Insect transmission of plant viruses: Multilayered interactions optimize viral propagation

Leafhopper salivary carboxylesterase suppresses JA-Ile synthesis to facilitate initial arbovirus transmission in rice phloem

Plant jasmonoyl-L-isoleucine (JA-Ile) is a major defense signal against insect feeding, but whether or how insect salivary effectors suppress JA-Ile synthesis and thus facilitate viral transmission in the plant phloem remains elusive. Insect carboxylesterases (CarEs) are the third major family of detoxification enzymes. Here, we identify a new leafhopper CarE, CarE10, that is specifically expressed in salivary glands and is secreted into the rice phloem as a saliva component. Leafhopper CarE10 directly binds to rice jasmonate resistant 1 (JAR1) and promotes its degradation by the proteasome system. Moreover, the direct association of CarE10 with JAR1 clearly impairs JAR1 enzyme activity for conversion of JA to JA-Ile in an in vitro JA-Ile synthesis system. A devastating rice reovirus activates and promotes the co-secretion of virions and CarE10 via virus-induced vesicles into the saliva-storing salivary cavities of the leafhopper vector and ultimately into the rice phloem to establish initial infection. Furthermore, a virus-mediated increase in CarE10 secretion or overexpression of CarE10 in transgenic rice plants causes reduced levels of JAR1 and thus suppresses JA-Ile synthesis, promoting host attractiveness to insect vectors and facilitating initial viral transmission. Our findings provide insight into how the insect salivary protein CarE10 suppresses host JA-Ile synthesis to promote initial virus transmission in the rice phloem.

Environmental Factors and the Symbiont Cardinium Influence the Bacterial Microbiome of Spider Mites Across the Landscape

Microbes play a key role in the biology, ecology, and evolution of arthropods. Despite accumulating data on microbial communities in arthropods that feed on plants using piercing-sucking mouthparts, we still lack a comprehensive understanding of the composition and assembly factors of the microbiota, particularly in field-collected spider mites. Here, we applied 16S rRNA amplicon sequencing to investigate the characters of the bacterial community in 140 samples representing 420 mite individuals, belonging to eight Tetranychus species (Acari: Tetranychidae) collected from 26 sites in China. The results showed that the bacterial composition of spider mites varied significantly among different species, locations, and plants. The environment showed a significant influence on the bacterial community of spider mites, with different relative contributions. Latitude and precipitation were found to be the main factors influencing the bacterial community composition. The dissimilarity of bacterial community and geographical distance between mite locations were significantly correlated. The assembly of spider mite bacterial communities seemed to be mainly influenced by stochastic processes. Furthermore, the symbiont Cardinium was found to be important in shaping the microbiota of many Tetranychus species. The relative abundance of Cardinium was > 50% in T. viennensis, T. urticae G, T. urticae R, and T. turkestani. Removing Cardinium reads from our analysis significantly changed Shannon diversity index and weighted beta diversity in these species. Altogether, this study provides novel insights into bacterial diversity patterns that contribute to our knowledge of the symbiotic relationships between arthropods and their bacterial communities.

Visualisation of Bluetongue Virus in the Salivary Apparatus of Culicoides Biting Midges Highlights the Accessory Glands as a Primary Arboviral Infection Site

BACKGROUND

Arthropods transmit a wide range of pathogens of importance for the global health of humans, animals, and plants. One group of these arthropod vectors, Culicoides biting midges (Diptera: Ceratopogonidae), is the biological vector of several human and animal pathogens, including economically important livestock viruses like bluetongue virus (BTV). Like other arthropod-borne viruses (arboviruses), Culicoides-borne viruses must reach and replicate in the salivary apparatus, from where they can be transmitted to susceptible hosts through the saliva during subsequent blood feeding. Despite the importance of the salivary gland apparatus for pathogen transmission to susceptible animals from the bite of infected Culicoides, these structures have received relatively little attention, perhaps due to the small size and fragility of these vectors.

RESULTS

In this study, we developed techniques to visualize the infection of the salivary glands and other soft tissues with BTV, in some of the smallest known arbovirus vectors, Culicoides biting midges, using three-dimensional immunofluorescence confocal microscopy. We showed BTV infection of specific structures of the salivary gland apparatus of female Culicoides vectors following oral virus uptake, related visualisation of viral infection in the salivary apparatus to high viral RNA copies in the body, and demonstrated for the first time, that the accessory glands are a primary site for BTV replication within the salivary apparatus.

CONCLUSIONS

Our work has revealed a novel site of virus-vector interactions, and a novel role of the accessory glands of Culicoides in arbovirus amplification and transmission. Our approach would also be applicable to a wide range of arbovirus vector groups including sand flies (Diptera: Psychodidae), as well as provide a powerful tool to investigate arbovirus infection and dissemination, particularly where there are practical challenges in the visualization of small size and delicate tissues of arthropods.

A systematic review of southern rice black-streaked dwarf virus in the age of omics

Southern rice black-streaked dwarf virus (SRBSDV) is one of the most damaging rice viruses. The virus decreases rice quality and yield, and poses a serious threat to food security. From this perspective, this review performed a survey of published studies in recent years to understand the current status of SRBSDV and white-backed planthopper (WBPH, Sogatella furcifera) transmission processes in rice. Recent studies have shown that the interactions between viral virulence proteins and rice susceptibility factors shape the transmission of SRBSDV. Moreover, the transmission of SRBSDV is influenced by the interactions between viral virulence proteins and S. furcifera susceptibility factors. This review focused on the molecular mechanisms of key genes or proteins associated with SRBSDV infection in rice via the S. furcifera vector, and the host defense response mechanisms against viral infection. A sustainable control strategy using RNAi was summarized to address this pest. Finally, we also present a model for screening anti-SRBSDV inhibitors using viral proteins as targets. © 2023 Society of Chemical Industry.

Plant and insect virus-like particles: emerging nanoparticles for agricultural pest management

Virus-like particles (VLPs) represent a biodegradable, biocompatible nanomaterial made from viral coat proteins that can improve the delivery of antigens, drugs, nucleic acids, and other substances, with most applications in human and veterinary medicine. Regarding agricultural viruses, many insect and plant virus coat proteins have been shown to assemble into VLPs accurately. In addition, some plant virus-based VLPs have been used in medical studies. However, to our knowledge, the potential application of plant/insect virus-based VLPs in agriculture remains largely underexplored. This review focuses on why and how to engineer coat proteins of plant/insect viruses as functionalized VLPs, and how to exploit VLPs in agricultural pest control. The first part of the review describes four different engineering strategies for loading cargo at the inner or the outer surface of VLPs depending on the type of cargo and purpose. Second, the literature on plant and insect viruses the coat proteins of which have been confirmed to self-assemble into VLPs is reviewed. These VLPs are good candidates for developing VLP-based agricultural pest control strategies. Lastly, the concepts of plant/insect virus-based VLPs for delivering insecticidal and antiviral components (e.g., double-stranded RNA, peptides, and chemicals) are discussed, which provides future prospects of VLP application in agricultural pest control. In addition, some concerns are raised about VLP production on a large scale and the short-term resistance of hosts to VLP uptake. Overall, this review is expected to stimulate interest and research exploring plant/insect virus-based VLP applications in agricultural pest management. © 2023 Society of Chemical Industry.

An Aphid-Transmitted Virus Reduces the Host Plant Response to Its Vector to Promote Its Transmission

The success of virus transmission by vectors relies on intricate trophic interactions between three partners, the host plant, the virus, and the vector. Despite numerous studies that showed the capacity of plant viruses to manipulate their host plant to their benefit, and potentially of their transmission, the molecular mechanisms sustaining this phenomenon has not yet been extensively analyzed at the molecular level. In this study, we focused on the deregulations induced in Arabidopsis thaliana by an aphid vector that were alleviated when the plants were infected with turnip yellows virus (TuYV), a polerovirus strictly transmitted by aphids in a circulative and nonpropagative mode. By setting up an experimental design mimicking the natural conditions of virus transmission, we analyzed the deregulations in plants infected with TuYV and infested with aphids by a dual transcriptomic and metabolomic approach. We observed that the virus infection alleviated most of the gene deregulations induced by the aphids in a noninfected plant at both time points analyzed (6 and 72 h) with a more pronounced effect at the later time point of infestation. The metabolic composition of the infected and infested plants was altered in a way that could be beneficial for the vector and the virus transmission. Importantly, these substantial modifications observed in infected and infested plants correlated with a higher TuYV transmission efficiency. This study revealed the capacity of TuYV to alter the plant nutritive content and the defense reaction against the aphid vector to promote the viral transmission.

Long-Term Anthropogenic Management and Associated Loss of Plant Diversity Deeply Impact Virome Richness and Composition of Poaceae Communities

Modern agriculture has influenced plant virus emergence through ecosystem simplification, introduction of new host species, and reduction in crop genetic diversity. Therefore, it is crucial to better understand virus distributions across cultivated and uncultivated communities in agro-ecological interfaces, as well as virus exchange among them. Here, we advance fundamental understanding in this area by characterizing the virome of three co-occurring replicated Poaceae community types that represent a gradient of grass species richness and management intensity, from highly managed crop monocultures to little-managed, species-rich grasslands. We performed a large-scale study on 950 wild and cultivated Poaceae over 2 years, combining untargeted virome analysis down to the virus species level with targeted detection of three plant viruses. Deep sequencing revealed (i) a diversified and largely unknown Poaceae virome (at least 51 virus species or taxa), with an abundance of so-called persistent viruses; (ii) an increase of virome richness with grass species richness within the community; (iii) stability of virome richness over time but a large viral intraspecific variability; and (iv) contrasting patterns of virus prevalence, coinfections, and spatial distribution among plant communities and species. Our findings highlight the complex structure of plant virus communities in nature and suggest the influence of anthropogenic management on viral distribution and prevalence. IMPORTANCE Because viruses have been mostly studied in cultivated plants, little is known about virus diversity and ecology in less-managed vegetation or about the influence of human management and agriculture on virome composition. Poaceae (grass family)-dominated communities provide invaluable opportunities to examine these ecological issues, as they are distributed worldwide across agro-ecological gradients, are essential for food security and conservation, and can be infected by numerous viruses. Here, we used multiple levels of analysis that considered plant communities, individual plants, virus species, and haplotypes to broaden understanding of the Poaceae virome and to evaluate host-parasite richness relationships within agro-ecological landscapes in our study area. We emphasized the influence of grass diversity and land use on the composition of viral communities and their life history strategies, and we demonstrated the complexity of plant-virus interactions in less-managed grass communities, such as the higher virus prevalence and overrepresentation of mixed virus infection compared to theoretical predictions.

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.

A New Perspective on the Co-Transmission of Plant Pathogens by Hemipterans

Co-infection of plants by pathogens is common in nature, and the interaction of the pathogens can affect the infection outcome. There are diverse ways in which viruses and bacteria are transmitted from infected to healthy plants, but insects are common vectors. The present review aims to highlight key findings of studies evaluating the co-transmission of plant pathogens by insects and identify challenges encountered in these studies. In this review, we evaluated whether similar pathogens might compete during co-transmission; whether the changes in the pathogen titer in the host, in particular associated with the co-infection, could influence its transmission; and finally, we discussed the pros and cons of the different approaches used to study co-transmission. At the end of the review, we highlighted areas of study that need to be addressed. This review shows that despite the recent development of techniques and methods to study the interactions between pathogens and their insect vectors, there are still gaps in the knowledge of pathogen transmission. Additional laboratory and field studies using different pathosystems will help elucidate the role of host co-infection and pathogen co-transmission in the ecology and evolution of infectious diseases.

Sugarcane multitrophic interactions: Integrating belowground and aboveground organisms

Sugarcane is a crop of major importance used mainly for sugar and biofuel production, and many additional applications of its byproducts are being developed. Sugarcane cultivation is plagued by many insect pests and pathogens that reduce sugarcane yields overall. Recently emerging studies have shown complex multitrophic interactions in cultivated areas, such as the induction of sugarcane defense-related proteins by insect herbivory that function against fungal pathogens that commonly appear after mechanical damage. Fungi and viruses infecting sugarcane also modulate insect behavior, for example, by causing changes in volatile compounds responsible for insect attraction or repelling natural vector enemies via a mechanism that increases pathogen dissemination from infected plants to healthy ones. Interestingly, the fungus Fusarium verticillioides is capable of being vertically transmitted to insect offspring, ensuring its persistence in the field. Understanding multitrophic complexes is important to develop better strategies for controlling pathosystems affecting sugarcane and other important crops and highlights the importance of not only studying binary interactions but also adding as many variables as possible to effectively translate laboratory research to real-life conditions.

Assessment of sublethal and transgenerational effects of spirotetramat, on population growth of cabbage aphid, Brevicoryne brassicae L. (Hemiptera: Aphididae)

The cabbage aphid (Brevicoryne brassicae L.) is a devastating pest of cruciferous crops causing economic damage worldwide and notably owing to its increasing resistance to commonly used pesticides. Such resistance prompts the development of integrated pest management (IPM) programs that include novel pesticides being effective against the aphids. Spirotetramat is a novel insecticide used against sap-sucking insect pests, particularly aphids. This study evaluated the toxicity of spirotetramat to adult apterous B. brassicae after 72 h using the leaf dipping method. According to the toxicity bioassay results, the LC₅₀ value of spirotetramat to B. brassicae was 1.304 mgL^-1. However, the sublethal concentrations (LC₅ and LC₁₅) and transgenerational effects of this novel insecticide on population growth parameters were estimated using the age-stage, two-sex life table theory method. The sublethal concentrations (LC₅; 0.125 mgL^-1 and LC₁₅; 0.298 mgL^-1) of spirotetramat reduced the adult longevity and fecundity of the parent generation (F₀). These concentrations prolonged the preadult developmental duration while decreasing preadult survival, adult longevity and reproduction of the F₁ generation. The adult pre-reproductive period was also extended by spirotetramat treatment groups. Subsequently, the population growth parameters such as the intrinsic rate of increase r, finite rate of increase λ and net reproductive rate R ₀ of the F₁ generation were decreased in spirotetramat treatment groups whereas, the mean generation time T of the F₁ generation was not affected when compared to the control. These results indicated the negative effect of sublethal concentrations of spirotetramat on the performance of B. brassicae by reducing its nymphal survival, extending the duration of some immature stages and suppressing the population growth of B. brassicae. Overall, we demonstrated that spirotetramat is a pesticide showing both sublethal activities, and transgenerational effects on cabbage aphid; it may be useful for implementation in IPM programs against this aphid pest.

Vector acquisition and co-inoculation of two plant viruses influences transmission, infection, and replication in new hosts

This study investigated the role of vector acquisition and transmission on the propagation of single and co-infections of tomato yellow leaf curl virus (TYLCV,) and tomato mottle virus (ToMoV) (Family: Geminiviridae, Genus: Begomovirus) by the whitefly vector Bemisia tabaci MEAM1 (Gennadius) in tomato. The aim of this research was to determine if the manner in which viruses are co-acquired and co-transmitted changes the probability of acquisition, transmission and new host infections. Whiteflies acquired virus by feeding on singly infected plants, co-infected plants, or by sequential feeding on singly infected plants. Viral titers were also quantified by qPCR in vector cohorts, in artificial diet, and plants after exposure to viruliferous vectors. Differences in transmission, infection status of plants, and titers of TYLCV and ToMoV were observed among treatments. All vector cohorts acquired both viruses, but co-acquisition/co-inoculation generally reduced transmission of both viruses as single and mixed infections. Co-inoculation of viruses by the vector also altered virus accumulation in plants regardless of whether one or both viruses were propagated in new hosts. These findings highlight the complex nature of vector-virus-plant interactions that influence the spread and replication of viruses as single and co-infections.

(Z)-3-hexenol primes callose deposition against whitefly-mediated begomovirus infection in tomato

Rapid callose accumulation has been shown to mediate defense in certain plant-virus interactions. Exposure to the green leaf volatile (Z)-3-hexenol (Z-3-HOL) can prime tomato (Solanum lycopersicum) for an enhanced defense against subsequent infection by whitefly-transmitted Tomato yellow leaf curl virus (TYLCV). However, the molecular mechanisms affecting Z-3-HOL-induced resistance are poorly understood. Here, we explored the mechanisms underlying Z-3-HOL-induced resistance against whitefly-transmitted TYLCV infection and the role of callose accumulation during this process. Tomato plants pre-treated with Z-3-HOL displayed callose priming upon whitefly infestation. The callose inhibitor 2-deoxy-d-glucose abolished Z-3-HOL-induced resistance, confirming the importance of callose in this induced resistance. We also found that Z-3-HOL pre-treatment enhanced salicylic acid levels and activated sugar signaling in tomato upon whitefly infestation, which increased the expression of the cell wall invertase gene Lin6 to trigger augmented callose deposition against TYLCV infection resulting from whitefly transmission. Using virus-induced gene silencing, we demonstrated the Lin6 expression is relevant for sugar accumulation mediated callose priming in restricting whitefly-transmitted TYLCV infection in plants that have been pre-treated with Z-3-HOL. Moreover, Lin6 induced the expression of the callose synthase gene Cals12, which is also required for Z-3-HOL-induced resistance of tomato against whitefly-transmitted TYLCV infection. These findings highlight the importance of sugar signaling in the priming of callose as a defense mechanism in Z-3-HOL-induced resistance of tomato against whitefly-transmitted TYLCV infection. The results will also increase our understanding of defense priming can be useful for the biological control of viral diseases.

Jingmenviruses: Ubiquitous, understudied, segmented flavi-like viruses

Jingmenviruses are a group of viruses identified recently, in 2014, and currently classified by the International Committee on Taxonomy of Viruses as unclassified Flaviviridae. These viruses closely related to flaviviruses are unique due to the segmented nature of their genome. The prototype jingmenvirus, Jingmen tick virus (JMTV), was discovered in Rhipicephalus microplus ticks collected from China in 2010. Jingmenviruses genomes are composed of four to five segments, encoding for up to seven structural proteins and two non-structural proteins, both of which display strong similarities with flaviviral non-structural proteins (NS2B/NS3 and NS5). Jingmenviruses are currently separated into two phylogenetic clades. One clade includes tick- and vertebrate-associated jingmenviruses, which have been detected in ticks and mosquitoes, as well as in humans, cattle, monkeys, bats, rodents, sheep, and tortoises. In addition to these molecular and serological detections, over a hundred human patients tested positive for jingmenviruses after developing febrile illness and flu-like symptoms in China and Serbia. The second phylogenetic clade includes insect-associated jingmenvirus sequences, which have been detected in a wide range of insect species, as well as in crustaceans, plants, and fungi. In addition to being found in various types of hosts, jingmenviruses are endemic, as they have been detected in a wide range of environments, all over the world. Taken together, all of these elements show that jingmenviruses correspond exactly to the definition of emerging viruses at risk of causing a pandemic, since they are already endemic, have a close association with arthropods, are found in animals in close contact with humans, and have caused sporadic cases of febrile illness in multiple patients. Despite these arguments, the vast majority of published data is from metagenomics studies and many aspects of jingmenvirus replication remain to be elucidated, such as their tropism, cycle of transmission, structure, and mechanisms of replication and restriction or epidemiology. It is therefore crucial to prioritize jingmenvirus research in the years to come, to be prepared for their emergence as human or veterinary pathogens.

A rhabdovirus accessory protein inhibits jasmonic acid signaling in plants to attract insect vectors

Plant rhabdoviruses heavily rely on insect vectors for transmission between sessile plants. However, little is known about the underlying mechanisms of insect attraction and transmission of plant rhabdoviruses. In this study, we used an arthropod-borne cytorhabdovirus, Barley yellow striate mosaic virus (BYSMV), to demonstrate the molecular mechanisms of a rhabdovirus accessory protein in improving plant attractiveness to insect vectors. Here, we found that BYSMV-infected barley (Hordeum vulgare L.) plants attracted more insect vectors than mock-treated plants. Interestingly, overexpression of BYSMV P6, an accessory protein, in transgenic wheat (Triticum aestivum L.) plants substantially increased host attractiveness to insect vectors through inhibiting the jasmonic acid (JA) signaling pathway. The BYSMV P6 protein interacted with the constitutive photomorphogenesis 9 signalosome subunit 5 (CSN5) of barley plants in vivo and in vitro, and negatively affected CSN5-mediated deRUBylation of cullin1 (CUL1). Consequently, the defective CUL1-based Skp1/Cullin1/F-box ubiquitin E3 ligases could not mediate degradation of jasmonate ZIM-domain proteins, resulting in compromised JA signaling and increased insect attraction. Overexpression of BYSMV P6 also inhibited JA signaling in transgenic Arabidopsis (Arabidopsis thaliana) plants to attract insects. Our results provide insight into how a plant cytorhabdovirus subverts plant JA signaling to attract insect vectors.

Increasing the activities of protective enzymes is an important strategy to improve resistance in cucumber to powdery mildew disease and melon aphid under different infection/infestation patterns

Powdery mildew, caused by Sphaerotheca fuliginea (Schlecht.) Poll., and melon aphids (Aphis gossypii Glover) are a typical disease and insect pest, respectively, that affect cucumber production. Powdery mildew and melon aphid often occur together in greenhouse production, resulting in a reduction in cucumber yield. At present there are no reports on the physiological and biochemical effects of the combined disease and pest infection/infestation on cucumber. This study explored how cucumbers can regulate photosynthesis, protective enzyme activity, and basic metabolism to resist the fungal disease and aphids. After powdery mildew infection, the chlorophyll and free proline contents in cucumber leaves decreased, while the activities of POD (peroxidase) and SOD (superoxide dismutase) and the soluble protein and MDA (malondialdehyde) contents increased. Cucumber plants resist aphid attack by increasing the rates of photosynthesis and basal metabolism, and also by increasing the activities of protective enzymes. The combination of powdery mildew infection and aphid infestation reduced photosynthesis and basal metabolism in cucumber plants, although the activities of several protective enzymes increased. Aphid attack after powdery mildew infection or powdery mildew infection after aphid attack had the opposite effect on photosynthesis, protective enzyme activity, and basal metabolism regulation. Azoxystrobin and imidacloprid increased the contents of chlorophyll, free proline, and soluble protein, increased SOD activity, and decreased the MDA content in cucumber leaves. However, these compounds had the opposite effect on the soluble sugar content and POD and CAT (catalase) activities. The mixed ratio of the two single agents could improve the resistance of cucumber to the combined infection of powdery mildew and aphids. These results show that cucumber can enhance its pest/pathogen resistance by changing physiological metabolism when exposed to a complex infection system of pathogenic microorganisms and insect pests.

Comparative Plant Transcriptome Profiling of Arabidopsis thaliana Col-0 and Camelina sativa var. Celine Infested with Myzus persicae Aphids Acquiring Circulative and Noncirculative Viruses Reveals Virus- and Plant-Specific Alterations Relevant to Aphid Feeding Behavior and Transmission

Evidence is accumulating that plant viruses alter host plant traits in ways that modify their insect vectors' behavior. These alterations often enhance virus transmission, which has led to the hypothesis that these effects are manipulations caused by viral adaptation. However, we lack a mechanistic understanding of the genetic basis of these indirect, plant-mediated effects on vectors, their dependence on the plant host, and their relation to the mode of virus transmission. Transcriptome profiling of Arabidopsis thaliana and Camelina sativa plants infected with turnip yellows virus (TuYV) or cauliflower mosaic virus (CaMV) and infested with the common aphid vector Myzus persicae revealed strong virus- and host-specific differences in gene expression patterns. CaMV infection caused more severe effects on the phenotype of both plant hosts than did TuYV infection, and the severity of symptoms correlated strongly with the proportion of differentially expressed genes, especially photosynthesis genes. Accordingly, CaMV infection modified aphid behavior and fecundity more strongly than did infection with TuYV. Overall, infection with CaMV, relying on the noncirculative transmission mode, tends to have effects on metabolic pathways, with strong potential implications for insect vector-plant host interactions (e.g., photosynthesis, jasmonic acid, ethylene, and glucosinolate biosynthetic processes), while TuYV, using the circulative transmission mode, alters these pathways only weakly. These virus-induced deregulations of genes that are related to plant physiology and defense responses might impact both aphid probing and feeding behavior on infected host plants, with potentially distinct effects on virus transmission. IMPORTANCE Plant viruses change the phenotype of their plant hosts. Some of the changes impact interactions of the plant with insects that feed on the plants and transmit these viruses. These modifications may result in better virus transmission. We examine here the transcriptomes of two plant species infected with two viruses with different transmission modes to work out whether there are plant species-specific and transmission mode-specific transcriptome changes. Our results show that both are the case.

Silencing the Autophagy-Related Genes ATG3 and ATG9 Promotes SRBSDV Propagation and Transmission in Sogatella furcifera

Autophagy plays diverse roles in the interaction among pathogen, vector, and host. In the plant virus and insect vector system, autophagy can be an antiviral/pro-viral factor to suppress/promote virus propagation and transmission. Here, we report the antiviral role of autophagy-related genes ATG3 and ATG9 in the white-backed planthopper (Sogatella furcifera) during the process of transmitting the southern rice black-streaked dwarf virus (SRBSDV). In this study, we annotated two autophagy-related genes, SfATG3 and SfATG9, from the female S. furcifera transcriptome. The cDNA of SfATG3 and SfATG9 comprised an open reading frame (ORF) of 999 bp and 2295 bp that encodes a protein of 332 and 764 amino acid residues, respectively. SfATG3 has two conserved domains and SfATG9 has one conserved domain. In S. furcifera females exposed to SRBSDV, expression of autophagy-related genes was significantly activated and shared similar temporal patterns to those of SRBSDV S9-1 and S10, all peaking at 4 d post viral exposure. Silencing the expression of SfATG3 and SfATG9 promoted SRBSDV propagation and transmission. This study provides evidence for the first time that S. furcifera autophagy-related genes ATG3 and ATG9 play an antiviral role to suppress SRBSDV propagation and transmission.

A Review on Transcriptional Responses of Interactions between Insect Vectors and Plant Viruses

This review provides a synopsis of transcriptional responses pertaining to interactions between plant viruses and the insect vectors that transmit them in diverse modes. In the process, it attempts to catalog differential gene expression pertinent to virus–vector interactions in vectors such as virus reception, virus cell entry, virus tissue tropism, virus multiplication, and vector immune responses. Whiteflies, leafhoppers, planthoppers, and thrips are the main insect groups reviewed, along with aphids and leaf beetles. Much of the focus on gene expression pertinent to vector–virus interactions has centered around whole-body RNA extraction, whereas data on virus-induced tissue-specific gene expression in vectors is limited. This review compares transcriptional responses in different insect groups following the acquisition of non-persistent, semi-persistent, and persistent (non-propagative and propagative) plant viruses and identifies parallels and divergences in gene expression patterns. Understanding virus-induced changes in vectors at a transcriptional level can aid in the identification of candidate genes for targeting with RNAi and/or CRISPR editing in insect vectors for management approaches.

A cytorhabdovirus-based expression vector in Nilaparvata lugens, Laodelphax striatellus, and Sogatella furcifera

The brown planthopper (BPH, Nilaparvata lugens), the small brown planthopper (SBPH, Laodelphax striatellus), and the white-backed planthopper (WBPH, Sogatella furcifera) are problematic insect pests and cause severe yield losses through phloem sap-sucking and virus transmission. Barley yellow striate mosaic virus (BYSMV), a plant cytorhabdovirus, has been developed as versatile expression platforms in SBPHs and cereal plants. However, bio-safe overexpression vectors based on recombinant BYSMV (rBYSMV) remain to be developed and applied to the three kinds of planthoppers. Here, we found that rBYSMV was able to infect SBPHs, BPHs and WBPHs through microinjection with crude extracts from rBYSMV-infected barley leaves. To ensure bio-safety of the rBYSMV vectors, we generated an rBYSMV mutant by deleting the accessory protein P3, a putative viral movement protein. As expected, the resulting mutant abolished viral systemic infection in barley plants but had no effects on BYSMV infectivity in insect vectors. Subsequently, we used the modified rBYSMV vector to overexpress iron transport peptide (ITP) in the three kinds of planthoppers and revealed the potential functions of ITP. Overall, our results provide bio-safe overexpression platforms to facilitate functional genomics studies of planthoppers.

Plant Viruses Can Alter Aphid-Triggered Calcium Elevations in Infected Leaves

Alighting aphids probe a new host plant by intracellular test punctures for suitability. These induce immediate calcium signals that emanate from the punctured sites and might be the first step in plant recognition of aphid feeding and the subsequent elicitation of plant defence responses. Calcium is also involved in the transmission of non-persistent plant viruses that are acquired by aphids during test punctures. Therefore, we wanted to determine whether viral infection alters calcium signalling. For this, calcium signals triggered by aphids were imaged on transgenic Arabidopsis plants expressing the cytosolic FRET-based calcium reporter YC3.6-NES and infected with the non-persistent viruses cauliflower mosaic (CaMV) and turnip mosaic (TuMV), or the persistent virus, turnip yellows (TuYV). Aphids were placed on infected leaves and calcium elevations were recorded by time-lapse fluorescence microscopy. Calcium signal velocities were significantly slower in plants infected with CaMV or TuMV and signal areas were smaller in CaMV-infected plants. Transmission tests using CaMV-infected Arabidopsis mutants impaired in pathogen perception or in the generation of calcium signals revealed no differences in transmission efficiency. A transcriptomic meta-analysis indicated significant changes in expression of receptor-like kinases in the BAK1 pathway as well as of calcium channels in CaMV- and TuMV-infected plants. Taken together, infection with CaMV and TuMV, but not with TuYV, impacts aphid-induced calcium signalling. This suggests that viruses can modify plant responses to aphids from the very first vector/host contact.

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.

Membrane association of importin α facilitates viral entry into salivary gland cells of vector insects

The importin α family belongs to the conserved nuclear transport pathway in eukaryotes. However, the biological functions of importin α in the plasma membrane are still elusive. Here, we report that importin α, as a plasma membrane-associated protein, is exploited by the rice stripe virus (RSV) to enter vector insect cells, especially salivary gland cells. When the expression of three importin α genes was simultaneously knocked down, few virions entered the salivary glands of the small brown planthopper, Laodelphax striatellus Through hemocoel inoculation of virions, only importin α2 was found to efficiently regulate viral entry into insect salivary-gland cells. Importin α2 bound the nucleocapsid protein of RSV with a relatively high affinity through its importin β-binding (IBB) domain, with a dissociation constant K _D of 9.1 μM. Furthermore, importin α2 and its IBB domain showed a distinct distribution in the plasma membrane through binding to heparin in heparan sulfate proteoglycan. When the expression of importin α2 was knocked down in viruliferous planthoppers or in nonviruliferous planthoppers before they acquired virions, the viral transmission efficiency of the vector insects in terms of the viral amount and disease incidence in rice was dramatically decreased. These findings not only reveal the specific function of the importin α family in the plasma membrane utilized by viruses, but also provide a promising target gene in vector insects for manipulation to efficiently control outbreaks of rice stripe disease.

MicroRNA-315-5p promotes rice black-streaked dwarf virus infection by targeting a melatonin receptor in the small brown planthopper

BACKGROUND

MicroRNAs (miRNAs), a class of small non-coding endogenous RNAs, play key roles in various biological processes. Most plant viruses are transmitted by insect vectors. However, little is known about the function of miRNAs on plant virus-insect host interaction.

RESULTS

We investigated the role of miR-315-5p in regulation of plant viral infection in insects using a rice black-streaked dwarf virus (RBSDV) and small brown planthopper (SBPH) interaction system. Our results showed that miR-315-5p had the highest expression level in 2nd-instar nymph, and was highly expressed in the salivary gland and midgut in SBPH. miR-315-5p was in response to and regulated RBSDV infection in SBPH. Injection of miR-315-5p mimic, agomir-315, significantly increased the RBSDV accumulation, whereas injection of miR-315-5p inhibitor, antagomir-315, reduced virus accumulation in SBPH. Furthermore, a melatonin receptor was identified as a target gene of miR-315-5p by the dual luciferase reporter assay. Knockdown of the melatonin receptor significantly increased the expression of RBSDV coat protein gene S10 and replication related genes, S5-1, S6, and S9-1. Furthermore, treatment with melatonin receptor antagonist luzindole and activator agomelatine significantly increased and reduced RBSDV accumulation in SBPH, respectively. Compared to the control, miR-315-5p did not affect the efficiency of RBSDV acquisition in SBPH. However, the efficiency of RBSDV transmission was significantly reduced after injecting antagomir-315.

CONCLUSION

Taken together, our data reveal that miR-315-5p is beneficial for RBSDV infection in its insect vector by directly targeting a melatonin receptor. These findings provide a new insight to the function of miRNAs in virus-insect vector interaction. © 2021 Society of Chemical Industry.

Silicon amendment to rice plants reduces the transmission of southern rice black-streaked dwarf virus by Sogatella furcifera

BACKGROUND

Plant viruses are transmitted mainly by piercing-sucking herbivores, and viral disease management relies on chemical control of vectors. Southern rice black-streaked dwarf virus (SRBSDV) is transmitted by the white-backed planthopper (WBPH), Sogatella furcifera. This study aimed to evaluate the potential of silicon (Si) amendment for reducing SRBSDV transmission.

RESULTS

The settling and ovipositional preferences of WBPH females decreased significantly by 14.6-43.7% for plants treated with either 0.16 g or 0.32 g SiO₂  kg^-1 soil during SRBSDV acquisition and by 26.2-28.3% for plants treated with 0.32 g SiO₂  kg^-1 soil during SRBSDV inoculation, compared with controls. Adding either 0.16 or 0.32 g SiO₂  kg^-1 soil significantly reduced SRBSDV inoculation rate by 31.3% and 45.3%, respectively, and acquisition rate by 25.5% and 66.0%, respectively. Silicification was intensified more in plants treated with 0.32 g SiO₂  kg^-1 soil than in controls. The nonprobing (np) duration increased, and the phloem sap ingestion (N4-b) duration decreased significantly in the WBPHs feeding on high-rate-Si-supplemented plants compared with control plants during both inoculation and acquisition access.

CONCLUSION

This study showed that Si amendment to rice plants decreased the WBPH settling and ovipositional preference and the SRBSDV acquisition and inoculation rates, thereby reducing SRBSDV transmission. The intensified plant silicification and the altered WBPH feeding behaviors (i.e. prolonged np and shortened N4-b) may explain the reduced SRBSDV transmission in Si-amended plants. © 2021 Society of Chemical Industry.

Responses of Passiflora spp. to cowpea aphid-borne mosaic virus reveal infection in asymptomatic plants and new species with probable immunity

Passion fruit woodiness disease (PWD), caused by cowpea aphid-borne mosaic virus (CABMV), produces socioeconomic problems in Brazil. The objectives of this study were to i) evaluate the temporal progression of PWD, ii) identify Passiflora genotypes with resistance to CABMV, and iii) detect virus infection in asymptomatic plants by reverse transcription quantitative polymerase chain reaction (RT-qPCR) in cases where standard RT-PCR detection failed. The experiment was conducted in a greenhouse using 128 genotypes belonging to 12 species and three hybrids (inter- and intraspecific) of Passiflora, evaluated at five time points after inoculation. Progression rates and disease severity were lower in P. cincinnata, P. gibertii, P. miersii, and P. mucronata than in P. edulis, P. alata, Passiflora sp., and hybrids. Of the genotypes tested, 20.31% were resistant, especially the accessions of P. suberosa, P. malacophylla, P. setacea, P. pohlii, and P. bahiensis, which remained asymptomatic throughout the experiment. The absence of symptoms does not imply immunity of plants to the virus, since RT-qPCR analysis confirmed infection by the virus in asymptomatic plants of P. cincinnata, P. gibertii, P. miersii, P. mucronata, P. setacea, P. malacophylla, and P. suberosa. Even after four inoculations, the virus was not detected by RT-qPCR in the upper leaves in plants of the species P. pohlii and P. bahiensis, indicating that these species are probably immune to CABMV.

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.

Imaging Techniques to Study Plant Virus Replication and Vertical Transmission

Plant viruses are obligate parasites that need to usurp plant cell metabolism in order to infect their hosts. Imaging techniques have been used for quite a long time to study plant virus-host interactions, making it possible to have major advances in the knowledge of plant virus infection cycles. The imaging techniques used to study plant-virus interactions have included light microscopy, confocal laser scanning microscopy, and scanning and transmission electron microscopies. Here, we review the use of these techniques in plant virology, illustrating recent advances in the area with examples from plant virus replication and virus plant-to-plant vertical transmission processes.

Vector Transmission of Tomato Yellow Leaf Curl Thailand Virus by the Whitefly Bemisia tabaci: Circulative or Propagative?

Viruses that cause tomato yellow leaf curl disease are part of a group of viruses of the genus Begomovirus, family Geminiviridae. Tomato-infecting begomoviruses cause epidemics in tomato crops in tropical, subtropical, and Mediterranean climates, and they are exclusively transmitted by Bemisia tabaci in the field. The objective of the present study was to examine the transmission biology of the tomato yellow leaf curl Thailand virus (TYLCTHV) by B. tabaci, including virus-infected tissues, virus translocation, virus replication, and transovarial transmission. The results demonstrated that the virus translocates from the alimentary gut to the salivary glands via the hemolymph, without apparent replication when acquired by B. tabaci. Furthermore, the virus was detected in 10% of the first-generation progeny of viruliferous females, but the progeny was unable to cause the viral infection of host plants. There was no evidence of transovarial transmission of TYLCTHV in B. tabaci. When combined with the current literature, our results suggest that B. tabaci transmits TYLCTHV in a persistent-circulative mode. The present study enhances our understanding of virus-vector interaction and the transmission biology of TYLCTHV in B. tabaci.

Meta-Analysis Suggests Differing Indirect Effects of Viral, Bacterial, and Fungal Plant Pathogens on the Natural Enemies of Insect Herbivores

Indirect effects are ubiquitous in nature, and have received much attention in terrestrial plant-insect herbivore-enemy systems. In such tritrophic systems, changes in plant quality can have consequential effects on the behavior and abundance of insect predators and parasitoids. Plant quality as perceived by insect herbivores may vary for a range of reasons, including because of infection by plant pathogens. However, plant diseases vary in their origin (viral, bacterial or fungal) and as a result may have differing effects on plant physiology. To investigate if the main groups of plant pathogens differ in their indirect effects on higher trophic levels, we performed a meta-analysis using 216 measured responses from 29 primary studies. There was no overall effect of plant pathogens on natural enemy traits as differences between pathogen types masked their effects. Infection by fungal plant pathogens showed indirect negative effects on the performance and preference of natural enemies via both chewing and piercing-sucking insect herbivore feeding guilds. Infection by bacterial plant pathogens had a positive effect on the natural enemies (parasitoids) of chewing herbivores. Infection by viral plant pathogens showed no clear effect, although parasitoid preference may be positively affected by their presence. It is important to note that given the limited volume of studies to date on such systems, this work should be considered exploratory. Plant pathogens are very common in nature, and tritrophic systems provide an elegant means to examine the consequences of indirect interactions in ecology. We suggest that further studies examining how plant pathogens affect higher trophic levels would be of considerable value.

Systematic Identification and Functional Analysis of Circular RNAs During Rice Black-Streaked Dwarf Virus Infection in the Laodelphax striatellus (Fallén) Midgut

Circular RNAs (circRNAs) are endogenous RNAs that have critical regulatory roles in numerous biological processes. However, it remains largely unknown whether circRNAs are induced in response to plant virus infection in the insect vector of the virus as well as whether the circRNAs regulate virus infection. Rice black-streaked dwarf virus (RBSDV) is transmitted by Laodelphax striatellus (Fallén) in a persistent propagative manner and causes severe losses in East Asian countries. To explore the expression and function of circRNAs in the regulation of virus infection, we determined the circRNA expression profile in RBSDV-free or RBSDV-infected L. striatellus midgut tissues by RNA-Seq. A total of 2,523 circRNAs were identified, of which thirteen circRNAs were differentially expressed after RBSDV infection. The functions of these differentially circRNAs were predicted by GO and KEGG pathway analyses. The expression changes of five differentially expressed circRNAs and eight parental genes were validated by RT-qPCR. The circRNAs-microRNAs (miRNAs) interaction networks were analyzed and two miRNAs, which were predicted to bind circRNAs, were differentially expressed after virus infection. CircRNA2030 was up-regulated after RBSDV infection in L. striatellus midgut. Knockdown of circRNA2030 by RNA interference inhibited the expression of its predicted parental gene phospholipid-transporting ATPase (PTA) and enhanced RBSDV infection in L. striatellus. However, none of the six miRNAs predicting to bind circRNA2030 was up-regulated after circRNA2030 knockdown. The results suggested that circRNA2030 might affect RBSDV infection via regulating PTA. Our results reveal the expression profile of circRNAs in L. striatellus midgut and provide new insight into the roles of circRNAs in virus-insect vector interaction.

Virome Analysis of Aphid Populations That Infest the Barley Field: The Discovery of Two Novel Groups of Nege/Kita-Like Viruses and Other Novel RNA Viruses

Aphids (order Hemiptera) are important insect pests of crops and are also vectors of many plant viruses. However, little is known about aphid-infecting viruses, particularly their diversity and relationship to plant viruses. To investigate the aphid viromes, we performed deep sequencing analyses of the aphid transcriptomes from infested barley plants in a field in Japan. We discovered virus-like sequences related to nege/kita-, flavi-, tombus-, phenui-, mononega-, narna-, chryso-, partiti-, and luteoviruses. Using RT-PCR and sequence analyses, we determined almost complete sequences of seven nege/kitavirus-like virus genomes; one of which was a variant of the Wuhan house centipede virus (WHCV-1). The other six seem to belong to four novel viruses distantly related to Wuhan insect virus 9 (WhIV-9) or Hubei nege-like virus 4 (HVLV-4). We designated the four viruses as barley aphid RNA virus 1 to 4 (BARV-1 to -4). Moreover, some nege/kitavirus-like sequences were found by searches on the transcriptome shotgun assembly (TSA) libraries of arthropods and plants. Phylogenetic analyses showed that BARV-1 forms a clade with WHCV-1 and HVLV-4, whereas BARV-2 to -4 clustered with WhIV-9 and an aphid virus, Aphis glycines virus 3. Both virus groups (tentatively designated as Centivirus and Aphiglyvirus, respectively), together with arthropod virus-like TSAs, fill the phylogenetic gaps between the negeviruses and kitaviruses lineages. We also characterized the flavi/jingmen-like and tombus-like virus sequences as well as other RNA viruses, including six putative novel viruses, designated as barley aphid RNA viruses 5 to 10. Interestingly, we also discovered that some aphid-associated viruses, including nege/kita-like viruses, were present in different aphid species, raising a speculation that these viruses might be distributed across different aphid species with plants being the reservoirs. This study provides novel information on the diversity and spread of nege/kitavirus-related viruses and other RNA viruses that are associated with aphids.

Whitefly HES1 binds to the intergenic region of Tomato yellow leaf curl China virus and promotes viral gene transcription

Intergenic region of begomovirus genome is vital to virus replication and viral gene transcription in plants. Previous studies have reported that Tomato yellow leaf curl China virus (TYLCCNV), a begomovirus, is able to accumulate and transcribe in its whitefly vector. However, the viral and host components that participate in begomovirus transcription in whiteflies are hitherto unknown. Using a yeast one-hybrid system, we identified >50 whitefly proteins that interacted with TYLCCNV intergenic region. Dual luciferase analysis revealed that one of the identified proteins, the hairy and enhancer of split homolog-1 (HES1), specifically bound to CACGTG motif in TYLCCNV intergenic region. Silencing HES1 decreased viral transcription, accumulation and transmission. These results demonstrate that the interactions between whitefly proteins and the intergenic region of TYLCCNV may contribute to viral transcription in the whitefly vector. Our findings offer valuable clues for the research and development of novel strategies to interfere with begomovirus transmission.

Molecular Insights into Host and Vector Manipulation by Plant Viruses

Plant viruses rely on both host plant and vectors for a successful infection. Essentially to simplify studies, transmission has been considered for decades as an interaction between two partners, virus and vector. This interaction has gained a third partner, the host plant, to establish a tripartite pathosystem in which the players can react with each other directly or indirectly through changes induced in/by the third partner. For instance, viruses can alter the plant metabolism or plant immune defence pathways to modify vector’s attraction, settling or feeding, in a way that can be conducive for virus propagation. Such changes in the plant physiology can also become favourable to the vector, establishing a mutualistic relationship. This review focuses on the recent molecular data on the interplay between viral and plant factors that provide some important clues to understand how viruses manipulate both the host plants and vectors in order to improve transmission conditions and thus ensuring their survival.

Impact of Mutations in Arabidopsis thaliana Metabolic Pathways on Polerovirus Accumulation, Aphid Performance, and Feeding Behavior

During the process of virus acquisition by aphids, plants respond to both the virus and the aphids by mobilizing different metabolic pathways. It is conceivable that the plant metabolic responses to both aggressors may be conducive to virus acquisition. To address this question, we analyze the accumulation of the phloem-limited polerovirus Turnip yellows virus (TuYV), which is strictly transmitted by aphids, and aphid's life traits in six Arabidopsis thaliana mutants (xth33, ss3-2, nata1, myc234, quad, atr1D, and pad4-1). We observed that mutations affecting the carbohydrate metabolism, the synthesis of a non-protein amino acid and the glucosinolate pathway had an effect on TuYV accumulation. However, the virus titer did not correlate with the virus transmission efficiency. Some mutations in A. thaliana affect the aphid feeding behavior but often only in infected plants. The duration of the phloem sap ingestion phase, together with the time preceding the first sap ingestion, affect the virus transmission rate more than the virus titer did. Our results also show that the aphids reared on infected mutant plants had a reduced biomass regardless of the mutation and the duration of the sap ingestion phase.

From foes to friends: Viral infections expand the limits of host phenotypic plasticity

Phenotypic plasticity enables organisms to survive in the face of unpredictable environmental stress. Intimately related to the notion of phenotypic plasticity is the concept of the reaction norm that places phenotypic plasticity in the context of a genotype-specific response to environmental gradients. Whether reaction norms themselves evolve and which factors might affect their shape has been the object of intense debates among evolutionary biologists along the years. Since their discovery, viruses have been considered as pathogens. However, new viromic techniques and a shift in conceptual paradigms are showing that viruses are mostly non-pathogenic ubiquitous entities. Recent studies have shown how viral infections can even be beneficial for their hosts. This may happen especially in the context of stressed hosts, where the virus infection can induce beneficial changes in the host's physiological homeostasis, hence changing the shape of the reaction norm. Despite the fact that underlying physiological mechanisms and evolutionary dynamics are still not well understood, such beneficial interactions are being discovered in a growing number of plant-virus systems. Here, we aim to review these disperse studies and place them into the context of phenotypic plasticity and the evolution of reaction norms. This is an emerging field that is posing many questions that still need to be properly answered. The answers would clearly interest virologists, plant pathologists and evolutionary biologists and likely they will suggest possible future biotechnological applications, including the development of crops with higher survival rates and yield under adverse environmental situations.

Plant-insect vector-virus interactions under environmental change

Insects play an important role in the spread of viruses from infected plants to healthy hosts through a variety of transmission strategies. Environmental factors continuously influence virus transmission and result in the establishment of infection or disease. Plant virus diseases become epidemic when viruses successfully dominate the surrounding ecosystem. Plant-insect vector-virus interactions influence each other; pushing each other for their benefit and survival. These interactions are modulated through environmental factors, though environmental influences are not readily predictable. This review focuses on exploiting the diverse relationships, embedded in the plant-insect vector-virus triangle by highlighting recent research findings. We examined the interactions between viruses, insect vectors, and host plants, and explored how these interactions affect their behavior.

Detection of Multiple Variants of Grapevine Fanleaf Virus in Single Xiphinema index Nematodes

Grapevine fanleaf virus (GFLV) is responsible for a widespread disease in vineyards worldwide. Its genome is composed of two single-stranded positive-sense RNAs, which both show a high genetic diversity. The virus is transmitted from grapevine to grapevine by the ectoparasitic nematode Xiphinema index. Grapevines in diseased vineyards are often infected by multiple genetic variants of GFLV but no information is available on the molecular composition of virus variants retained in X. index following nematodes feeding on roots. In this work, aviruliferous X. index were fed on three naturally GFLV-infected grapevines for which the virome was characterized by RNAseq. Six RNA-1 and four RNA-2 molecules were assembled segregating into four and three distinct phylogenetic clades of RNA-1 and RNA-2, respectively. After 19 months of rearing, single and pools of 30 X. index tested positive for GFLV. Additionally, either pooled or single X. index carried multiple variants of the two GFLV genomic RNAs. However, the full viral genetic diversity found in the leaves of infected grapevines was not detected in viruliferous nematodes, indicating a genetic bottleneck. Our results provide new insights into the complexity of GFLV populations and the putative role of X. index as reservoirs of virus diversity.

Maize Iranian mosaic virus (family Rhabdoviridae) improves biological traits of its vector Laodelphax striatellus

Plant viruses can alter the behavior or performance of their arthropod vectors, either indirectly (through effects of virus infection on the host plant) or directly (from virus acquisition by the vector). Given the diversity of plant viruses and their arthropod vectors, the effects for any specific system are not possible to predict. Here, we present experimental evidence that acquisition of maize Iranian mosaic virus (MIMV, genus Nucleorhabdovirus, family Rhabdoviridae) modifies the biological traits of its insect vector, the small brown planthopper (SBPH) Laodelphax striatellus. MIMV is an economically important virus of maize and several other grass species. It is transmitted by SBPHs in a persistent-propagative manner. We evaluated the effects of MIMV acquisition by SBPH on its life history when reared on healthy barley plants (Hordeum vulgare). We conclude that 1) MIMV acquisition by SBPHs increases female fecundity, duration of the nymph stage, adult longevity, and survival of SBPHs, (2) the mortality rate and female-to-male sex ratio are reduced in MIMV-infected planthoppers, and (3) MIMV infection increases the concentration of some biochemical components of the infected plants, including carbohydrates, some amino acids, and total protein, which might influence the life traits of its insect vector. The results indicate the potential of MIMV to improve the ecological fitness of its vector, SBPH, through direct or indirect effects, with the potential to increase the spread of the virus.

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.

Evolution and ecology of plant viruses

The discovery of the first non-cellular infectious agent, later determined to be tobacco mosaic virus, paved the way for the field of virology. In the ensuing decades, research focused on discovering and eliminating viral threats to plant and animal health. However, recent conceptual and methodological revolutions have made it clear that viruses are not merely agents of destruction but essential components of global ecosystems. As plants make up over 80% of the biomass on Earth, plant viruses likely have a larger impact on ecosystem stability and function than viruses of other kingdoms. Besides preventing overgrowth of genetically homogeneous plant populations such as crop plants, some plant viruses might also promote the adaptation of their hosts to changing environments. However, estimates of the extent and frequencies of such mutualistic interactions remain controversial. In this Review, we focus on the origins of plant viruses and the evolution of interactions between these viruses and both their hosts and transmission vectors. We also identify currently unknown aspects of plant virus ecology and evolution that are of practical importance and that should be resolvable in the near future through viral metagenomics.

Viruses in the phytobiome

The phytobiome, defined as plants and all the entities that interact with them, is rich in viruses, but with the exception of plant viruses of crop plants, most of the phytobiome viruses remain very understudied. This review focuses on the neglected portions of the phytobiome, including viruses of other microbes interacting with plants, viruses in the soil, viruses of wild plants, and relationships between viruses and the vectors of plant viruses.

Pharmacological analysis of transmission activation of two aphid-vectored plant viruses, turnip mosaic virus and cauliflower mosaic virus

Turnip mosaic virus (TuMV, family Potyviridae) and cauliflower mosaic virus (CaMV, family Caulimoviridae) are transmitted by aphid vectors. They are the only viruses shown so far to undergo transmission activation (TA) immediately preceding plant-to-plant propagation. TA is a recently described phenomenon where viruses respond to the presence of vectors on the host by rapidly and transiently forming transmissible complexes that are efficiently acquired and transmitted. Very little is known about the mechanisms of TA and on whether such mechanisms are alike or distinct in different viral species. We use here a pharmacological approach to initiate the comparison of TA of TuMV and CaMV. Our results show that both viruses rely on calcium signaling and reactive oxygen species (ROS) for TA. However, whereas application of the thiol-reactive compound N-ethylmaleimide (NEM) inhibited, as previously shown, TuMV transmission it did not alter CaMV transmission. On the other hand, sodium azide, which boosts CaMV transmission, strongly inhibited TuMV transmission. Finally, wounding stress inhibited CaMV transmission and increased TuMV transmission. Taken together, the results suggest that transmission activation of TuMV and CaMV depends on initial calcium and ROS signaling that are generated during the plant's immediate responses to aphid manifestation. Interestingly, downstream events in TA of each virus appear to diverge, as shown by the differential effects of NEM, azide and wounding on TuMV and CaMV transmission, suggesting that these two viruses have evolved analogous TA mechanisms.

Progress and challenges in identifying molecular mechanisms underlying host and vector manipulation by plant viruses

Plant virus infection fundamentally alters chemical and behavioral phenotypes of hosts and vectors. These alterations often enhance virus transmission, leading researchers to surmise that such effects are manipulations caused by virus adaptations and not just by-products of pathology. But identification of the virus components behind manipulation is missing from most studies performed to date. Here, we evaluate causative empirical evidence that virus components are the drivers of manipulated host and vector phenotypes. To do so, we link findings and methodologies on virus pathology with observational and functional genomics studies on virus manipulation. Our synthesis provides an overview of progress, areas of synergy, and new approaches that will lead to an improved mechanistic understanding of host and vector manipulation by plant viruses.

Preference of the aphid Myzus persicae (Hemiptera: Aphididae) for tobacco plants at specific stages of potato virus Y infection

Potato virus Y (PVY) is a common pathogen affecting agricultural production worldwide and is mainly transmitted by Myzus persicae in a non-persistent manner. Insect-borne plant viruses can modify the abundance, performance, and behavior of their vectors by altering host plant features; however, most studies have overlooked the fact that the dynamic progression of virus infection in plants can have variable effects on their vectors. We addressed this point in the present study by dividing the PVY infection process in tobacco into three stages (early state, steady state and late state); delineated by viral copy number. We then compared the differential effects of PVY-infected tobacco (Nicotiana tabacum) plants on the host selection and feeding behavior of M. persicae. We used Y-shaped olfactory apparatus and electrical penetration graph (EPG) methods to evaluate host selection and feeding behavior, respectively. Interestingly, we found that PVY-infected plants at the steady state attracted more aphids than healthy plants, whereas no differences were observed for those at the early and late states. In terms of feeding behavior, intracellular punctures (closely related to PVY acquisition and transmission) were more abundant on PVY-infected tobacco plants at the early and steady states of infection than in uninfected plants. These results indicate that PVY-infected host plants can alter the host selection and feeding behavior of aphids in a stage-dependent manner, which is an important consideration when studying the interactions among host plants, viruses, and insect vectors.

Turnip Mosaic Virus Is a Second Example of a Virus Using Transmission Activation for Plant-to-Plant Propagation by Aphids

Cauliflower mosaic virus (CaMV; family Caulimoviridae) responds to the presence of aphid vectors on infected plants by forming specific transmission morphs. This phenomenon, coined transmission activation (TA), controls plant-to-plant propagation of CaMV. A fundamental question is whether other viruses rely on TA. Here, we demonstrate that transmission of the unrelated turnip mosaic virus (TuMV; family Potyviridae) is activated by the reactive oxygen species H₂O₂ and inhibited by the calcium channel blocker LaCl₃ H₂O₂-triggered TA manifested itself by the induction of intermolecular cysteine bonds between viral helper component protease (HC-Pro) molecules and by the formation of viral transmission complexes, composed of TuMV particles and HC-Pro that mediates vector binding. Consistently, LaCl₃ inhibited intermolecular HC-Pro cysteine bonds and HC-Pro interaction with viral particles. These results show that TuMV is a second virus using TA for transmission but using an entirely different mechanism than CaMV. We propose that TuMV TA requires reactive oxygen species (ROS) and calcium signaling and that it is operated by a redox switch.IMPORTANCE Transmission activation, i.e., a viral response to the presence of vectors on infected hosts that regulates virus acquisition and thus transmission, is an only recently described phenomenon. It implies that viruses contribute actively to their transmission, something that has been shown before for many other pathogens but not for viruses. However, transmission activation has been described so far for only one virus, and it was unknown whether other viruses also rely on transmission activation. Here we present evidence that a second virus uses transmission activation, suggesting that it is a general transmission strategy.

Mikania Micrantha Wilt Virus Alters Insect Vector's Host Preference to Enhance Its Own Spread

As an invasive weed, Mikaniamicrantha Kunth has caused serious damage to natural forest ecosystems in South China in recent years. Mikania micrantha wilt virus (MMWV), an isolate of the Gentian mosaic virus (GeMV), is transmitted by Myzuspersicae (Sulzer) in a non-persistent manner and can effectively inhibit the growth of M. micrantha. To explore the MMWV-M. micrantha-M. persicae interaction and its impact on the invasion of M. micrantha, volatile compounds (VOCs) emitted from healthy, mock-inoculated, and MMWV-infected plants were collected, and effects on host preference of the apterous and alate aphids were assessed with Y-shaped olfactometers. Gas chromatography-mass spectrometry (GC-MS) analysis indicated that MMWV infection changed the VOC profiles, rendering plants more attractive to aphids. Clip-cages were used to document the population growth rate of M.persicae fed on healthy, mock-inoculated, or MMWV-infected plants. Compared to those reared on healthy plants, the population growth of M. persicae drastically decreased on the MMWV-infected plants. Plant host choice tests based on visual and contact cues were also conducted using alate M.persicae. Interestingly, the initial attractiveness of MMWV-infected plants diminished, and more alate M. persicae moved to healthy plants. Taken together, MMWV appeared to be able to manipulate its plant host to first attract insect vectors to infected plants but then repel viruliferous vectors to promote its own dispersal. Its potential application for invasive weed management is discussed.

Tenuivirus utilizes its glycoprotein as a helper component to overcome insect midgut barriers for its circulative and propagative transmission

Many persistent transmitted plant viruses, including rice stripe virus (RSV), cause serious damage to crop production worldwide. Although many reports have indicated that a successful insect-mediated virus transmission depends on a proper interaction between the virus and its insect vector, the mechanism(s) controlling this interaction remained poorly understood. In this study, we used RSV and its small brown planthopper (SBPH) vector as a working model to elucidate the molecular mechanisms underlying the entrance of RSV virions into SBPH midgut cells for virus circulative and propagative transmission. We have determined that this non-enveloped tenuivirus uses its non-structural glycoprotein NSvc2 as a helper component to overcome the midgut barrier(s) for RSV replication and transmission. In the absence of this glycoprotein, purified RSV virions were unable to enter SBPH midgut cells. In the RSV-infected cells, this glycoprotein was processed into two mature proteins: an amino-terminal protein (NSvc2-N) and a carboxyl-terminal protein (NSvc2-C). Both NSvc2-N and NSvc2-C interact with RSV virions. Our results showed that the NSvc2-N could bind directly to the surface of midgut lumen via its N-glycosylation sites. Upon recognition, the midgut cells underwent endocytosis followed by compartmentalization of RSV virions and NSvc2 into early and then late endosomes. The NSvc2-C triggered cell membrane fusion via its highly conserved fusion loop motifs under the acidic condition inside the late endosomes, leading to the release of RSV virions from endosomes into cytosol. In summary, our results showed for the first time that a rice tenuivirus utilized its glycoprotein NSvc2 as a helper component to ensure a proper interaction between its virions and SBPH midgut cells for its circulative and propagative transmission.

Over 75% of the known plant viruses are insect transmitted. Understanding how plant viruses interact with their insect vectors during virus transmission is a key step towards the successful management of plant viruses worldwide. Several models for the direct or indirect virus–insect vector interactions have been proposed for the non-persistent or semi-persistent virus transmissions. However, the mechanisms controlling the interactions between viruses and their insect vector midgut barriers are poorly understood. In this study, we demonstrated that the circulative and propagative transmitted rice stripe virus (RSV) utilized its glycoprotein NSvc2 as a helper component to ensure a specific interaction between its virions and SBPH midgut cells to overcome the midgut barriers inside this vector. This is the first report of a viral helper component mediated mechanism for persistent-propagative virus transmission. Our new findings and working model should expand our knowledge on the molecular mechanism(s) controlling the interaction between virus and its insect vector during virus circulative and propagative transmission in nature.

The Epidemiology of Xylella fastidiosa; A Perspective on Current Knowledge and Framework to Investigate Plant Host-Vector-Pathogen Interactions

Insect-transmitted plant diseases caused by viruses, phytoplasmas, and bacteria share many features in common regardless of the causal agent. This perspective aims to show how a model framework, developed originally for plant virus diseases, can be modified for the case of diseases incited by Xylella fastidiosa. In particular, the model framework enables the specification of a simple but quite general invasion criterion defined in terms of key plant, pathogen, and vector parameters and, importantly, their interactions, which determine whether or not an incursion or isolated outbreak of a pathogen will lead to establishment, persistence, and subsequent epidemic development. Hence, this approach is applicable to the wide range of X. fastidiosa-incited diseases that have recently emerged in southern Europe, each with differing host plant, pathogen subspecies, and vector identities. Of particular importance are parameters relating to vector abundance and activity, transmission characteristics, and behavior in relation to preferences for host infection status. Some gaps in knowledge with regard to the developing situation in Europe are noted.

Host Plant Infection by Soybean Mosaic Virus Reduces the Fitness of Its Vector, Aphis glycines (Hemiptera: Aphididae)

Coevolutionary interactions between pathogens and their insect vectors can dramatically impact the fitness of herbivorous insects and patterns of plant disease transmission. Soybean mosaic virus (SMV) is a common disease in soybean production worldwide. Infected seed is the primary source of inoculum in fields and the virus is secondarily spread among plants by the soybean aphid, Aphis glycines Matsumura (Hemiptera: Aphididae), in a nonpersistent manner. In this study, we compared the biological fitness of A. glycines colonizing both SMV-infected and uninfected soybean plants. Aphids feeding on SMV-infected soybean seedlings were significantly smaller and lighter than those feeding on uninfected plants across life stages. SMV infection caused delayed development of aphid nymphs on soybean seedlings, but this was more than compensated by a reduction in the pre-reproductive period of apterous adults. The fecundity of A. glycines was reduced when feeding on SMV-infected seedlings, resulting in a lower reproductive rate, a longer generation time, and a slower population doubling time. A smaller proportion of aphid offspring developed into alatae when feeding on SMV-infected soybean seedling, and these took longer to mature than their counterparts on uninfected plants. We infer that SMV infection has significantly negative effects on the biological performance of A. glycines, which may be consistent with the long-term coevolution of SMV, soybean, and A. glycines in the transmission cycle of SMV.