Apple scar skin viroid naked RNA is actively transmitted by the whitefly Trialeurodes vaporariorum

Cellular roadmaps of viroid infection

Viroids are single-stranded circular noncoding RNAs that infect plants. According to the International Committee on Taxonomy of Viruses, there are 44 viroids known to date. Notably, more than 20 000 distinct viroid-like RNA sequences have recently been identified in existing sequencing datasets, suggesting an unprecedented complexity in biological roles of viroids and viroid-like RNAs. Interestingly, a human pathogen, hepatitis delta virus (HDV), also replicates via a rolling circle mechanism like viroids. Therefore, knowledge of viroid infection is informative for research on HDV and other viroid-like RNAs reported from various organisms. Here, we summarize recent advancements in understanding viroid shuttling among subcellular compartments for completing replication cycles, emphasizing regulatory roles of RNA motifs and structural dynamics in diverse biological processes. We also compare the knowledge of viroid intracellular trafficking with known pathways governing cellular RNA movement in cells. Future investigations on regulatory RNA structures and cognate factors in regulating viroid subcellular trafficking and replication will likely provide new insights into RNA structure-function relationships and facilitate the development of strategies controlling RNA localization and function in cells.

Small Heat Shock Protein (sHsp22.98) from Trialeurodes vaporariorum Plays Important Role in Apple Scar Skin Viroid Transmission

Trialeurodes vaporariorum, commonly known as the greenhouse whitefly, severely infests important crops and serves as a vector for apple scar skin viroid (ASSVd). This vector-mediated transmission may cause the spread of infection to other herbaceous crops. For effective management of ASSVd, it is important to explore the whitefly's proteins, which interact with ASSVd RNA and are thereby involved in its transmission. In this study, it was found that a small heat shock protein (sHsp) from T. vaporariorum, which is expressed under stress, binds to ASSVd RNA. The sHsp gene is 606 bp in length and encodes for 202 amino acids, with a molecular weight of 22.98 kDa and an isoelectric point of 8.95. Intermolecular interaction was confirmed through in silico analysis, using electrophoretic mobility shift assays (EMSAs) and northwestern assays. The sHsp22.98 protein was found to exist in both monomeric and dimeric forms, and both forms showed strong binding to ASSVd RNA. To investigate the role of sHsp22.98 during ASSVd infection, transient silencing of sHsp22.98 was conducted, using a tobacco rattle virus (TRV)-based virus-induced gene silencing system. The sHsp22.98-silenced whiteflies showed an approximate 50% decrease in ASSVd transmission. These results suggest that sHsp22.98 from T. vaporariorum is associated with viroid RNA and plays a significant role in transmission.

Natural Cross-Kingdom Spread of Apple Scar Skin Viroid from Apple Trees to Fungi

Viroids are the smallest known infectious agents that are thought to only infect plants. Here, we reveal that several species of plant pathogenic fungi that were isolated from apple trees infected with apple scar skin viroid (ASSVd) carried ASSVd naturally. This finding indicates the spread of viroids to fungi under natural conditions and further suggests the possible existence of mycoviroids in nature. A total of 117 fungal isolates were isolated from ASSVd-infected apple trees, with the majority (85.5%) being an ascomycete Alternaria alternata and the remaining isolates being other plant-pathogenic or -endophytic fungi. Out of the examined samples, viroids were detected in 81 isolates (69.2%) including A. alternata as well as other fungal species. The phenotypic comparison of ASSVd-free specimens developed by single-spore isolation and ASSVd-infected fungal isogenic lines showed that ASSVd affected the growth and pathogenicity of certain fungal species. ASSVd confers hypovirulence on ascomycete Epicoccum nigrum. The mycobiome analysis of apple tree-associated fungi showed that ASSVd infection did not generally affect the diversity and structure of fungal communities but specifically increased the abundance of Alternaria species. Taken together, these data reveal the occurrence of the natural spread of viroids to plants; additionally, as an integral component of the ecosystem, viroids may affect the abundance of certain fungal species in plants. Moreover, this study provides further evidence that viroid infection could induce symptoms in certain filamentous fungi.

Modes of Viroid Transmission

Studies on the ways in which viroids are transmitted are important for understanding their epidemiology and for developing effective control measures for viroid diseases. Viroids may be spread via vegetative propagules, mechanical damage, seed, pollen, or biological vectors. Vegetative propagation is the most prevalent mode of spread at the global, national and local level while further dissemination can readily occur by mechanical transmission through crop handling with viroid-contaminated hands or pruning and harvesting tools. The current knowledge of seed and pollen transmission of viroids in different crops is described. Biological vectors shown to transmit viroids include certain insects, parasitic plants, and goats. Under laboratory conditions, viroids were also shown to replicate in and be transmitted by phytopathogenic ascomycete fungi; therefore, fungi possibly serve as biological vectors of viroids in nature. The term “mycoviroids or fungal viroids” has been introduced in order to denote these viroids. Experimentally, known sequence variants of viroids can be transmitted as recombinant infectious cDNA clones or transcripts. In this review, we endeavor to provide a comprehensive overview of the modes of viroid transmission under both natural and experimental situations. A special focus is the key findings which can be applied to the control of viroid diseases.

An Inside Look into Biological Miniatures: Molecular Mechanisms of Viroids

Viroids are tiny single-stranded circular RNA pathogens that infect plants. Viroids do not encode any proteins, yet cause an assortment of symptoms. The following review describes viroid classification, molecular biology and spread. The review also discusses viroid pathogenesis, host interactions and detection. The review concludes with a description of future prospects in viroid research.

Transmission studies of the newly described apple chlorotic fruit spot viroid using a combined RT-qPCR and droplet digital PCR approach

The transmission of the apscaviroid tentatively named apple chlorotic fruit spot viroid (ACFSVd) was investigated using a one-step reverse-transcription (RT) droplet digital PCR assay for absolute quantification of the viroid, followed by quantification of relative standard curves by RT-qPCR. Our results indicate that ACFSVd is effectively transmitted by grafting, budding and seeds. No transmission has yet been observed to the viroid-inoculated pome fruit species Pyrus sp. and Cydonia sp. ACFSVd was detected in viruliferous aphids (Myzus persicae, Dysaphis plantaginea) and in codling moths (Cydia pomonella). The viroid was also detected systemically in the infected hemiparasitic plant Viscum album subsp. album (mistletoe).

Phloem transport of structured RNAs: A widening repertoire of trafficking signals and protein factors

The conducting sieve tubes of the phloem consist of sieve elements (SEs), which are enucleate cells incapable of transcription and translation. Nevertheless, SEs contain a large variety of RNAs, and long-distance RNA trafficking via the phloem has been documented. The phloem transport of certain RNAs, as well as the further unloading of these RNAs at target tissues, is essential for plant individual development and responses to environmental cues. The translocation of such RNAs via the phloem is believed to be directed by RNA structural elements serving as phloem transport signals (PTSs), which are recognized by proteins that direct the PTS-containing RNAs into the phloem translocation pathway. The ability of phloem transport has been reported for several classes of structured RNAs including viroids, genuine tRNAs, mRNAs with tRNA sequences embedded into mRNA untranslated regions, tRNA-like structures in the genomic RNAs of plant viruses, and micro-RNA (miRNA) precursors (pri-miRNA). Here, three distinct types of such RNAs are discussed, along with the proteins that may specifically interact with these structures in the phloem. Three-dimensional (3D) motifs, which are characteristic of imperfect RNA duplexes, are discussed as elements of phloem-mobile structured RNAs specifically recognized by proteins involved in phloem transport, thus serving as PTSs.

An aphid RNA transcript migrates systemically within plants and is a virulence factor

Aphids are sap-feeding insects that colonize a broad range of plant species and often cause feeding damage and transmit plant pathogens, including bacteria, viruses, and viroids. These insects feed from the plant vascular tissue, predominantly the phloem. However, it remains largely unknown how aphids, and other sap-feeding insects, establish intimate long-term interactions with plants. To identify aphid virulence factors, we took advantage of the ability of the green peach aphid Myzus persicae to colonize divergent plant species. We found that a M. persicae clone of near-identical females established stable colonies on nine plant species of five representative plant eudicot and monocot families that span the angiosperm phylogeny. Members of the novel aphid gene family Ya are differentially expressed in aphids on the nine plant species and are coregulated and organized as tandem repeats in aphid genomes. Aphids translocate Ya transcripts into plants, and some transcripts migrate to distal leaves within several plant species. RNAi-mediated knockdown of Ya genes reduces M. persicae fecundity, and M. persicae produces more progeny on transgenic plants that heterologously produce one of the systemically migrating Ya transcripts as a long noncoding (lnc) RNA. Taken together, our findings show that beyond a range of pathogens, M. persicae aphids translocate their own transcripts into plants, including a Ya lncRNA that migrates to distal locations within plants, promotes aphid fecundity, and is a member of a previously undescribed host-responsive aphid gene family that operate as virulence factors.

Current overview on viroid-host interactions

Viroids are one of the most enigmatic highly structured, circular, single-stranded RNA phytopathogens. Although they are not known to code for any peptide, viroids induce visible symptoms in susceptible host plants that resemble those associated with many plant viruses. It is known that viroids induce disease symptoms by direct interaction with host factors; however, the precise mechanism by which this occurs remains poorly understood. Studies on the host's responses to viroid infection, host susceptibility and nonhost resistance have been underway for several years, but much remains to be done in order to fully understand the complex nature of viroid-host interactions. Recent progress using molecular biology techniques combined with computational algorithms, in particular evidence of the role of viroid-derived small RNAs in the RNA silencing pathways of a disease network, has widened the knowledge of viroid pathogenicity. The complexity of viroid-host interactions has been revealed in the past decades to include, but not be limited to, the involvement of host factors, viroid structural complexity, and viroid-induced ribosomal stress, which is further boosted by the discovery of long noncoding RNAs (lncRNAs). In this review, the current understanding of the viroid-host interaction has been summarized with the goal of simplifying the complexity of viroid biology for future research. This article is categorized under: RNA in Disease and Development > RNA in Disease.

Pest categorisation of non-EU viruses and viroids of Cydonia Mill., Malus Mill. and Pyrus L

Following a request from the EU Commission, the Panel on Plant Health performed a pest categorisation of 17 viruses and viroids, herein called viruses, of Cydonia Mill., Malus Mill. and Pyrus L. determined as being either non‐EU or of undetermined standing in a previous EFSA opinion. These viruses belong to different genera and are heterogeneous in their biology. They can be detected by available methods and are efficiently transmitted by vegetative propagation techniques, with plants for planting representing a major long‐distance spread mechanism and, potentially, a major entry pathway. Depending on the viruses, additional pathway(s) can also be seed, pollen and/or vector transmission. Most of the viruses categorised here are known to infect only one of few related plant genera, but some of them have a wider host range, thus extending the possible entry pathways. Three viruses (apple necrotic mosaic virus, cherry rasp leaf virus, temperate fruit decay‐associated virus) and one viroid (apple fruit crinkle viroid) satisfy all the criteria to be considered as Union quarantine pests. Five viruses (apple green crinkle‐associated virus, blackberry chlorotic ringspot virus, eggplant mottled crinkle virus, tobacco ringspot virus and tomato ringspot virus) and one viroid (apple scar skin viroid), satisfy the criteria to be considered as Union quarantine pests with the possible exception of being absent from the EU territory or having a restricted presence and being under official control. The remaining six viruses (apple geminivirus, apple latent spherical virus, apple‐associated luteovirus, Pyrus pyrifolia cryptic virus, Pyrus pyrifolia partitivirus 2 and Tulare apple mosaic virus) and one viroid (apple hammerhead viroid) were not found to satisfy one or more of these criteria. The Panel highlights that for several viruses, especially those recently discovered, the categorisation is associated with high uncertainties mainly linked to the absence of data on biology and distribution. Since this opinion addresses specifically the non‐EU viruses, in general these viruses do not meet the criteria assessed by EFSA to qualify as a potential Union regulated non‐quarantine pests.

This publication is linked to the following EFSA Journal articles: http://onlinelibrary.wiley.com/doi/10.2903/j.efsa.2019.5501/full, http://onlinelibrary.wiley.com/doi/10.2903/j.efsa.2019.5669/full, http://onlinelibrary.wiley.com/doi/10.2903/j.efsa.2019.5735/full, http://onlinelibrary.wiley.com/doi/10.2903/j.efsa.2019.5766/full

Pest categorisation of non-EU viruses and viroids of Prunus L

Following a request from the EU Commission, the Panel on Plant Health addressed the pest categorisation of the viruses and viroids of Prunus L. determined as being either non‐EU or of undetermined standing in a previous EFSA opinion. These infectious agents belong to different genera and are heterogeneous in their biology. With the exclusion of Ilarvirus S1 and Ilarvirus S2, for which very limited information exists, the pest categorisation was completed for 26 viruses and 1 viroid having acknowledged identities and available detection methods. All these viruses are efficiently transmitted by vegetative plant propagation techniques, with plants for planting representing the major pathway for long‐distance dispersal and thus considered as the major pathway for entry. Depending on the virus, additional pathway(s) can also be Prunus seeds, pollen and/or vector(s). Most of the viruses categorised here are known to infect only one or few plant genera, but some of them have a wide host range, thus extending the possible entry pathways. Apple scar skin viroid, American plum line pattern virus, cherry mottle leaf virus, cherry rasp leaf virus, cherry rosette virus, cherry rusty mottle‐associated virus, cherry twisted leaf‐associated virus, peach enation virus, peach mosaic virus, peach rosette mosaic virus, tobacco ringspot virus and tomato ringspot virus meet all the criteria evaluated by EFSA to qualify as potential Union quarantine pests (QPs). With the exception of impact in the EU territory, on which the Panel was unable to conclude, apricot vein clearing virus, Asian prunus virus 1, Asian prunus virus 2, Asian prunus virus 3, Caucasus prunus virus, cherry virus B, Mume virus A, nectarine stem pitting‐associated virus, nectarine virus M, peach chlorotic mottle virus, peach leaf pitting‐associated virus, peach virus D, prunus virus F and prunus virus T satisfy all the other criteria to be considered as potential Union QPs. Prunus geminivirus A does not meet the criterion of having negative impact in the EU. For several viruses, especially those recently discovered, the categorisation is associated with high uncertainties mainly because of the absence of data on their biology, distribution and impact. Since this opinion addresses specifically the non‐EU viruses, in general these viruses do not meet the criteria assessed by EFSA to qualify as potential Union regulated non‐quarantine pests.

This publication is linked to the following EFSA Journal articles: http://onlinelibrary.wiley.com/doi/10.2903/j.efsa.2019.5501/full, http://onlinelibrary.wiley.com/doi/10.2903/j.efsa.2019.5590/full, http://onlinelibrary.wiley.com/doi/10.2903/j.efsa.2019.5669/full, http://onlinelibrary.wiley.com/doi/10.2903/j.efsa.2019.5766/full

Pest categorisation of non-EU viruses and viroids of Vitis L

Following a request from the EU Commission, the Panel on Plant Health addressed the pest categorisation of the viruses and viroids of Vitis L. determined as being either non-EU or of undetermined standing in a previous EFSA opinion. These infectious agents belong to different genera and are heterogeneous in their biology. With the exclusion of grapevine virus 101-14.N.23.9.1/South Africa/2009 for which very limited information exists, the pest categorisation was completed for 30 viruses or viroids having acknowledged identities and available detection methods. All these viruses are efficiently transmitted by vegetative propagation techniques, with plants for planting representing the major pathway for long-distance dispersal and thus considered as the major pathway for potential entry. Depending on the virus, additional pathway(s) can also be seeds, pollen and/or vector(s). Most of the viruses categorised here are known to infect only one or few plant genera, but some of them have a wide host range, thus extending the possible entry pathways. Grapevine yellow speckle viroid 2, blueberry leaf mottle virus, grapevine Ajinashika virus, grapevine Anatolian ringspot virus, grapevine berry inner necrosis virus, grapevine deformation virus, grapevine fabavirus, grapevine red blotch virus, grapevine stunt virus, grapevine Tunisian ringspot virus, grapevine vein-clearing virus, temperate fruit decay-associated virus, peach rosette mosaic virus, tobacco ringspot virus, tomato ringspot virus meet all the criteria evaluated by EFSA to qualify as potential Union quarantine pests (QPs). With the exception of impact for the EU territory, on which the Panel was unable to conclude, blackberry virus S, grapevine geminivirus A, grapevine leafroll-associated virus 7, grapevine leafroll-associated virus 13, grapevine satellite virus, grapevine virus E, grapevine virus I, grapevine virus J, grapevine virus S, summer grape enamovirus, summer grape latent virus satisfy all the other criteria to be considered as potential Union QPs. Australian grapevine viroid, grapevine cryptic virus 1, grapevine endophyte endornavirus and wild vitis virus 1 do not meet all the criteria evaluated by EFSA to be regarded as potential Union QPs because they are not known to cause an impact on Vitis. For several viruses, especially those recently discovered, the categorisation is associated with high uncertainties mainly because of the absence of data on their biology, distribution and impact. Since this opinion addresses specifically non-EU viruses, in general these viruses do not meet the criteria assessed by EFSA to qualify as a potential Union regulated non-quarantine pests.

RNA Back and Forth: Looking through Ribozyme and Viroid Motifs

Current cellular facts allow us to follow the link from chemical to biochemical metabolites, from the ancient to the modern world. In this context, the "RNA world" hypothesis proposes that early in the evolution of life, the ribozyme was responsible for the storage and transfer of genetic information and for the catalysis of biochemical reactions. Accordingly, the hammerhead ribozyme (HHR) and the hairpin ribozyme belong to a family of endonucleolytic RNAs performing self-cleavage that might occur during replication. Furthermore, regarding the widespread occurrence of HHRs in several genomes of modern organisms (from mammals to small parasites and elsewhere), these small ribozymes have been regarded as living fossils of a primitive RNA world. They fold into 3D structures that generally require long-range intramolecular interactions to adopt the catalytically active conformation under specific physicochemical conditions. By studying viroids as plausible remains of ancient RNA, we recently demonstrated that they replicate in non-specific hosts, emphasizing their adaptability to different environments, which enhanced their survival probability over the ages. All these results exemplify ubiquitous features of life. Those are the structural and functional versatility of small RNAs, ribozymes, and viroids, as well as their diversity and adaptability to various extreme conditions. All these traits must have originated in early life to generate novel RNA populations.

Poly-adenine-Coupled LAMP Barcoding to Detect Apple Scar Skin Viroid

Apple Scar Skin Viroid (ASSVd), a nonprotein coding, circular RNA pathogen is relatively difficult to detect by immunoassay. We report here a one-step reverse transcription loop-mediated isothermal amplification (RT-LAMP) assay to improve selectivity for diagnostic use in detecting ASSVd in plants. ASSVd RT-LAMP was accelerated using loop primers and was found to be highly sensitive with a detection limit of 10⁴ copies of cDNA-ASSVd within 30 min. Real-time LAMP and melting curve analysis could differentiate between the true-positive LAMP amplicons and false-positive nonspecific primer amplification products. The optimized RT-LAMP was then followed by the addition of nonthiolated AuNP:poly-adenine (A10)-ASSVd LAMP barcodes, showing a high authentication capacity with colorimetric changes. This type of barcoding assay is a potential alternative for rapid and multiple viroid diagnosis, providing for visible sensing in the field that can be applied to viroid-free planting.