Transmission of a New Polerovirus Infecting Pepper by the Whitefly Bemisia tabaci

New persistent plant RNA virus carries mutations to weaken viral suppression of antiviral RNA interference

Persistent plant viruses are widespread in natural ecosystems. However, little is known about why persistent infection with these viruses may cause little or no harm to their host. Here, we discovered a new polerovirus that persistently infected wild rice plants by deep sequencing and assembly of virus-derived small-interfering RNAs (siRNAs). The new virus was named Rice tiller inhibition virus 2 (RTIV2) based on the symptoms developed in cultivated rice varieties following Agrobacterium-mediated inoculation with an infectious RTIV2 clone. We showed that RTIV2 infection induced antiviral RNA interference (RNAi) in both the wild and cultivated rice plants as well as Nicotiana benthamiana. It is known that virulent virus infection in plants depends on effective suppression of antiviral RNAi by viral suppressors of RNAi (VSRs). Notably, the P0 protein of RTIV2 exhibited weak VSR activity and carries alanine substitutions of two amino acids broadly conserved among diverse poleroviruses. Mixed infection with umbraviruses enhanced RTIV2 accumulation and/or enabled its mechanical transmission in N. benthamiana. Moreover, replacing the alanine at either one or both positions of RTIV2 P0 enhanced the VSR activity in a co-infiltration assay, and RTIV2 mutants carrying the corresponding substitutions replicated to significantly higher levels in both rice and N. benthamiana plants. Together, our findings show that as a persistent plant virus, RTIV2 carries specific mutations in its VSR gene to weaken viral suppression of antiviral RNAi. Our work reveals a new strategy for persistent viruses to maintain long-term infection by weak suppression of the host defence response.

Exploring the Global Trends of Bacillus, Trichoderma and Entomopathogenic Fungi for Pathogen and Pest Control in Chili Cultivation

Chili, renowned globally and deeply ingrained in various cultures. Regrettably, the onset of diseases instigated by pests and pathogens has inflicted substantial losses on chili crops, with some farms experiencing complete production decimation. Challenges confronting chili cultivation include threats from pathogenic microbes like Xanthomonas, Fusarium, Phytophthora, Verticillium, Rhizoctonia, Colletotrichium and Viruses, alongside pests such as whiteflies, mites, thrips, aphids, and fruit flies. While conventional farming practices often resort to chemical pesticides to combat these challenges, their utilization poses substantial risks to both human health and the environment. In response to this pressing issue, this review aims to evaluate the potential of microbe-based biological control as eco-friendly alternatives to chemical pesticides for chili cultivation. Biocontrol agents such as Bacillus spp., Trichoderma spp., and entomopathogenic fungi present safer and more environmentally sustainable alternatives to chemical pesticides. However, despite the recognised potential of biocontrol agents, research on their efficacy in controlling the array of pests and pathogens affecting chili farming remains limited. This review addresses this gap by evaluating the efficiency of biocontrol agents, drawing insights from existing studies conducted in other crop systems, regarding pest and pathogen management. Notably, an analysis of Scopus publications revealed fewer than 30 publications in 2023 focused on these three microbial agents. Intriguingly, India, as the world's largest chili producer, leads in the number of publications concerning Bacillus spp., Trichoderma spp., and entomopathogenic fungi in chili cultivation. Further research on microbial agents is imperative to mitigate infections and reduce reliance on chemical pesticides for sustainable chili production.

Identification and Characterization of Hibiscus mutabilis Varieties Resistant to Bemisia tabaci and Their Resistance Mechanisms

Hibiscus mutabilis, the city flower of Chengdu, is culturally significant and has nutritional and medicinal benefits. However, frequent infestations of Bemisia tabaci have caused economic losses. This study aimed to identify insect-resistant H. mutabilis varieties. Over two years, varieties like Jinqiusong, Zuiyun, and Zuifurong showed moderate to high resistance based on reproductive indices. Assessments of antixenosis and developmental impacts revealed that adult B. tabaci exhibited low selectivity toward these resistant varieties, indicating a strong repellent effect. Gas chromatography-mass spectrometry analysis identified volatile organic compounds, such as alcohols, alkanes, and terpenes. Notably, 2-ethylhexanol and 6-methylheptanol exhibited repellent properties. Using nontargeted metabolomics, this study compared the metabolite profiles of the insect-resistant variety Jinqiusong (JQS), moderately resistant Bairihuacai (BRHC), and highly susceptible Chongbanbai (CBB) post B. tabaci infestation. Fifteen key metabolites were linked to resistance, emphasizing the phenylpropanoid biosynthesis pathway as crucial in defense. These findings offer a theoretical foundation for breeding insect-resistant H. mutabilis varieties and developing eco-friendly strategies against B. tabaci infestations.

A comprehensive framework for the delimitation of species within the Bemisia tabaci cryptic complex, a global pest-species group

Identifying cryptic species poses a substantial challenge to both biologists and naturalists due to morphological similarities. Bemisia tabaci is a cryptic species complex containing more than 44 putative species; several of which are currently among the world's most destructive crop pests. Interpreting and delimiting the evolution of this species complex has proved problematic. To develop a comprehensive framework for species delimitation and identification, we evaluated the performance of distinct data sources both individually and in combination among numerous samples of the B. tabaci species complex acquired worldwide. Distinct datasets include full mitogenomes, single-copy nuclear genes, restriction site-associated DNA sequencing, geographic range, host speciation, and reproductive compatibility datasets. Phylogenetically, our well-supported topologies generated from three dense molecular markers highlighted the evolutionary divergence of species of the B. tabaci complex and suggested that the nuclear markers serve as a more accurate representation of B. tabaci species diversity. Reproductive compatibility datasets facilitated the identification of at least 17 different cryptic species within our samples. Native geographic range information provides a complementary assessment of species recognition, while the host range datasets provide low rate of delimiting resolution. We further summarized different data performances in species classification when compared with reproductive compatibility, indicating that combination of mtCOI divergence, nuclear markers, geographic range provide a complementary assessment of species recognition. Finally, we represent a model for understanding and untangling the cryptic species complexes based on the evidence from this study and previously published articles.

Genomic variation in pepper vein yellows viruses in Australia, including a new putative variant, PeVYV-10

Since the first identification and full sequence of the polerovirus pepper vein yellows virus in Australia in 2016, virus surveys of crops and weeds have sporadically identified PeVYV in different hosts and locations. Genomic comparisons of 14 PeVYV-like isolates using RT-PCR products spanning the 3' end of the RdRp region (ORF 2), the intergenic region, ORF 3a, ORF 4, and ORF 3 (1388 nt) showed that four of the PeVYV isolates might be a new variant or PeVYV-like virus. From six PeVYV-positive plants, eight PeVYV-like sequences were obtained by high-throughput sequencing, as two hosts, 5352 and 5634, contained two slightly different PeVYV-like isolates. Three of the PeVYV-like isolates were most closely related to PeVYV-6 and PeVYV-5, and two isolates were closely related to PeVYV-9 and PeVYV-2. The other three isolates shared only 69-74% nucleotide sequence identity across the whole genome with any of the other PeVYVs, despite sharing 73-98%, 87-91%, and 84-87% amino acid sequence identity in ORF 3a, ORF 3, and the RdRp (ORF 2), respectively, suggesting that this virus is a new PeVYV-like virus, which we have tentatively called PeVYV-10. This is also the first report of a PeVYV-like virus infecting garlic.

Transcriptomic and Proteomic Analyses of Myzus persicae Carrying Brassica Yellows Virus

Viruses in the genus Polerovirus infect a wide range of crop plants and cause severe economic crop losses. BrYV belongs to the genus Polerovirus and is transmitted by Myzus persicae. However, the changes in transcriptome and proteome profiles of M. persicae during viral infection are unclear. Here, RNA-Seq and TMT-based quantitative proteomic analysis were performed to compare the differences between viruliferous and nonviruliferous aphids. In total, 1266 DEGs were identified at the level of transcription with 980 DEGs being upregulated and 286 downregulated in viruliferous aphids. At the protein level, among the 18 DEPs identified, the number of upregulated proteins in viruliferous aphids was twice that of the downregulated DEPs. Enrichment analysis indicated that these DEGs and DEPs were mainly involved in epidermal protein synthesis, phosphorylation, and various metabolic processes. Interestingly, the expressions of a number of cuticle proteins and tubulins were upregulated in viruliferous aphids. Taken together, our study revealed the complex regulatory network between BrYV and its vector M. persicae from the perspective of omics. These findings should be of great benefit to screening key factors involved in the process of virus circulation in aphids and provide new insights for BrYV prevention via vector control in the field.

A C2H2 zinc finger transcription factor of the whitefly Bemisia tabaci interacts with the capsid proteins of begomoviruses and inhibits virus retention

Begomoviruses are a group of ssDNA viruses exclusively transmitted by the whitefly Bemisia tabaci and constrain vegetable production in the old and new worlds. Although multiple molecular determinants governing the transmission of begomoviruses by whiteflies have been unravelled, factors critical for transmission majorly remain unknown. In this study, a whitefly C2H2 zinc finger (ZF) protein, 100% identical to the vascular endothelial ZF-like gene (vezf) protein was confirmed to interact with the CP of both old- and new-world begomoviruses. This was achieved by a yeast two-hybrid (Y2H) system screening of a whitefly cDNA library using capsid protein (CP) of TYLCV as a bait. In silico annotation of vezf protein revealed that it contains a N-terminal ZF-associated domain (ZAD) alongside multiple C2H2 ZF domains on the C-terminal end. ZAD-ZF proteins form the most abundant class of transcription factors within insects. Herein, we validated the interaction of vezf with four diverse begomoviruses and its functional role in begomovirus transmission. Silencing of the vezf gene of B. tabaci led to increased retention of three diverse begomoviruses tested. Vezf is the first insect transcription factor identified to interact with plant viruses and can be crucial to understand the possible mechanisms by which plant viruses modulate transcription of their insect vectors during transmission.

Begomoviruses: what is the secret(s) of their success?

Begomoviruses constitute an extremely successful group of emerging plant viruses transmitted by whiteflies of the Bemisia tabaci complex. Hosts include important vegetable, root, and fiber crops grown in the tropics and subtropics. Factors contributing to the ever-increasing diversity and success of begomoviruses include their predisposition to recombine their genomes, interaction with DNA satellites recruited throughout their evolution, presence of wild plants as a virus reservoir and a source of speciation, and extreme polyphagia and continuous movement of the insect vectors to temperate regions. These features as well as some controversial issues (replication in the insect vector, putative seed transmission, transmission by insects other than B. tabaci, and expansion of the host range to monocotyledonous plants) will be analyzed in this review.

Discovery of Brassica Yellows Virus and Porcine Reproductive and Respiratory Syndrome Virus in Diaphorina citri and Changes in Virome Due to Infection with 'Ca. L. asiaticus'

Detection of new viruses or new virus hosts is essential for the protection of economically important agroecosystems and human health. Increasingly, metatranscriptomic data are being used to facilitate this process. Such data were obtained from adult Asian citrus psyllids (ACP) (Diaphorina citri Kuwayama) that fed solely on mandarin (Citrus ×aurantium L.) plants grafted with buds infected with 'Candidatus Liberibacter asiaticus' (CLas), a phloem-limited bacterium associated with the severe Asian variant of huanglongbing (HLB), the most destructive disease of citrus. Brassica yellows virus (BrYV), the causative agent of yellowing or leafroll symptoms in brassicaceous plants, and its associated RNA (named as BrYVaRNA) were detected in ACP. In addition, the porcine reproductive and respiratory syndrome virus (PRRSV), which affects pigs and is economically important to pig production, was also found in ACP. These viruses were not detected in insects feeding on plants grafted with CLas-free buds. Changes in the concentrations of insect-specific viruses within the psyllid were caused by coinfection with CLas. IMPORTANCE The cross transmission of pathogenic viruses between different farming systems or plant communities is a major threat to plants and animals and, potentially, human health. The use of metagenomics is an effective approach to discover viruses and vectors. Here, we collected buds from the CLas-infected and CLas-free mandarin (Citrus ×aurantium L. [Rutaceae: Aurantioideae: Aurantieae]) trees from a commercial orchard and grafted them onto CLas-free mandarin plants under laboratory conditions. Through metatranscriptome sequencing, we first identified the Asian citrus psyllids feeding on plants grafted with CLas-infected buds carried the plant pathogen, brassica yellows virus and its associated RNA, and the swine pathogen, porcine reproductive and respiratory syndrome virus. These discoveries indicate that both viruses can be transmitted by grafting and acquired by ACP from CLas+ mandarin seedlings.

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.

Assessment of the RNA Silencing Suppressor Activity of Protein P0 of Pepper Vein Yellows Virus 5: Uncovering Natural Variability, Relevant Motifs and Underlying Mechanism

Pepper vein yellows virus 5 (PeVYV-5) belongs to a group of emerging poleroviruses (family Solemoviridae) which pose a risk to pepper cultivation worldwide. Since its first detection in Spain in 2013 and the determination of the complete genome sequence of an isolate in 2018, little is known on the presence, genomic variation and molecular properties of this pathogen. As other members of genus Polerovirus, PeVYV-5 encodes a P0 protein that was predicted to act as viral suppressor of RNA silencing (VSR), one of the major antiviral defense mechanisms in plants. The results of the present work have indicated that PeVYV-5 P0 is a potent VSR, which is able to induce the degradation of Argonaute (AGO) endonucleases, the main effectors of RNA silencing. New viral isolates have been identified in samples collected in 2020-2021 and sequencing of their P0 gene has revealed limited heterogeneity, suggesting that the protein is under negative selection. Analysis of natural and engineered P0 variants has pinpointed distinct protein motifs as critical for the VSR role. Moreover, a positive correlation between the VSR activity of the protein and its capability to promote AGO degradation could be established, supporting that such activity essentially relies on the clearance of core components of the RNA silencing machinery.

Detection of Apple Hammerhead Viroid, Apple Luteovirus 1 and Citrus Concave Gum-Associated Virus in Apple Propagation Materials and Orchards in the Czech Republic and Hungary

Grafting cultivars onto rootstocks is a widely used practice by the apple industry predominantly aimed at faster fruit bearing. Using high-throughput sequencing, we revealed the presence of recently described viral agents, namely apple hammerhead viroid (AHVd), apple luteovirus 1 (ALV-1), and citrus concave gum-associated virus (CCGaV), in germplasm collections and production orchards in the Czech Republic and Hungary. The HTS results were validated with RT-(q)PCR, and Northern blotting techniques. To obtain further insight about the presence of these agents, RT-PCR based surveys were carried out and showed their widespread presence alone or in mixed infections. The pathogens were present both in production areas and in feral samples. In addition, rootstock-to-scion transmission of ALV-1 and CCGaV was confirmed using commercial rootstock materials. Phylogenetic relationships based on partial sequences of distinct variants were also investigated. Furthermore, the rosy apple aphid was found to be ALV-1-positive, suggesting that it might be a potential vector of the virus.

Mitochondrial Genetic Diversity of Bemisia tabaci (Gennadius) (Hemiptera: Aleyrodidae) Associated with Cassava in Lao PDR

Cassava Mosaic Disease (CMD) caused by Sri Lankan cassava mosaic virus (SLCMV), has rapidly spread in Southeast Asia (SEA) since 2016. Recently it has been documented in Lao PDR. Previous reports have identified whitefly species of B. tabaci as potential vectors of CMD in SEA, but their occurrence and distribution in cassava fields is not well known. We conducted a countrywide survey in Lao PDR for adult whiteflies in cassava fields, and determined the abundance and genetic diversity of the B. tabaci species complex using mitochondrial cytochrome oxidase I (mtCOI) sequencing. In order to expedite the process, PCR amplifications were performed directly on whitefly adults without DNA extraction, and mtCOI sequences obtained using nanopore portable-sequencing technology. Low whitefly abundances and two cryptic species of the B. tabaci complex, Asia II 1 and Asia II 6, were identified. This is the first work on abundance and genetic identification of whiteflies associated with cassava in Lao PDR. This study indicates currently only a secondary role for Asia II in spreading CMD or as a pest. Routine monitoring and transmission studies on Asia II 6 should be carried out to establish its potential role as a vector of SLCMV in this region.

Comparative Analysis of Volatiles Emitted from Tomato and Pepper Plants in Response to Infection by Two Whitefly-Transmitted Persistent Viruses

The whitefly Bemisia tabaci is one of the most important agricultural pests due to its extreme invasiveness, insecticide resistance, and ability to transmit hundreds of plant viruses. Among these, Begomoviruses and recombinant whitefly-borne Poleroviruses are transmitted persistently. Several studies have shown that upon infection, plant viruses manipulate plant-emitted volatile organic compounds (VOCs), which have important roles in communication with insects. In this study, we profiled and compared the VOCs emitted by tomato and pepper plant leaves after infection with the Tomato yellow leaf curl virus (TYLCV) (Bogomoviruses) and the newly discovered Pepper whitefly-borne vein yellows virus (PeWBVYV) (Poleroviruses), respectively. The results identified shared emitted VOCs but also uncovered unique VOC signatures for each virus and for whitefly infestation (i.e., without virus infection) independently. The results suggest that plants have general defense responses; however, they are also able to respond individually to infection with specific viruses or infestation with an insect pest. The results are important to enhance our understanding of virus- and insect vector-induced alteration in the emission of plant VOCs. These volatiles can eventually be used for the management of virus diseases/insect vectors by either monitoring or disrupting insect-plant interactions.

Transcriptomic Changes of Bemisia tabaci Asia II 1 Induced by Chilli Leaf Curl Virus Trigger Infection and Circulation in Its Vector

Bemisia tabaci (Hemiptera: Aleyrodidae) is a highly efficient vector in the spread of chilli leaf curl virus (ChiLCV, Begomovirus) which is a major constraint in the production of chilli in South Asia. Transcriptome analysis of B. tabaci post-6 h acquisition of ChiLCV showed differential expression of 80 (29 upregulated and 51 downregulated) genes. The maximum number of DEGs are categorized under the biological processes category followed by cellular components and molecular functions. KEGG analysis of DEGs showed that the genes are involved in the functions like metabolism, signaling pathways, cellular processes, and organismal systems. The expression of highly expressed 20 genes post-ChiLCV acquisition was validated in RT-qPCR. DEGs such as cytosolic carboxypeptidase 3, dual-specificity protein phosphatase 10, 15, dynein axonemal heavy chain 17, fasciclin 2, inhibin beta chain, replication factor A protein 1, and Tob1 were found enriched and favored the virus infection and circulation in B. tabaci. The present study provides an improved understanding of the networks of molecular interactions between B. tabaci and ChiLCV. The candidate genes of B. tabaci involved in ChiLCV transmission would be novel targets for the management of the B. tabaci-begomovirus complex.

Effector-mediated plant-virus-vector interactions

Hemipterans (such as aphids, whiteflies, and leafhoppers) are some of the most devastating insect pests due to the numerous plant pathogens they transmit as vectors, which are primarily viral. Over the past decade, tremendous progress has been made in broadening our understanding of plant-virus-vector interactions, yet on the molecular level, viruses and vectors have typically been studied in isolation of each other until recently. From that work, it is clear that both hemipteran vectors and viruses use effectors to manipulate host physiology and successfully colonize a plant and that co-evolutionary dynamics have resulted in effective host immune responses, as well as diverse mechanisms of counterattack by both challengers. In this review, we focus on advances in effector-mediated plant-virus-vector interactions and the underlying mechanisms. We propose that molecular synergisms in vector-virus interactions occur in cases where both the virus and vector benefit from the interaction (mutualism). To support this view, we show that mutualisms are common in virus-vector interactions and that virus and vector effectors target conserved mechanisms of plant immunity, including plant transcription factors, and plant protein degradation pathways. Finally, we outline ways to identify true effector synergisms in the future and propose future research directions concerning the roles effectors play in plant-virus-vector interactions.

Construction of an Agrobacterium-mediated infectious cDNA clone of melon aphid-borne yellows virus

Melon aphid-borne yellows virus (MABYV), a member of the genus Polerovirus in the family Solemoviridae, has widely spread in recent years and cause yellowing disease on cucurbits. Here, we obtained the complete genome sequence of MABYV bottle guard (Lagenaria siceraria) isolate MABYV-KF, and constructed its infectious cDNA clone under the control of the cauliflower mosaic virus (CaMV) 35S promoter by Gibson assembly. The 5,677 nt of its genome shared more than 94.00% sequence identity with the two known MABYV isolates. The inoculation results showed that MABYV infectious cDNA clone could systemically infect bottle guard, cucumber and muskmelon plants, and cause typical yellowing symptom. The virus progeny from the infectious clone could be transmitted between bottle guard plants by aphid. Further analyses revealed that point mutations in the F-box-like motif (Pro57) and C-terminal conserved sequence (Phe211) of P0 cause low viral accumulations in systematic leaves and failed to induce symptom. The infectious clone will be potentially a tool in the investigation of viral pathogenesis, virus-virus interaction and virus-host/-vector interactions.

Topical Spray of dsRNA Induces Mortality and Inhibits Chilli Leaf Curl Virus Transmission by Bemisia tabaci Asia II 1

Chilli leaf curl virus (ChiLCV; genus: Begomovirus), transmitted by Bemisia tabaci (Gennadius) (Hemiptera: Aleyrodidae) in a persistent-circulative manner, is a major constraint in chilli production. The present study demonstrates for the first time that a topical spray of naked double-stranded RNA (dsRNA) on chilli plants causes mortality and inability to acquire and transmit ChiLCV in B. tabaci. dsRNA targeting heat shock protein 70 (hsp70) and fasciclin 2 (fas2) of B. tabaci Asia II 1 was first assessed under controlled conditions through oral delivery. Hsp70 and fas2 dsRNA resulted in up to 82.22% and 72% mortality of B. tabaci and around 12.4- and 8.5-fold decreases in mRNA levels, respectively, 24 h post-ingestion. ChiLCV copies in hsp70 dsRNA-fed B. tabaci steadily decreased with an increase in dsRNA concentration and were undetectable at a higher concentration of dsRNA. However, ChiLCV copies significantly increased in fas2 dsRNA-fed B. tabaci. Transmission of ChiLCV by B. tabaci was completely inhibited post-24 h feeding on hsp70 dsRNA at 3 μg/mL. Naked hsp70 dsRNA was topically sprayed on ChiLCV-infected chilli plants like an insecticide. 67.77% mortality of B. tabaci, 4.6-fold downregulation of hsp70 mRNA, and 1.34 × 10¹⁵-fold decreased ChiLCV copies in B. tabaci were recorded when adults were exposed to the dsRNA-treated plants under semi-field conditions. Foliar application of naked dsRNA reduced the ChiLCV transmission by 75% without any visible symptoms in the inoculated plants. A total of 2 consecutive sprays of dsRNA provided significant protection to B. tabaci for up to 20 days under semi-field conditions.

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.

Viruses Infecting Greenhood Orchids (Pterostylidinae) in Eastern Australia

The Australasian biogeographic realm is a major centre of diversity for orchids, with every subfamily of the Orchidaceae represented and high levels of endemism at the species rank. It is hypothesised that there is a commensurate diversity of viruses infecting this group of plants. In this study, we have utilised high-throughput sequencing to survey for viruses infecting greenhood orchids (Pterostylidinae) in New South Wales and the Australian Capital Territory. The main aim of this study was to characterise Pterostylis blotch virus (PtBV), a previously reported but uncharacterised virus that had been tentatively classified in the genus Orthotospovirus. This classification was confirmed by genome sequencing, and phylogenetic analyses suggested that PtBV is representative of a new species that is possibly indigenous to Australia as it does not belong to either the American or Eurasian clades of orthotospoviruses. Apart from PtBV, putative new viruses in the genera Alphaendornavirus, Amalgavirus, Polerovirus and Totivirus were discovered, and complete genome sequences were obtained for each virus. It is concluded that the polerovirus is likely an example of an introduced virus infecting a native plant species in its natural habitat, as this virus is probably vectored by an aphid, and Australia has a depauperate native aphid fauna that does not include any species that are host-adapted to orchids.

High Incidence of Strawberry Polerovirus 1 in the Czech Republic and Its Vectors, Genetic Variability and Recombination

In total, 332 strawberry plants from 33 different locations in the Czech Republic with or without disease symptoms were screened by RT-PCR for the presence of strawberry polerovirus 1 (SPV1) and five other viruses: strawberry mottle virus, strawberry crinkle virus, strawberry mild yellow edge virus, strawberry vein banding virus, and strawberry virus 1. SPV1 was detected in 115 tested strawberry plants (35%), including 89 mixed infections. No correlation between symptoms and the detected viruses was found. To identify potential invertebrate SPV1 vectors, strawberry-associated invertebrate species were screened by RT-PCR, and the virus was found in the aphids Aphis forbesi, A. gossypii, A. ruborum, A.sanquisorbae, Aulacorthum solani, Chaetosiphon fragaefolii, Myzus ascalonicus, and several other non-aphid invertebrate species. SPV1 was also detected in aphid honeydew. Subsequent tests of C. fragaefolii and A.gossypii virus transmission ability showed that at least 4 h of acquisition time were needed to acquire the virus. However, 1 day was sufficient for inoculation using C. fragaefolii. In conclusion, being aphid-transmitted like other tested viruses SPV1 was nevertheless the most frequently detected agent. Czech SPV1 isolates belonged to at least two phylogenetic clusters. The sequence analysis also indicated that recombination events influence evolution of SPV1 genomes.

Whole-genome single nucleotide polymorphism and mating compatibility studies reveal the presence of distinct species in sub-Saharan Africa Bemisia tabaci whiteflies

In sub-Saharan Africa cassava growing areas, two members of the Bemisia tabaci species complex termed sub-Saharan Africa 1 (SSA1) and SSA2 have been reported as the prevalent whiteflies associated with the spread of viruses that cause cassava mosaic disease (CMD) and cassava brown streak disease (CBSD) pandemics. At the peak of CMD pandemic in the late 1990s, SSA2 was the prevalent whitefly, although its numbers have diminished over the last two decades with the resurgence of SSA1 whiteflies. Three SSA1 subgroups (SG1 to SG3) are the predominant whiteflies in East Africa and vary in distribution and biological properties. Mating compatibility between SSA1 subgroups and SSA2 whiteflies was reported as the possible driver for the resurgence of SSA1 whiteflies. In this study, a combination of both phylogenomic methods and reciprocal crossing experiments were applied to determine species status of SSA1 subgroups and SSA2 whitefly populations. Phylogenomic analyses conducted with 26 548 205 bp whole genome single nucleotide polymorphisms (SNPs) and the full mitogenomes clustered SSA1 subgroups together and separate from SSA2 species. Mating incompatibility between SSA1 subgroups and SSA2 further demonstrated their distinctiveness from each other. Phylogenomic analyses conducted with SNPs and mitogenomes also revealed different genetic relationships among SSA1 subgroups. The former clustered SSA1-SG1 and SSA1-SG2 together but separate from SSA1-SG3, while the latter clustered SSA1-SG2 and SSA1-SG3 together but separate from SSA1-SG1. Mating compatibility was observed between SSA1-SG1 and SSA1-SG2, while incompatibility occurred between SSA1-SG1 and SSA1-SG3, and SSA1-SG2 and SSA1-SG3. Mating results among SSA1 subgroups were coherent with phylogenomics results based on SNPs but not the full mitogenomes. Furthermore, this study revealed that the secondary endosymbiont-Wolbachia-did not mediate reproductive success in the crossing assays carried out. Overall, using genome wide SNPs together with reciprocal crossings assays, this study established accurate genetic relationships among cassava-colonizing populations, illustrating that SSA1 and SSA2 are distinct species while at least two species occur within SSA1 species.

First evidence for thermal tolerance benefits of the bacterial symbiont Cardinium in an invasive whitefly, Bemisia tabaci

BACKGROUD

Cardinium symbiont is a maternally inherited bacterial endosymbiont and widely spreads in arthropods including Bemisia tabaci (Hemiptera: Aleyrodidae). However, the potential role of Cardinium played in the biology of their hosts is largely unknown. In two genetic lines (i.e. LS and SG lines) of B. tabaci MED, collected from different locations in China, we tested the effects of Cardinium on the performance of the host whitefly under a constant high temperature (31 °C) using the age-stage two-sex life table method, and explored the genes influenced by Cardinium-infection by RNA-sequencing.

RESULTS

We found that Cardinium did provide protection of B. tabaci against heat stress under 31 °C. However, there was a significant connection between Cardinium-infection and whitefly genetic backgrounds. Performance revealed that Cardinium infection can increase the longevity of both female and male adults and oviposition periods in both lines, but it also conferred benefits of fecundity and pre-adult period to LS line. Additionally, the population parameters such as intrinsic rate of increase (r), finite rate of increase (λ) and mean generation time (T) demonstrated that Cardinium infection conferred fitness benefits to LS line but not to SG line. Transcriptome analysis indicated that several genes related to homeostasis and metamorphosis such as ubiquitin-related genes were highly expressed in Cardinium-infected B. tabaci.

CONCLUSION

The research provided the first evidence that Cardinium can increase the thermal tolerance of whitefly, which may be associated with host genetic background.

Factors Determining Transmission of Persistent Viruses by Bemisia tabaci and Emergence of New Virus-Vector Relationships

Many plant viruses depend on insect vectors for their transmission and dissemination. The whitefly Bemisia tabaci (Hemiptera: Aleyrodidae) is one of the most important virus vectors, transmitting more than four hundred virus species, the majority belonging to begomoviruses (Geminiviridae), with their ssDNA genomes. Begomoviruses are transmitted by B. tabaci in a persistent, circulative manner, during which the virus breaches barriers in the digestive, hemolymph, and salivary systems, and interacts with insect proteins along the transmission pathway. These interactions and the tissue tropism in the vector body determine the efficiency and specificity of the transmission. This review describes the mechanisms involved in circulative begomovirus transmission by B. tabaci, focusing on the most studied virus in this regard, namely the tomato yellow leaf curl virus (TYLCV) and its closely related isolates. Additionally, the review aims at drawing attention to the recent knowhow of unorthodox virus—B. tabaci interactions. The recent knowledge of whitefly-mediated transmission of two recombinant poleroviruses (Luteoviridae), a virus group with an ssRNA genome and known to be strictly transmitted with aphids, is discussed with its broader context in the emergence of new whitefly-driven virus diseases.

Assessing the diversity of whiteflies infesting cassava in Brazil

Background

The necessity of a competent vector for transmission is a primary ecological factor driving the host range expansion of plant arthropod-borne viruses, with vectors playing an essential role in disease emergence. Cassava begomoviruses severely constrain cassava production in Africa. Curiously, begomoviruses have never been reported in cassava in South America, the center of origin for this crop. It has been hypothesized that the absence of a competent vector in cassava is the reason why begomoviruses have not emerged in South America.

Methods

We performed a country-wide whitefly diversity study in cassava in Brazil. Adults and/or nymphs of whiteflies were collected from sixty-six cassava fields in the main agroecological zones of the country. A total of 1,385 individuals were genotyped based on mitochondrial cytochrome oxidase I sequences.

Results

A high species richness was observed, with five previously described species and two putative new ones. The prevalent species were Tetraleurodes acaciae and Bemisia tuberculata, representing over 75% of the analyzed individuals. Although we detected, for the first time, the presence of Bemisia tabaci Middle East-Asia Minor 1 (BtMEAM1) colonizing cassava in Brazil, it was not prevalent. The species composition varied across regions, with fields in the Northeast region showing a higher diversity. These results expand our knowledge of whitefly diversity in cassava and support the hypothesis that begomovirus epidemics have not occurred in cassava in Brazil due to the absence of competent vector populations. However, they indicate an ongoing adaptation process of BtMEAM1 to cassava, increasing the likelihood of begomovirus emergence in this crop.

Genomic characterization of two novel viruses infecting Barleria cristata L. from the genera Orthotospovirus and Polerovirus

Barleria cristata L. has become naturalized in South Africa, where it is commonly used as an ornamental. In 2019, plants of B. cristata showing putative viral symptoms were collected from two locations in Gauteng, South Africa. RNAtag-seq libraries were prepared and sequenced using an Illumina HiSeq 2500 platform. De novo assembly of the resulting data revealed the presence of a novel member of the family Tospoviridae associated with the plants from both locations, and this virus was given the tentative name "barleria chlorosis-associated virus". Segments L, M, and S have lengths of 8752, 4760, and 2906 nt, respectively. Additionally, one of the samples was associated with a novel polerovirus, provisionally named "barleria polerovirus 1", with a complete genome length of 6096 nt. This is the first study to show the association of viruses with a member of the genus Barleria.

Competitive Interactions Between Whitefly- and Aphid-Transmitted Poleroviruses Within the Plant Host and the Insect Vectors

Pepper cultivation in Israel has been constrained by two sympatric poleroviruses, Pepper vein yellows virus-2 (PeVYV-2) and Pepper whitefly-borne vein yellows virus (PeWBVYV), which are transmitted specifically by aphids and whiteflies, respectively. This study investigated the interaction between PeVYV-2 and PeWBVYV inside the host plant and the insect vectors. Our results show that PeVYV-2 and PeWBVYV compete against each other inside the host plant and also inside aphids. PeWBVYV was the weaker competitor inside the host plant, with diminished transmission rates when inoculated simultaneously or successively after PeVYV-2 and could only be transmitted efficiently when inoculated first and then challenged by PeVYV-2. Successive inoculations of plants with viruliferous whiteflies with PeWBVYV followed by viruliferous aphids with PeVYV-2 led to a coinfection rate of 60%, but with severely reduced titers of PeWBVYV in the coinfected plants compared with singly infected plants. In contrast, PeVYV-2 was the weaker competitor inside the insect vector, with reduced quantities of the acquired virus and a reduced transmission rate by aphids when given prior acquisition on PeWBVYV. However, we also show that the transmission efficiency of PeVYV-2 and PeWBVYV from coinfected plants by whiteflies and aphids remained comparable to that from singly infected plants. This is likely attributable to the reduced titers of PeWBVYV inside coinfected plants causing lesser impact on transmission of PeVYV-2 by aphids and the stronger competitiveness of PeWBVYV inside the whitefly. Competitive interactions between PeVYV-2 and PeWBVYV inside the host plant and insect vector can thus be beneficial for their coexistence.

Transmission parameters of pepper whitefly-borne vein yellows virus (PeWBVYV) by Bemisia tabaci and identification of an insect protein with a putative role in polerovirus transmission

Pepper crops in Israel are infected by poleroviruses, Pepper vein yellows virus 2 (PeVYV-2) and Pepper whitefly-borne vein yellows virus (PeWBVYV). Herein we characterize the transmission of PeWBVYV and the aphid-transmitted PeVYV-2, and show that PeWBVYV is specifically transmitted by MEAM1 species of the whitefly Bemisia tabaci, with a minimum latency period of 120 h, and not by the Mediterranean (MED). PeWBVYV and PeVYV-2 were detected in the hemolymph of MED and MEAM1, respectively, however, amounts of PeWBVYV in the hemolymph of MED or PeVYV-2 in MEAM1 were much lower than PeWBVYV in hemolymph of MEAM1. Moreover, we show that PeWBVYV does not interact with the GroEL protein of the symbiont Hamiltonella and thus does not account for the non-transmissibility by MED. An insect glycoprotein, C1QBP, interacting in vitro with the capsid proteins of both PeWBVYV and PeVYV-2 is reported which suggests a putative functional role in polerovirus transmission.

Pod pepper vein yellows virus, a new recombinant polerovirus infecting Capsicum frutescens in Yunnan province, China

Pepper vein yellows viruses (PeVYV) are phloem-restricted viruses in the genus Polerovirus, family Luteoviridae. Typical viral symptoms of PeVYV including interveinal yellowing of leaves and upward leaf curling were observed in pod pepper plants (Capsicum frutescens) growing in Wenshan city, Yunnan province, China. The complete genome sequence of a virus from a sample of these plants was determined by next-generation sequencing and RT-PCR. Pod pepper vein yellows virus (PoPeVYV) (MT188667) has a genome of 6015 nucleotides, and the characteristic genome organization of a member of the genus Polerovirus. In the 5′ half of its genome (encoding P0 to P4), PoPeVYV is most similar (93.1% nt identity) to PeVYV-3 (Pepper vein yellows virus 3) (KP326573) but diverges greatly in the 3′-part encoding P5, where it is most similar (91.7% nt identity) to tobacco vein distorting virus (TVDV, EF529624) suggesting a recombinant origin. Recombination analysis predicted a single recombination event affecting nucleotide positions 4126 to 5192 nt, with PeVYV-3 as the major parent but with the region 4126–5192 nt derived from TVDV as the minor parent. A full-length clone of PoPeVYV was constructed and shown to be infectious in C. frutescens by RT-PCR and the presence of icosahedral viral particles.

Win by Quantity: a Striking Rickettsia-Bias Symbiont Community Revealed by Seasonal Tracking in the Whitefly Bemisia tabaci

Maintaining an adaptive seasonality is a basic ecological requisite for cold-blooded organism insects which usually harbor various symbionts. However, how coexisting symbionts coordinate in insects during seasonal progress is still unknown. The whitefly Bemisia tabaci in China harbors the obligate symbiont Portiera that infects each individual, as well as various facultative symbionts. In this study, we investigated whitefly populations in cucumber and cotton fields from May to December 2019, aiming to reveal the fluctuations of symbiont infection frequencies, symbiont coordination in multiple infected individuals, and host plants effects on symbiont infections. The results indicated that the facultative symbionts Hamiltonella (H), Rickettsia (R), and Cardinium (C) exist in field whiteflies, with single (H) and double (HC and HR) infections occurring frequently. Infection frequencies of Hamiltonella (always 100%) and Cardinium (29.50-34.38%) remained steady during seasonal progression. Rickettsia infection frequency in the cucumber whitefly population decreased from 64.47% in summer to 35.29% in winter. Significantly lower Rickettsia infection frequency (15.55%) was identified in cotton whitefly populations and was not subject to seasonal fluctuation. Nevertheless, Rickettsia had a significantly quantitative advantage in the symbiont community of whitefly individuals and populations from both cucumber and cotton field all through the seasons. Moreover, higher Portiera and Hamiltonella densities were found in HC and HR whitefly than in H whitefly, suggesting these symbionts may contribute to producing nutrients for their symbiont partners. These results provide ample cues to further explore the interactions between coexisting symbionts, the coevolutionary relationship between symbionts and host symbiont-induced effects on host plant use.

First report of cDNA clone-launched infection of maize plants with the polerovirus maize yellow mosaic virus (MaYMV)

An East African isolate of the maize-associated polerovirus, maize yellow mosaic virus (MaYMV) was previously shown to cause leaf reddening on singly infected maize plants (Zea mays). Here we describe the construction of a full-length infectious clone of an East African isolate and, for the first time, show infectivity of clone-derived transcripts in the primary host, maize, through vascular puncture inoculation (VPI), as well as in the dicotyledonous research model plant species, Nicotiana benthamiana, through agrobacterium inoculation. Characteristic leaf reddening symptoms were observed in a subset of maize plants inoculated with clone-derived transcripts, and infection was confirmed by RT-PCR and Northern blot analyses. In N. benthamiana plants, infections were entirely asymptomatic even at high virus titers, as was also reported for the cloned Chinese isolate. In this study, however, we demonstrated that N. benthamiana can serve as a clone launching platform for maize infection, as VPI of sap of infected N. benthamiana plants into maize kernels resulted in infection and the typical red leaf symptoms. We further demonstrated that the cloned East African isolate virus was aphid transmissible to maize, with experimental transmission rates up to 97 %, comparable to that shown previously for the native virus. Interestingly, our data additionally showed a definitive correlation of leaf reddening symptoms with increased expression of chalcone synthase, thus suggesting upregulation of the flavonoid biosynthesis pathway as the molecular basis for symptom induction in maize. As the first report of experimental infection of maize with transcripts from a cloned polerovirus, this work constitutes a breakthrough for studies on molecular maize-polerovirus-aphid interactions.

Genetic diversity of whitefly (Bemisia spp.) on crop and uncultivated plants in Uganda: implications for the control of this devastating pest species complex in Africa

Over the past three decades, highly increased whitefly (Bemisia tabaci) populations have been observed on the staple food crop cassava in eastern Africa and associated with ensuing viral disease pandemics and food insecurity. Increased whitefly numbers have also been observed in other key agricultural crops and weeds. Factors behind the population surges on different crops and their interrelationships are unclear, although in cassava they have been associated with specific populations within the Bemisia tabaci species complex known to infest cassava crops in Africa. This study carried out an in-depth survey to understand the distribution of B. tabaci populations infesting crops and uncultivated plant hosts in Uganda, a centre of origin for this pest complex. Whitefly samples were collected from 59 identified plant species and 25 unidentified weeds in a countrywide survey. Identities of 870 individual adult whiteflies were determined through mitochondrial cytochrome oxidase 1 sequences (651 bp) in the 3' barcode region used for B. tabaci systematics. Sixteen B. tabaci and five related whitefly putative species were identified based on > 4.0% nucleotide divergence, of which three are proposed as novel B. tabaci putative species and four as novel closely related whitefly species. The most prevalent whiteflies were classified as B. tabaci MED-ASL (30.5% of samples), sub-Saharan Africa 1 (SSA1, 22.7%) and Bemisia Uganda1 (12.1%). These species were also indicated to be the most polyphagous occurring on 33, 40 and 25 identified plant species, respectively. Multiple (≥ 3) whitefly species occurred on specific crops (bean, eggplant, pumpkin and tomato) and weeds (Sida acuta and Ocimum gratissimum). These plants may have increased potential to act as reservoirs for mixed infections of whitefly-vectored viruses. Management of whitefly pest populations in eastern Africa will require an integration of approaches that consider their degree of polyphagy and a climate that enables the continuous presence of crop and uncultivated plant hosts.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s10340-021-01355-6.

Population Dynamics of Whiteflies and Associated Viruses in South America: Research Progress and Perspectives

Simple Summary

Whiteflies are one of the most important and widespread pests in the world. In South America, the currently most important species occurring are Bemisia afer,Trialeurodes vaporariorum, and the cryptic species Middle East-Asia Minor 1, Mediterranean, and New World, from Bemisia tabaci complex. The present review compiles information from several studies conducted in South America regarding these insects, providing data related to the dynamics and distribution of whiteflies, the associated viruses, and the management strategies to keep whiteflies under the economic damage threshold.

Abstract

By having an extensive territory and suitable climate conditions, South America is one of the most important agricultural regions in the world, providing different kinds of vegetable products to different regions of the world. However, such favorable conditions for plant production also allow the development of several pests, increasing production costs. Among them, whiteflies (Hemiptera: Aleyrodidae) stand out for their potential for infesting several crops and for being resistant to insecticides, having high rates of reproduction and dispersal, besides their efficient activity as virus vectors. Currently, the most important species occurring in South America are Bemisia afer, Trialeurodes vaporariorum, and the cryptic species Middle East-Asia Minor 1, Mediterranean, and New World, from Bemisia tabaci complex. In this review, a series of studies performed in South America were compiled in an attempt to unify the advances that have been developed in whitefly management in this continent. At first, a background of the current whitefly distribution in South American countries as well as factors affecting them are shown, followed by a background of the whitefly transmitted viruses in South America, addressing their location and association with whiteflies in each country. Afterwards, a series of management strategies are proposed to be implemented in South American fields, including cultural practices and biological and chemical control, finalizing with a section containing future perspectives and directions for further research.

A proteomic approach reveals possible molecular mechanisms and roles for endosymbiotic bacteria in begomovirus transmission by whiteflies

BACKGROUND

Many plant viruses are vector-borne and depend on arthropods for transmission between host plants. Begomoviruses, the largest, most damaging and emerging group of plant viruses, infect hundreds of plant species, and new virus species of the group are discovered each year. Begomoviruses are transmitted by members of the whitefly Bemisia tabaci species complex in a persistent-circulative manner. Tomato yellow leaf curl virus (TYLCV) is one of the most devastating begomoviruses worldwide and causes major losses in tomato crops, as well as in many agriculturally important plant species. Different B. tabaci populations vary in their virus transmission abilities; however, the causes for these variations are attributed among others to genetic differences among vector populations, as well as to differences in the bacterial symbionts housed within B. tabaci.

RESULTS

Here, we performed discovery proteomic analyses in 9 whitefly populations from both Middle East Asia Minor I (MEAM1, formerly known as B biotype) and Mediterranean (MED, formerly known as Q biotype) species. We analysed our proteomic results on the basis of the different TYLCV transmission abilities of the various populations included in the study. The results provide the first comprehensive list of candidate insect and bacterial symbiont (mainly Rickettsia) proteins associated with virus transmission.

CONCLUSIONS

Our data demonstrate that the proteomic signatures of better vector populations differ considerably when compared with less efficient vector populations in the 2 whitefly species tested in this study. While MEAM1 efficient vector populations have a more lenient immune system, the Q efficient vector populations have higher abundance of proteins possibly implicated in virus passage through cells. Both species show a strong link of the facultative symbiont Rickettsia to virus transmission.

Pepper vein yellows virus 9: a novel polerovirus isolated from chili pepper in Indonesia

In 2017, a leaf sample from a single chili pepper (Capsicum annuum) plant exhibiting yellowing was collected from Aceh province, Indonesia. Total RNA was extracted from this sample, and RNA-Seq analysis was conducted. Putative infecting viruses were detected by mapping the obtained reads to the full-length viral genome sequences available in the GenBank database (7457 sequences) and the de novo-assembled contigs. RNA-Seq analysis detected polerovirus, begomovirus, and amalgavirus sequences, and the polerovirus-like sequences showed strong similarity to those of previously reported pepper vein yellows viruses (PeVYVs). The complete viral genome sequence obtained by RT-PCR had a length of 6023 nt, had the typical genome organization of a polerovirus and showed a high degree of sequence similarity to PeVYV-2 from Israel. Moreover, the predicted amino acid sequence of the P0 protein of the Indonesian isolate was 85.1% to 88.8% identical to those of other PeVYVs. In accordance with the polerovirus species demarcation criteria, this isolate should be assigned to a new polerovirus species, and we propose the name "pepper vein yellows virus 9" (PeVYV-9) for this virus.

Transmission of the Bean-Associated Cytorhabdovirus by the Whitefly Bemisia tabaci MEAM1

The knowledge of genomic data of new plant viruses is increasing exponentially; however, some aspects of their biology, such as vectors and host range, remain mostly unknown. This information is crucial for the understanding of virus–plant interactions, control strategies, and mechanisms to prevent outbreaks. Typically, rhabdoviruses infect monocot and dicot plants and are vectored in nature by hemipteran sap-sucking insects, including aphids, leafhoppers, and planthoppers. However, several strains of a potentially whitefly-transmitted virus, papaya cytorhabdovirus, were recently described: (i) bean-associated cytorhabdovirus (BaCV) in Brazil, (ii) papaya virus E (PpVE) in Ecuador, and (iii) citrus-associated rhabdovirus (CiaRV) in China. Here, we examine the potential of the Bemisia tabaci Middle East-Asia Minor 1 (MEAM1) to transmit BaCV, its morphological and cytopathological characteristics, and assess the incidence of BaCV across bean producing areas in Brazil. Our results show that BaCV is efficiently transmitted, in experimental conditions, by B. tabaci MEAM1 to bean cultivars, and with lower efficiency to cowpea and soybean. Moreover, we detected BaCV RNA in viruliferous whiteflies but we were unable to visualize viral particles or viroplasm in the whitefly tissues. BaCV could not be singly isolated for pathogenicity tests, identification of the induced symptoms, and the transmission assay. BaCV was detected in five out of the seven states in Brazil included in our study, suggesting that it is widely distributed throughout bean producing areas in the country. This is the first report of a whitefly-transmitted rhabdovirus.

New features on the genomic organization of a novel dicistrovirus identified from the sweet potato whitefly Bemisia tabaci

The whitefly Bemisia tabaci is an agricultural pest causing large economic losses worldwide. We analysed the genomic sequence of a new viral member of the family Dicistroviridae identified by high-throughput sequencing of total RNA extracted from whiteflies. The virus, tentatively named Bemisia-associated dicistrovirus 2 (BaDV-2), has a genome of 8012 nucleotides with a polyadenylated 3' end. In contrast to typical dicistroviruses, BaDV-2 has a genome containing three open reading frames (ORFs) encoding predicted proteins of 1078 (ORF1a), 481 (ORF1b) and 834 (ORF2) amino acids, which correspond to replicase A (containing helicase and cysteine protease domains), replicase B (a domain of an RNA-dependent RNA polymerase - RdRP) and capsid proteins, respectively. The 3' end of ORF1a contains a potential frameshift signal, suggesting that ORF1a and ORF1b may be expressed as a single polyprotein (replicase^FS), corresponding to other dicistroviruses. The BaDV-2 genomic sequence shares the highest nucleotide identity (61.1 %) with Bemisia-associated dicistrovirus 1 (BaDV-1), another dicistrovirus identified from whiteflies. The full BaDV-2 replicase^FS polyprotein clustered with aparaviruses, whereas the capsid polyprotein clustered with cripaviruses in phylogenetic analyses, as with BaDV-1. The intergenic region (IGR) between ORF1b and ORF2 is predicted to adopt a secondary structure with atypical features that resembles the dicistrovirus IGR IRES structure. Our analyses indicate that BaDV-2 is a novel dicistrovirus and that BaDV-2 together with BaDV-1 may not be appropriately grouped in any of the three currently accepted dicistrovirus genera.

Molecular and Biological Characterization of a New World Mono-/Bipartite Begomovirus/Deltasatellite Complex Infecting Corchorus siliquosus

The genus Begomovirus (family Geminiviridae) is the largest genus in the entire virosphere, with more than 400 species recognized. Begomoviruses are single-stranded DNA plant viruses transmitted by whiteflies of the Bemisia tabaci complex and are considered one of the most important groups of emerging plant viruses in tropical and subtropical regions. Several types of DNA satellites have been described to be associated with begomoviruses: betasatellites, alphasatellites, and deltasatellites. Recently, a family of single-stranded DNA satellites associated with begomoviruses has been created, Tolecusatellitidae, including the genera Betasatellite and Deltasatellite. In this work, we analyzed the population of begomoviruses and associated DNA satellites present in Corchorus siliquosus, a malvaceous plant growing wild in Central America, southeastern North America and the Caribbean, collected in Cuba. The genomes of isolates of two New World begomoviruses [(Desmodium leaf distortion virus (DesLDV) and Corchorus yellow vein Cuba virus (CoYVCUV)] and two deltasatellites [tomato yellow leaf distortion deltasatellite 2 (TYLDD2) and Desmodium leaf distortion deltasatellite (DesLDD)] have been cloned and sequenced from plants showing yellow vein symptoms. Isolates of one of the begomoviruses, CoYVCUV, and one of the deltasatellites, DesLDD, represent novel species. Experiments with infectious clones showed the monopartite nature of CoYVCUV and that DesLDD utilizes the bipartite DesLDV as helper virus, but not the monopartite CoYVCUV. Also, CoYVCUV was shown to infect common bean in addition to Nicotiana benthamiana. This is the first time that (i) a monopartite New World begomovirus is found in a host other than tomato and (ii) deltasatellites have been found in C. siliquosus, thus extending the host and helper virus ranges of this recently recognized class of DNA satellites.

A Recently Discovered Maize Polerovirus Causes Leaf Reddening Symptoms in Several Maize Genotypes and is Transmitted by Both the Corn Leaf Aphid (Rhopalosiphum maidis) and the Bird Cherry-Oat Aphid (Rhopalosiphum padi)

A maize-infecting polerovirus variously named maize yellow dwarf virus RMV2 (MYDV-RMV2) and maize yellow mosaic virus (MaYMV) has been discovered and previously described in East Africa, Asia, and South America. It was identified in virus surveys in these locations instigated by outbreaks of maize lethal necrosis (MLN), known to be caused by coinfections of unrelated maize chlorotic mottle virus (MCMV) and any of several maize-infecting potyviruses, and was often found in coinfections with MLN viruses. Although sequenced in many locations globally and named for symptoms of related or coinfecting viruses, and with an infectious clone reported that experimentally infects Nicotiana benthamiana, rudimentary biological characterization of MaYMV in maize, including insect vector(s) and symptoms in single infections, has not been reported until now. We report isolation from other viruses and leaf tip reddening symptoms in several maize genotypes, along with transmission by two aphids, Rhopalosiphum padi and Rhopalosiphum maidis. This is important information distinguishing this virus and demonstrating that in single infections it causes symptoms distinct from those of potyviruses or MCMV in maize, and identification of vectors provides an important framework for determination of potential disease impact and management.

Levels of the endosymbiont Rickettsia in the whitefly Bemisia tabaci are influenced by the expression of vitellogenin

Bacterial endosymbionts play essential roles in the biology of their arthropod hosts by interacting with internal factors in the host. The whitefly Bemisia tabaci is a worldwide agricultural pest and a supervector for more than 100 plant viruses. Like many other arthropods, Be. tabaci harbours a primary endosymbiont, Porteira aleyrodidarum, and an array of secondary endosymbionts that coexist with Portiera inside bacteriocyte cells. Unlike all of the other secondary symbionts that infect Be. tabaci, Rickettsia has been shown to be an exception by infecting insect organs and not colocalizing with Portiera, and has been shown to significantly impact the insect biology and its interactions with the environment. Little is known about the molecular interactions that underlie insect-symbiont interactions in general, and particularly Be. tabaci-Rickettsia interactions. Here we performed transcriptomic analysis and identified vitellogenin as an important protein that influences the levels of Rickettsia in Be. tabaci. Vitellogenin expression levels were lower in whole insects, but higher in midguts of Rickettsia-infected insects. Immunocapture-PCR assay showed interaction between vitellogenin and Rickettsia, whereas silencing of vitellogenin resulted in nearly complete disappearance of Rickettsia from midguts. Altogether, these results suggest that vitellogenin plays an important role in influencing the levels of Rickettsia in Be. tabaci.

Transmission of Begomoviruses and Other Whitefly-Borne Viruses: Dependence on the Vector Species

Most plant viruses require a biological vector to spread from plant to plant in nature. Among biological vectors for plant viruses, hemipteroid insects are the most common, including phloem-feeding aphids, whiteflies, mealybugs, planthoppers, and leafhoppers. A majority of the emerging diseases challenging agriculture worldwide are insect borne, with those transmitted by whiteflies (Hemiptera: Aleyrodidae) topping the list. Most damaging whitefly-transmitted viruses include begomoviruses (Geminiviridae), criniviruses (Closteroviridae), and torradoviruses (Secoviridae). Among the whitefly vectors, Bemisia tabaci, now recognized as a complex of cryptic species, is the most harmful in terms of virus transmission. Here, we review the available information on the differential transmission efficiency of begomoviruses and other whitefly-borne viruses by different species of whiteflies, including the cryptic species of the B. tabaci complex. In addition, we summarize the factors affecting transmission of viruses by whiteflies and point out some future research prospects.

Complete Genome Sequence of a Putative Densovirus Infecting the Carrot Psyllid Bactericera trigonica

Here, we report the genome of a putative densovirus infecting the carrot psyllid Bactericera trigonica, obtained by inverse PCR and named Bactericera trigonica densovirus (BtDNV). The ambisense genome of BtDNV is identical in structure to those of the ambidensoviruses, and its encoded proteins share the highest sequence identity with the Asian citrus psyllid Diaphorina citri densovirus.

Here, we report the genome of a putative densovirus infecting the carrot psyllid Bactericera trigonica, obtained by inverse PCR and named Bactericera trigonica densovirus (BtDNV). The ambisense genome of BtDNV is identical in structure to those of the ambidensoviruses, and its encoded proteins share the highest sequence identity with the Asian citrus psyllid Diaphorina citri densovirus.