Viroids are naked nucleic acids that do not code for any proteins and yet are able to be replicated, processed, moved cell-to-cell and systemically through their host plants, as well as resist plant ...defense response and be transmitted from plant to plant, without a protective coat. All of the information specifying these functions lies within their nucleotide sequence and the RNA structures they form. This review examines what information about these processes has been acquired since 2008. Sequences involved in viroid replication and movement within the plant have been identified, in particular for the nuclear-associated (
Pospiviroidae
) viroids, as have sequences of one chloroplast-associated viroid (
Avsunviroidae
) involved in chloroplast uptake. The enzymes involved in ligation of viroids of either of the above two types also have been identified. Viroid sequences that are involved in pathogenicity through the RNA silencing system and the target of their viroid-specific small RNAs also have been identified. Effects of viroid infection on plant gene expression have been assessed for several viroids, and further specific interactions between viroids and host proteins have been identified. The variation in sequence of natural or passaged populations of viroids in various host species has been examined, and the effects of the host have been evaluated. New approaches to obtaining resistance to viroid infection have been examined or implemented, as have combinations of approaches to control viroid infection, and to better understand how viroids are transmitted. Finally, new viroids have also been discovered and characterized.
ICTV Virus Taxonomy Profile: Avsunviroidae Di Serio, Francesco; Li, Shi-Fang; Matoušek, Jaroslav ...
Journal of general virology,
05/2018, Volume:
99, Issue:
5
Journal Article
Peer reviewed
Open access
Members of the family Avsunviroidae have a single-stranded circular RNA genome that adopts a rod-like or branched conformation and can form, in the strands of either polarity, hammerhead ribozymes ...involved in their replication in plastids through a symmetrical RNA-RNA rolling-circle mechanism. These viroids lack the central conserved region typical of members of the family Pospiviroidae. The family Avsunviroidae includes three genera, Avsunviroid, Pelamoviroid and Elaviroid, with a total of four species. This is a summary of the ICTV Report on the taxonomy of the family Avsunviroidae, which is available at http://www.ictv.global/report/avsunviroidae.
Viroids – ancient plant-pathogenic long noncoding RNAs – have developed a singular evolutionary strategy based on reprogramming specific phases of host-metabolism to ensure that their infection cycle ...can be completed in infected cells. However, the molecular aspects governing this transregulatory phenomenon remain elusive.
Here, we use immunoprecipitation assays and bisulfite sequencing of rDNA to shown that, in infected cucumber and Nicotiana benthamina plants, Hop stunt viroid (HSVd) recruits and functionally subverts Histone Deacetylase 6 (HDA6) to promote host-epigenetic alterations that trigger the transcriptional alterations observed during viroid pathogenesis.
This notion is supported by the demonstration that, during infection, the HSVd–HDA6 complex occurs in vivo and that endogenous HDA6 expression is increased in HSVd-infected cells. Moreover, transient overexpression of recombinant HDA6 reverts the hypomethylation status of rDNA observed in HSVd-infected plants and reduces viroid accumulation. We hypothesize that the host-transcriptional alterations induced as a consequence of viroid-mediated HDA6 recruitment favor spurious recognition of HSVd-RNA as an RNA Pol II template, thereby improving viroid replication.
Our results constitute the first description of a physical and functional interaction between a pathogenic RNA and a component of the host RNA silencing mechanism, providing novel evidence of the potential of these pathogenic lncRNAs to physically redesign the host-cell environment and reprogram their regulatory mechanisms.
The management of plant diseases relies on the accurate identification of pathogens that requires a robust and validated tool in terms of specificity, sensitivity, repeatability, and reproducibility. ...High-throughput sequencing (HTS) has become the method of choice for virus detection when either a complete viral status of a plant is required in a single assay or if an unknown viral agent is expected. To ensure that the most accurate diagnosis is made from an HTS data analysis, a standardized protocol per pathosystem is required. This chapter presents a detailed protocol for the detection of viruses and viroids infecting citrus using HTS. The protocol describes all the steps from sample processing, nucleic acid extraction, and bioinformatic analyses validated to be an efficient method for detection in this pathosystem. The protocol also includes a section on citrus tristeza virus (CTV) genotype differentiation using HTS data.
Hop latent viroid (HLVd) is the biggest concern for cannabis and hop growers worldwide. Although most HLVd-infected plants remain asymptomatic, research on hops has demonstrated a decrease in both ...the α-bitter acid and terpene content of hop cones, which affects their economic value. The HLVd-associated "dudding" or "duds" disease of cannabis was first reported in 2019 in California. Since then, the disease has become widespread in cannabis-growing facilities across North America. Although severe yield loss associated with duds disease has been recorded, little scientific information is available to growers in order to contain HLVd. Consequently, this review aims to summarise all of the scientific information available on HLVd so as to be able to understand the effect of HLVd on yield loss, cannabinoid content, terpene profile, disease management and inform crop protection strategies.
Eukaryotic organisms exposed to adverse conditions are required to show a certain degree of transcriptional plasticity in order to cope successfully with stress. Epigenetic regulation of the genome ...is a key regulatory mechanism allowing dynamic changes of the transcriptional status of the plant in response to stress. The Hop stunt viroid (HSVd) induces the demethylation of ribosomal RNA (rRNA) in cucumber (Cucumis sativus) leaves, leading to increasing transcription rates of rRNA. In addition to the clear alterations observed in vegetative tissues, HSVd infection is also associated with drastic changes in gametophyte development. To examine the basis of viroid-induced alterations in reproductive tissues, we analysed the cellular and molecular consequences of HSVd infection in the male gametophyte of cucumber plants. Our results indicate that in the pollen grain, accumulation of HSVd RNA induces a decondensation of the generative nucleus that correlates with a dynamic demethylation of repetitive regions in the cucumber genome that include rRNA genes and transposable elements (TEs). We therefore propose that HSVd infection impairs the epigenetic control of rRNA genes and TEs in gametic cells of cucumber, a phenomenon thus far unknown to occur in this reproductive tissue as a consequence of pathogen infection.
BACKGROUND: Two citrus viroids, Citrus exocortis viroid (CEVd) and Hop stunt viroid (HSVd), have been reported and become potential threats to the citrus industry in Taiwan. The distributions and ...infection rates of two viroids have not been investigated since the two diseases were presented decades ago. The genetic diversities and evolutionary relationships of two viroids also remain unclear in the mix citrus planted region. METHODS: Multiplex RT-PCR was used to detect the two viroids for the first time in seven main cultivars of citrus. Multiplex real-time RT-PCR quantified the distributions of two viroids in four citrus tissues. Sequence alignment and phylogenetic analysis were performed using the ClustalW and MEGA6 (neighbor-joining with p-distance model), respectively. RESULTS: HSVd was found more prevalent than CEVd (32.2% vs. 30.4%). Both CEVd and HSVd were commonly found simultaneously in the different citrus cultivars (up to 55%). Results of the multiplex quantitative analysis suggested that uneven distributions of both viroids with twig bark as the most appropriate material for studies involving viroid sampling such as quarantine inspection. Sequence alignment against Taiwanese isolates, along with analysis of secondary structure, revealed the existence of 10 and 5 major mutation sites in CEVd and HSVd, respectively. The mutation sites in CEVd were located at both ends of terminal and variability domains, whereas those in HSVd were situated in left terminal and pathogenicity domains. A phylogenetic analysis incorporating worldwide viroid isolates indicated three and two clusters for the Taiwanese isolates of CEVd and HSVd, respectively. CONCLUSIONS: Moderately high infection and co-infection rates of two viroids in certain citrus cultivars suggest that different citrus cultivars may play important roles in viroid infection and evolution. These data also demonstrate that two multiplex molecular detection methods developed in the present study provide powerful tools to understand the genetic diversities among viroid isolates and quantify viroids in citrus host. Our field survey can help clarify citrus-viroid relationships as well as develop proper prevention strategies.
To date, viroids have been found to naturally infect only plants, resulting in substantial losses for some crops. Whether viroids or viroid‐like RNAs naturally infect non‐plant hosts remains unknown. ...Here the existence of a set of exogenous, single‐stranded circular RNAs, ranging in size from 157 to 450 nucleotides, isolated from the fungus Botryosphaeria dothidea and nominated B. dothidea RNAs (BdcRNAs) is reported. BdcRNAs replicate autonomously in the nucleus via a rolling‐circle mechanism following a symmetric pathway. BdcRNA infection induces symptoms, because BdcRNAs can apparently modulate, to different degrees, specific biological traits (e.g., alter morphology, decrease growth rate, attenuate virulence, and increase or decrease tolerance to osmotic and oxidative stress) of the host fungus. Overall, BdcRNAs have genome characteristics similar to those of viroids and exhibit pathogenic effects on fungal hosts. It is proposed that these novel fungus infecting RNAs should be termed mycoviroids. BdcRNA(s) may be considered additional inhabitants at the frontier of life in terms of genomic complexity, and represent a new class of acellular entities endowed with regulatory functions, and novel epigenomic carriers of biological information.
A novel class of viroid‐like RNAs, termed as mycoviroids, naturally infects a filamentous fungus Botryosphaeria dothidea responsible for fungal attenuated virulence and growth rate. These mycoviroids range in size between 157 and 450 nucleotides, replicate autonomously in the nucleus via a rolling‐circle replication mechanism following symmetric pathways, and regulate the metabolic pathways of the host fungus.
► Generic detection of all known pospiviroids is described using real-time RT-PCR. ► The assay is suitable for large-scale testing of various crops. ► Validation shows high sensitivity (relative ...infection rate of 0.13%), specificity and robustness.
In many countries phytosanitary regulations apply to Potato spindle tuber viroid, because it can cause serious diseases in potato and tomato crops. Other pospiviroids, some of which are distributed widely in ornamental crops, can cause similar diseases. Consequently, there is a need for a reliable and cost-effective generic testing method.
An assay was developed that detects all known species of the genus Pospiviroid, using real-time RT-PCR based on TaqMan technology. This GenPospi assay consists of two reactions running in parallel, the first targeting all pospiviroids, except Columnea latent viroid, the second specifically targeting the latter viroid (already published). To monitor the RNA extraction a nad5 internal control was included. Method validation on tomato leaves showed that the GenPospi assay detects all pospiviroids up to a relative infection rate of 0.13% (equals 770 times dilution). The assay was specific because no cross reactivity was observed with other viroids, viruses or nucleic acid from plant hosts. Repeatability and reproducibility were 100% and the assay appeared robust in an inter-laboratory comparison. The GenPospi assay has been shown to be a suitable tool for large-scale screening for all known pospiviroids. Although it has been validated for tomato leaves it can potentially be used for any crop.
Abstract Infection by viroids, non-protein-coding circular RNAs, occurs with the accumulation of 21–24 nt viroid-derived small RNAs (vd-sRNAs) with characteristic properties of small interfering RNAs ...(siRNAs) associated to RNA silencing. The vd-sRNAs most likely derive from dicer-like (DCL) enzymes acting on viroid-specific dsRNA, the key elicitor of RNA silencing, or on the highly structured genomic RNA. Previously, viral dsRNAs delivered mechanically or agroinoculated have been shown to interfere with virus infection in a sequence-specific manner. Here, we report similar results with members of the two families of nuclear- and chloroplast-replicating viroids. Moreover, homologous vd-sRNAs co-delivered mechanically also interfered with one of the viroids examined. The interference was sequence-specific, temperature-dependent and, in some cases, also dependent on the dose of the co-inoculated dsRNA or vd-sRNAs. The sequence-specific nature of these effects suggests the involvement of the RNA induced silencing complex (RISC), which provides sequence specificity to RNA silencing machinery. Therefore, viroid titer in natural infections might be regulated by the concerted action of DCL and RISC. Viroids could have evolved their secondary structure as a compromise between resistance to DCL and RISC, which act preferentially against RNAs with compact and relaxed secondary structures, respectively. In addition, compartmentation, association with proteins or active replication might also help viroids to elude their host RNA silencing machinery.
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