A single amino acid position in the helper component of cauliflower mosaic virus can change the spectrum of transmitting vector species

The cauliflower mosaic virus transmission helper protein P2 modifies directly the probing behavior of the aphid vector Myzus persicae to facilitate transmission

There is growing evidence that plant viruses manipulate their hosts and vectors in ways that increase transmission. However, to date only few viral components underlying these phenomena have been identified. Here we show that cauliflower mosaic virus (CaMV) protein P2 modifies the feeding behavior of its aphid vector. P2 is necessary for CaMV transmission because it mediates binding of virus particles to the aphid mouthparts. We compared aphid feeding behavior on plants infected with the wild-type CaMV strain Cabb B-JI or with a deletion mutant strain, Cabb B-JIΔP2, which does not produce P2. Only aphids probing Cabb B-JI infected plants doubled the number of test punctures during the first contact with the plant, indicating a role of P2. Membrane feeding assays with purified P2 and virus particles confirmed that these viral products alone are sufficient to cause the changes in aphid probing. The behavior modifications were not observed on plants infected with a CaMV mutant expressing P2Rev5, unable to bind to the mouthparts. These results are in favor of a virus manipulation, where attachment of P2 to a specific region in the aphid stylets-the acrostyle-exercises a direct effect on vector behavior at a crucial moment, the first vector contact with the infected plant, which is essential for virus acquisition.

Cauliflower mosaic virus protein P6-TAV plays a major role in alteration of aphid vector feeding behaviour but not performance on infected Arabidopsis

Emerging evidence suggests that viral infection modifies host plant traits that in turn alter behaviour and performance of vectors colonizing the plants in a way conducive for transmission of both nonpersistent and persistent viruses. Similar evidence for semipersistent viruses like cauliflower mosaic virus (CaMV) is scarce. Here we compared the effects of Arabidopsis infection with mild (CM) and severe (JI) CaMV isolates on the feeding behaviour (recorded by the electrical penetration graph technique) and fecundity of the aphid vector Myzus persicae. Compared to mock-inoculated plants, feeding behaviour was altered similarly on CM- and JI-infected plants, but only aphids on JI-infected plants had reduced fecundity. To evaluate the role of the multifunctional CaMV protein P6-TAV, aphid feeding behaviour and fecundity were tested on transgenic Arabidopsis plants expressing wild-type (wt) and mutant versions of P6-TAV. In contrast to viral infection, aphid fecundity was unchanged on all transgenic lines, suggesting that other viral factors compromise fecundity. Aphid feeding behaviour was modified on wt P6-CM-, but not on wt P6-JI-expressing plants. Analysis of plants expressing P6 mutants identified N-terminal P6 domains contributing to modification of feeding behaviour. Taken together, we show that CaMV infection can modify both aphid fecundity and feeding behaviour and that P6 is only involved in the latter.

The N-terminus of the cauliflower mosaic virus aphid transmission protein P2 is involved in transmission body formation and microtubule interaction

Cauliflower mosaic virus (CaMV) is transmitted by aphids using the non-circulative transmission mode: when the insects feed on infected leaves, virus particles from infected cells attach rapidly to their stylets and are transmitted to a new host when the aphids change plants. Mandatory for CaMV transmission, the viral helper protein P2 mediates as a molecular linker binding of the virus particles to the aphid stylets. P2 is available in infected plant cells in a viral inclusion that is specialized for transmission and named the transmission body (TB). When puncturing an infected leaf cell, the aphid triggers an ultra-rapid viral response, necessary for virus acquisition and called transmission activation: The TB disrupts and P2 is redistributed onto cortical microtubules, together with virus particles that are simultaneously set free from virus factories and join P2 on the microtubules to form the so-called mixed networks (MNs). The MNs are the predominant structure from which CaMV is acquired by aphids. However, the P2 domains involved in microtubule interaction are not known. To identify P2 regions involved in its functions, we generated a set of P2 mutants by alanine scanning and analyzed them in the viral context for their capacity to form a TB, to interact with microtubules and to transmit CaMV. Our results show that contrary to the previously characterized P2-P2 and P2-virion binding sites in its C-terminus, the microtubule binding site is contained in the N-terminal half of P2. Further, this region is important for TB formation since some P2 mutant proteins did not accumulate in TBs but were retained in the viral factories where P2 is translated. Taken together, the N-terminus of P2 is not only involved in vector interaction as previously reported, but also in interaction with microtubules and in formation of TBs.

Route of a Multipartite Nanovirus across the Body of Its Aphid Vector

Vector transmission plays a primary role in the life cycle of viruses, and insects are the most common vectors. An important mode of vector transmission, reported only for plant viruses, is circulative nonpropagative transmission whereby the virus cycles within the body of its insect vector, from gut to salivary glands and saliva, without replicating. This mode of transmission has been extensively studied in the viral families Luteoviridae and Geminiviridae and is also reported for Nanoviridae The biology of viruses within these three families is different, and whether the viruses have evolved similar molecular/cellular virus-vector interactions is unclear. In particular, nanoviruses have a multipartite genome organization, and how the distinct genome segments encapsidated individually transit through the insect body is unknown. Here, using a combination of fluorescent in situ hybridization and immunofluorescence, we monitor distinct proteins and genome segments of the nanovirus Faba bean necrotic stunt virus (FBNSV) during transcytosis through the gut and salivary gland cells of its aphid vector Acyrthosiphon pisum FBNSV specifically transits through cells of the anterior midgut and principal salivary gland cells, a route similar to that of geminiviruses but distinct from that of luteoviruses. Our results further demonstrate that a large number of virus particles enter every single susceptible cell so that distinct genome segments always remain together. Finally, we confirm that the success of nanovirus-vector interaction depends on a nonstructural helper component, the viral protein nuclear shuttle protein (NSP), which is shown to be mandatory for viral accumulation within gut cells.IMPORTANCE An intriguing mode of vector transmission described only for plant viruses is circulative nonpropagative transmission, whereby the virus passes through the gut and salivary glands of the insect vector without replicating. Three plant virus families are transmitted this way, but details of the molecular/cellular mechanisms of the virus-vector interaction are missing. This is striking for nanoviruses that are believed to interact with aphid vectors in ways similar to those of luteoviruses or geminiviruses but for which empirical evidence is scarce. We here confirm that nanoviruses follow a within-vector route similar to that of geminiviruses but distinct from that of luteoviruses. We show that they produce a nonstructural protein mandatory for viral entry into gut cells, a unique phenomenon for this mode of transmission. Finally, noting that nanoviruses are multipartite viruses, we demonstrate that a large number of viral particles penetrate susceptible cells of the vector, allowing distinct genome segments to remain together.

Cuticular Structure Proteomics in the Pea Aphid Acyrthosiphon pisum Reveals New Plant Virus Receptor Candidates at the Tip of Maxillary Stylets

Aphids are phloem-feeding insects known as major pests in agriculture that are able to transmit hundreds of plant viruses. The majority of these viruses, classified as noncirculative, are retained and transported on the inner surface of the cuticle of the needle-like mouthparts while the aphids move from plant to plant. Identification of receptors of viruses within insect vectors is a key challenge because they are promising targets for alternative control strategies. The acrostyle, an organ discovered earlier within the common food/salivary canal at the tip of aphid maxillary stylets, displays proteins at the cuticle-fluid interface, some of which are receptors of noncirculative viruses. To assess the presence of stylet- and acrostyle-specific proteins and identify putative receptors, we have developed a comprehensive comparative analysis of the proteomes of four cuticular anatomical structures of the pea aphid, stylets, antennae, legs, and wings. In addition, we performed systematic immunolabeling detection of the cuticular proteins identified by mass spectrometry in dissected stylets. We thereby establish the first proteome of stylets of an insect and determine the minimal repertoire of the cuticular proteins composing the acrostyle. Most importantly, we propose a short list of plant virus receptor candidates, among which RR-1 proteins are remarkably predominant. The data are available via ProteomeXchange (PXD016517).

The Use of Engineered Plant Viruses in a Trans-Kingdom Silencing Strategy Against Their Insect Vectors

Plants, plant viruses, and their vectors are co-evolving actors that co-exist and interact in nature. Insects are the most important vectors of plant viruses, serving as both carriers and hosts for the virus. This trans-kingdom interaction can be harnessed for the production of recombinant plant viruses designed to target insect genes via the RNAi machinery. The selection of the adequate viruses is important since they must infect and preferentially replicate in both the host plant and the insect vector. The routes of transmission that determine the extent of the infection inside the insect vary among different plant viruses. In the context of the proposed strategy, plant viruses that are capable of transversing the insect gut-hemocoel barrier and replicating in insect tissues are attractive candidates. Thus, the transmission of such viruses in a persistent and propagative manner is considered as a prerequisite for this strategy to be feasible, a characteristic that is found in viruses from the families Bunyaviridae, Reoviridae, and Rhabdoviridae. In addition, several RNA viruses are known that replicate in both plant and insect tissues via a yet unclarified transmission route. In this review, advances in knowledge of trans-kingdom transmission of plant viruses and future perspectives for their engineering as silencing vectors are thoroughly discussed.

Molecular dynamics investigations for the prediction of molecular interaction of cauliflower mosaic virus transmission helper component protein complex with Myzus persicae stylet's cuticular protein and its docking studies with annosquamosin-A encapsulated in nano-porous Silica

Large numbers of bioactive natural products from plant species such as alkaloids, phenolics, terpenoids etc. are remaining unexplored for their potential as plant protective agents as inhibitors for viral and other pathogenic infections of plant. Myzus aphids are important plant pests and vectors for several plant viruses. Cauliflower mosaic virus (CaMV) belongs to the plant virus family Caulimoviridae which is transmitted "non-circulative" from plant to plant through an interaction with aphid insect vectors. This viral transmission process most likely involves a protein-protein binding interaction between aphid stylet receptor cuticular protein and viral proteins namely, CaMV aphid transmission Helper Component protein and virion associated protein. Aphid stylets are made of cuticle and little is known about the structure of cuticle protein of this insect group. The present study reports the molecular modeling of the structures of Myzus persicae aphid stylet's cuticular protein (MpsCP) and cauliflower mosaic virus aphid transmission Helper component protein (CaMV HCP). Protein-protein docking studies and molecular dynamics simulations are performed to establish the mode of binding of MpsCP with CaMV HCP. Molecular docking and molecular dynamics investigations of terpenoids Annosquamosin-A from Annona squamosa complex with CaMV transmitting aphid M. persicae stylet's cuticular protein revealed their means of interaction perhaps relates to restrain viral binding and transmission. QM/MM optimization of mesoporous silica nanopores composite with Annosquamosin-A for smart and safe delivery of bioactive is carried out to study their electronic parameters such as heat of formation, total energy, electronic energy, Ionization potential, Highest Occupied Molecular Orbital, Lowest Un-occupied Molecular Orbital and energy gaps.

Identification of Plant Virus Receptor Candidates in the Stylets of Their Aphid Vectors

Plant viruses transmitted by insects cause tremendous losses in most important crops around the world. The identification of receptors of plant viruses within their insect vectors is a key challenge to understanding the mechanisms of transmission and offers an avenue for future alternative control strategies to limit viral spread. We here report the identification of two cuticular proteins within aphid mouthparts, and we provide experimental support for the role of one of them in the transmission of a noncirculative virus. These two proteins, named Stylin-01 and Stylin-02, belong to the RR-1 cuticular protein subfamily and are highly conserved among aphid species. Using an immunolabeling approach, they were localized in the maxillary stylets of the pea aphid Acyrthosiphon pisum and the green peach aphid Myzus persicae, in the acrostyle, an organ earlier shown to harbor receptors of a noncirculative virus. A peptide motif present at the C termini of both Stylin-01 and Stylin-02 is readily accessible all over the surface of the acrostyle. Competition for in vitro binding to the acrostyle was observed between an antibody targeting this peptide and the helper component protein P2 of Cauliflower mosaic virus Furthermore, silencing the stylin-01 but not stylin-02 gene through RNA interference decreased the efficiency of Cauliflower mosaic virus transmission by Myzus persicae These results identify the first cuticular proteins ever reported within arthropod mouthparts and distinguish Stylin-01 as the best candidate receptor for the aphid transmission of noncirculative plant viruses.IMPORTANCE Most noncirculative plant viruses transmitted by insect vectors bind to their mouthparts. They are acquired and inoculated within seconds when insects hop from plant to plant. The receptors involved remain totally elusive due to a long-standing technical bottleneck in working with insect cuticle. Here we characterize the role of the two first cuticular proteins ever identified in arthropod mouthparts. A domain of these proteins is directly accessible at the surface of the cuticle of the acrostyle, an organ at the tip of aphid stylets. The acrostyle has been shown to bind a plant virus, and we consistently demonstrated that one of the identified proteins is involved in viral transmission. Our findings provide an approach to identify proteins in insect mouthparts and point at an unprecedented gene candidate for a plant virus receptor.

Insect transmission of plant viruses: Multilayered interactions optimize viral propagation

By serving as vectors of transmission, insects play a key role in the infection cycle of many plant viruses. Viruses use sophisticated transmission strategies to overcome the spatial barrier separating plants and the impediment imposed by the plant cell wall. Interactions among insect vectors, viruses, and host plants mediate transmission by integrating all organizational levels, from molecules to populations. Best-examined on the molecular scale are two basic transmission modes wherein virus-vector interactions have been well characterized. Whereas association of virus particles with specific sites in the vector's mouthparts or in alimentary tract regions immediately posterior to them is required for noncirculative transmission, the cycle of particles through the vector body is necessary for circulative transmission. Virus transmission is also determined by interactions that are associated with changes in vector feeding behaviors and with alterations in plant host's morphology and/or metabolism that favor the attraction or deterrence of vectors. A recent concept in virus-host-vector interactions proposes that when vectors land on infected plants, vector elicitors and effectors "inform" the plants of the confluence of interacting entities and trigger signaling pathways and plant defenses. Simultaneously, the plant responses may also influence virus acquisition and inoculation by vectors. Overall, a picture is emerging where transmission depends on multilayered virus-vector-host interactions that define the route of a virus through the vector, and on the manipulation of the host and the vector. These interactions guarantee virus propagation until one or more of the interactants undergo changes through evolution or are halted by environmental interventions.

Localizing viruses in their insect vectors

The mechanisms and impacts of the transmission of plant viruses by insect vectors have been studied for more than a century. The virus route within the insect vector is amply documented in many cases, but the identity, the biochemical properties, and the structure of the actual molecules (or molecule domains) ensuring compatibility between them remain obscure. Increased efforts are required both to identify receptors of plant viruses at various sites in the vector body and to design competing compounds capable of hindering transmission. Recent trends in the field are opening questions on the diversity and sophistication of viral adaptations that optimize transmission, from the manipulation of plants and vectors ultimately increasing the chances of acquisition and inoculation, to specific "sensing" of the vector by the virus while still in the host plant and the subsequent transition to a transmission-enhanced state.

An experimental test of the transmission-virulence trade-off hypothesis in a plant virus

The transmission-virulence trade-off hypothesis is one of the few adaptive explanations of virulence evolution, and assumes that there is an overall positive correlation between parasite transmission and virulence. The shape of the transmission-virulence relationship predicts whether virulence should evolve toward either a maximum or to an intermediate optimum. A positive correlation between each of these traits and within-host growth is often suggested to underlie the relationship between virulence and transmission. There are few experimental tests of this hypothesis; this study reports on the first empirical test on a plant pathogen. We infected Brassica rapa plants with nine natural isolates of Cauliflower mosaic virus and then estimated three traits: transmission, virulence, and within-host viral accumulation. As predicted by the trade-off hypothesis, we observed a positive correlation between transmission and virulence, suggestive of the existence of an intermediate optimum. We discovered the unexpected existence of two groups of within-host accumulation, differing by at least an order of magnitude. When accumulation groups were not accounted for, within-host accumulation was correlated neither to virulence nor transmission, although our results suggest that within each group these correlations exist.

Co-infection patterns and geographic distribution of a complex pathosystem targeted by pathogen-resistant plants

Increasingly, pathogen-resistant (PR) plants are being developed to reduce the agricultural impacts of disease. However PR plants also have the potential to result in increased invasiveness of nontarget host populations and so pose a potential threat to nontarget ecosystems. In this paper we use a new framework to investigate geographical variation in the potential risk associated with unintended release of genetically modified alfalfa mosaic virus (AMV)-resistant Trifolium repens (white clover) into nontarget host populations containing AMV, clover yellow vein virus (ClYVV), and white clover mosaic virus (WCIMV) in southeastern Australia. Surveys of 213 sites in 37 habitat types over a 300 000-km2 study region showed that T. repens is a significant weed of many high-conservation-value habitats in southeastern Australia and that AMV, ClYVV, and WClMV occur in 15-97% of nontarget host populations. However, T. repens abundance varied with site disturbance, habitat conservation value, and proximity to cropping, and all viral pathogens had distinct geographic distributions and infection patterns. Virus species frequently co-infected host plants and displayed nonindependent distributions within host populations, although co-infection patterns varied across the study region. Our results clearly illustrate the complexity of conducting environmental risk assessments that involve geographically widespread, invasive pasture species and demonstrate the general need for targeted, habitat- and pathosystem-specific studies prior to the process of tiered risk assessment.

Structural insights into viral determinants of nematode mediated Grapevine fanleaf virus transmission

Many animal and plant viruses rely on vectors for their transmission from host to host. Grapevine fanleaf virus (GFLV), a picorna-like virus from plants, is transmitted specifically by the ectoparasitic nematode Xiphinema index. The icosahedral capsid of GFLV, which consists of 60 identical coat protein subunits (CP), carries the determinants of this specificity. Here, we provide novel insight into GFLV transmission by nematodes through a comparative structural and functional analysis of two GFLV variants. We isolated a mutant GFLV strain (GFLV-TD) poorly transmissible by nematodes, and showed that the transmission defect is due to a glycine to aspartate mutation at position 297 (Gly297Asp) in the CP. We next determined the crystal structures of the wild-type GFLV strain F13 at 3.0 Å and of GFLV-TD at 2.7 Å resolution. The Gly297Asp mutation mapped to an exposed loop at the outer surface of the capsid and did not affect the conformation of the assembled capsid, nor of individual CP molecules. The loop is part of a positively charged pocket that includes a previously identified determinant of transmission. We propose that this pocket is a ligand-binding site with essential function in GFLV transmission by X. index. Our data suggest that perturbation of the electrostatic landscape of this pocket affects the interaction of the virion with specific receptors of the nematode's feeding apparatus, and thereby severely diminishes its transmission efficiency. These data provide a first structural insight into the interactions between a plant virus and a nematode vector.

Numerous pathogenic viruses from animals and plants rely on vectors such as insects, worms or other organisms for their transmission from host to host. The reasons why certain vectors transmit some viruses but not others remain poorly understood. In plants, Grapevine fanleaf virus (GFLV), a major pathogen of grapes worldwide and its specific vector, the dagger nematode Xiphinema index, provides a well-established model illustrating this specificity. Here, we determined the high-resolution structures of two GFLV isolates that differ in their transmissibility. We show that this difference is due to a single mutation in a region exposed at the outer surface of the viral particles. This mutation does not alter the conformation of the particles but modifies the distribution of charges within a positively-charged pocket at the outer surface of virions which likely affects particle retention by X. index and, thereby also transmission efficiency. Therefore, we propose that this pocket is involved in the specific recognition of GFLV by its nematode vector. This work paves the way towards the characterization of the specific compound(s) within the nematodes that trigger vector specificity and provides novel perspectives to interfere with virus transmission.

Dynamics of the multiplicity of cellular infection in a plant virus

Recombination, complementation and competition profoundly influence virus evolution and epidemiology. Since viruses are intracellular parasites, the basic parameter determining the potential for such interactions is the multiplicity of cellular infection (cellular MOI), i.e. the number of viral genome units that effectively infect a cell. The cellular MOI values that prevail in host organisms have rarely been investigated, and whether they remain constant or change widely during host invasion is totally unknown. Here, we fill this experimental gap by presenting the first detailed analysis of the dynamics of the cellular MOI during colonization of a host plant by a virus. Our results reveal ample variations between different leaf levels during the course of infection, with values starting close to 2 and increasing up to 13 before decreasing to initial levels in the latest infection stages. By revealing wide dynamic changes throughout a single infection, we here illustrate the existence of complex scenarios where the opportunity for recombination, complementation and competition among viral genomes changes greatly at different infection phases and at different locations within a multi-cellular host.

Viruses are fast evolving organisms for which changes in fitness and virulence are driven by interactions between genomes such as recombination, functional complementation, and competition. Viruses being intra-cellular parasites, one basic parameter determines the potential for such interactions: the cellular multiplicity of infection (cellular MOI), defined as the number of genome units actually penetrating and co-replicating within individual cells of the host. Despite its importance for virus evolution, this trait has scarcely been investigated. For example, there are only three point estimates for eukaryote-infecting viruses while the possibility that the cellular MOI may vary during the infection or across organs of a given host individual has never been conclusively addressed. By monitoring the cellular MOI in plants infected by the Cauliflower mosaic virus we found remarkably ample variations during the development of the infection process in successive leaf levels. Our results reveal that the opportunities for recombination, complementation and competition among viral genomes can greatly change at different infection phases and at different locations within a multi-cellular host.

A stretch of 11 amino acids in the betaB-betaC loop of the coat protein of grapevine fanleaf virus is essential for transmission by the nematode Xiphinema index

Grapevine fanleaf virus (GFLV) and Arabis mosaic virus (ArMV) from the genus Nepovirus, family Secoviridae, cause a severe degeneration of grapevines. GFLV and ArMV have a bipartite RNA genome and are transmitted specifically by the ectoparasitic nematodes Xiphinema index and Xiphinema diversicaudatum, respectively. The transmission specificity of both viruses maps to their respective RNA2-encoded coat protein (CP). To further delineate the GFLV CP determinants of transmission specificity, three-dimensional (3D) homology structure models of virions and CP subunits were constructed based on the crystal structure of Tobacco ringspot virus, the type member of the genus Nepovirus. The 3D models were examined to predict amino acids that are exposed at the external virion surface, highly conserved among GFLV isolates but divergent between GFLV and ArMV. Five short amino acid stretches that matched these topographical and sequence conservation criteria were selected and substituted in single and multiple combinations by their ArMV counterparts in a GFLV RNA2 cDNA clone. Among the 21 chimeric RNA2 molecules engineered, transcripts of only three of them induced systemic plant infection in the presence of GFLV RNA1. Nematode transmission assays of the three viable recombinant viruses showed that swapping a stretch of (i) 11 residues in the betaB-betaC loop near the icosahedral 3-fold axis abolished transmission by X. index but was insufficient to restore transmission by X. diversicaudatum and (ii) 7 residues in the betaE-alphaB loop did not interfere with transmission by the two Xiphinema species. This study provides new insights into GFLV CP determinants of nematode transmission.

Aphids as transport devices for plant viruses

Plant viruses have evolved a wide array of strategies to ensure efficient transfer from one host to the next. Any organism feeding on infected plants and traveling between plants can potentially act as a virus transport device. Such organisms, designated vectors, are found among parasitic fungi, root nematodes and plant-feeding arthropods, particularly insects. Due to their extremely specialized feeding behavior - exploring and sampling all plant tissues, from the epidermis to the phloem and xylem - aphids are by far the most important vectors, transmitting nearly 30% of all plant virus species described to date. Several different interaction patterns have evolved between viruses and aphid vectors and, over the past century, a tremendous number of studies have provided details of the underlying mechanisms. This article presents an overview of the different types of virus-aphid relationships, state-of-the-art knowledge of the molecular processes underlying these interactions, and the remaining black boxes waiting to be opened in the near future.

Structural insights into the molecular mechanisms of cauliflower mosaic virus transmission by its insect vector

Cauliflower mosaic virus (CaMV) is transmitted from plant to plant through a seemingly simple interaction with insect vectors. This process involves an aphid receptor and two viral proteins, P2 and P3. P2 binds to both the aphid receptor and P3, itself tightly associated with the virus particle, with the ensemble forming a transmissible viral complex. Here, we describe the conformations of both unliganded CaMV P3 protein and its virion-associated form. X-ray crystallography revealed that the N-terminal domain of unliganded P3 is a tetrameric parallel coiled coil with a unique organization showing two successive four-stranded subdomains with opposite supercoiling handedness stabilized by a ring of interchain disulfide bridges. A structural model of virus-liganded P3 proteins, folding as an antiparallel coiled-coil network coating the virus surface, was derived from molecular modeling. Our results highlight the structural and biological versatility of this coiled-coil structure and provide new insights into the molecular mechanisms involved in CaMV acquisition and transmission by the insect vector.

Early progress in aphid genomics and consequences for plant-aphid interactions studies

Aphids occupy a niche comprising two conceptual realms: a micron-scale feeding site beneath the plant surface, in which a syringe-like appendage mediates chemical exchange with a specific plant cell type; and the larger realm of a metazoan with sensory organs, a nervous system, and behavior, all responsive to the condition of the host plant and the broader environment. The biology that connects these realms is not well understood, but new details are emerging with the help of genomic tools. The power of these tools is set to increase substantially now that the first genome of an aphid is being sequenced and annotated. This has been possible because a community of aphid researchers focused their efforts to develop and share genomic resources through an international consortium. This complete genome sequence, along with other resources, should permit major advances in understanding the complex and peculiar biological traits responsible for aphids' evolutionary success and their damaging effects on agriculture. This review highlights early progress in the application of aphid genomics and identifies key issues of plant-aphid interactions likely to benefit as molecular tools are further developed. Use of this new knowledge could make significant contributions to crop protection against these and other phloem-feeding insects.

A protein key to plant virus transmission at the tip of the insect vector stylet

Hundreds of species of plant viruses, many of them economically important, are transmitted by noncirculative vector transmission (acquisition by attachment of virions to vector mouthparts and inoculation by subsequent release), but virus receptors within the vector remain elusive. Here we report evidence for the existence, precise location, and chemical nature of the first receptor for a noncirculative virus, cauliflower mosaic virus, in its insect vector. Electron microscopy revealed virus-like particles in a previously undescribed anatomical zone at the extreme tip of the aphid maxillary stylets. A novel in vitro interaction assay characterized binding of cauliflower mosaic virus protein P2 (which mediates virus-vector interaction) to dissected aphid stylets. A P2-GFP fusion exclusively labeled a tiny cuticular domain located in the bottom-bed of the common food/salivary duct. No binding to stylets of a non-vector species was observed, and a point mutation abolishing P2 transmission activity correlated with impaired stylet binding. The novel receptor appears to be a nonglycosylated protein deeply embedded in the chitin matrix. Insight into such insect receptor molecules will begin to open the major black box of this scientific field and might lead to new strategies to combat viral spread.

Virus-vector interactions mediating nonpersistent and semipersistent transmission of plant viruses

Most plant viruses are absolutely dependent on a vector for plant-to-plant spread. Although a number of different types of organisms are vectors for different plant viruses, phloem-feeding Hemipterans are the most common and transmit the great majority of plant viruses. The complex and specific interactions between Hemipteran vectors and the viruses they transmit have been studied intensely, and two general strategies, the capsid and helper strategies, are recognized. Both strategies are found for plant viruses that are transmitted by aphids in a nonpersistent manner. Evidence suggests that these strategies are found also for viruses transmitted in a semipersistent manner. Recent applications of molecular and cell biology techniques have helped to elucidate the mechanisms underlying the vector transmission of several plant viruses. This review examines the fundamental contributions and recent developments in this area.