Plant virus proteins involved in natural vector transmission

Biochemical and nanotechnological approaches to combat phytoparasitic nematodes

The foundation of most food production systems underpinning global food security is the careful management of soil resources. Embedded in the concept of soil health is the impact of diverse soil-borne pests and pathogens, and phytoparasitic nematodes represent a particular challenge. Root-knot nematodes and cyst nematodes are severe threats to agriculture, accounting for annual yield losses of US$157 billion. The control of soil-borne phytoparasitic nematodes conventionally relies on the use of chemical nematicides, which can have adverse effects on the environment and human health due to their persistence in soil, plants, and water. Nematode-resistant plants offer a promising alternative, but genetic resistance is species-dependent, limited to a few crops, and breeding and deploying resistant cultivars often takes years. Novel approaches for the control of phytoparasitic nematodes are therefore required, those that specifically target these parasites in the ground whilst minimizing the impact on the environment, agricultural ecosystems, and human health. In addition to the development of next-generation, environmentally safer nematicides, promising biochemical strategies include the combination of RNA interference (RNAi) with nanomaterials that ensure the targeted delivery and controlled release of double-stranded RNA. Genome sequencing has identified more than 75 genes in root knot and cyst nematodes that have been targeted with RNAi so far. But despite encouraging results, the delivery of dsRNA to nematodes in the soil remains inefficient. In this review article, we describe the state-of-the-art RNAi approaches targeting phytoparasitic nematodes and consider the potential benefits of nanotechnology to improve dsRNA delivery.

Unlocking Cowpea's Defense Responses: Conserved Transcriptional Signatures in the Battle against CABMV and CPSMV Viruses

Cowpea aphid-borne mosaic virus (CABMV) and Cowpea severe mosaic virus (CPSMV) threaten cowpea commercial production. This study aimed to analyze Conserved Transcriptional Signatures (CTS) in cowpea's genotypes that are resistant to these viruses. CTS covered up- (UR) or down-regulated (DR) cowpea transcripts in response to CABMV and CPSMV mechanical inoculations. The conservation of cowpea's UR defense response was primarily observed with the one hpi treatments, with decreased CTS representatives as time elapsed. This suggests that cowpea utilizes generic mechanisms during its early interaction with the studied viruses, and subsequently employs more specialized strategies for each viral agent. The potential action of the CTS-UR emphasizes the importance of redox balance, ethylene and jasmonic acid pathways. Additionally, the CTS-UR provides evidence for the involvement of R genes, PR proteins, and PRRs receptors-extensively investigated in combating bacterial and fungal pathogens-in the defense against viral inoculation. AP2-ERF, WRKY, and MYB transcription factors, as well as PIP aquaporins and MAPK cascades, also emerged as significant molecular players. The presented work represents the first study investigating conserved mechanisms in the cowpea defense response to viral inoculations, highlighting relevant processes for initial defense responses.

Delivery of Nematicides Using TMGMV-Derived Spherical Nanoparticles

Spherical nanoparticles (SNPs) from tobacco mild green mosaic virus (TMGMV) were developed and characterized, and their application for agrochemical delivery was demonstrated. Specifically, we set out to develop a platform for pesticide delivery targeting nematodes in the rhizosphere. SNPs were obtained by thermal shape-switching of the TMGMV. We demonstrated that cargo can be loaded into the SNPs during thermal shape-switching, enabling the one-pot synthesis of functionalized nanocarriers. Cyanine 5 and ivermectin were encapsulated into SNPs to achieve 10% mass loading. SNPs demonstrated good mobility and soil retention slightly higher than that of TMGMV rods. Ivermectin delivery to Caenorhabditis elegans using SNPs was determined after passing the formulations through soil. Using a gel burrowing assay, we demonstrate the potent efficacy of SNP-delivered ivermectin against nematodes. Like many pesticides, free ivermectin is adsorbed in the soil and did not show efficacy. The SNP nanotechnology offers good soil mobility and a platform technology for pesticide delivery to the rhizosphere.

Decoyinine Induced Resistance in Rice against Small Brown Planthopper Laodelphax striatellus

Induced resistance against SBPH via microbial pesticides is considered as an eco-friendly and promising management approach. In this study, the induced resistance against SBPH in rice seedling by a new potential microbial pesticide, decoyinine (DCY), a secondary metabolite produced by Streptomyces hygroscopicus, was evaluated to investigate the effects of DCY on SBPH's biological and population parameters along with defense-related physiological and biochemical indices in rice against SBPH feeding. We found that DCY has potential to improve rice resistance and significantly reduced the fecundity of SBPH. Laboratory results revealed that DCY treated rice significantly changed SBPH's fecundity and population life table parameters. The concentrations of hydrogen peroxide (H2O2), soluble sugars and malondialdehyde (MDA) were significantly lower in DCY treated rice plants against SBPH infestation at 24, 48 and 96 hours post infestation (hpi), respectively. The concentrations of antioxidant enzymes, catalase (CAT) was significantly higher at 72 hpi, while super oxidase dismutase (SOD) and peroxidase (POD) concentrations were recorded higher at 96 hpi. The concentrations of synthases enzymes, phenyl alanine ammonia-lyase (PAL) was higher at 48 hpi, whereas polyphenol oxidase (PPO) concentration was maximum at 72 hpi against SBPH infestation. The results imply that DCY has unique properties to enhance rice resistance against SBPH by stimulating plant defensive responses. Microbial pesticides may be developed as an alternative to chemical pest control.

Integrative proteomics to understand the transmission mechanism of Barley yellow dwarf virus-GPV by its insect vector Rhopalosiphum padi

Barley yellow dwarf virus-GPV (BYDV-GPV) is transmitted by Rhopalosiphum padi and Schizaphis graminum in a persistent nonpropagative manner. To improve our understanding of its transmission mechanism by aphid vectors, we used two approaches, isobaric tags for relative and absolute quantitation (iTRAQ) and yeast two-hybrid (YTH) system, to identify proteins in R. padi that may interact with or direct the spread of BYDV-GPV along the circulative transmission pathway. Thirty-three differential aphid proteins in viruliferous and nonviruliferous insects were identified using iTRAQ coupled to 2DLC-MS/MS. With the yeast two-hybrid system, 25 prey proteins were identified as interacting with the readthrough protein (RTP) and eight with the coat protein (CP), which are encoded by BYDV-GPV. Among the aphid proteins identified, most were involved in primary energy metabolism, synaptic vesicle cycle, the proteasome pathway and the cell cytoskeleton organization pathway. In a systematic comparison of the two methods, we found that the information generated by the two methods was complementary. Taken together, our findings provide useful information on the interactions between BYDV-GPV and its vector R. padi to further our understanding of the mechanisms regulating circulative transmission in aphid vectors.

Endogenous antiviral mechanisms of RNA interference: a comparative biology perspective

RNA interference (RNAi) is a natural process that occurs in many organisms ranging from plants to mammals. In this process, double-stranded RNA or hairpin RNA is cleaved by a RNaseIII-type enzyme called Dicer into small interfering RNA duplex. This then directs sequence-specific, homology-dependent, posttranscriptional gene silencing by binding to its complementary RNA and triggering its elimination through degradation or by inducing translational inhibition. In plants, worms, and insects, RNAi is a strong antiviral defense mechanism. Although, at present, it is unclear whether RNA silencing naturally restricts viral infection in vertebrates, there are signs that this is certainly the case. In a relatively short period, RNAi has progressed to become an important experimental tool both in vitro and in vivo for the analysis of gene function and target validation in mammalian systems. In addition, RNA silencing has subsequently been found to be involved in translational repression, transcriptional inhibition, and DNA degradation. In this article we review the literature in this field, which may open doors to the many uses to which this important technology is being put, including the potential of RNAi as a therapeutic strategy for gene regulation to modulate host-pathogen interactions.

Electron microscopy studies on luteovirid transmission by aphids

Transmission electron microscopy (TEM) observations have been extensively applied to follow the route of luteovirids in their vectors. Luteovirids are icosahedral plant viruses which are phloem-limited and strictly transmitted in a circulative manner by aphids. Virus particles, acquired by aphids while feeding on an infected plant, circulate in the aphid's body without replication and are internalized during this process in two different cell types (intestinal and accessory salivary gland cells). The endocytosis mechanism at the gut level seems to rely on a clathrin-mediated entry process and virions are observed in the aphid's gut cells in various vesicular structures. After exocytosis from intestinal cells, virions are released in the aphid's body cavity where they are thought to bind to symbionin, an endosymbiotic protein. Transcytosis of the accessory salivary gland cells occurs similarly as at the gut level but in the reverse direction. Using engineered virus mutants, viral proteins required for transmission and involved in virus retention in the hemocoel have been identified. Virus mutants poorly or non aphid-transmitted have also been localized in the aphid's body by TEM. These observations reveal the crucial role of the minor capsid protein in gut internalization. While not strictly required, this protein seems to play an important role in the efficiency of this process by interacting with putative virus receptors localized on the gut apical membrane. More recently, some aphid proteins have also been shown to exhibit in vitro virus binding capacity and could potentially be components of the endocytotic apparatus.

A simple wax-embedding method for isolation of aphid hemolymph for detection of luteoviruses in the hemocoel

A protocol for isolating hemolymph from viruliferous aphids has been developed. This method uses warm melted wax to immobilize the aphid. Following removal of a hind leg, the hemolymph can be collected readily. Flushing with RNase-free water allows for collection of sufficient hemolymph for RNA extraction from individual aphids. The extracted RNA was successfully used for detection of barley yellow dwarf virus (BYDV) and pea enation mosaic virus (PEMV) from individual viruliferous Rhopalosiphum padi and Acyrthosiphon pisum aphids, respectively. A TaqMan real-time RT-PCR protocol for quantitation of PEMV in the hemolymph of individual aphids was developed. The wax-embedding hemolymph collection technique provides a useful tool for studying molecular interactions between persistent and circulative plant viruses and their insect vectors.

Anti-viral RNA silencing: do we look like plants?

The anti-viral function of RNA silencing was first discovered in plants as a natural manifestation of the artificial 'co-suppression', which refers to the extinction of endogenous gene induced by homologous transgene. Because silencing components are conserved among most, if not all, eukaryotes, the question rapidly arose as to determine whether this process fulfils anti-viral functions in animals, such as insects and mammals. It appears that, whereas the anti-viral process seems to be similarly conserved from plants to insects, even in worms, RNA silencing does influence the replication of mammalian viruses but in a particular mode: micro(mi)RNAs, endogenous small RNAs naturally implicated in translational control, rather than virus-derived small interfering (si)RNAs like in other organisms, are involved. In fact, these recent studies even suggest that RNA silencing may be beneficial for viral replication. Accordingly, several large DNA mammalian viruses have been shown to encode their own miRNAs. Here, we summarize the seminal studies that have implicated RNA silencing in viral infection and compare the different eukaryotic responses.

Evidence that vector transmission of a plant virus requires conformational change in virus particles

Transmission of Cucumber necrosis virus (CNV) by zoospores of its fungal vector, Olpidium bornovanus, involves specific adsorption of virus particles onto the zoospore plasmalemma prior to infestation of cucumber roots by virus-bound zoospores. Previous work has shown that specific components of both CNV and zoospores are required for successful CNV/zoospore recognition. Here, we show that limited trypsin digestion of CNV following in vitro CNV/zoospore binding assays, results in the production of specific proteolytic digestion products under conditions where native CNV is resistant. The proteolytic digestion pattern of zoospore-bound CNV was found to be similar to that of swollen CNV particles produced in vitro, suggesting that zoospore-bound CNV is in an altered conformational state, perhaps similar to that of swollen CNV. We show that an engineered CNV mutant (Pro73Gly) in which a conserved proline residue (Pro73) in the beta-annulus of the CP arm is changed to glycine is resistant to proteolysis following in vitro zoospore binding assays. Moreover, Pro73Gly particles are transmitted only poorly by O.bornovanus. Together, the results of these studies suggest that CNV undergoes conformational change upon zoospore binding and that the conformational change is important for CNV transmissibility.

A conserved capsid protein surface domain of Cucumber mosaic virus is essential for efficient aphid vector transmission

A prominent feature on the surfaces of virions of Cucumber mosaic virus (CMV) is a negatively charged loop structure (the beta H-beta I loop). Six of 8 amino acids in this capsid protein loop are highly conserved among strains of CMV and other cucumoviruses. Five of these amino acids were individually changed to alanine or lysine (an amino acid of opposite charge) to create nine mutants (the D191A, D191K, D192A, D192K, L194A, E195A, E195K, D197A, and D197K mutants). Transcripts of cDNA clones were infectious when they were mechanically inoculated onto tobacco, giving rise to symptoms of a mottle-mosaic typical of the wild-type virus (the D191A, D191K, D192A, E195A, E195K, and D197A mutants), a systemic necrosis (the D192K mutant), or an atypical chlorosis with necrotic flecking (the L194A mutant). The mutants formed virions and accumulated to wild-type levels, but eight of the nine mutants were defective in aphid vector transmission. The aspartate-to-lysine mutation at position 197 interfered with infection; the only recovered progeny (the D197K(*) mutant) harbored a second-site mutation (denoted by the asterisk) of alanine to glutamate at position 193, a proximal site in the beta H-beta I loop. Since the disruption of charged amino acid residues in the beta H-beta I loop reduces or eliminates vector transmissibility without grossly affecting infectivity or virion formation, we hypothesize that this sequence or structure has been conserved to facilitate aphid vector transmission.

Identification of specific cucumber necrosis virus coat protein amino acids affecting fungus transmission and zoospore attachment

Cucumber necrosis virus (CNV) is naturally transmitted in the soil by zoospores of the fungal vector Olpidium bornovanus. Successful transmission requires that virus particles attach to the surface of zoospores prior to zoospore encystment on host roots. Mechanically passaged CNV was screened for mutants deficient in fungus transmission. We found six such mutants, exhibiting transmission efficiencies ranging from approximately 14 to 76% of that of wild-type (WT) CNV. Results of in vitro virus-zoospore binding assays show that each mutant binds to zoospores less efficiently than WT CNV (21 to 68%), suggesting that defects in transmission for these mutants are at least partially due to inefficient zoospore binding. Analysis of the structure of the CNV coat protein subunit and trimer indicates that affected amino acids in all of the mutants are located in the shell or protruding domain and that five of six of them are potentially exposed on the surface of the virus particle. In addition, several of the mutated sites, along with a previously identified site in a region of subunit-subunit interaction in the coat protein shell domain (M. A. Robbins, R. D. Reade, and D. M. Rochon, Virology 234:138-146, 1997), are located on the particle quasi-threefold axis, suggesting that this region of the capsid may be important in recognition of a putative zoospore receptor. The individual sites may directly affect attachment to a receptor or could indirectly affect attachment via changes in virion conformation.

The multifunctional capsid proteins of plant RNA viruses

This article summarizes studies of viral coat (capsid) proteins (CPs) of RNA plant viruses. In addition, we discuss and seek to interpret the knowledge accumulated to data. CPs are named for their primary function; to encapsidate viral genomic nucleic acids. However, encapsidation is only one feature of an extremely diverse array of structural, functional, and ecological roles played during viral infection and spread. Herein, we consider the evolution of viral CPs and their multitude of interactions with factors encoded by the virus, host plant, or viral vector (biological transmission agent) that influence the infection and epidemiological facets of plant disease. In addition, applications of today's understanding of CPs in the protection of crops from viral infection and use in the manufacture of valuable compounds are considered.

The GroEL protein of the whitefly Bemisia tabaci interacts with the coat protein of transmissible and nontransmissible begomoviruses in the yeast two-hybrid system

We have previously suggested that a GroEL homolog produced by the whitefly Bemisia tabaci endosymbiotic bacteria interacts in the insect hemolymph with particles of Tomato yellow leaf curl virus from Israel (TYLCV-Is), ensuring the safe circulative transmission of the virus. We have now addressed the question of whether the nontransmissibility of Abutilon mosaic virus from Israel (AbMV-Is) is related to a lack of association between GroEL and the virus coat protein (CP). Translocation analysis has shown that, whereas TYLCV-Is DNA is conspicuous in the digestive tract, hemolymph, and salivary glands of B. tabaci 8 h after acquisition feeding started, AbMV-Is DNA was detected only in the insect digestive tract, even after 96 h. To determine whether AbMV-Is particles were rapidly degraded in the hemolymph as a result of their inability to interact with GroEL, we have isolated a GroEL gene from B. tabaci and used a yeast two-hybrid assay to compare binding of the CP of TYLCV-Is and AbMV-Is to the insect GroEL. The yeast assay showed that the CPs of the two viruses are able to bind efficiently to GroEL. We therefore suggest that, although GroEL-CP interaction in the hemolymph is a necessary condition for circulative transmission, the nontransmissibility of AbMV-Is is not the result of lack of binding to GroEL in the B. tabaci hemolymph, but most likely results from an inability to cross the gut/hemolymph barrier.

Cucumber mosaic virus mutants with altered physical properties and defective in aphid vector transmission

Two mutant strains of cucumber mosaic virus (CMV) were investigated with respect to virion stability and molecular determinants of aphid vector transmission. The mutant 2A1-MT-60x, derived from the mechanically passaged wild type 2A1-AT, is poorly transmissible by the aphid Aphis gossypii and not transmissible by the aphid Myzus persicae, whereas the wild type virus is transmissible by both aphid species. The mutant phenotype was shown to be conferred by a single encoded amino acid change of alanine to threonine at position 162 of the coat protein (CP). Modifying the mutant CP gene to encode the wild type sequence (alanine) at position 162 restored aphid transmission. To test for a correspondence between changes in the physical stability of virions and defects in aphid transmission, a urea disruption assay was developed. Virions of aphid-transmissible strains 2A1-AT and CMV-Fny were stable with treatments of up to between 3 and 4 M urea. In this assay mutant viruses 2A1-MT-60x and CMV-M were less stable, as they were completely disrupted at urea concentrations of 2 and 1 M urea, respectively. The mutant 2A1-MT-60x also accumulated at a reduced level in infected squash relative to the wild type virus. These studies suggest that a primary factor in the loss of aphid transmissibility of some strains of CMV is a reduction in virion stability.

The structure of cucumber mosaic virus and comparison to cowpea chlorotic mottle virus

The structure of cucumber mosaic virus (CMV; strain Fny) has been determined to a 3.2-A resolution using X-ray crystallography. Despite the fact that CMV has only 19% capsid protein sequence identity (34% similarity) to cowpea chlorotic mottle virus (CCMV), the core structures of these two members of the Bromoviridae family are highly homologous. As suggested by a previous low-resolution structural study, the 305-A diameter (maximum) of CMV is approximately 12 A larger than that of CCMV. In CCMV, the structures of the A, B, and C subunits are nearly identical except in their N termini. In contrast, the structures of two loops in subunit A of CMV differ from those in B and C. These loops are 6 and 7 residues longer than the analogous regions in CCMV. Unlike that of CCMV, the capsid of CMV does not undergo swelling at pH 7.0 and is stable at pH 9.0. This may be partly due to the fact that the N termini of the B and C subunits form a unique bundle of six amphipathic helices oriented down into the virion core at the threefold axes. In addition, while CCMV has a cluster of aspartic acid residues at the quasi-threefold axis that are proposed to bind metal in a pH-dependent manner, this cluster is replaced by complementing acids and bases in CMV. Finally, this structure clearly demonstrates that the residues important for aphid transmission lie at the outermost portion of the betaH-betaI loop and yields details of the portions of the virus that are hypothesized to mediate binding to aphid mouthparts.

Nonstructural proteins of Tobacco rattle virus which have a role in nematode-transmission: expression pattern and interaction with viral coat protein

RNA 2 of Tobacco rattle virus isolate PpK20 encodes the viral coat protein (CP) and two nonstructural proteins of 40 kDa ('40K protein') and 32.8 kDa ('32.8K'). The 40K protein is required for transmission of the virus by the vector nematode Paratrichodorus pachydermus whereas the 32.8K protein may be involved in transmission by other vector nematode species. An antiserum was raised against the 40K protein expressed in E. coli and used to study the expression and subcellular localization of this protein in infected Nicotiana benthamiana plants. The time-course of the expression of the 40K protein in leaves and roots was similar to that of CP and both proteins were similarly distributed over the 1000 g pellet, 30000 g pellet and 30000 g supernatant fractions of leaf and root homogenates. Using the yeast two-hybrid system, a strong interaction between CP subunits was observed and weaker interactions between CP and the 32.8K protein and between CP and the 40K protein were detected. A deletion of the C-terminal 19 amino acids of CP interfered with the CP-40K interaction but not with CP-32.8K or CP-CP interactions, whereas a C-terminal deletion of 79 amino acids interfered with CP-40K and CP-32.8K interactions but not with the CP-CP interaction. As the C terminus of CP is known to be involved in nematode-transmission of tobraviruses, the data support the hypothesis that interactions between CP and RNA 2-encoded nonstructural proteins play a role in the transmission process.

A GroEL homologue from endosymbiotic bacteria of the whitefly Bemisia tabaci is implicated in the circulative transmission of tomato yellow leaf curl virus

Evidence for the involvement of a Bemisia tabaci GroEL homologue in the transmission of tomato yellow leaf curl geminivirus (TYLCV) is presented. A approximately 63-kDa protein was identified in B. tabaci whole-body extracts using an antiserum raised against aphid Buchnera GroEL. The GroEL homologue was immunolocalized to a coccoid-shaped whitefly endosymbiont. The 30 N-terminal amino acids of the whitefly GroEL homologue showed 80% homology with that from different aphid species and GroEL from Escherichia coli. Purified GroEL from B. tabaci exhibited ultrastructural similarities to that of the endosymbiont from aphids and E. coli. In vitro ligand assays showed that tomato yellow leaf curl virus (TYLCV) particles displayed a specific affinity for the B. tabaci 63-kDa GroEL homologue. Feeding whiteflies anti-Buchnera GroEL antiserum before the acquisition of virions reduced TYLCV transmission to tomato test plants by >80%. In the haemolymph of these whiteflies, TYLCV DNA was reduced to amounts below the threshold of detection by Southern blot hybridization. Active antibodies were recovered from the insect haemolymph suggesting that by complexing the GoEL homologue, the antibody disturbed interaction with TYLCV, leading to degradation of the virus. We propose that GroEL of B. tabaci protects the virus from destruction during its passage through the haemolymph.

Mechanisms of arthropod transmission of plant and animal viruses

A majority of the plant-infecting viruses and many of the animal-infecting viruses are dependent upon arthropod vectors for transmission between hosts and/or as alternative hosts. The viruses have evolved specific associations with their vectors, and we are beginning to understand the underlying mechanisms that regulate the virus transmission process. A majority of plant viruses are carried on the cuticle lining of a vector's mouthparts or foregut. This initially appeared to be simple mechanical contamination, but it is now known to be a biologically complex interaction between specific virus proteins and as yet unidentified vector cuticle-associated compounds. Numerous other plant viruses and the majority of animal viruses are carried within the body of the vector. These viruses have evolved specific mechanisms to enable them to be transported through multiple tissues and to evade vector defenses. In response, vector species have evolved so that not all individuals within a species are susceptible to virus infection or can serve as a competent vector. Not only are the virus components of the transmission process being identified, but also the genetic and physiological components of the vectors which determine their ability to be used successfully by the virus are being elucidated. The mechanisms of arthropod-virus associations are many and complex, but common themes are beginning to emerge which may allow the development of novel strategies to ultimately control epidemics caused by arthropod-borne viruses.

Transmission of Pineapple Mealybug Wilt-Associated Virus by Two Species of Mealybug (Dysmicoccus spp.)

ABSTRACT Closterovirus-like particles associated with mealybug wilt of pineapple were acquired and transmitted by the pink pineapple mealybug, Dysmicoccus brevipes, and the gray pineapple mealybug, D. neobrevipes. Mealybugs acquired pineapple mealybug wilt-associated virus (PMWaV) from infected pineapple plants or detached leaves. The virus was detected in plants by tissue blot immunoassay and confirmed by immunosorbent electron microscopy. Plants exposed to mealybugs reared on PMWaV-free pineapple tissue remained uninfected. The presence of ants was correlated with an increased rate of virus spread when caged with D. brevipes. All stages of D. neobrevipes acquired PMWaV, although vector efficiency decreased significantly in older adult females. The probability of a single third-instar immature transmitting the virus was 0.04. Both species of mealybug acquired and transmitted PMWaV from infected pineapple material that had been clonally propagated for decades, and both species acquired PMWaV from sources previously infected with the virus by the other mealybug species.

Amino acid changes in the coat protein of cucumber mosaic virus differentially affect transmission by the aphids Myzus persicae and Aphis gossypii

Cucumber mosaic virus (CMV) transmission by two aphid species was investigated using strains of the virus with modified coat proteins. Chimeric viral RNA 3 cDNA constructs were designed with coat protein-encoding sequences from the efficiently aphid-transmitted Fny-CMV and the very poorly aphid-transmitted M-CMV. Different regions of the viral coat protein variably influenced transmission; the magnitude of effects depended on the aphid species used as vector. Five amino acid changes in the coat protein of M-CMV (positions 25, 129, 162, 168, and 214) were required to restore efficient transmission by Myzus persicae. This contrasts with previously demonstrated, more limited changes required for restoration of transmission by Aphis gossypii. A construct with modified amino acids 129, 162, and 168 was efficiently transmitted by A. gossypii, while remaining poorly transmissible by M. persicae. Transmission of Fny-CMV by single aphids of A. gossypii and M. persicae was 54.0% and 27.9%, respectively. Single coat protein amino acid changes in Fny-CMV dramatically reduced transmission by M. persicae, while showing little effect on transmission by A. gossypii.

A cucumber necrosis virus variant deficient in fungal transmissibility contains an altered coat protein shell domain

Little is currently known regarding the specific interactions that govern transmission of plant viruses by their vectors. A cucumber necrosis virus (CNV) variant (LL5) deficient in fungal transmissibility has been isolated from mechanically passaged CNV and characterized. Although LL5 accumulates to wild-type (WT) levels, is capable of rapid systemic infection, and produces stable, highly infectious particles, it is only inefficiently transmitted by Olpidium bornovanus zoospores. The LL5 coat protein (CP) gene was amplified by RT-PCR and cloned in place of the WT CNV CP gene in an infectious CNV cDNA clone. Particles derived from this construct also failed to be efficiently transmitted. The LL5 CP gene was sequenced and found to contain two amino acid substitutions relative to WT CNV CP. One substitution (Phe to Cys) occurred in the arm region and another (Glu to Lys) in the shell domain. These amino acid changes were separately introduced into the WT CNV genome through in vitro mutagenesis and it was found that the Glu to Lys change in the LL5 CP shell domain is largely responsible for the loss of transmissibility. In vitro binding assays were developed to determine if the defect in transmissibility was due to a defect in binding zoospores. LL5 particles were found to bind less efficiently than WT CNV. Furthermore, the nontransmissible tomato bushy stunt virus did not detectably bind zoospores. These binding studies suggest that the specificity of CNV transmission by O. bornovanus occurs through specific recognition of a putative zoospore receptor.

A specific interaction between coat protein and helper component correlates with aphid transmission of a potyvirus

Specific binding between the coat protein (CP) and the helper component (HC) of the tobacco vein mottling potyvirus (TVMV) was characterized using a protein blotting-overlay protocol. In this in vitro assay, HC interacted with either virions or CP monomers originating from the aphid-transmissible TVMV-AT but not from the non-aphid-transmissible TVMV-NAT. There was a strong correlation between the aphid transmissibility of a series of TVMV variants having mutations in the DAG motif of the CP and their ability to bind HC. Expression of TVMV CP derivatives in bacteria allowed a precise determination of the minimum domain mediating HC binding. This domain is composed of seven amino acids, including the DAG motif (DTVDAGK), located in the N-terminus of the TVMV CP at amino acid positions 2 to 8.

Barley yellow dwarf viruses

Barley yellow dwarf viruses represent one of the most economically important and ubiquitous groups of plant viruses. This review focuses primarily on four research areas in which progress has been most rapid. These include (a) evidence supporting reclassification of BYDVs into two genera; (b) elucidation of gene function and novel mechanisms controlling gene expression; (c) initial forays into understanding the complex interactions between BYDV virions and their aphid vectors; and (d) replication of a BYDV satellite RNA. Economic losses, symptomatology, and means of control of BYD are also discussed.

Mechanisms of plant virus evolution

Plant viruses utilize several mechanisms to generate the large amount of genetic diversity found both within and between species. Plant RNA viruses and pararetroviruses probably have highly error prone replication mechanisms, that result in numerous mutations and a quasispecies nature. The plant DNA viruses also exhibit diversity, but the source of this is less clear. Plant viruses frequently use recombination and reassortment as driving forces in evolution, and, occasionally, other mechanisms such as gene duplication and overprinting. The amount of variation found in different species of plant viruses is remarkably different, even though there is no evidence that the mutation rate varies. The origin of plant viruses is uncertain, but several possible theories are proposed. The relationships between some plant and animal viruses suggests a common origin, possibly an insect virus. The propensity for rapid adaptation makes tracing the evolutionary history of viruses difficult, and long term control of virus disease nearly impossible, but it provides an excellent model system for studying general mechanisms of molecular evolution.