Helper component proteinase of the genus Potyvirus is an interaction partner of translation initiation factors eIF(iso)4E and eIF4E and contains a 4E binding motif

Cloning of the Arabidopsis rwm1 gene for resistance to Watermelon mosaic virus points to a new function for natural virus resistance genes

Arabidopsis thaliana represents a valuable and efficient model to understand mechanisms underlying plant susceptibility to viral diseases. Here, we describe the identification and molecular cloning of a new gene responsible for recessive resistance to several isolates of Watermelon mosaic virus (WMV, genus Potyvirus) in the Arabidopsis Cvi-0 accession. rwm1 acts at an early stage of infection by impairing viral accumulation in initially infected leaf tissues. Map-based cloning delimited rwm1 on chromosome 1 in a 114-kb region containing 30 annotated genes. Positional and functional candidate gene analysis suggested that rwm1 encodes cPGK2 (At1g56190), an evolutionary conserved nucleus-encoded chloroplast phosphoglycerate kinase with a key role in cell metabolism. Comparative sequence analysis indicates that a single amino acid substitution (S78G) in the N-terminal domain of cPGK2 is involved in rwm1-mediated resistance. This mutation may have functional consequences because it targets a highly conserved residue, affects a putative phosphorylation site and occurs within a predicted nuclear localization signal. Transgenic complementation in Arabidopsis together with virus-induced gene silencing in Nicotiana benthamiana confirmed that cPGK2 corresponds to rwm1 and that the protein is required for efficient WMV infection. This work uncovers new insight into natural plant resistance mechanisms that may provide interesting opportunities for the genetic control of plant virus diseases.

A novel role of the potyviral helper component proteinase contributes to enhance the yield of viral particles

The helper component proteinase (HCPro) is an indispensable, multifunctional protein of members of the genus Potyvirus and other viruses of the family Potyviridae. This viral factor is directly involved in diverse steps of viral infection, such as aphid transmission, polyprotein processing, and suppression of host antiviral RNA silencing. In this paper, we show that although a chimeric virus based on the potyvirus Plum pox virus lacking HCPro, which was replaced by a heterologous silencing suppressor, caused an efficient infection in Nicotiana benthamiana plants, its viral progeny had very reduced infectivity. Making use of different approaches, here, we provide direct evidence of a previously unknown function of HCPro in which the viral factor enhances the stability of its cognate capsid protein (CP), positively affecting the yield of virions and consequently improving the infectivity of the viral progeny. Site-directed mutagenesis revealed that the ability of HCPro to stabilize CP and enhance the yield of infectious viral particles is not linked to any of its previously known activities and helped us to delimit the region of HCPro involved in this function in the central region of the protein. Moreover, the function is highly specific and cannot be fulfilled by the HCPro of a heterologous potyvirus. The importance of this novel requirement in regulating the sorting of the viral genome to be subjected to replication, translation, and encapsidation, thus contributing to the synchronization of these viral processes, is discussed.

IMPORTANCE

Potyviruses form one of the most numerous groups of plant viruses and are a major cause of crop loss worldwide. It is well known that these pathogens make use of virus-derived multitasking proteins, as well as dedicated host factors, to successfully infect their hosts. Here, we describe a novel requirement for the proper yield and infectivity of potyviral progeny. In this case, such a function is performed by the extensively studied viral factor HCPro, which seems to use an unknown mechanism that is not linked to its previously described activities. To our knowledge, this is the first time that a factor different from capsid protein (CP) has been shown to be directly involved in the yield of potyviral particles. Based on the data presented here, we hypothesize that this capacity of HCPro might be involved in the coordination of mutually exclusive activities of the viral genome by controlling correct assembly of CP in stable virions.

Tobacco etch virus protein P1 traffics to the nucleolus and associates with the host 60S ribosomal subunits during infection

The genus Potyvirus comprises a large group of positive-strand RNA plant viruses whose genome encodes a large polyprotein processed by three viral proteinases. P1 protein, the most amino-terminal product of the polyprotein, is an accessory factor stimulating viral genome amplification whose role during infection is not well understood. We infected plants with Tobacco etch virus (TEV; genus Potyvirus) clones in which P1 was tagged with a fluorescent protein to track its expression and subcellular localization or with an affinity tag to identify host proteins involved in complexes in which P1 also takes part during infection. Our results showed that TEV P1 exclusively accumulates in infected cells at an early stage of infection and that the protein displays a dynamic subcellular localization, trafficking in and out of the nucleus and nucleolus during infection. Inside the nucleolus, P1 particularly targets the dense granular component. Consistently, we found functional nucleolar localization and nuclear export signals in TEV P1 sequence. Our results also indicated that TEV P1 physically interacts with the host 80S cytoplasmic ribosomes and specifically binds to the 60S ribosomal subunits during infection. In vitro translation assays of reporter proteins suggested that TEV P1 stimulates protein translation, particularly when driven from the TEV internal ribosome entry site. These in vitro assays also suggested that TEV helper-component proteinase (HC-Pro) inhibits protein translation. Based on these findings, we propose that TEV P1 stimulates translation of viral proteins in infected cells.

IMPORTANCE

In this work, we researched the role during infection of tobacco etch virus P1 protease. P1 is the most mysterious protein of potyviruses, a relevant group of RNA viruses infecting plants. Our experiments showed that the viral P1 protein exclusively accumulates in infected cells at an early stage of infection and moves in and out of the nucleus of infected cells, particularly targeting the nucleolus. Our experiments also showed that P1 protein binds host ribosomes during infection. Based on these findings and other in vitro experiments we propose that P1 protein stimulates translation of viral proteins during infection.

Molecular and cellular mechanisms underlying potyvirus infection

Potyviruses represent one of the most economically important and widely distributed groups of plant viruses. Despite considerable progress towards understanding the cellular and molecular basis of their pathogenicity, many questions remain about the mechanisms by which potyviruses suppress host defences and create an optimal intracellular environment for viral translation, replication, assembly and spread. The review focuses on the multifunctional roles of potyviral proteins and their interplay with various host factors in different compartments of the infected cell. We place special emphasis on the recently discovered and currently putative mechanisms by which potyviruses subvert the normal functions of different cellular organelles in order to establish an efficient and productive infection.

Intracellular coordination of potyviral RNA functions in infection

Establishment of an infection cycle requires mechanisms to allocate the genomes of (+)-stranded RNA viruses in a balanced ratio to translation, replication, encapsidation, and movement, as well as mechanisms to prevent translocation of viral RNA (vRNA) to cellular RNA degradation pathways. The ratio of vRNA allocated to various functions is likely balanced by the availability of regulatory proteins or competition of the interaction sites within regulatory ribonucleoprotein complexes. Due to the transient nature of viral processes and the interdependency between vRNA pathways, it is technically demanding to work out the exact molecular mechanisms underlying vRNA regulation. A substantial number of viral and host proteins have been identified that facilitate the steps that lead to the assembly of a functional potyviral RNA replication complex and their fusion with chloroplasts. Simultaneously with on-going viral replication, part of the replicated potyviral RNA enters movement pathways. Although not much is known about the processes of potyviral RNA release from viral replication complexes, the molecular interactions involved in these processes determine the fate of the replicated vRNA. Some viral and host cell proteins have been described that direct replicated potyviral RNA to translation to enable potyviral gene expression and productive infection. The antiviral defense of the cell causes vRNA degradation by RNA silencing. We hypothesize that also plant pathways involved in mRNA decay may have a role in the coordination of potyviral RNA expression. In this review, we discuss the roles of different potyviral and host proteins in the coordination of various potyviral RNA functions.

The Potyviridae cylindrical inclusion helicase: a key multipartner and multifunctional protein

A unique feature shared by all plant viruses of the Potyviridae family is the induction of characteristic pinwheel-shaped inclusion bodies in the cytoplasm of infected cells. These cylindrical inclusions are composed of the viral-encoded cylindrical inclusion helicase (CI protein). Its helicase activity was characterized and its involvement in replication demonstrated through different reverse genetics approaches. In addition to replication, the CI protein is also involved in cell-to-cell and long-distance movements, possibly through interactions with the recently discovered viral P3N-PIPO protein. Studies over the past two decades demonstrate that the CI protein is present in several cellular compartments interacting with viral and plant protein partners likely involved in its various roles in different steps of viral infection. Furthermore, the CI protein acts as an avirulence factor in gene-for-gene interactions with dominant-resistance host genes and as a recessive-resistance overcoming factor. Although a significant amount of data concerning the potential functions and subcellular localization of this protein has been published, no synthetic review is available on this important multifunctional protein. In this review, we compile and integrate all information relevant to the current understanding of this viral protein structure and function and present a mode of action for CI, combining replication and movement.

Improved silencing suppression and enhanced heterologous protein expression are achieved using an engineered viral helper component proteinase

RNA silencing limits transient expression of heterologous proteins in plants. Co-expression of viral silencing suppressor proteins can increase and prolong protein expression, but highly efficient silencing suppressors may stress plant tissue and be detrimental to protein yields. Little is known whether silencing suppression could be improved without harm to plant tissues. This study reports development of enhanced silencing suppressors by engineering the helper component proteinase (HCpro) of Potato virus A (PVA). Mutations were introduced to a short region of HCpro (positions 330-335 in PVA HCpro), which is hypervariable among potyviruses. Three out of the four HCpro mutants suppressed RNA silencing more efficiently and sustained expression of co-expressed jellyfish green fluorescent protein for a longer time than wild-type HCpro in agroinfiltrated leaves of Nicotiana benthamiana. Leaf tissues remained healthy-looking without any visible signs of stress.

Interaction of the microtubule-associated host protein HIP2 with viral helper component proteinase is important in infection with potato virus A

Microtubules (MT) outline and maintain the overall shape of cells and can reorganize cellular membranes to serve as sites of RNA virus replication. Here, we provide data on involvement of an MT-associated protein in infection of plants with a potyvirus, Potato virus A (PVA), representing the largest family of plant-infecting RNA viruses. Our results showed that helper-component proteinase (HCpro)-interacting protein 2 (HIP2) of potato (Solanum tuberosum) is an MT-associated protein similar to Arabidopsis SPR2. Virus-induced silencing of HIP2 in Nicotiana benthamiana resulted in a spiral-like growth phenotype, similar to the Arabidopsis spr2 mutant, and the spr2 phenotype in Arabidopsis was complemented with potato HIP2. HCpro of PVA interacted with HIP2 of potato and tobacco (Nicotiana tabacum). The interaction was detected by bimolecular fluorescence complementation in PVA-infected leaves on MT and MT intersections at the cell cortex. HIP2-HCpro interaction was determined by the C-proximal α-helix-rich domain of HIP2, whereas the N-proximal putative TOG domain and the central coiled-coil domain of HIP2 controlled HIP2 dimerization and binding to MT. Accumulation of PVA was significantly reduced in the HIP2-silenced leaves of N. benthamiana, which indicates that HIP2-HCpro interactions are important for virus infection.

Mutation of a Short Variable Region in HCpro Protein of Potato virus A Affects Interactions with a Microtubule-Associated Protein and Induces Necrotic Responses in Tobacco

Helper component proteinase (HCpro) is a multifunctional protein of potyviruses (genus Potyvirus). HCpro of Potato virus A (PVA) interacts with the microtubule-associated protein HIP2 in host cells, and depletion of HIP2 reduces virus accumulation. This study shows that HCpro of Potato virus Y and Tobacco etch virus also interact with HIP2. The C-proximal portion of PVA HCpro determines the interaction with HIP2 and was found to contain a stretch of six residues comprising a highly variable region (HVR) in potyviruses. Mutations in HVR reduced PVA accumulation in tobacco plants and induced necrotic symptoms novel to PVA. Microarray and quantitative reverse transcription polymerase chain reaction analyses revealed induction of many defense-related genes including ethylene- and jasmonic acid-inducible pathways in systemically infected leaves at necrosis onset. Salicylic acid-mediated signaling was dispensable for the response. Genes related to microtubule functions were down-regulated. Structural modeling of HCpro suggested that all mutations in HVR caused conformational changes in adjacent regions containing functionally important motifs conserved in potyviruses. Those mutations, which also caused conformational changes in HVR, led to the greatest reduction of fitness. Our results implicate HVR in the regulation of HCpro conformation and virus-host interactions and suggest that mutation of HVR induces host defense.

The influence of cis-acting P1 protein and translational elements on the expression of Potato virus Y helper-component proteinase (HCPro) in heterologous systems and its suppression of silencing activity

In the Potyvirus genus, the P1 protein is the first N-terminal product processed from the viral polyprotein, followed by the helper-component proteinase (HCPro). In silencing suppression patch assays, we found that Potato virus Y (PVY) HCPro expressed from a P1-HCPro sequence increased the accumulation of a reporter gene, whereas protein expressed from an HCPro sequence did not, even with P1 supplied in trans. This enhancing effect of P1 has been noted in other potyviruses, but has remained unexplained. We analysed the accumulation of PVY HCPro in infiltrated tissues and found that it was higher when expressed from P1-HCPro than from HCPro sequences. Co-expression of heterologous suppressors increased the steady-state level of mRNA expressed from the HCPro sequence, but not that of protein. This suggests that, in the absence of P1 upstream, either HCPro acquires a conformation that affects negatively its activity or stability, or that its translation is reduced. To test these options, we purified HCPro expressed in the presence or absence of upstream P1, and found no difference in purification pattern and final soluble state. By contrast, alteration of the Kozak context in the HCPro mRNA sequence to favour translation increased partially suppressor accumulation and activity. Furthermore, protein activity was not lower than in protein expressed from P1-HCPro sequences. Thus, a direct role for P1 on HCPro suppressor activity or stability, by influencing its conformation during translation, can be excluded. However, P1 could still have an indirect effect favouring HCPro accumulation. Our data highlight the relevance of cis-acting translational elements in the heterologous expression of HCPro.

Potato virus Y: a major crop pathogen that has provided major insights into the evolution of viral pathogenicity

TAXONOMY

Potato virus Y (PVY) is the type member of the genus Potyvirus in the family Potyviridae. VIRION AND GENOME PROPERTIES: PVY virions have a filamentous, flexuous form, with a length of 730 nm and a diameter of 12 nm. The genomic RNA is single stranded, messenger sense, with a length of 9.7 kb, covalently linked to a viral-encoded protein (VPg) at the 5' end and to a 3' polyadenylated tail. The genome is expressed as a polyprotein of approximately 3062 amino acid residues, processed by three virus-specific proteases into 11 mature proteins.

HOSTS

PVY is distributed worldwide and has a broad host range, consisting of cultivated solanaceous species and many solanaceous and nonsolanaceous weeds. It is one of the most economically important plant pathogens and causes severe diseases in cultivated hosts, such as potato, tobacco, tomato and pepper, as well as in ornamental plants.

TRANSMISSION

PVY is transmitted from plant to plant by more than 40 aphid species in a nonpersistent manner and, in potato, by planting contaminated seed tubers. DIVERSITY: Five major clades, named C1, C2, Chile, N and O, have been described within the PVY species. In recent decades, a strong increase in prevalence of N × O recombinant isolates has been observed worldwide. A correlation has been observed between PVY phylogeny and certain pathogenicity traits. GENETIC CONTROL OF PVY: Resistance genes against PVY have been used widely in breeding programmes and deployed in the field. These resistance genes show a large diversity of spectrum of action, durability and genetic determinism. Notably, recessive and dominant major resistance genes show highly contrasting patterns of interaction with PVY populations, displaying rapid co-evolution or stable relationships, respectively.

Translation elongation factor 1B (eEF1B) is an essential host factor for Tobacco mosaic virus infection in plants

Identifying host factors provides an important clue to understand virus infection. We selected 10 host factor candidate genes and each gene was silenced in Nicotiana benthamiana (N. benthamiana) to investigate their roles in virus infection. The resulting plants were infected with Tobacco mosaic virus (TMV). The accumulation of viral coat protein and the spread of virus were greatly reduced in the plants that eukaryotic translation elongation factor 1A (eEF1A) or 1B (eEF1B) was silenced. These results suggest both eEF1A and eEF1B are required for TMV infection. We also tested for interactions between the eEFs and viral proteins of TMV. Both eEF1A and eEF1B proteins interacted directly with the methyltransferase (MT) domain of the TMV RNA-dependent RNA polymerase (RdRp). eEF1A and eEF1B also interacted with each other in vivo. Our data suggest that eEF1B may be a component of the TMV replication complex which interacts with MT domain of TMV RdRp and eEF1A.

Tyrosine phosphorylation of the triple gene block protein 3 regulates cell-to-cell movement and protein interactions of Potato mop-top virus

Functions of viral proteins can be regulated through phosphorylation by serine/threonine kinases in plants, but little is known about the involvement of tyrosine kinases in plant virus infection. In this study, TGBp3, one of the three movement proteins encoded by a triple gene block (TGB) of Potato mop-top virus (PMTV), was detected for the first time in PMTV-infected plants and found to be tyrosine phosphorylated. Phosphorylation sites (Tyr(87-89) and Tyr(120)) were located in two amino acid motifs conserved in the TGB-containing, rod-shaped plant viruses. Substitution of these tyrosine residues in both motifs was needed to abolish tyrosine phosphorylation of TGBp3. Substitution of Tyr(87-89) with alanine residues enhanced the interaction between TGBp3 and TGBp2 and inhibited cell-to-cell movement of PMTV. On the other hand, substitution of Tyr(120) with alanine resulted in no alteration in the interaction of TGBp3 with TGBp2, but the mutant virus was not infectious. The results suggest that tyrosine phosphorylation is a mechanism regulating the functions of plant virus movement proteins.

An isoform of eukaryotic initiation factor 4E from Chrysanthemum morifolium interacts with Chrysanthemum virus B coat protein

Background

Eukaryotic translation initiation factor 4E (eIF4E) plays an important role in plant virus infection as well as the regulation of gene translation.

Methodology/Principal Findings

Here, we describe the isolation of a cDNA encoding CmeIF(iso)4E (GenBank accession no. JQ904592), an isoform of eIF4E from chrysanthemum, using RACE PCR. We used the CmeIF(iso)4E cDNA for expression profiling and to analyze the interaction between CmeIF(iso)4E and the Chrysanthemum virus B coat protein (CVBCP). Multiple sequence alignment and phylogenetic tree analysis showed that the sequence similarity of CmeIF(iso)4E with other reported plant eIF(iso)4E sequences varied between 69.12% and 89.18%, indicating that CmeIF(iso)4E belongs to the eIF(iso)4E subfamily of the eIF4E family. CmeIF(iso)4E was present in all chrysanthemum organs, but was particularly abundant in the roots and flowers. Confocal microscopy showed that a transiently transfected CmeIF(iso)4E-GFP fusion protein distributed throughout the whole cell in onion epidermis cells. A yeast two hybrid assay showed CVBCP interacted with CmeIF(iso)4E but not with CmeIF4E. BiFC assay further demonstrated the interaction between CmeIF(iso)4E and CVBCP. Luminescence assay showed that CVBCP increased the RLU of Luc-CVB, suggesting CVBCP might participate in the translation of viral proteins.

Conclusions/Significance

These results inferred that CmeIF(iso)4E as the cap-binding subunit eIF(iso)4F may be involved in Chrysanthemum Virus B infection in chrysanthemum through its interaction with CVBCP in spatial.

Genetic determinants of Potato virus Y required to overcome or trigger hypersensitive resistance to PVY strain group O controlled by the gene Ny in potato

Potato virus Y (PVY) (genus Potyvirus) is the most economically damaging and widely distributed virus in potato. Spread of PVY in the field is controlled by growing resistant cultivars. The dominant potato gene Ny(tbr) for hypersensitive resistance (HR) controls ordinary PVY strains (PVY(O)) but is overcome by PVY(N) strains. Studies with infectious PVY chimeras and mutants indicated that the viral determinants necessary and sufficient to overcome Ny(tbr) reside within the helper component proteinase (HC-Pro) (residues 227 to 327). Specifically, eight residues and the modeled three-dimensional conformation of this HC-Pro region distinguish PVY(N) from PVY(O) strains. According to the model, the conserved IGN and CCCT motifs implicated in potyvirus replication and movement, respectively, are situated in a coiled structure and an α-helix, respectively, within this region in PVY(O); however, their locations are reversed in PVY(N). Two residues (R269 and K270) are crucial for the predicted PVY(O)-specific HC-Pro conformation. Two viral chimeras triggered Ny(tbr) and induced veinal necrosis in tobacco, which is novel for PVY. One chimera belonged to strain group PVY(E). Our results suggest a structure-function relationship in recognition of PVY(O) HC-Pro by Ny(tbr), reveal HC-Pro amino acid signatures specific to PVY(O) and PVY(N), and facilitate identification of PVY strains overcoming Ny(tbr).

Identification of new Potato virus Y (PVY) molecular determinants for the induction of vein necrosis in tobacco

Two tobacco vein necrosis (TVN) determinants, the residues K(400) and E(419) , have been identified previously in the helper component-protease (HC-Pro) protein sequence of Potato virus Y (PVY). However, since their description, non-necrotic PVY isolates with both K(400) and E(419) necrotic determinants have been reported in the literature. This suggests the presence in the viral genome of other, as yet uncharacterized, TVN determinant(s). The identification of PVY(N) pathogenicity determinants was approached through the replacement of genomic regions of the necrotic PVY(N) -605 infectious clone by corresponding sequences from the non-necrotic PVY(O) -139 isolate. Series of PVY(N/O) chimeras and site-directed PVY mutants were constructed to test the involvement of different parts of the PVY genome (from nucleotide 421 to nucleotide 9629) in the induction of TVN symptoms. The analysis of both the genomic characteristics and biological properties of these mutants made it possible to highlight the involvement, in addition to residues K(400) and E(419), of the residue N(339) of the HC-Pro protein and two regions in the cytoplasmic inclusion (CI) protein to nuclear inclusion protein a-protease (NIa-Pro) sequence (nucleotides 5496-5932 and 6233-6444) in the induction of vein necrosis in tobacco infected by PVY isolates.

Towards an integrated molecular model of plant-virus interactions

The application in recent years of network theory methods to the study of host-virus interactions is providing a new perspective to the way viruses manipulate the host to promote their own replication. An integrated molecular model of such pathosystems require three detailed maps describing, firstly, the interactions between viral elements, secondly, the interactions between host elements, and thirdly, the cross-interactions between viral and host elements. Here, we compile available information for Potyvirus infecting Arabidopsis thaliana. With an integrated model, it is possible to analyze the mode of virus action and how the perturbation of the virus targets propagates along the network. These studies suggest that viral pathogenicity results not only from the alteration of individual elements but it is a systemic property.

Eukaryotic translation initiation factor 4E-mediated recessive resistance to plant viruses and its utility in crop improvement

The use of genetic resistance is considered to be the most effective and sustainable approach to the control of plant pathogens. Although most of the known natural resistance genes are monogenic dominant R genes that are predominant against fungi and bacteria, more and more recessive resistance genes against viruses have been cloned in the last decade. Interestingly, of the 14 natural recessive resistance genes against plant viruses that have been cloned from diverse plant species thus far, 12 encode the eukaryotic translation initiation factor 4E (eIF4E) or its isoform eIF(iso)4E. This review is intended to summarize the current state of knowledge about eIF4E and the possible mechanisms underlying its essential role in virus infection, and to discuss recent progress and the potential of eIF4E as a target gene in the development of genetic resistance to viruses for crop improvement.

Heterologous RNA-silencing suppressors from both plant- and animal-infecting viruses support plum pox virus infection

HCPro, the RNA-silencing suppressor (RSS) of viruses belonging to the genus Potyvirus in the family Potyviridae, is a multifunctional protein presumably involved in all essential steps of the viral infection cycle. Recent studies have shown that plum pox potyvirus (PPV) HCPro can be replaced successfully by cucumber vein yellowing ipomovirus P1b, a sequence-unrelated RSS from a virus of the same family. In order to gain insight into the requirement of a particular RSS to establish a successful potyviral infection, we tested the ability of different heterologous RSSs from both plant- and animal-infecting viruses to substitute for HCPro. Making use of engineered PPV chimeras, we show that PPV HCPro can be replaced functionally by some, but not all, unrelated RSSs, including the NS1 protein of the mammal-infecting influenza A virus. Interestingly, the capacity of a particular RSS to replace HCPro does not correlate strictly with its RNA silencing-suppression strength. Altogether, our results suggest that not all suppression strategies are equally suitable for efficient escape of PPV from the RNA-silencing machinery. The approach followed here, based on using PPV chimeras in which an under-consideration RSS substitutes for HCPro, could further help to study the function of diverse RSSs in a 'highly sensitive' RNA-silencing context, such as that taking place in plant cells during the process of a viral infection.

The Cucumber vein yellowing virus silencing suppressor P1b can functionally replace HCPro in Plum pox virus infection in a host-specific manner

Plant viruses of the genera Potyvirus and Ipomovirus (Potyviridae family) use unrelated RNA silencing suppressors (RSS) to counteract antiviral RNA silencing responses. HCPro is the RSS of Potyvirus spp., and its activity is enhanced by the upstream P1 protein. Distinctively, the ipomovirus Cucumber vein yellowing virus (CVYV) lacks HCPro but contains two P1 copies in tandem (P1aP1b), the second of which functions as RSS. Using chimeras based on the potyvirus Plum pox virus (PPV), we found that P1b can functionally replace HCPro in potyviral infections of Nicotiana plants. Interestingly, P1a, the CVYV protein homologous to potyviral P1, disrupted the silencing suppression activity of P1b and reduced the infection efficiency of PPV in Nicotiana benthamiana. Testing the influence of RSS in host specificity, we found that a P1b-expressing chimera poorly infected PPV's natural host, Prunus persica. Conversely, P1b conferred on PPV chimeras the ability to replicate locally in cucumber, CVYV's natural host. The deleterious effect of P1a on PPV infection is host dependent, because the P1aP1b-expressing PPV chimera accumulated in cucumber to higher levels than PPV expressing P1b alone. These results demonstrate that a potyvirus can use different RSS, and that particular RSS and upstream P1-like proteins contribute to defining the virus host range.

The C terminus of lettuce mosaic potyvirus cylindrical inclusion helicase interacts with the viral VPg and with lettuce translation eukaryotic initiation factor 4E

Recessive resistance to lettuce mosaic virus (LMV) is conferred in lettuce by the mo1 gene, encoding the eukaryotic translation initiation factor 4E (eIF4E). The C terminus of the viral cylindrical inclusion helicase (CI-Cter), together with the VPg, is involved directly in overcoming mo1 resistance. In this study, recombinant LMV VPg and CI-Cter proteins from wild-type or resistance-breaking isolates were expressed and purified from Escherichia coli. The allelic forms of eIF4E from susceptible or resistant lettuce cultivars were produced similarly and these proteins were used in ELISA-based assays to demonstrate the in vitro binding of the various forms of LMV CI-Cter to both lettuce eIF4E and LMV VPg proteins. All combinations tested displayed significant and specific interactions, and the interaction between the C-terminal part of the LMV CI and eIF4E was confirmed in vivo in bimolecular fluorescence complementation assays. Higher interaction signals for both CI-eIF4E and CI-VPg were observed for LMV-E, indicating that the eIF4E interaction network involving CI and VPg appears to be stronger in the case of this resistance-breaking isolate. This could suggest the need for a minimal interaction threshold for infection success in resistant lettuce, but more precise measurement of the interaction parameters linking eIF4E, VPg and CI is needed in order to reinforce such a hypothesis.

Simultaneous mutations in multi-viral proteins are required for soybean mosaic virus to gain virulence on soybean genotypes carrying different R genes

BACKGROUND

Genetic resistance is the most effective and sustainable approach to the control of plant pathogens that are a major constraint to agriculture worldwide. In soybean, three dominant R genes, i.e., Rsv1, Rsv3 and Rsv4, have been identified and deployed against Soybean mosaic virus (SMV) with strain-specificities. Molecular identification of virulent determinants of SMV on these resistance genes will provide essential information for the proper utilization of these resistance genes to protect soybean against SMV, and advance knowledge of virus-host interactions in general.

METHODOLOGY/PRINCIPAL FINDINGS

To study the gain and loss of SMV virulence on all the three resistance loci, SMV strains G7 and two G2 isolates L and LRB were used as parental viruses. SMV chimeras and mutants were created by partial genome swapping and point mutagenesis and then assessed for virulence on soybean cultivars PI96983 (Rsv1), L-29 (Rsv3), V94-5152 (Rsv4) and Williams 82 (rsv). It was found that P3 played an essential role in virulence determination on all three resistance loci and CI was required for virulence on Rsv1- and Rsv3-genotype soybeans. In addition, essential mutations in HC-Pro were also required for the gain of virulence on Rsv1-genotype soybean. To our best knowledge, this is the first report that CI and P3 are involved in virulence on Rsv1- and Rsv3-mediated resistance, respectively.

CONCLUSIONS/SIGNIFICANCE

Multiple viral proteins, i.e., HC-Pro, P3 and CI, are involved in virulence on the three resistance loci and simultaneous mutations at essential positions of different viral proteins are required for an avirulent SMV strain to gain virulence on all three resistance loci. The likelihood of such mutations occurring naturally and concurrently on multiple viral proteins is low. Thus, incorporation of all three resistance genes in a soybean cultivar through gene pyramiding may provide durable resistance to SMV.

The genome-linked protein VPg of plant viruses-a protein with many partners

For some plant positive-sense RNA viruses, a protein known as VPg (short for virus protein, genome linked) is covalently linked to the 5' end of the viral RNA. The VPg is an intrinsically disordered protein, and this property would confer an ability to bind several proteins. Accordingly, the potyvirus VPg interacts with many proteins, notably host factors involved in protein synthesis within viral replication factories or within the nucleus. The number of protein partners, the clustering of the various interactions centering around it, the biological importance for some of these interactions (e.g. VPg-eIF4E) and the intrinsically disordered state of the protein are all elements that support the notion that VPg is a hub protein that controls many processes leading to virus production and spread.