marginal relationship between the nidoviruses and closteroviruses, their similar properties likely have evolved convergently, along with the increase in the genome size.

Probing into the Effects of Grapevine Leafroll-Associated Viruses on the Physiology, Fruit Quality and Gene Expression of Grapes

Grapevine leafroll is one of the most widespread and highly destructive grapevine diseases that is responsible for great economic losses to the grape and wine industries throughout the world. Six distinct viruses have been implicated in this disease complex. They belong to three genera, all in the family Closteroviridae. For the sake of convenience, these viruses are named as grapevine leafroll-associated viruses (GLRaV-1, -2, -3, -4, -7, and -13). However, their etiological role in the disease has yet to be established. Furthermore, how infections with each GLRaV induce the characteristic disease symptoms remains unresolved. Here, we first provide a brief overview on each of these GLRaVs with a focus on genome structure, expression strategies and gene functions, where available. We then provide a review on the effects of GLRaV infection on the physiology, fruit quality, fruit chemical composition, and gene expression of grapevine based on the limited information so far reported in the literature. We outline key methodologies that have been used to study how GLRaV infections alter gene expression in the grapevine host at the transcriptomic level. Finally, we present a working model as an initial attempt to explain how infections with GLRaVs lead to the characteristic symptoms of grapevine leafroll disease: leaf discoloration and downward rolling. It is our hope that this review will serve as a starting point for grapevine virology and the related research community to tackle this vastly important and yet virtually uncharted territory in virus-host interactions involving woody and perennial fruit crops.

Comparative analysis identifies amino acids critical for citrus tristeza virus (T36CA) encoded proteins involved in suppression of RNA silencing and differential systemic infection in two plant species

Complementary (c)DNA clones corresponding to the full-length genome of T36CA (a Californian isolate of Citrus tristeza virus with the T36 genotype), which shares 99.1% identity with that of T36FL (a T36 isolate from Florida), were made into a vector system to express the green fluorescent protein (GFP). Agroinfiltration of two prototype T36CA-based vectors (pT36CA) to Nicotiana benthamiana plants resulted in local but not systemic GFP expression/viral infection. This contrasted with agroinfiltration of the T36FL-based vector (pT36FL), which resulted in both local and systemic GFP expression/viral infection. A prototype T36CA systemically infected RNA silencing-defective N. benthamiana lines, demonstrating that a genetic basis for its defective systemic infection was RNA silencing. We evaluated the in planta bioactivity of chimeric pT36CA-pT36FL constructs and the results suggested that nucleotide variants in several open reading frames of the prototype T36CA could be responsible for its defective systemic infection. A single amino acid substitution in each of two silencing suppressors, p20 (S107G) and p25 (G36D), of prototype T36CA facilitated its systemic infectivity in N. benthamiana (albeit with reduced titre relative to that of T36FL) but not in Citrus macrophylla plants. Enhanced virus accumulation and, remarkably, robust systemic infection of T36CA in N. benthamiana and C. macrophylla plants, respectively, required two additional amino acid substitutions engineered in p65 (N118S and S158L), a putative closterovirus movement protein. The availability of pT36CA provides a unique opportunity for comparative analysis to identify viral coding and noncoding nucleotides or sequences involved in functions that are vital for in planta infection.

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.

A Non-Conserved p33 Protein of Citrus Tristeza Virus Interacts with Multiple Viral Partners

The RNA genome of citrus tristeza virus (CTV), one of the most damaging viral pathogens of citrus, contains 12 open reading frames resulting in production of at least 19 proteins. Previous studies on the intraviral interactome of CTV revealed self-interaction of the viral RNA-dependent RNA polymerase, the major coat protein (CP), p20, p23, and p33 proteins, while heterologous interactions between the CTV proteins have not been characterized. In this work, we examined interactions between the p33 protein, a nonconserved protein of CTV, which performs multiple functions in the virus infection cycle and is needed for virus ability to infect the extended host range, with other CTV proteins shown to mediate virus interactions with its plant hosts. Using yeast two-hybrid, bimolecular fluorescence complementation, and coimmunoprecipitation assays, we demonstrated that p33 interacts with three viral proteins, i.e., CP, p20, and p23, in vivo and in planta. Coexpression of p33, which is an integral membrane protein, resulted in a shift in the localization of the p20 and p23 proteins toward the subcellular crude-membrane fraction. Upon CTV infection, the four proteins colocalized in the CTV replication factories. In addition, three of them, CP, p20, and p23, were found in the p33-formed membranous structures. Using bioinformatic analyses and mutagenesis, we found that the N-terminus of p33 is involved in the interactions with all three protein partners. A potential role of these interactions in virus ability to infect the extended host range is discussed.

Design, Synthesis, and Anti-ToCV Activity of Novel 4(3H)-Quinazolinone Derivatives Bearing Dithioacetal Moiety

Tomato chlorosis virus (ToCV) has caused great harm to the production of tomato worldwide. To develop efficient anti-ToCV agents, some novel 4(3H)-quinazolinone derivatives containing dithioacetal were designed and synthesized, and their anti-ToCV activities were evaluated by microscale thermophoresis (MST) using ToCV coat protein (ToCV-CP) as a new target. The results showed that some compounds had a strong binding capacity to ToCV-CP. In particular, compounds C5 and C22 have an excellent binding capacity to ToCV-CP, with binding constant values of 0.24 and 0.25 μM, respectively. Additionally, reduced ToCV-CP gene expression levels of 81.05 and 87.59% could be achieved when tomato was treated with compounds C5 and C22, respectively, which were obviously higher than the levels after ningnanmycin (NNM) treatment (43.88%) and lead compound Xiangcaoliusuobingmi (XCLSBM) treatment (63.56%). Therefore, this work indicates that 4(3H)-quinazolinone derivatives containing dithioacetal moiety can be used as novel anti-ToCV agents.

Discovery of Potent and Novel Quinazolinone Sulfide Inhibitors with Anti-ToCV Activity

A series of novel quinazolinone sulfide derivatives containing a dithioacetal moiety were designed and synthesized using Tomato chlorosis virus coat protein (ToCVCP) as a potential drug target, and the inhibitory effect of ToCV was systematically evaluated in vitro and in vivo. The experimental results showed that most of the compounds presented a strong affinity. Notably, the binding abilities of compounds D8 and D16 to ToCVCP both reached a micromolar level, which were 0.19 and 0.83 μM, respectively. The relative expression level of ToCVCP gene was detected using real-time quantitative polymerase chain reaction in Nicotiana benthamiana. Compounds D8 and D16 significantly reduced the relative expression level of ToCVCP gene by 93.34 and 83.47%, respectively, which were better than those of conventional antiviral agents. This study lays a good foundation for the structural design and modification of quinazolinone sulfide derivatives as anti-ToCV drugs.

Tomato chlorosis virus, an emergent plant virus still expanding its geographical and host ranges

Tomato chlorosis virus (ToCV) causes an important disease that primarily affects tomato, although it has been found infecting other economically important vegetable crops and a wide range of wild plants. First described in Florida (USA) and associated with a 'yellow leaf disorder' in the mid-1990s, ToCV has been found in 35 countries and territories to date, constituting a paradigmatic example of an emergent plant pathogen. ToCV is transmitted semipersistently by whiteflies (Hemiptera: Aleyrodidae) belonging to the genera Bemisia and Trialeurodes. Whitefly transmission is highly efficient and cases of 100% infection are frequently observed in the field. To date, no resistant or tolerant tomato plants are commercially available and the control of the disease relies primarily on the control of the insect vector.

TAXONOMY

Tomato chlorosis virus is one of the 14 accepted species in the genus Crinivirus, one of the four genera in the family Closteroviridae of plant viruses.

VIRION AND GENOME PROPERTIES

The genome of ToCV is composed of two molecules of single-stranded positive-sense RNA, named RNA1 and RNA2, separately encapsidated in long, flexuous, rod-like virions. As has been shown for other closterovirids, ToCV virions are believed to have a bipolar structure. RNA1 contains four open reading frames (ORFs) encoding proteins associated with virus replication and suppression of gene silencing, whereas RNA2 contains nine ORFs encoding proteins putatively involved in encapsidation, cell-to-cell movement, gene silencing suppression and whitefly transmission.

HOST RANGE

In addition to tomato, ToCV has been found to infect 84 dicot plant species belonging to 25 botanical families, including economically important crops.

TRANSMISSION

Like all species within the genus Crinivirus, ToCV is semipersistently transmitted by whiteflies, being one of only two criniviruses transmitted by members of the genera Bemisia and Trialeurodes.

DISEASE SYMPTOMS

Tomato 'yellow leaf disorder' syndrome includes interveinal yellowing and thickening of leaves. Symptoms first develop on lower leaves and then advance towards the upper part of the plant. Bronzing and necrosis of the older leaves are accompanied by a decline in vigour and reduction in fruit yield. In other hosts the most common symptoms include interveinal chlorosis and mild yellowing on older leaves.

CONTROL

Control of the disease caused by ToCV is based on the use of healthy seedlings for transplanting, limiting accessibility of alternate host plants that can serve as virus reservoirs and the spraying of insecticides for vector control. Although several wild tomato species have been shown to contain genotypes resistant to ToCV, there are no commercially available resistant or tolerant tomato varieties to date.

Lettuce Chlorosis Virus Disease: A New Threat to Cannabis Production

In a survey conducted in Cannabis sativa L. (cannabis) authorized farms in Israel, plants showed disease symptoms characteristic of nutrition deprivation. Interveinal chlorosis, brittleness, and occasional necrosis were observed in older leaves. Next generation sequencing analysis of RNA extracted from symptomatic leaves revealed the presence of lettuce chlorosis virus (LCV), a crinivirus that belongs to the Closteroviridae family. The complete viral genome sequence was obtained using RT-PCR and Rapid Amplification of cDNA Ends (RACE) PCR followed by Sanger sequencing. The two LCV RNA genome segments shared 85-99% nucleotide sequence identity with LCV isolates from GenBank database. The whitefly Bemisia tabaci Middle Eastern Asia Minor1 (MEAM1) biotype transmitted the disease from symptomatic cannabis plants to un-infected 'healthy' cannabis, Lactuca sativa, and Catharanthus roseus plants. Shoots from symptomatic cannabis plants, used for plant propagation, constituted a primary inoculum of the disease. To the best of our knowledge, this is the first report of cannabis plant disease caused by LCV.

Key checkpoints in the movement of plant viruses through the host

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

First Anti-ToCV Activity Evaluation of Glucopyranoside Derivatives Containing a Dithioacetal Moiety through a Novel ToCVCP-Oriented Screening Method

Tomato chlorosis virus (ToCV) is a newly reported plant virus that has rapidly spread to all parts of the world, resulting in a serious decline in tomato quality and yield due to the lack of effective control agents. In this study, the ToCV coat protein (ToCVCP) was expressed and purified in Escherichia coli, and a series of novel glucopyranoside derivatives containing a dithioacetal moiety was designed and synthesized. The binding affinity of these compounds to ToCVCP was determined using microscale thermophoresis. Results revealed that compounds 6b and 8a interacted with ToCVCP with K_d values of 0.12 and 0.21 μM, respectively. Quantitative reverse transcription polymerase chain reaction was used to evaluate the anti-ToCV activity of 6b and 8a in vivo, and both significantly reduced the expression level of ToCVCP gene in tomato compared with commercial antivirus agents. This study provides an efficient and convenient screening method for anti-ToCV agents and reliable support for the development of novel agrochemicals for ToCV.

Identification of cucurbit chlorotic yellows virus P4.9 as a possible movement protein

BACKGROUND

Cucurbit chlorotic yellows virus (CCYV) is a bipartite cucurbit-infecting crinivirus within the family Closteroviridae. The crinivirus genome varies among genera. P4.9 is the first protein encoded by CCYV RNA2. P5, which is encoded by LIYV, is necessary for efficient viral infectivity in plants; however, it remains unknown whether CCYV P4.9 is involved in movement.

FINDING

In this study, we used green fluorescent protein (GFP) to examine the intracellular distribution of P4.9-GFP in plant cells, and observed fluorescence in the cytoplasm and nucleus. Transient expression of P4.9 was localized to the plasmodesmata. Co-infiltration of agrobacterium carrying binary plasmids of P4.9 and GFP facilitated GFP diffusion between cells. Besides P4.9 was able to spread by itself to neighboring cells, and co-localized with a marker specific to the endoplasmic reticulum, HDEL-mCherry, but not with the Golgi marker Man49-mCherry.

CONCLUSIONS

Together, these results demonstrate that CCYV P4.9 is involved in cell-cell movement.

Molecular characterization and detection of a new closterovirus identified from blackcurrant by high-throughput sequencing

Two large contigs with high sequence similarities to several closteroviruses were identified by high-throughput sequencing from a blackcurrant plant. The complete genome of this new virus was determined to be 17,320 nucleotides. Its genome contains ten open reading frames (ORF) that include, in the 5'-3' direction, a large ORF encoding a putative viral polyprotein (ORF 1a) and nine ORFs that encode RNA-dependent RNA polymerase (RdRp, ORF 1b), p6 (ORF 2), heat shock protein 70-like protein (Hsp70h, ORF 3), Hsp-90-like protein (p61, ORF 4), CP minor (ORF 5), CP (ORF 6), p17 (ORF 7), p11 (ORF 8), and p26 (ORF 9), respectively. BCCV-1 shares nucleotide sequence identities of 43-45% with other 9 closteroviruses at genome sequences. The amino acid sequence identities between BCCV-1 and the closteroviruses were 49-55% (RdRp), 37-41% (Hsp70h), 19-33% (p61), 26-38% (CPm), and 19-28% (CP), respectively. Phylogenetic analysis of Hsp70h sequences placed the new virus with members of genus Closterovirus in the same group. The results indicate that this new virus, which is provisionally named as Blackcurrant closterovirus 1, should represent a new species of the genus Closterovirus. A RT-PCR was developed and used to detect BCCV-1 in more germplasm accessions of Ribes spp.

Non-replicative Integral Membrane Proteins Encoded by Plant Alpha-Like Viruses: Emergence of Diverse Orphan ORFs and Movement Protein Genes

Fast accumulation of sequencing data on plant virus genomes and plant transcriptomes demands periodic re-evaluation of current views on the genome evolution of viruses. Here, we substantiate and further detail our previously mostly speculative model on the origin and evolution of triple gene block (TGB) encoding plant virus movement proteins TGB1, TGB2, and TGB3. Recent experimental data on functional competence of transport gene modules consisting of two proteins related to TGB1 and TGB2, as well as sequence analysis data on similarity of TGB2 and TGB3 encoded by a viral genome and virus-like RNAs identified in a plant transcriptomes, suggest that TGB evolution involved events of gene duplication and gene transfer between viruses. In addition, our analysis identified that plant RNA-seq data assembled into RNA virus-like contigs encode a significant variety of hydrophobic proteins. Functions of these orphan proteins are still obscure; however, some of them are obviously related to hydrophobic virion proteins of recently sequenced invertebrate (mostly insect) viruses, therefore supporting the current view on a common origin for many groups of plant and insect RNA-containing viruses. Moreover, these findings may suggest that the function of at least some orphan hydrophobic proteins is to provide plant viruses with the ability to infect insect hosts. In general, our observations emphasize that comparison of RNA virus sequences in a large variety of land plants and algae isolated geographically and ecologically may lead to experimental confirmation of previously purely speculative schemes of evolution of single genes, gene modules, and whole genomes.

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

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

Self-interaction of Citrus tristeza virus p33 protein via N-terminal helix

Citrus tristeza virus (CTV), the most economically important viral pathogen of citrus, encodes a unique protein, p33. CTV p33 shows no similarity with other known proteins, yet plays an important role in viral pathogenesis: it extends the virus host range and mediates virus ability to exclude superinfection by other variants of the virus. Previously we demonstrated that p33 is an integral membrane protein and appears to share characteristics of viral movement proteins. In this study, we show that the p33 protein self-interacts in vitro and in vivo using co-immunoprecipitation, yeast two hybrid, and bimolecular fluorescence complementation assays. Furthermore, a helix located at the N-terminus of the protein is required and sufficient for the protein self-interaction.

Biotechnology Applications of Grapevine Viruses

Plant virus genomes are engineered as vectors for functional genomics and production of foreign proteins. The application of plant virus vectors is of potential interest to the worldwide, multibillion dollar, grape and wine industries. These applications include grapevine functional genomics, pathogen control, and production of beneficial proteins such as vaccines and enzymes. However, grapevine virus biology exerts certain limitations on the utility of the virus-derived gene expression and RNA interference vectors. As is typical for viruses infecting woody plants, several grapevine viruses exhibit prolonged infection cycles and relatively low overall accumulation levels, mainly because of their phloem-specific pattern of systemic infection. Here we consider the biotechnology potential of grapevine virus vectors with a special emphasis on members of the families Closteroviridae and Betaflexiviridae.

Structure and Noncanonical Activities of Coat Proteins of Helical Plant Viruses

The main function of virus coat protein is formation of the capsid that protects the virus genome against degradation. However, besides the structural function, coat proteins have many additional important activities in the infection cycle of the virus and in the defense response of host plants to viral infection. This review focuses on noncanonical functions of coat proteins of helical RNA-containing plant viruses with positive genome polarity. Analysis of data on the structural organization of coat proteins of helical viruses has demonstrated that the presence of intrinsically disordered regions within the protein structure plays an important role in implementation of nonstructural functions and largely determines the multifunctionality of coat proteins.

Complete genome sequence of tobacco virus 1, a closterovirus from Nicotiana tabacum

The complete genome sequence of a novel virus, provisionally named tobacco virus 1 (TV1), was determined, and this virus was identified in leaves of tobacco (Nicotiana tabacum) exhibiting leaf mosaic and yellowing symptoms in Anhui Province, China. The genome sequence of TV1 consists of 15,395 nucleotides with 61.6 % nucleotide sequence identity to mint virus 1 (MV1). Its genome organization is similar to that of MV1, containing nine open reading frames (ORFs) that potentially encode proteins with putative functions in virion assembly, cell-to-cell movement and suppression of RNA silencing. Phylogenetic analysis of the heat shock protein 70 homolog (HSP70h) placed TV1 alongside members of the genus Closterovirus in the family Closteroviridae. To our knowledge, this study is the first report of the complete genome sequence of a closterovirus identified in tobacco.

The conundrum of a unique protein encoded by citrus tristeza virus that is dispensable for infection of most hosts yet shows characteristics of a viral movement protein

Citrus tristeza virus (CTV), one of the most economically important viruses, produces a unique protein, p33, which is encoded only in the genomes of isolates of CTV. Recently, we demonstrated that membrane association of the p33 protein confers virus ability to extend its host range. In this work we show that p33 shares characteristics of viral movement proteins. Upon expression in a host cell, the protein localizes to plasmodesmata and displays the ability to form extracellular tubules. Furthermore, p33 appears to traffic via the cellular secretory pathway and the actin network to plasmodesmata locations and is likely being recycled through the endocytic pathway. Finally, our study reveals that p33 colocalizes with a putative movement protein of CTV, the p6 protein. These results suggest a potential role of p33 as a noncanonical viral movement protein, which mediates virus translocation in the specific hosts.

Deep sequencing reveals a novel closterovirus associated with wild rose leaf rosette disease

A bizarre virus-like symptom of a leaf rosette formed by dense small leaves on branches of wild roses (Rosa multiflora Thunb.), designated as 'wild rose leaf rosette disease' (WRLRD), was observed in China. To investigate the presumed causal virus, a wild rose sample affected by WRLRD was subjected to deep sequencing of small interfering RNAs (siRNAs) for a complete survey of the infecting viruses and viroids. The assembly of siRNAs led to the reconstruction of the complete genomes of three known viruses, namely Apple stem grooving virus (ASGV), Blackberry chlorotic ringspot virus (BCRV) and Prunus necrotic ringspot virus (PNRSV), and of a novel virus provisionally named 'rose leaf rosette-associated virus' (RLRaV). Phylogenetic analysis clearly placed RLRaV alongside members of the genus Closterovirus, family Closteroviridae. Genome organization of RLRaV RNA (17,653 nucleotides) showed 13 open reading frames (ORFs), except ORF1 and the quintuple gene block, most of which showed no significant similarities with known viral proteins, but, instead, had detectable identities to fungal or bacterial proteins. Additional novel molecular features indicated that RLRaV seems to be the most complex virus among the known genus members. To our knowledge, this is the first report of WRLRD and its associated closterovirus, as well as two ilarviruses and one capilovirus, infecting wild roses. Our findings present novel information about the closterovirus and the aetiology of this rose disease which should facilitate its control. More importantly, the novel features of RLRaV help to clarify the molecular and evolutionary features of the closterovirus.

Membrane association of a nonconserved viral protein confers virus ability to extend its host range

Citrus tristeza virus (CTV), the largest and most complex member of the family Closteroviridae, encodes a unique protein, p33, which shows no homology with other known proteins, however, plays an important role in virus pathogenesis. In this study, we examined some of the characteristics of p33. We show that p33 is a membrane-associated protein that is inserted into the membrane via a transmembrane helix formed by hydrophobic amino acid residues at the C-terminal end of the protein. Removal of this transmembrane domain (TMD) dramatically altered the intracellular localization of p33. Moreover, the TMD alone was sufficient to confer membrane localization of an unrelated protein. Finally, a CTV variant that produced a truncated p33 lacking the TMD was unable to infect sour orange, one of the selected virus hosts, which infection requires p33, suggesting that membrane association of p33 is important for the ability of CTV to extend its host range.

Development of a single-replicon miniBYV vector for co-expression of heterologous proteins

In planta production of recombinant proteins, including vaccine antigens and monoclonal antibodies, continues gaining acceptance. With the broadening range of target proteins, the need for vectors with higher performance is increasing. Here, we have developed a single-replicon vector based on beet yellows virus (BYV) that enables co-delivery of two target genes into the same host cell, resulting in transient expression of each target. This BYV vector maintained genetic stability during systemic spread throughout the host plant, Nicotiana benthamiana. Furthermore, we have engineered a miniBYV vector carrying the sequences encoding heavy and light chains of a monoclonal antibody (mAb) against protective antigen (PA) of Bacillius anthracis, and achieved the expression of the full-length functional anti-PA mAb at ~300 mg/kg of fresh leaf tissue. To demonstrate co-expression and functionality of two independent proteins, we cloned the sequences of the Pfs48/45 protein of Plasmodium falciparum and endoglycosidase F (PNGase F) from Flavobacterium meningosepticum into the miniBYV vector under the control of two subgenomic RNA promoters. Agroinfiltration of N. benthamiana with this miniBYV vector resulted in accumulation of biologically active Pfs48/45 that was devoid of N-linked glycosylation and had correct conformation and epitope display. Overall, our findings demonstrate that the new BYV-based vector is capable of co-expressing two functionally active recombinant proteins within the same host cell.

Grapevine leafroll disease and associated viruses: a unique pathosystem

Grapevine leafroll is the most complex and intriguing viral disease of grapevine (Vitis spp.). Several monopartite closteroviruses (family Closteroviridae) from grapevines have been molecularly characterized, yet their role in disease etiology is not completely resolved. Hence, these viruses are currently designated under the umbrella term of Grapevine leafroll-associated viruses (GLRaVs). This review examines our current understanding of the genetically divergent GLRaVs and highlights the emerging picture of several unique aspects of the leafroll disease pathosystem. A systems biology approach using contemporary technologies in molecular biology, -omics, and cell biology aids in exploring the comparative molecular biology of GLRaVs and deciphering the complex network of host-virus-vector interactions to bridge the gap between genomics and phenomics of leafroll disease. In addition, grapevine-infecting closteroviruses have a great potential as designer viruses to pursue functional genomics and for the rational design of novel disease intervention strategies in this agriculturally important perennial fruit crop.

Genetic variability and evolutionary implications of RNA silencing suppressor genes in RNA1 of sweet potato chlorotic stunt virus isolates infecting sweetpotato and related wild species

BACKGROUND

The bipartite single-stranded RNA genome of Sweet potato chlorotic stunt virus (SPCSV, genus Crinivirus; Closteroviridae) encodes a Class 1 RNase III (RNase3), a putative hydrophobic protein (p7) and a 22-kDa protein (p22) from genes located in RNA1. RNase3 and p22 suppress RNA silencing, the basal antiviral defence mechanism in plants. RNase3 is sufficient to render sweetpotato (Ipomoea batatas) virus-susceptible and predisposes it to development of severe diseases following infection with unrelated virus. The incidence, strains and gene content of SPCSV infecting wild plant species have not been studied.

METHODOLOGY/PRINCIPAL FINDINGS

Thirty SPCSV isolates were characterized from 10 wild Ipomoea species, Hewittia sublobata or Lepistemon owariensis (family Convolvulaceae) in Uganda and compared with 34 local SPCSV isolates infecting sweetpotatoes. All isolates belonged to the East African (EA) strain of SPCSV and contained RNase3 and p7, but p22 was not detected in six isolates. The three genes showed only limited genetic variability and the proteins were under purifying selection. SPCSV isolates lacking p22 synergized with Sweet potato feathery mottle virus (SPFMV, genus potyvirus; Potyviridae) and caused severe symptoms in co-infected sweetpotato plants. One SPCSV isolate enhanced accumulation of SPFMV, but no severe symptoms developed. A new whitefly-transmitted virus (KML33b) encoding an RNase3 homolog (<56% identity to SPCSV RNase3) able to suppresses sense-mediated RNA silencing was detected in I. sinensis.

CONCLUSIONS/SIGNIFICANCE

SPCSV isolates infecting wild species and sweetpotato in Uganda were genetically undifferentiated, suggesting inter-species transmission of SPCSV. Most isolates in Uganda contained p22, unlike SPCSV isolates characterized from other countries and continents. Enhanced accumulation of SPFMV and increased disease severity were found to be uncoupled phenotypic outcomes of RNase3-mediated viral synergism in sweetpotato. A second virus encoding an RNase3-like RNA silencing suppressor was detected. Overall, results provided many novel and important insights into evolutionary biology of SPCSV.

Viral and cellular factors involved in Phloem transport of plant viruses

Phloem transport of plant viruses is an essential step in the setting-up of a complete infection of a host plant. After an initial replication step in the first cells, viruses spread from cell-to-cell through mesophyll cells, until they reach the vasculature where they rapidly move to distant sites in order to establish the infection of the whole plant. This last step is referred to as systemic transport, or long-distance movement, and involves virus crossings through several cellular barriers: bundle sheath, vascular parenchyma, and companion cells for virus loading into sieve elements (SE). Viruses are then passively transported within the source-to-sink flow of photoassimilates and are unloaded from SE into sink tissues. However, the molecular mechanisms governing virus long-distance movement are far from being understood. While most viruses seem to move systemically as virus particles, some viruses are transported in SE as viral ribonucleoprotein complexes (RNP). The nature of the cellular and viral factors constituting these RNPs is still poorly known. The topic of this review will mainly focus on the host and viral factors that facilitate or restrict virus long-distance movement.

Grapevine leafroll-associated virus 3

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

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

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

Parallels and distinctions in the direct cell-to-cell spread of the plant and animal viruses

The paradigm that viruses can move directly, and in some cases covertly, between contacting target cells is now well established for several virus families. The underlying mechanisms of cell-to-cell spread, however, remain to be fully elucidated and may differ substantially depending on the viral exit/entry route and the cellular tropism. Here, two divergent cell-to-cell spread mechanisms are exemplified: firstly by human retroviruses, which rely upon transient adhesive structures that form between polarized immune cells termed virological synapses, and secondly by herpesviruses that depend predominantly on pre-existing stable cellular contacts, but may also form virological synapses. Plant viruses can also spread directly between contacting cells, but are obliged by the rigid host cell wall to move across pore structures termed plasmodesmata. This review will focus primarily on recent advances in our understanding of animal virus cell-to-cell spread using examples from these two virus families to highlight differences and similarities, and will conclude by comparing and contrasting the cell-to-cell spread of animal and plant viruses.

Agroinoculation of Citrus tristeza virus causes systemic infection and symptoms in the presumed nonhost Nicotiana benthamiana

Citrus tristeza virus (CTV) naturally infects only some citrus species and relatives and within these it only invades phloem tissues. Failure to agroinfect citrus plants and the lack of an experimental herbaceous host hindered development of a workable genetic system. A full-genome cDNA of CTV isolate T36 was cloned in binary plasmids and was used to agroinfiltrate Nicotiana benthamiana leaves, with or without coinfiltration with plasmids expressing different silencing-suppressor proteins. A time course analysis in agroinfiltrated leaves indicated that CTV accumulates and moves cell-to-cell for at least three weeks postinoculation (wpi), and then, it moves systemically and infects the upper leaves with symptom expression. Silencing suppressors expedited systemic infection and often increased infectivity. In systemically infected Nicotiana benthamiana plants, CTV invaded first the phloem, but after 7 wpi, it was also found in other tissues and reached a high viral titer in upper leaves, thus allowing efficient transmission to citrus by stem-slash inoculation. Infected citrus plants showed the symptoms, virion morphology, and phloem restriction characteristic of the wild T36 isolate. Therefore, agroinfiltration of Nicotiana benthamiana provided the first experimental herbaceous host for CTV and an easy and efficient genetic system for this closterovirus.

Intracellular transport of plant viruses: finding the door out of the cell

Plant viruses are a class of plant pathogens that specialize in movement from cell to cell. As part of their arsenal for infection of plants, every virus encodes a movement protein (MP), a protein dedicated to enlarging the pore size of plasmodesmata (PD) and actively transporting the viral nucleic acid into the adjacent cell. As our knowledge of intercellular transport has increased, it has become apparent that viruses must also use an active mechanism to target the virus from their site of replication within the cell to the PD. Just as viruses are too large to fit through an unmodified plasmodesma, they are also too large to be freely diffused through the cytoplasm of the cell. Evidence has accumulated now for the involvement of other categories of viral proteins in intracellular movement in addition to the MP, including viral proteins originally associated with replication or gene expression. In this review, we will discuss the strategies that viruses use for intracellular movement from the replication site to the PD, in particular focusing on the role of host membranes for intracellular transport and the coordinated interactions between virus proteins within cells that are necessary for successful virus spread.

A mutation in the Lettuce infectious yellows virus minor coat protein disrupts whitefly transmission but not in planta systemic movement

The Lettuce infectious yellows virus (LIYV) RNA 2 mutant p1-5b was previously isolated from Bemisia tabaci-transmitted virus maintained in Chenopodium murale plants. p1-5b RNA 2 contains a single-nucleotide deletion in the minor coat protein (CPm) open reading frame (ORF) that is predicted to result in a frameshift and premature termination of the protein. Using the recently developed agroinoculation system for LIYV, we tested RNA 2 containing the p1-5b CPm mutant genotype (agro-pR6-5b) in Nicotiana benthamiana plants. We showed that plant infection triggered by agro-pR6-5b spread systemically and resulted in the formation of virions similar to those produced in p1-5b-inoculated protoplasts. However, virions derived from these mutant CPm genotypes were not transmitted by whiteflies, even though virion concentrations were above the typical transmission thresholds. In contrast, and as demonstrated for the first time, an engineered restoration mutant (agro-pR6-5bM1) was capable of both systemic movement in plants and whitefly transmission. These results provide strong molecular evidence that the full-length LIYV-encoded CPm is dispensable for systemic plant movement but is required for whitefly transmission.

Further complexity of the genus Crinivirus revealed by the complete genome sequence of Lettuce chlorosis virus (LCV) and the similar temporal accumulation of LCV genomic RNAs 1 and 2

The sequence of Lettuce chlorosis virus (LCV) (genus Crinivirus) was determined and found to contain unique open reading frames (ORFs) and ORFs similar to those of other criniviruses, as well as 3' non-coding regions that shared a high degree of identity. Northern blot analysis of RNA extracted from LCV-infected plants identified subgenomic RNAs corresponding to six prominent internal ORFs and detected several novel LCV-single stranded RNA species. Virus replication in tobacco protoplasts was investigated and results indicated that LCV replication proceeded with novel crinivirus RNA accumulation kinetics, wherein viral genomic RNAs exhibited a temporally similar expression pattern early in the infection. This was noticeably distinct from the asynchronous RNA accumulation pattern previously observed for Lettuce infectious yellows virus (LIYV), the type member of the genus, suggesting that replication of the two viruses likely operate via dissimilar mechanisms.

The complete nucleotide sequence and genome organization of tomato infectious chlorosis virus: a distinct crinivirus most closely related to lettuce infectious yellows virus

The complete nucleotide sequence of tomato infectious chlorosis virus (TICV) was determined and compared with those of other members of the genus Crinivirus. RNA 1 is 8,271 nucleotides long with three open reading frames and encodes proteins involved in replication. RNA 2 is 7,913 nucleotides long and encodes eight proteins common within the genus Crinivirus that are involved in genome protection, movement and other functions yet to be identified. Similarity between TICV and other criniviruses varies throughout the genome but TICV is related more closely to lettuce infectious yellows virus than to any other crinivirus, thus identifying a third group within the genus.

Lettuce infectious yellows virus-encoded P26 induces plasmalemma deposit cytopathology

Lettuce infectious yellows virus (LIYV) encodes a 26 kDa protein (P26) previously shown to associate with plasmalemma deposits (PLDs), unique LIYV-induced cytopathologies located at the plasmalemma over plasmodesmata pit fields in companion cells and phloem parenchyma. To further characterize the relationship of P26 and PLDs, we assessed localization and cytopathology induction of P26 expressed from either LIYV or a heterologous Tobacco mosaic virus (TMV) vector using green fluorescent protein (GFP) fusions, immunofluorescence microscopy, biochemical fractionation, and transmission electron microscopy (TEM). TEM analyses demonstrated that P26 not only associated with, but induced formation of PLDs in the absence of other LIYV proteins. Interestingly, PLDs induced by P26-expressing TMV were no longer confined to phloem cells. Putative P26 orthologs from two other members of the genus Crinivirus which do not induce conspicuous PLDs exhibited fractionation properties similar to LIYV P26 but were not associated with any PLD-like cytopathology.

The complete nucleotide sequence of the RNA2 of the crinivirus tomato infectious chlorosis virus: isolates from North America and Europe are essentially identical

The complete nucleotide sequences of the RNA2 of two isolates of Tomato infectious chlorosis virus (TICV, genus Crinivirus, family Closteroviridae) from the United States and Spain, respectively, were determined. The sequences of both isolates were found to be nearly identical. TICV RNA2 consisted of 7,914 nucleotides in both isolates and contains eight open reading frames that encompass the Closteroviridae hallmark gene array represented by a heat shock protein 70 family homologue, a protein of 59 kDa, the major coat protein, and a divergent copy of the coat protein. Phylogenetic analysis suggested that TICV is most similar to Lettuce infectious yellows virus (LIYV), the type species of the genus Crinivirus.

Tandem leader proteases of Grapevine leafroll-associated virus-2: host-specific functions in the infection cycle

Several viruses in the genus Closterovirus including Grapevine leafroll-associated virus-2 (GLRaV-2), encode a tandem of papain-like leader proteases (L1 and L2) whose functional profiles remained largely uncharacterized. We generated a series of the full-length, reporter-tagged, clones of GLRaV-2 and demonstrated that they are systemically infectious upon agroinfection of an experimental host plant Nicotiana benthamiana. These clones and corresponding minireplicon derivatives were used to address L1 and L2 functions in GLRaV-2 infection cycle. It was found that the deletion of genome region encoding the entire L1-L2 tandem resulted in a ~100-fold reduction in minireplicon RNA accumulation. Five-fold reduction in RNA level was observed upon deletion of L1 coding region. In contrast, deletion of L2 coding region did not affect RNA accumulation. It was also found that the autocatalytic cleavage by L2 but not by L1 is essential for genome replication. Analysis of the corresponding mutants in the context of N. benthamiana infection launched by the full-length GLRaV-2 clone revealed that L1 or its coding region is essential for virus ability to establish infection, while L2 plays an accessory role in the viral systemic transport. Strikingly, when tagged minireplicon variants were used for the leaf agroinfiltration of the GLRaV-2 natural host, Vitis vinifera, deletion of either L1 or L2 resulted in a dramatic reduction of minireplicon ability to establish infection attesting to a host-specific requirement for tandem proteases in the virus infection cycle.

Three genes of Citrus tristeza virus are dispensable for infection and movement throughout some varieties of citrus trees

Citrus tristeza virus (CTV), a member of the Closteroviridae, possesses a 19.3-kb positive-stranded RNA genome that is organized into twelve open reading frames (ORFs). The CTV genome contains two sets of conserved genes, which are characteristic of this virus group, the replication gene block (ORF 1a and 1b) and the quintuple gene block (p6, HSP70 h, p61, CPm, and CP). With the exception of the p6 gene, they are required for replication and virion assembly. CTV contains five additional genes, p33, p18, p13, p20 and p23, in the 3' half of the genome, some of which (p33, p18 and p13) are not conserved among other members of this virus group, and have been proposed to have evolved for specific interactions with the citrus host. In the present study, the requirements for systemic infection of citrus trees of p33, p6, p18, p13 and p20 were examined. Viral mutants with a deletion in the p6 or the p20 ORF failed to infect citrus plants systemically, suggesting their possible roles in virus translocation/systemic infection. However, we found that deletions within the p33, p18 or p13 ORF individually resulted in no significant loss of ability of the virus to infect, multiply, and spread throughout citrus trees. Furthermore, deletions in the p33, p18 and p13 genes in all possible combinations including deletions in all three genes allowed the virus to systemically invade citrus trees. Green fluorescent protein-tagged CTV variants with deletions in the p33 ORF or the p33, p18 and p13 ORFs demonstrated that the movement and distribution of these deletion mutants were similar to that of the wild-type virus.

Citrus tristeza virus: survival at the edge of the movement continuum

Systemic invasion of plants by viruses is thought to involve two processes: cell-to-cell movement between adjacent cells and long-distance movement that allows the virus to rapidly move through sieve elements and unload at the growing parts of the plant. There is a continuum of proportions of these processes that determines the degrees of systemic infection of different plants by different viruses. We examined the systemic distribution of Citrus tristeza virus (CTV) in citrus species with a range of susceptibilities. By using a "pure" culture of CTV from a cDNA clone and green fluorescent protein-labeled virus we show that both cell-to-cell and long-distance movement are unusually limited, and the degree of limitation varies depending on the citrus host. In the more-susceptible hosts CTV infected only a small portion of phloem-associated cells, and moreover, the number of infection sites in less-susceptible citrus species was substantially decreased further, indicating that long-distance movement was reduced in those hosts. Analysis of infection foci in the two most differential citrus species, Citrus macrophylla and sour orange, revealed that in the more-susceptible host the infection foci were composed of a cluster of multiple cells, while in the less-susceptible host infection foci were usually single cells, suggesting that essentially no cell-to-cell movement occurred in the latter host. Thus, CTV in sour orange represents a pattern of systemic infection in which the virus appears to function with only the long-distance movement mechanism, yet is able to survive in nature.

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

The Hsp70 homolog (Hsp70h) of Beet yellows virus (BYV) functions in virion assembly and cell-to-cell movement and is autonomously targeted to plasmodesmata in association with the actomyosin motility system (A. I. Prokhnevsky, V. V. Peremyslov, and V. V. Dolja, J. Virol. 79:14421-14428, 2005). Myosins are a diverse category of molecular motors that possess a motor domain and a tail domain involved in cargo binding. Plants have two classes of myosins, VIII and XI, whose specific functions are poorly understood. We used dominant negative inhibition to identify myosins required for Hsp70h localization to plasmodesmata. Six full-length myosin cDNAs from the BYV host plant Nicotiana benthamiana were sequenced and shown to encode apparent orthologs of the Arabidopsis thaliana myosins VIII-1, VIII-2, VIII-B, XI-2, XI-F, and XI-K. We found that the ectopic expression of the tail domains of each of the class VIII, but not the class XI, myosins inhibited the plasmodesmatal localization of Hsp70h. In contrast, the overexpression of the motor domains or the entire molecules of the class VIII myosins did not affect Hsp70h targeting. Further mapping revealed that the minimal cargo-binding part of the myosin VIII tails was both essential and sufficient for the inhibition of the proper Hsp70h localization. Interestingly, plasmodesmatal localization of the Tobacco mosaic virus movement protein and Arabidopsis protein RGP2 was not affected by myosin VIII tail overexpression. Collectively, our data implicate class VIII myosins in protein delivery to plasmodesmata and suggest that more than one mechanism of such delivery exist in plants.

Complete nucleotide sequence and genome organization of pineapple mealybug wilt-associated virus-1

Pineapple mealybug wilt-associated virus-1 (PMWaV-1; family Closteroviridae, genus Ampelovirus) belongs to a complex of mealybug-transmissible viruses found in pineapple worldwide. In this study, the complete genome of PMWaV-1 was sequenced and found to be 13.1 kb in length, making it the smallest in the family. The genome encoded seven open reading frames (ORFs) and was unusual for an ampelovirus due to the lack of an intergenic region between the RdRp and p6 ORFs, an ORF encoding a relatively small coat protein (CP), and the absence of an ORF encoding a coat protein duplicate (CPd). Phylogenetic analyses placed PMWaV-1, plum bark necrosis stem pitting-associated virus and some grapevine leafroll-associated viruses in a distinct clade within the genus Ampelovirus.

Completion of the mapping of transcription start sites for the five-gene block subgenomic RNAs of Beet yellows Closterovirus and identification of putative subgenomic promoters

In the positive-sense RNA genome of Beet yellows Closterovirus (BYV), the 3'-terminal open reading frames (ORFs) 2-8 are expressed as a nested set of subgenomic (sg) RNAs. ORFs 2-6, coding for the structural and movement proteins, form a 'five-gene block' conserved in closteroviruses. We mapped the 5'-end of the ORF 4 sgRNA, which encodes the p64 protein, at adenosine-11169 in the BYV genome. This completes the mapping of the transcription start sites for the five-gene block sgRNAs of BYV. Computer-assisted analysis of the sequences upstream of BYV ORFs 2, 3, 4, 5, and 6 revealed two conserved motifs, which might constitute the subgenomic promoter elements. These motifs are conserved in the equivalent positions upstream of three orthologous genes of Citrus tristeza Closterovirus and two orthologous genes of Beet yellow stunt Closterovirus.