Brevipalpus yothersi Baker (Tenuipalpidae) development in sweet orange plants is influenced by previous mite infestation and the presence of shelters.

Citrus leprosis is the most important viral disease affecting citrus. The disease is caused predominantly by CiLV-C and is transmitted by Brevipalpus yothersi Baker mites. This study brings some insight into the colonization of B. yothersi in citrus [(Citrus × sinensis (L.) Osbeck (Rutaceae)] previously infested by viruliferous or non-viruliferous B. yothersi. It also assesses the putative role of shelters on the behavior of B. yothersi. Expression of PR1 and PR4 genes, markers of plant defense mechanisms, were evaluated by RT-qPCR to correlate the role of the plant hormonal changes during the tri-trophic virus-mite-plant interplay. A previous infestation with either non-viruliferous and viruliferous mites positively influenced oviposition and the number of adult individuals in the resulting populations. Mite populations were higher on branches that had received a previous mite infestation than branches that did not. There was an increase in the expression of PR4, a marker gene in the jasmonic acid (JA) pathway, in the treatment with non-viruliferous mites, indicating a response from the plant to their feeding. Conversely, an induced expression of PR1, a marker gene in the salicylic acid (SA) pathway, was observed mainly in the treatment with viruliferous mites, which suggests the activation of a plant response against the pathogen. The earlier mite infestation, as well as the presence of leprosis lesions and a gypsum mixture as artificial shelters, all fostered the growth of the B. yothersi populations after the second infestation, regardless of the presence or absence of CiLV-C. Furthermore, it is suggested that B. yothersi feeding actually induces the JA pathway in plants. At the same time, the CiLV-C represses the JA pathway and induces the SA pathway, which benefits the mite vector.

ICTV Virus Taxonomy Profile: Rhabdoviridae 2022.

The family Rhabdoviridae comprises viruses with negative-sense (-) RNA genomes of 10-16 kb. Virions are typically enveloped with bullet-shaped or bacilliform morphology but can also be non-enveloped filaments. Rhabdoviruses infect plants or animals, including mammals, birds, reptiles, amphibians or fish, as well as arthropods, which serve as single hosts or act as biological vectors for transmission to animals or plants. Rhabdoviruses include important pathogens of humans, livestock, fish or agricultural crops. This is a summary of the International Committee on Taxonomy of Viruses (ICTV) Report on the family Rhabdoviridae, which is available at ictv.global/report/rhabdoviridae.

Viruses representing two new genomovirus species identified in citrus from Tunisia.

Using a high-throughput sequencing approach, we identified four genomoviruses (family Genomoviridae) associated with a sweet orange (Citrus sinensis) plant collected in Tunisia. The ssDNA genomes of these genomoviruses, which were amplified, cloned and Sanger sequenced, range in size from 2156 to 2191 nt. Three of these viruses share > 99% full-genome pairwise sequence identity and are referred to as citrus Tunisia genomovirus 1 (CTNGmV-1). The CTNGmV-1 isolates share < 62% genome-wide pairwise nucleotide sequence identity with other genomoviruses and belong to the genus Gemykolovirus. The genome of the fourth virus, which was called CTNGmV-2, shares < 68% nucleotide sequence identity with other genomoviruses and belongs to the genus Gemycircularvirus. Based on the species demarcation criteria for members of the family Genomoviridae, CTNGmV-1 and -2 would each represent a new species. Although found associated with Citrus sp. plants, it is likely that these viruses infect fungi or other organisms associated with the plants.

Unveiling the complete genome sequence of clerodendrum chlorotic spot virus, a putative dichorhavirus infecting ornamental plants.

The genus Dichorhavirus includes plant-infecting rhabdoviruses with bisegmented genomes that are horizontally transmitted by false spider mites of the genus Brevipalpus. The complete genome sequences of three isolates of the putative dichorhavirus clerodendrum chlorotic spot virus were determined using next-generation sequencing (Illumina) and traditional RT-PCR. Their genome organization, sequence similarity and phylogenetic relationship to other viruses, and transmissibility by Brevipalpus yothersi mites support the assignment of these viruses to a new species of dichorhavirus, as suggested previously. New data are discussed stressing the reliability of the current rules for species demarcation and taxonomic status criteria within the genus Dichorhavirus.

Citrus leprosis virus N: A New Dichorhavirus Causing Citrus Leprosis Disease.

Citrus leprosis (CL) is a viral disease endemic to the Western Hemisphere that produces local necrotic and chlorotic lesions on leaves, branches, and fruit and causes serious yield reduction in citrus orchards. Samples of sweet orange (Citrus × sinensis) trees showing CL symptoms were collected during a survey in noncommercial citrus areas in the southeast region of Brazil in 2013 to 2016. Transmission electron microscopy analyses of foliar lesions confirmed the presence of rod-like viral particles commonly associated with CL in the nucleus and cytoplasm of infected cells. However, every attempt to identify these particles by reverse-transcription polymerase chain reaction tests failed, even though all described primers for the detection of known CL-causing cileviruses and dichorhaviruses were used. Next-generation sequencing of total RNA extracts from three symptomatic samples revealed the genome of distinct, although highly related (>92% nucleotide sequence identity), viruses whose genetic organization is similar to that of dichorhaviruses. The genome sequence of these viruses showed <62% nucleotide sequence identity with those of orchid fleck virus and coffee ringspot virus. Globally, the deduced amino acid sequences of the open reading frames they encode share 32.7 to 63.8% identity with the proteins of the dichorhavirids. Mites collected from both the naturally infected citrus trees and those used for the transmission of one of the characterized isolates to Arabidopsis plants were anatomically recognized as Brevipalpus phoenicis sensu stricto. Molecular and biological features indicate that the identified viruses belong to a new species of CL-associated dichorhavirus, which we propose to call Citrus leprosis N dichorhavirus. Our results, while emphasizing the increasing diversity of viruses causing CL disease, lead to a reevaluation of the nomenclature of those viruses assigned to the genus Dichorhavirus. In this regard, a comprehensive discussion is presented.

Two Novel Betarhabdovirins Infecting Ornamental Plants and the Peculiar Intracellular Behavior of the Cytorhabdovirus in the Liana Aristolochia gibertii.

Two novel members of the subfamily Betarhabdovirinae, family Rhabdoviridae, were identified in Brazil. Overall, their genomes have the typical organization 3'-N-P-P3-M-G-L-5' observed in mono-segmented plant-infecting rhabdoviruses. In aristolochia-associated cytorhabdovirus (AaCV), found in the liana aristolochia (Aristolochia gibertii Hook), an additional short orphan ORF encoding a transmembrane helix was detected between P3 and M. The AaCV genome and inferred encoded proteins share the highest identity values, consistently < 60%, with their counterparts of the yerba mate chlorosis-associated virus (Cytorhabdovirus flaviyerbamate). The second virus, false jalap virus (FaJV), was detected in the herbaceous plant false jalap (Mirabilis jalapa L.) and represents together with tomato betanucleorhabdovirus 2, originally found in tomato plants in Slovenia, a tentative new species of the genus Betanucleorhabdovirus. FaJV particles accumulate in the perinuclear space, and electron-lucent viroplasms were observed in the nuclei of the infected cells. Notably, distinct from typical rhabdoviruses, most virions of AaCV were observed to be non-enclosed within membrane-bounded cavities. Instead, they were frequently seen in close association with surfaces of mitochondria or peroxisomes. Unlike FaJV, AaCV was successfully graft-transmitted to healthy plants of three species of the genus Aristolochia, while mechanical and seed transmission proved unsuccessful for both viruses. Data suggest that these viruses belong to two new tentative species within the subfamily Betarhabdovirinae.

Conserved untranslated regions of multipartite viruses: Natural markers of novel viral genomic components and tags of viral evolution.

Viruses with split genomes are classified as being either segmented or multipartite based on whether their genomic segments occur within a single virion or across different virions. Despite variations in number and sequence during evolution, the genomic segments of many viruses are conserved within the untranslated regions (UTRs). In this study, we present a methodology that combines RNA sequencing with iterative BLASTn of UTRs (https://github.com/qq371260/Iterative-blast-v.1.0) to identify new viral genomic segments. Some novel multipartite-like viruses related to the phylum Kitrinoviricota were annotated using sequencing data from field plant samples and public databases. We identified potentially plant-infecting jingmen-related viruses (order Amarillovirales) and jivi-related viruses (order Martellivirales) with at least six genomic components. The number of RNA molecules associated with a genome of the novel viruses in the families Closteroviridae, Kitaviridae, and Virgaviridae within the order Martellivirales reached five. Several of these viruses seem to represent new taxa at the subgenus, genus, and family levels. The diversity of novel genomic components and the multiple duplication of proteins or protein domains within single or multiple genomic components emphasize the evolutionary roles of genetic recombination (horizontal gene transfer), reassortment, and deletion. The relatively conserved UTRs at the genome level might explain the relationships between monopartite and multipartite viruses, as well as how subviral agents such as defective RNAs and satellite viruses can coexist with their helper viruses.

Kitaviruses: A Window to Atypical Plant Viruses Causing Nonsystemic Diseases.

Kitaviridae is a family of plant-infecting viruses that have multiple positive-sense, single-stranded RNA genomic segments. Kitaviruses are assigned into the genera Cilevirus, Higrevirus, and Blunervirus, mainly on the basis of the diversity of their genomic organization. Cell-to-cell movement of most kitaviruses is provided by the 30K family of proteins or the binary movement block, considered an alternative movement module among plant viruses. Kitaviruses stand out for producing conspicuously unusual locally restricted infections and showing deficient or nonsystemic movement likely resulting from incompatible or suboptimal interactions with their hosts. Transmission of kitaviruses is mediated by mites of many species of the genus Brevipalpus and at least one species of eriophyids. Kitavirus genomes encode numerous orphan open reading frames but RNA-dependent RNA polymerase and the transmembrane helix-containing protein, generically called SP24, typify a close phylogenetic link with arthropod viruses. Kitaviruses infect a large range of host plants and cause diseases of economic concern in crops such as citrus, tomato, passion fruit, tea, and blueberry.

Citrus Bright Spot Virus: A New Dichorhavirus, Transmitted by Brevipalpus azores, Causing Citrus Leprosis Disease in Brazil.

Citrus leprosis (CL) is the main viral disease affecting the Brazilian citriculture. Sweet orange (Citrus sinensis L. Osbeck) trees affected by CL were identified in small orchards in Southern Brazil. Rod-like particles of 40 × 100 nm and electron lucent viroplasm were observed in the nucleus of infected cells in symptomatic tissues. RNA extracts from three plants, which proved negative by RT-PCR for known CL-causing viruses, were analyzed by high throughput sequencing and Sanger sequencing after RT-PCR. The genomes of bi-segmented ss(-)RNA viruses, with ORFs in a typical organization of members of the genus Dichorhavirus, were recovered. These genomes shared 98-99% nt sequence identity among them but <73% with those of known dichorhavirids, a value below the threshold for new species demarcation within that genus. Phylogenetically, the three haplotypes of the new virus called citrus bright spot virus (CiBSV) are clustered with citrus leprosis virus N, which is a dichorhavirus transmitted by Brevipalpus phoenicis sensu stricto. In CiBSV-infected citrus plants, B. papayensis and B. azores were found, but the virus could only be transmitted to Arabidopsis plants by B. azores. The study provides the first evidence of the role of B. azores as a viral vector and supports the assignment of CiBSV to the tentative new species Dichorhavirus australis.

Circulative Transmission of Cileviruses in Brevipalpus Mites May Involve the Paracellular Movement of Virions.

Plant viruses transmitted by mites of the genus Brevipalpus are members of the genera Cilevirus, family Kitaviridae, or Dichorhavirus, family Rhabdoviridae. They produce non-systemic infections that typically display necrotic and/or chlorotic lesions around the inoculation loci. The cilevirus citrus leprosis virus C (CiLV-C) causes citrus leprosis, rated as one of the most destructive diseases affecting this crop in the Americas. CiLV-C is vectored in a persistent manner by the flat mite Brevipalpus yothersi. Upon the ingestion of viral particles with the content of the infected plant cell, virions must pass through the midgut epithelium and the anterior podocephalic gland of the mites. Following the duct from this gland, virions reach the salivary canal before their inoculation into a new plant cell through the stylet canal. It is still unclear whether CiLV-C multiplies in mite cells and what mechanisms contribute to its movement through mite tissues. In this study, based on direct observation of histological sections from viruliferous mites using the transmission electron microscope, we posit the hypothesis of the paracellular movement of CiLV-C in mites which may involve the manipulation of septate junctions. We detail the presence of viral particles aligned in the intercellular spaces between cells and the gastrovascular system of Brevipalpus mites. Accordingly, we propose putative genes that could control either active or passive paracellular circulation of viral particles inside the mites.

First detection of orchid fleck virus in orchids in Mexico.

For the first time, an isolate of the dichorhavirus orchid fleck virus (OFV, family Rhabdoviridae) was found infecting an orchid plant in Mexico. The infected sample of Epidendrum veroscriptum was collected in a nursery in Lagunillas, municipality of Zihuateutla, Edo. Puebla. Mites gathered on this plant were analyzed by light and scanning electron microscopy, which consistently indicated the presence of adults of the species Brevipalpus californicus, the common vector of OFV. Viral identification was based on symptoms, cytopathology, and reverse transcriptase-PCR/sequencing of genome fragments of the RNA1 and 2 molecules. Since isolates of OFV causing citrus leprosis have been previously detected in the Mexican states of Chiapas, Querétaro, and Jalisco, we promote a pertinent discussion and thought-provoking questions regarding the epidemiology and putative evolution of OFV.

Plant Immune System Activation Upon Citrus Leprosis Virus C Infection Is Mimicked by the Ectopic Expression of the P61 Viral Protein.

Citrus leprosis virus C (CiLV-C, genus Cilevirus, family Kitaviridae) is an atypical virus that does not spread systemically in its plant hosts. Upon its inoculation by Brevipalpus mites, only localized lesions occur, and the infection remains limited to cells around mite feeding sites. Here, we aimed to gain insights into the putative causes of viral unfitness in plants by expanding the limited knowledge of the molecular mechanisms underlying plant/kitavirid interactions. Firstly, we quantified the CiLV-C viral RNAs during the infection in Arabidopsis thaliana plants using RT-qPCR and systematized it by defining three stages of distinguishing subgenomic and genomic RNA accumulation: i) 0-24 h after infestation, ii) 2-4 days after infestation (dai), and iii) 6-10 dai. Accordingly, the global plant response to CiLV-C infection was assessed by RNA-Seq at each period. Results indicated a progressive reprogramming of the plant transcriptome in parallel to the increasing viral loads. Gene ontology enrichment analysis revealed the induction of cell growth-related processes at the early stages of the infection and the triggering of the SA-mediated pathway, ROS burst and hypersensitive response (HR) at the presymptomatic stage. Conversely, infected plants downregulated JA/ET-mediated pathways and processes involved in the primary metabolism including photosynthesis. Marker genes of unfolded protein response were also induced, suggesting a contribution of the endoplasmic reticulum stress to the cell death caused by the viral infection. Finally, we transiently expressed CiLV-C proteins in Nicotiana benthamiana plants to undertake their roles in the elicited plant responses. Expression of the CiLV-C P61 protein consistently triggered ROS burst, upregulated SA- and HR-related genes, increased SA levels, reduced JA levels, and caused cell death. Mimicry of responses typically observed during CiLV-C-plant interaction indicates P61 as a putative viral effector causing the HR-like symptoms associated with the infection. Our data strengthen the hypothesis that symptoms of CiLV-C infection might be the outcome of a hypersensitive-like response during an incompatible interaction. Consequently, the locally restricted infection of CiLV-C, commonly observed across infections by kitavirids, supports the thesis that these viruses, likely arising from an ancestral arthropod-infecting virus, are unable to fully circumvent plant defenses.

Passion Fruit Green Spot Virus Genome Harbors a New Orphan ORF and Highlights the Flexibility of the 5'-End of the RNA2 Segment Across Cileviruses.

Passion fruit green spot and passion fruit sudden death are two reportedly distinct viral diseases that recurrently affect passion fruit (Passiflora spp.) groves in Brazil. Here we used a systematic approach that interconnects symptoms, transmission electron microscopy, RT-PCR detection assays followed by Sanger sequencing, and high-throughput sequencing of the RNA of affected passion fruit plants to gain insights about these diseases. Our data confirmed not only the involvement of cileviruses in these two pathologies, as previously suggested, but also that these viruses belong to the same tentative species: passion fruit green spot virus (PfGSV). Results revealed that PfGSV has a positive-sense RNA genome split into two molecules of approximately 9 kb (RNA1) and 5 kb (RNA2), which share about 50-70% nucleotide sequence identity with other viruses in the genus Cilevirus. Genome sequences of five PfGSV isolates suggest that they have more conserved RNA1 (<5% of nucleotide sequence variability) compared to RNA2 (up to 7% of variability) molecules. The highest nucleotide sequence divergence among PfGSV isolates and other cileviruses is in the genomic segment covering from the 5'-end of the RNA2 until the 5'-end of the open reading frame (ORF) p61, which includes the ORF p15 and the intergenic region. This genomic stretch also harbors a novel orphan ORF encoding a 13 kDa protein presenting a cysteine-rich domain. High variability of 5'-end of the RNA2 in cileviruses is discussed in an evolutionary context assuming that they share putative common ancestors with unclassified arthropod-infecting single-strand positive RNA viruses, including mosquito-specific viruses of the group Negevirus (clades Nelorpivirus and Sandwavirus), and other viruses in the family Kitaviridae.

Brevipalpus-transmitted viruses: parallelism beyond a common vector or convergent evolution of distantly related pathogens?

Although diseases caused by Brevipalpus-transmitted viruses (BTV) became relevant for agriculture a century ago, their causal agents have been only recently characterized and classified in two new genera of plant-infecting viruses: Cilevirus and Dichorhavirus. In this review, we highlight both similarities and differences between these viruses emphasizing their current taxonomy and historical classification, phylogeny, genomic organization, gene expression, and the latest research developments on BTVs. Additionally, we stress particular features of interactions with their mite vectors and plant hosts that support, from an evolutionary perspective, the potential convergence of both viral groups.

Phylogenetic and Molecular Variability Studies Reveal a New Genetic Clade of Citrus leprosis virus C.

Citrus leprosis virus C (CiLV-C) causes a severe disease affecting citrus orchards in the Western hemisphere. This study reveals the molecular variability of the virus by analyzing four genomic regions (p29, p15, MP and RNA2-intergenic region) distributed over its two RNAs. Nucleotide diversity (π) values were relatively low but statistically different over the analyzed genes and subpopulations, indicating their distinct evolutionary history. Values of πp29 and πMP were higher than those of πp15 and πRNA2-IR, whereas πMP was increased due to novel discovered isolates phylogenetically clustered in a divergent clade that we called SJP. Isolate BR_SP_SJP_01 RNA1 and RNA2 sequences, clade SJP, showed an identity of 85.6% and 88.4%, respectively, with those corresponding to CiLV-C, the type member of the genus Cilevirus, and its RNA2 5'-proximal region was revealed as a minor donor in a putative inter-clade recombination event. In addition to citrus, BR_SP_SJP_01 naturally infects the weed Commelina benghalensis and is efficiently transmitted by Brevipalpus yothersi mites. Our data demonstrated that negative selection was the major force operating in the evaluated viral coding regions and defined amino acids putatively relevant for the biological function of cilevirus proteins. This work provides molecular tools and sets up a framework for further epidemiological studies.