Anna E. Whitfield

Insect Vector Interactions with Persistently Transmitted Viruses

The majority of described plant viruses are transmitted by insects of the Hemipteroid assemblage that includes aphids, whiteflies, leafhoppers, planthoppers, and thrips. In this review we highlight progress made in research on vector interactions of the more than 200 plant viruses that are transmitted by hemipteroid insects beginning a few hours or days after acquisition and for up to the life of the insect, i.e., in a persistent-circulative or persistent-propagative mode. These plant viruses move through the insect vector, from the gut lumen into the hemolymph or other tissues and finally into the salivary glands, from which these viruses are introduced back into the plant host during insect feeding. The movement and/or replication of the viruses in the insect vectors require specific interactions between virus and vector components. Recent investigations have resulted in a better understanding of the replication sites and tissue tropism of several plant viruses that propagate in insect vectors. Furthermore, virus and insect proteins involved in overcoming transmission barriers in the vector have been identified for some virus-vector combinations.

Taxonomy of the order Mononegavirales: update 2016

In 2016, the order Mononegavirales was emended through the addition of two new families (Mymonaviridae and Sunviridae), the elevation of the paramyxoviral subfamily Pneumovirinae to family status (Pneumoviridae), the addition of five free-floating genera (Anphevirus, Arlivirus, Chengtivirus, Crustavirus, and Wastrivirus), and several other changes at the genus and species levels. This article presents the updated taxonomy of the order Mononegavirales as now accepted by the International Committee on Taxonomy of Viruses (ICTV).

Cellular and Molecular Aspects of Rhabdovirus Interactions with Insect and Plant Hosts

The rhabdoviruses form a large family (Rhabdoviridae) whose host ranges include humans, other vertebrates, invertebrates, and plants. There are at least 90 plant-infecting rhabdoviruses, several of which are economically important pathogens of various crops. All definitive plant-infecting and many vertebrate-infecting rhabdoviruses are persistently transmitted by insect vectors, and a few putative plant rhabdoviruses are transmitted by mites. Plant rhabdoviruses replicate in their plant and arthropod hosts, and transmission by vectors is highly specific, with each virus species transmitted by one or a few related insect species, mainly aphids, leafhoppers, or planthoppers. Here, we provide an overview of plant rhabdovirus interactions with their insect hosts and of how these interactions compare with those of vertebrate-infecting viruses and with the Sigma rhabdovirus that infects Drosophila flies. We focus on cellular and molecular aspects of vector/host specificity, transmission barriers, and virus receptors in the vectors. In addition, we briefly discuss recent advances in understanding rhabdovirus-plant interactions.

Insect vector-mediated transmission of plant viruses

The majority of plant-infecting viruses are transmitted to their host plants by vectors. The interactions between viruses and vector vary in duration and specificity but some common themes in vector transmission have emerged: 1) plant viruses encode structural proteins on the surface of the virion that are essential for transmission, and in some cases additional non-structural helper proteins that act to bridge the virion to the vector binding site; 2) viruses bind to specific sites in or on vectors and are retained there until they are transmitted to their plant hosts; and 3) viral determinants of vector transmission are promising candidates for translational research aimed at disrupting transmission or decreasing vector populations. In this review, we focus on well-characterized insect vector-transmitted viruses in the following genera: Caulimovirus, Crinivirus, Luteovirus, Geminiviridae, Reovirus, Tospovirus, and Tenuivirus. New discoveries regarding these genera have increased our understanding of the basic mechanisms of virus transmission by arthropods, which in turn have enabled the development of innovative strategies for breaking the transmission cycle.

Thrips as vectors of tospoviruses

Publisher Summary This chapter reviews the thrips– tospovirus pathosystem and the cellular and molecular determinants of thrips acquisition of tospoviruses. Viruses in the genus Tospovirus (family Bunyaviridae) are transmitted by thrips and have become an ever increasing problem for the producers of agricultural and horticultural crops worldwide. The genus Tospovirus is the genus within the Bunyaviridae containing plant-infecting viruses. Tomato spotted wilt virus ( TSWV) is the type species of this genus. Thrips cause significant direct damage to plants, but it is their transmission of tospoviruses that is most difficult to control and frequently causes the most severe damage to crops. At least ten species of thrips transmit tospoviruses, all of which are in the Thysanopteran family Thripidae. Most thrips vector species deposit their eggs into plant tissue and the eggs hatch after 2–3 days, depending on temperature and plant host. For tospoviruses to be transmitted by thrips, they must be acquired by the larvae. Thus, only immature thrips that acquire tospoviruses or adults arising from such immatures are important to the transmission of the virus. This concept is extremely important in managing tospoviruses, because only the plants that serve as hosts for both the insect and the virus are important in epidemics.

Variation in Tomato spotted wilt virus titer in Frankliniella occidentalis and its association with frequency of transmission.

Tomato spotted wilt virus (TSWV) is transmitted in a persistent propagative manner by Frankliniella occidentalis, the western flower thrips. While it is well established that vector competence depends on TSWV acquisition by young larvae and virus replication within the insect, the biological factors associated with frequency of transmission have not been well characterized. We hypothesized that the number of transmission events by a single adult thrips is determined, in part, by the amount of virus harbored (titer) by the insect. Transmission time-course experiments were conducted using a leaf disk assay to determine the efficiency and frequency of TSWV transmission following 2-day inoculation access periods (IAPs). Virus titer in individual adult thrips was determined by real-time quantitative reverse transcriptase-PCR (qRT-PCR) at the end of the experiments. On average, 59% of adults transmitted the virus during the first IAP (2 to 3 days post adult-eclosion). Male thrips were more efficient at transmitting TSWV multiple times compared with female thrips of the same cohort. However, females harbored two to three times more copies of TSWV-N RNA per insect, indicating that factors other than absolute virus titer in the insect contribute to a successful transmission event. Examination of virus titer in individual insects at the end of the third IAP (7 days post adult-eclosion) revealed significant and consistent positive associations between frequency of transmission and virus titer. Our data support the hypothesis that a viruliferous thrips is more likely to transmit multiple times if it harbors a high titer of virus. This quantitative relationship provides new insights into the biological parameters that may influence the spread of TSWV by thrips.

Plant-virus interactions and the agro-ecological interface

As a result of human activities, an ever-increasing portion of Earth’s natural landscapes now lie adjacent to agricultural lands. This border between wild and agricultural communities represents an agro-ecological interface, which may be populated with crop plants, weeds of crop systems, and non-crop plants that vary from exotic to native in origin. Plant viruses are important components of the agro-ecological interface because of their ubiquity, dispersal by arthropod vectors, and ability to colonize both crop and wild species. Here we provide an overview of research on plant-virus dynamics across this interface and suggest three research priorities: (1) an increased effort to identify and describe plant virus diversity and distribution in its entirety across agricultural and ecological boundaries; (2) multi-scale studies of virus transmission to develop predictive power in estimating virus propagation across landscapes; and (3) quantitative evaluation of the influence of viruses on plant fitness and populations in environmental contexts beyond crop fields. We close by emphasizing that agro-ecological interfaces are dynamic, influenced by the human-mediated redistribution of plants, vectors, and viruses around the world, climate change, and the development of new crops. Consideration of virus interactions within these environmentally complex systems promises new insight into virus, plant, and vector dynamics from molecular mechanisms to ecological consequences.

Thrips transmission of tospoviruses.

One hundred years ago, the disease tomato spotted wilt was first described in Australia. Since that time, knowledge of this disease caused by Tomato spotted wilt virus (TSWV) and transmitted by thrips (insects in the order Thysanoptera) has revealed a complex relationship between the virus, vector, plant host, and environment. Numerous tospoviruses and thrips vectors have been described, revealing diversity in plant host range and geographical distributions. Advances in characterization of the tripartite interaction between the virus, vector, and plant host have provided insight into molecular and ecological relationships. Comparison to animal-infecting viruses in the family Bunyaviridae has enabled the identification of commonalities between tospoviruses and other bunyaviruses in transmission by arthropod vectors and molecular interactions with hosts. This review provides a special emphasis on TSWV and Frankliniella occidentalis, the model tospovirus-thrips pathosystem. However, other virus-vector combinations are also of importance and where possible, comparisons are made between different viruses and thrips vectors.

Dichorhavirus: a proposed new genus for Brevipalpus mite-transmitted, nuclear, bacilliform, bipartite, negative-strand RNA plant viruses

Abstract Orchid fleck virus (OFV) is an unassigned negative-sense, single-stranded (−)ssRNA plant virus that was previously suggested to be included in the family Rhabdoviridae, order Mononegavirales. Although OFV shares some biological characteristics, including nuclear cytopathological effects, gene order, and sequence similarities, with nucleorhabdoviruses, its taxonomic status is unclear because unlike all mononegaviruses, OFV has a segmented genome and its particles are not enveloped. This article analyses the available biological, physico-chemical, and nucleotide sequence evidence that seems to indicate that OFV and several other Brevipalpus mite-transmitted short bacilliform (−)ssRNA viruses are likely related and may be classified taxonomically in novel species in a new free-floating genus Dichorhavirus.

Expression and Characterization of a Soluble Form of Tomato Spotted Wilt Virus Glycoprotein GN

ABSTRACT Tomato spotted wilt virus (TSWV), a member of the Tospovirus genus within the Bunyaviridae, is an economically important plant pathogen with a worldwide distribution. TSWV is transmitted to plants via thrips (Thysanoptera: Thripidae), which transmit the virus in a persistent propagative manner. The envelope glycoproteins, GN and GC, are critical for the infection of thrips, but they are not required for the initial infection of plants. Thus, it is assumed that the envelope glycoproteins play important roles in the entry of TSWV into the insect midgut, the first site of infection. To directly test the hypothesis that GN plays a role in TSWV acquisition by thrips, we expressed and purified a soluble, recombinant form of the GN protein (GN-S). The expression of GN-S allowed us to examine the function of GN in the absence of other viral proteins. We detected specific binding to thrips midguts when purified GN-S was fed to thrips in an in vivo binding assay. The TSWV nucleocapsid protein and human cytomegalovirus glycoprotein B did not bind to thrips midguts, indicating that the GN-S-thrips midgut interaction is specific. TSWV acquisition inhibition assays revealed that thrips that were concomitantly fed purified TSWV and GN-S had reduced amounts of virus in their midguts compared to thrips that were fed TSWV only. Our findings that GN-S binds to larval thrips guts and decreases TSWV acquisition provide evidence that GN may serve as a viral ligand that mediates the attachment of TSWV to receptors displayed on the epithelial cells of the thrips midgut.