A structured viroid RNA serves as a substrate for dicer-like cleavage to produce biologically active small RNAs but is resistant to RNA-induced silencing complex-mediated degradation

Predicting symptom severity in PSTVd-infected tomato plants using the PSTVd genome sequence

Viroids, one of the smallest known infectious agents, induce symptoms of varying severity, ranging from latent to severe, based on the combination of viroid isolates and host plant species. Because viroids are transmissible between plant species, asymptomatic viroid-infected plants may serve as latent sources of infection for other species that could exhibit severe symptoms, occasionally leading to agricultural and economic losses. Therefore, predicting the symptoms induced by viroids in host plants without biological experiments could remarkably enhance control measures against viroid damage. Here, we developed an algorithm using unsupervised machine learning to predict the severity of disease symptoms caused by viroids (e.g., potato spindle tuber viroid; PSTVd) in host plants (e.g., tomato). This algorithm, mimicking the RNA silencing mechanism thought to be linked to viroid pathogenicity, requires only the genome sequences of the viroids and host plants. It involves three steps: alignment of synthetic short sequences of the viroids to the host plant genome, calculation of the alignment coverage, and clustering of the viroids based on coverage using UMAP and DBSCAN. Validation through inoculation experiments confirmed the effectiveness of the algorithm in predicting the severity of disease symptoms induced by viroids. As the algorithm only requires the genome sequence data, it may be applied to any viroid and plant combination. These findings underscore a correlation between viroid pathogenicity and the genome sequences of viroid isolates and host plants, potentially aiding in the prevention of viroid outbreaks and the breeding of viroid-resistant crops.

Understanding Citrus Viroid Interactions: Experience and Prospects

Citrus is the natural host of at least eight viroid species, providing a natural platform for studying interactions among viroids. The latter manifests as antagonistic or synergistic phenomena. The antagonistic effect among citrus viroids intuitively leads to reduced symptoms caused by citrus viroids, while the synergistic effect leads to an increase in symptom severity. The interaction phenomenon is complex and interesting, and a deep understanding of the underlying mechanisms induced during this viroid interaction is of great significance for the prevention and control of viroid diseases. This paper summarizes the research progress of citrus viroids in recent years, focusing on the interaction phenomenon and analyzing their interaction mechanisms. It points out the core role of the host RNA silencing mechanism and viroid-derived siRNA (vd-siRNA), and provides suggestions for future research directions.

Knockout of SlDCL2b attenuates the resistance of tomato to potato spindle tuber viroid infection

RNA interference, or RNA silencing, is an important defence mechanism against viroid infection in plants. Plants encode multiple DICER-LIKE (DCL) proteins that are key components of the RNA silencing pathway. However, the roles of different DCLs in defence responses against viroid infection remain unclear. Here, we determined the function of tomato DCL2b (SlDCL2b) in defence responses against potato spindle tuber viroid (PSTVd) infection using SlDCL2b loss-of-function tomato mutant plants. Compared with wild-type plants, mutant plants were more susceptible to PSTVd infection, developing more severe symptoms earlier and accumulating higher levels of PSTVd RNAs. Moreover, we verified the feedback mechanism for the regulation of SlDCL2b expression by miR6026. Functional blocking of tomato miR6026, by expressing its target mimics, can enhance resistance to PSTVd infection in tomato plants. These findings deepen the current understanding of RNAi-based resistance against viroid infection and provide a potentially new strategy for viroid control.

Effects of RNA silencing during antagonism between citrus exocortis viroid and citrus bark cracking viroid in Etrog citron (Citrus medica)

Citrus exocortis viroid (CEVd) and citrus bark cracking viroid (CBCVd) are two important viroids that infect citrus plants and frequently occur as mixed infections in orchards. However, the mechanism of antagonism between the two viroids in mixed infections remains unclear. The CEVd/CBCVd-citron system and small RNA sequencing (sRNA-seq) were used to study the antagonism. When CBCVd was inoculated before CEVd, the CEVd titre was significantly reduced and the symptoms were attenuated. Viroid-derived sRNAs (vd-sRNAs) from CEVd and CBCVd were predominantly 21-nucleotide (nt) and 22-nt in length and had similar 5' base biases. Homologous sequences of the two viroids in the terminal right (TR) region are rich in vd-sRNAs, and the high frequency vd-sRNAs selected from the CBCVd TR region can be used to degrade the transcripts of CEVd in vivo directly. These results suggest that RNA silencing may play an important role in the antagonism of the two viroids, thus deepening our understanding of the molecular interaction of long noncoding RNAs in woody plants.

PSTVd infection in Nicotiana benthamiana plants has a minor yet detectable effect on CG methylation

Viroids are small circular RNAs infecting a wide range of plants. They do not code for any protein or peptide and therefore rely on their structure for their biological cycle. Observed phenotypes of viroid infected plants are thought to occur through changes at the transcriptional/translational level of the host. A mechanism involved in such changes is RNA-directed DNA methylation (RdDM). Till today, there are contradictory works about viroids interference of RdDM. In this study, we investigated the epigenetic effect of viroid infection in Nicotiana benthamiana plants. Using potato spindle tuber viroid (PSTVd) as the triggering pathogen and via bioinformatic analyses, we identified endogenous gene promoters and transposable elements targeted by 24 nt host siRNAs that differentially accumulated in PSTVd-infected and healthy plants. The methylation status of these targets was evaluated following digestion with methylation-sensitive restriction enzymes coupled with PCR amplification, and bisulfite sequencing. In addition, we used Methylation Sensitive Amplification Polymorphism (MSAP) followed by sequencing (MSAP-seq) to study genomic DNA methylation of 5-methylcytosine (5mC) in CG sites upon viroid infection. In this study we identified a limited number of target loci differentially methylated upon PSTVd infection. These results enhance our understanding of the epigenetic host changes as a result of pospiviroid infection.

Integrative time-scale and multi-omics analysis of host responses to viroid infection

Viroids are circular RNAs of minimal complexity compelled to subvert plant-regulatory networks to accomplish their infectious process. Studies focused on the response to viroid-infection have mostly addressed specific regulatory levels and considered specifics infection-times. Thus, much remains to be done to understand the temporal evolution and complex nature of viroid-host interactions. Here we present an integrative analysis of the temporal evolution of the genome-wide alterations in cucumber plants infected with hop stunt viroid (HSVd) by integrating differential host transcriptome, sRNAnome and methylome. Our results support that HSVd promotes the redesign of the cucumber regulatory-pathways predominantly affecting specific regulatory layers at different infection-phases. The initial response was characterised by a reconfiguration of the host-transcriptome by differential exon-usage, followed by a progressive transcriptional downregulation modulated by epigenetic changes. Regarding endogenous small RNAs, the alterations were limited and mainly occurred at the late stage. Significant host-alterations were predominantly related to the downregulation of transcripts involved in plant-defence mechanisms, the restriction of pathogen-movement and the systemic spreading of defence signals. We expect that these data constituting the first comprehensive temporal-map of the plant-regulatory alterations associated with HSVd infection could contribute to elucidate the molecular basis of the yet poorly known host-response to viroid-induced pathogenesis.

Strategies for Engineering Virus Resistance in Potato

Potato (Solanum tuberosum L.) is an important vegetable crop that plays a pivotal role in the world, especially given its potential to feed the world population and to act as the major staple food in many developing countries. Every year, significant crop loss is caused by viral diseases due to a lack of effective agrochemical treatments, since only transmission by insect vectors can be combated with the use of insecticides, and this has been an important factor hindering potato production. With the rapid development of molecular biology and plant genetic engineering technology, transgenic approaches and non-transgenic techniques (RNA interference and CRISPR-cas9) have been effectively employed to improve potato protection against devastating viruses. Moreover, the availability of viral sequences, potato genome sequences, and host immune mechanisms has remarkably facilitated potato genetic engineering. In this study, we summarize the progress of antiviral strategies applied in potato through engineering either virus-derived or plant-derived genes. These recent molecular insights into engineering approaches provide the necessary framework to develop viral resistance in potato in order to provide durable and broad-spectrum protection against important viral diseases of solanaceous crops.

Viroids: Non-Coding Circular RNAs Able to Autonomously Replicate and Infect Higher Plants

Viroids are a unique type of infectious agent, exclusively composed of a relatively small (246-430 nt), highly base-paired, circular, non-coding RNA. Despite the small size and non-coding nature, the more-than-thirty currently known viroid species infectious of higher plants are able to autonomously replicate and move systemically through the host, thereby inducing disease in some plants. After recalling viroid discovery back in the late 60s and early 70s of last century and discussing current hypotheses about their evolutionary origin, this article reviews our current knowledge about these peculiar infectious agents. We describe the highly base-paired viroid molecules that fold in rod-like or branched structures and viroid taxonomic classification in two families, Pospiviroidae and Avsunviroidae, likely gathering nuclear and chloroplastic viroids, respectively. We review current knowledge about viroid replication through RNA-to-RNA rolling-circle mechanisms in which host factors, notably RNA transporters, RNA polymerases, RNases, and RNA ligases, are involved. Systemic movement through the infected plant, plant-to-plant transmission and host range are also discussed. Finally, we focus on the mechanisms of viroid pathogenesis, in which RNA silencing has acquired remarkable importance, and also for the initiation of potential biotechnological applications of viroid molecules.

Role of RNA silencing in plant-viroid interactions and in viroid pathogenesis

Viroids are small, single-stranded, non-protein coding and circular RNAs able to infect host plants in the absence of any helper virus. They may elicit symptoms in their hosts, but the underlying molecular pathways are only partially known. Here we address the role of post-transcriptional RNA silencing in plant-viroid-interplay, with major emphasis on the involvement of this sequence-specific RNA degradation mechanism in both plant antiviroid defence and viroid pathogenesis. This review is a tribute to the memory of Dr. Ricardo Flores, who largely contributed to elucidate this and other molecular mechanisms involved in plant-viroid interactions.

Critical points for the design and application of RNA silencing constructs for plant virus resistance

Control of plant virus diseases is a big challenge in agriculture as is resistance in plant lines to infection by viruses. Recent progress using advanced technologies has provided fast and durable alternatives. One of the most promising techniques against plant viruses that is cost-effective and environmentally safe is RNA silencing or RNA interference (RNAi), a technology that could be used alone or along with other control methods. To achieve the goals of fast and durable resistance, the expressed and target RNAs have been examined in many studies, with regard to the variability in silencing efficiency, which is regulated by various factors such as target sequences, target accessibility, RNA secondary structures, sequence variation in matching positions, and other intrinsic characteristics of various small RNAs. Developing a comprehensive and applicable toolbox for the prediction and construction of RNAi helps researchers to achieve the acceptable performance level of silencing elements. Although the attainment of complete prediction of RNAi robustness is not possible, as it also depends on the cellular genetic background and the nature of the target sequences, some important critical points have been discerned. Thus, the efficiency and robustness of RNA silencing against viruses can be improved by considering the various parameters of the target sequence and the construct design. In this review, we provide a comprehensive treatise regarding past, present and future prospective developments toward designing and applying RNAi constructs for resistance to plant viruses.

A circular RNA vector for targeted plant gene silencing based on an asymptomatic viroid

Gene silencing for functional studies in plants has been largely facilitated by manipulating viral genomes with inserts from host genes to trigger virus-induced gene silencing (VIGS) against the corresponding mRNAs. However, viral genomes encode multiple proteins and can disrupt plant homeostasis by interfering with endogenous cell mechanisms. To try to circumvent this functional limitation, we have developed a silencing method based on the minimal autonomously-infectious nucleic acids currently known: viroids, which lack proven coding capability. The genome of Eggplant latent viroid, an asymptomatic viroid, was manipulated with insertions ranging between 21 and 42 nucleotides. Our results show that, although larger insertions might be tolerated, the maintenance of the secondary structure appears to be critical for viroid genome stability. Remarkably, these modified ELVd molecules are able to induce systemic infection promoting the silencing of target genes in eggplant. Inspired by the design of artificial microRNAs, we have developed a simple and standardized procedure to generate stable insertions into the ELVd genome capable of silencing a specific target gene. Analogously to VIGS, we have termed our approach viroid-induced gene silencing, and demonstrate that it is a promising tool for dissecting gene functions in eggplant.

Emerging value of the viroid model in molecular biology and beyond

Viroids are single-stranded circular noncoding RNAs that infect plants. Research in the past five decades has deciphered the viroid genome structures, viroid replication cycles, numerous host factors for viroid infection, viroid motifs for intracellular and intercellular trafficking, interactions with host defense machinery, etc. In this review, we mainly focus on some significant questions that remain to be tackled, centered around (1) how the RNA polymerase II machinery performs transcription on RNA templates of nuclear-replicating viroids, (2) how viroid RNAs coordinate multiple structural elements for diverse functions, and (3) how viroid RNAs activate plant immunity. Research on viroids has led to seminal discoveries in biology, and we expect the research directions outlined in this review to continue providing key knowledge inspiring other areas of biology.

Plant RNA-mediated gene regulatory network

Not all transcribed RNAs are protein-coding RNAs. Many of them are non-protein-coding RNAs in diverse eukaryotes. However, some of them seem to be non-functional and are resulted from spurious transcription. A lot of non-protein-coding transcripts have a significant function in the translation process. Gene expressions depend on complex networks of diverse gene regulatory pathways. Several non-protein-coding RNAs regulate gene expression in a sequence-specific system either at the transcriptional level or post-transcriptional level. They include a significant part of the gene expression regulatory network. RNA-mediated gene regulation machinery is evolutionarily ancient. They well-evolved during the evolutionary time and are becoming much more complex than had been expected. In this review, we are trying to summarizing the current knowledge in the field of RNA-mediated gene silencing.

Advances in Viroid-Host Interactions

Viroids are small, single-stranded, circular RNAs infecting plants. Composed of only a few hundred nucleotides and being unable to code for proteins, viroids represent the lowest level of complexity for an infectious agent, even below that of the smallest known viruses. Despite the relatively small size, viroids contain RNA structural elements embracing all the information needed to interact with host factors involved in their infectious cycle, thus providing models for studying structure-function relationships of RNA. Viroids are specifically targeted to nuclei (family Pospiviroidae) or chloroplasts (family Avsunviroidae), where replication based on rolling-circle mechanisms takes place. They move locally and systemically through plasmodesmata and phloem, respectively, and may elicit symptoms in the infected host, with pathogenic pathways linked to RNA silencing and other plant defense responses. In this review, recent advances in the dissection of the complex interplay between viroids and plants are presented, highlighting knowledge gaps and perspectives for future research. Expected final online publication date for the Annual Review of Virology, Volume 8 is September 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Viroids as a Tool to Study RNA-Directed DNA Methylation in Plants

Viroids are plant pathogenic, circular, non-coding, single-stranded RNAs (ssRNAs). Members of the Pospiviroidae family replicate in the nucleus of plant cells through double-stranded RNA (dsRNA) intermediates, thus triggering the host’s RNA interference (RNAi) machinery. In plants, the two RNAi pillars are Post-Transcriptional Gene Silencing (PTGS) and RNA-directed DNA Methylation (RdDM), and the latter has the potential to trigger Transcriptional Gene Silencing (TGS). Over the last three decades, the employment of viroid-based systems has immensely contributed to our understanding of both of these RNAi facets. In this review, we highlight the role of Pospiviroidae in the discovery of RdDM, expound the gradual elucidation through the years of the diverse array of RdDM’s mechanistic details and propose a revised RdDM model based on the cumulative amount of evidence from viroid and non-viroid systems.

Next-Generation Sequencing Identification and Characterization of MicroRNAs in Dwarfed Citrus Trees Infected With Citrus Dwarfing Viroid in High-Density Plantings

Citrus dwarfing viroid (CDVd) induces stunting on sweet orange trees [Citrus sinensis (L.) Osbeck], propagated on trifoliate orange rootstock [Citrus trifoliata (L.), syn. Poncirus trifoliata (L.) Raf.]. MicroRNAs (miRNAs) are a class of non-coding small RNAs (sRNAs) that play important roles in the regulation of tree gene expression. To identify miRNAs in dwarfed citrus trees, grown in high-density plantings, and their response to CDVd infection, sRNA next-generation sequencing was performed on CDVd-infected and non-infected controls. A total of 1,290 and 628 miRNAs were identified in stem and root tissues, respectively, and among those, 60 were conserved in each of these two tissue types. Three conserved miRNAs (csi-miR479, csi-miR171b, and csi-miR156) were significantly downregulated (adjusted p-value < 0.05) in the stems of CDVd-infected trees compared to the non-infected controls. The three stem downregulated miRNAs are known to be involved in various physiological and developmental processes some of which may be related to the characteristic dwarfed phenotype displayed by CDVd-infected C. sinensis on C. trifoliata rootstock field trees. Only one miRNA (csi-miR535) was significantly downregulated in CDVd-infected roots and it was predicted to target genes controlling a wide range of cellular functions. Reverse transcription quantitative polymerase chain reaction analysis performed on selected miRNA targets validated the negative correlation between the expression levels of these targets and their corresponding miRNAs in CDVd-infected trees. Our results indicate that CDVd-responsive plant miRNAs play a role in regulating important citrus growth and developmental processes that may participate in the cellular changes leading to the observed citrus dwarf phenotype.

Induction of Plant Resistance in Tobacco (Nicotiana tabacum) against Tomato Spotted Wilt Orthotospovirus through Foliar Application of dsRNA

Thrips-transmitted tomato spotted wilt orthotospovirus (TSWV) continues to be a constraint to peanut, pepper, tobacco, and tomato production in Georgia and elsewhere. TSWV is being managed by an integrated disease management strategy that includes a combination of cultural practices, vector management, and growing virus-resistant varieties where available. We used a non-transgenic strategy to induce RNA interference (RNAi)-mediated resistance in tobacco (Nicotiana tabacum) plants against TSWV. Double-stranded RNA (dsRNA) molecules for the NSs (silencing suppressor) and N (nucleoprotein) genes were produced by a two-step PCR approach followed by in vitro transcription. When topically applied to tobacco leaves, both molecules elicited a resistance response. Host response to the treatments was measured by determining the time to symptom expression, and the level of resistance by absolute quantification of the virus. We also show the systemic movement of dsRNA_N from the inoculated leaves to younger, non-inoculated leaves. Post-application, viral siRNAs were detected for up to nine days in inoculated leaves and up to six days in non-inoculated leaves. The topical application of dsRNAs to induce RNAi represents an environmentally safe and efficient way to manage TSWV in tobacco crops and could be applicable to other TSWV-susceptible crops.

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.

Viroid pathogenesis: a critical appraisal of the role of RNA silencing in triggering the initial molecular lesion

The initial molecular lesions through which viroids, satellite RNAs and viruses trigger signal cascades resulting in plant diseases are hotly debated. Since viroids are circular non-protein-coding RNAs of ∼250-430 nucleotides, they appear very convenient to address this issue. Viroids are targeted by their host RNA silencing defense, generating viroid-derived small RNAs (vd-sRNAs) that are presumed to direct Argonaute (AGO) proteins to inactivate messenger RNAs, thus initiating disease. Here, we review the existing evidence. Viroid-induced symptoms reveal a distinction. Those attributed to vd-sRNAs from potato spindle tuber viroid and members of the family Pospiviroidae (replicating in the nucleus) are late, non-specific and systemic. In contrast, those attributed to vd-sRNAs from peach latent mosaic viroid (PLMVd) and other members of the family Avsunviroidae (replicating in plastids) are early, specific and local. Remarkably, leaf sectors expressing different PLMVd-induced chloroses accumulate viroid variants with specific pathogenic determinants. Some vd-sRNAs containing such determinant guide AGO1-mediated cleavage of mRNAs that code for proteins regulating chloroplast biogenesis/development. Therefore, the initial lesions and the expected phenotypes are connected by short signal cascades, hence supporting a cause-effect relationship. Intriguingly, one virus satellite RNA initiates disease through a similar mechanism, whereas in the Pospiviroidae and in plant viruses the situation remains uncertain.

Hop stunt viroid: A polyphagous pathogenic RNA that has shed light on viroid-host interactions

TAXONOMY

Hop stunt viroid (HSVd) is the type species of the genus Hostuviroid (family Pospiviroidae). The other species of this genus is Dahlia latent viroid, which presents an identical central conserved region (CCR) but lacks other structural hallmarks present in Hop stunt viroid. HSVd replication occurs in the nucleus through an asymmetric rolling-circle model as in the other members of the family Pospiviroidae, which also includes the genera Pospiviroid, Cocadviroid, Apscaviroid, and Coleoviroid.

PHYSICAL PROPERTIES

Hop stunt viroid consists of a single-stranded, circular RNA of 295-303 nucleotides depending on isolates and sequence variants. The most stable secondary structure is a rod-like or quasi-rod-like conformation with two characteristic domains: a CCR and a terminal conserved hairpin similar to that of cocadviroids. HSVd lacks a terminal conserved region.

HOSTS AND SYMPTOMS

HSVd infects a very broad range of natural hosts and has been reported to be the causal agent of five different diseases (citrus cachexia, cucumber pale fruit, peach and plum apple apricot distortion, and hop stunt). It is distributed worldwide.

TRANSMISSION

HSVd is transmitted mechanically and by seed.

Progress in 50 years of viroid research-Molecular structure, pathogenicity, and host adaptation

Viroids are non-encapsidated, single-stranded, circular RNAs consisting of 246-434 nucleotides. Despite their non-protein-encoding RNA nature, viroids replicate autonomously in host cells. To date, more than 25 diseases in more than 15 crops, including vegetables, fruit trees, and flowers, have been reported. Some are pathogenic but others replicate without eliciting disease. Viroids were shown to have one of the fundamental attributes of life to adapt to environments according to Darwinian selection, and they are likely to be living fossils that have survived from the pre-cellular RNA world. In 50 years of research since their discovery, it was revealed that viroids invade host cells, replicate in nuclei or chloroplasts, and undergo nucleotide mutation in the process of adapting to new host environments. It was also demonstrated that structural motifs in viroid RNAs exert different levels of pathogenicity by interacting with various host factors. Despite their small size, the molecular mechanism of viroid pathogenicity turned out to be more complex than first thought.

Impact of Nucleic Acid Sequencing on Viroid Biology

The early 1970s marked two breakthroughs in the field of biology: (i) The development of nucleotide sequencing technology; and, (ii) the discovery of the viroids. The first DNA sequences were obtained by two-dimensional chromatography which was later replaced by sequencing using electrophoresis technique. The subsequent development of fluorescence-based sequencing method which made DNA sequencing not only easier, but many orders of magnitude faster. The knowledge of DNA sequences has become an indispensable tool for both basic and applied research. It has shed light biology of viroids, the highly structured, circular, single-stranded non-coding RNA molecules that infect numerous economically important plants. Our understanding of viroid molecular biology and biochemistry has been intimately associated with the evolution of nucleic acid sequencing technologies. With the development of the next-generation sequence method, viroid research exponentially progressed, notably in the areas of the molecular mechanisms of viroids and viroid diseases, viroid pathogenesis, viroid quasi-species, viroid adaptability, and viroid–host interactions, to name a few examples. In this review, the progress in the understanding of viroid biology in conjunction with the improvements in nucleotide sequencing technology is summarized. The future of viroid research with respect to the use of third-generation sequencing technology is also briefly envisaged.

Sequence Similarities between Viroids and Human MicroRNAs

Viroids are minute unencapsidated non-coding circular RNAs known to be present and to cause diseases only in plants. Infections were associated with the occurrence of specific single-stranded RNAs similar in size to miRNAs and endogenous small interfering RNAs, and viroid pathogenicity is suspected to occur through RNA interference. We looked for sequence similarities between viroids and the seed region of human microRNAs (hsa-miRNAs). Viroid genomes were retrieved from GenBank and mature hsa-mi-RNAs were retrieved from miRBase. Two hundred 300-nucleotide-long sequences were randomly generated as controls. BLAST searches were performed using viroids as queries and hsa-miRNAs as subjects with relaxed parameters, and matches involving hsa-miRNA seed regions were considered. A total of 81,021 matches were found, and 1,501 that showed 100% identity with whole hsa-miRNA seed regions were selected. The most frequent matches involved Chrysanthemum stunt viroid or Hop stunt viroidspecies with hsa-miR-4286, in 365 and 207 cases, respectively. Three hsa-mi-RNAs (miR-4286, miR-6808-5p, and miR-3622a-3p) were involved in 47% of all matches between viroids and hsa-mi-RNAs. Taken together, these findings warrant further investigation on the potential of viroids and their derived small RNAs to cross kingdoms and interact with nucleic acids in humans.

Insights Into Potato Spindle Tuber Viroid Quasi-Species From Infection to Disease

Viroids are non-coding RNA plant pathogens that are characterized by their possession of a high mutation level. Although the sequence heterogeneity in viroid infected plants is well understood, shifts in viroid population dynamics due to mutations over the course of infection remain poorly understood. In this study, the ten most abundant sequence variants of potato spindle tuber viroid RG1 (PSTVd) expressed at different time intervals in PSTVd infected tomato plants were identified by high-throughput sequencing. The sequence variants, forming a quasi-species, were subjected to both the identification of the regions favoring mutations and the effect of the mutations on viroid secondary structure and viroid derived small RNAs (vd-sRNA). At week 1 of PSTVd infection, 25% of the sequence variants were similar to the "master" sequence (i.e., the sequence used for inoculation). The frequency of the master sequence within the population increased to 70% at week 2 after PSTVd infection, and then stabilized for the rest of the disease cycle (i.e., weeks 3 and 4). While some sequence variants were abundant at week 1 after PSTVd infection, they tended to decrease in frequency over time. For example, the variants with insertions at positions 253 or 254, positions that could affect the Loop E as well as the metastable hairpin I structure that has been shown important during replication and viroid infectivity, resulted in decreased frequency. Data obtained by in silico analysis of the viroid derived small RNAs (vd-sRNA) was also analyzed. A few mutants had the potential of positively affecting the viroid's accumulation by inducing the RNA silencing of the host's defense related genes. Variants with mutations that could negatively affect viroid abundance were also identified because their derived vd-sRNA were no longer capable of targeting any host mRNA or of changing its target sequence from a host defense gene to some other non-important host gene. Together, these findings open avenues into understanding the biological role of sequence variants, this viroid's interaction with host components, stable and metastable structures generated by mutants during the course of infection, and the influence of sequence variants on stabilizing viroid population dynamics.

Viral and subviral derived small RNAs as pathogenic determinants in plants and insects

The phenotypic manifestations of disease induced by viruses and subviral infectious entities are the result of complex molecular interactions between host and viral factors. The viral determinants of the diseased phenotype have traditionally been sought at the level of structural or non-structural proteins. However, the discovery of RNA silencing mechanisms has led to speculations that determinants of the diseased phenotype are caused by viral nucleic acid sequences in addition to proteins. RNA silencing is a gene regulation mechanism conserved within eukaryotic kingdoms (with the exception of some yeast species), and in plants and insects it also functions as an antiviral mechanism. Non-coding RNAs of viral origin, ranging in size from 21 to 24 nucleotides (viral small interfering RNAs, vsiRNAs) accumulate in virus-infected tissues and organs, in some cases to comparable levels as the entire complement of host-encoded small interfering RNAs. Upon incorporation into RNA-induced silencing complexes, vsiRNAs can mediate cleavage or induce translational inhibition of nucleic acid targets in a sequence-specific manner. This review focuses on recent findings that suggest an increased complexity of small RNA-based interactions between virus and host. We mainly address plant viruses, but where applicable discuss insect viruses as well. Prominence is given to studies that have indisputably demonstrated that vsiRNAs determine diseased phenotype by either carrying sequence determinants or, indirectly, by altering host-gene regulatory pathways. Results from these studies suggest biotechnological applications, which are also discussed.

Evidence Supporting That RNA Polymerase II Catalyzes De Novo Transcription Using Potato Spindle Tuber Viroid Circular RNA Templates

Transcription is a fundamental process that mediates the interplay between genetic information and phenotype. Emerging evidence indicates that RNA polymerase II (Pol II) can catalyze transcription using both DNA and RNA templates. It is well established that Pol II initiates de novo transcription on DNA templates. However, it is unclear whether Pol II performs de novo transcription or relies on primers for initiation (primed transcription) on RNA templates. Using potato spindle tuber viroid (PSTVd) as a model, we presented evidence showing that circular PSTVd templates are critical for the synthesis of longer-than-unit-length (-)-strand products, which supports the de novo transcription based on the asymmetric rolling circle model of PSTVd replication. We further showed that the crucial factor for primed transcription, transcription factor IIS (TFIIS), is dispensable for PSTVd replication in cells. Together, our data support the de novo transcription on PSTVd RNA templates catalyzed by Pol II. This result has significant implications in understanding the mechanism and machinery underlying Pol II-catalyzed transcription using other RNA templates.

Genetically Modified Organism-Free RNA Interference: Exogenous Application of RNA Molecules in Plants

The latest advances in the field exogenous application of RNA molecules in plants help to protect and modify them through RNA interference (RNAi).

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.

RNAi-Mediated Down-Regulation of Dicer-Like 2 and 4 Changes the Response of 'Moneymaker' Tomato to Potato Spindle Tuber Viroid Infection from Tolerance to Lethal Systemic Necrosis, Accompanied by Up-Regulation of miR398, 398a-3p and Production of Excessive Amount of Reactive Oxygen Species

To examine the role of RNA silencing in plant defenses against viroids, a Dicer-like 2 and 4 (DCL2&4)–double knockdown transgenic tomato plant line, 72E, was created. The expression of endogenous SlDCL2s and SlDCL4 in line 72E decreased to about a half that of the empty cassette line, EC. When challenged with potato spindle tuber viroid (PSTVd), line 72E showed significantly higher levels of PSTVd accumulation early in the course of the infection and lethal systemic necrosis late in the infection. The size distribution of PSTVd-derived small RNAs was significantly different with the number of RNAs of 21 and 22 nucleotides (nt) in line 72E, at approximately 66.7% and 5% of those in line EC, respectively. Conversely, the numbers of 24 nt species increased by 1100%. Furthermore, expression of the stress-responsive microRNA species miR398 and miR398a-3p increased 770% and 868% in the PSTVd-infected line 72E compared with the PSTVd-infected EC. At the same time, the expression of cytosolic and chloroplast-localized Cu/Zn-superoxide dismutase 1 and 2 (SOD1 and SOD2) and the copper chaperon for SOD (CCS1) mRNAs, potential targets of miR398 or 398a-3p, decreased significantly in the PSTVd-infected line 72E leaves, showing necrosis. In concert with miR398 and 398a-3p, SODs control the detoxification of reactive oxygen species (ROS) generated in cells. Since high levels of ROS production were observed in PSTVd-infected line 72E plants, it is likely that the lack of full dicer-likes (DCL) activity in these plants made them unable to control excessive ROS production after PSTVd infection, as disruption in the ability of miR398 and miR398a-3p to regulate SODs resulted in the development of lethal systemic necrosis.

Combining a Simple Method for DNA/RNA/Protein Co-Purification and Arabidopsis Protoplast Assay to Facilitate Viroid Research

Plant–viroid interactions represent a valuable model for delineating structure–function relationships of noncoding RNAs. For various functional studies, it is desirable to minimize sample variations by using DNA, RNA, and proteins co-purified from the same samples. Currently, most of the co-purification protocols rely on TRI Reagent (Trizol as a common representative) and require protein precipitation and dissolving steps, which render difficulties in experimental handling and high-throughput analyses. Here, we established a simple and robust method to minimize the precipitation steps and yield ready-to-use RNA and protein in solutions. This method can be applied to samples in small quantities, such as protoplasts. Given the ease and the robustness of this new method, it will have broad applications in virology and other disciplines in molecular biology.

SERRATE, a miRNA biogenesis factor, affects viroid infection in Nicotiana benthamiana and Nicotiana tabacum

Viroids are plant infecting, non - coding RNA molecules of economic importance. Potato spindle tuber viroid (PSTVd), the type species of Pospiviroidae family, has been shown to be affected by specific RNA silencing pathways. Dicer like 1 (DCL1), a key player in micro RNA (miRNA) pathway has been previously linked with PSTVd infectivity. In this report we aim to further dissect the interaction between the miRNA pathway and Pospiviroid virulence. We mainly focused on the Zinc-finger protein SERRATE (SE) a co-factor of DCL1 and core component of miRNA pathway. We generated Nicotiana tabacum and Nicotiana benthamiana SE knock-down plants exhibiting considerable miRNA reduction and strong phenotypic abnormalities. PSTVd infection of SE suppressed plants resulted in a significant viroid reduction, especially at the initial infection stages. This positive correlation between SE levels and viroid infectivity underlines its role in PSTVd life cycle and reveals the importance of the miRNA pathway upon viroid infection.

Pospiviroid Infection of Tomato Regulates the Expression of Genes Involved in Flower and Fruit Development

Viroids are unencapsidated, single-stranded, covalently-closed circular, highly structured, noncoding RNAs of 239–401 nucleotides that cause disease in several economically important crop plants. In tomato (Solanum lycopersicum cv. Rutgers), symptoms of pospiviroid infection include stunting, reduced vigor, flower abortion, and reduced size and number of fruits, resulting in significant crop losses. Dramatic alterations in plant development triggered by viroid infection are the result of differential gene expression; in our study, we focused on the effect of tomato planta macho viroid (TPMVd) and Mexican papita viroid (MPVd) infection on gene networks associated with the regulation of flower and fruit development. The expression of several of the genes were previously reported to be affected by viroid infection, but two genes not previously studied were included. Changes in gene expression of SlBIGPETAL1 (bHLH transcription factor) and SlOVA6 (proline-like tRNA synthetase) are involved in petal morphology and fertility, respectively. Expression of SlOVA6 was down-regulated in flowers of TPMVd- and MPVd-infected plants, while expression of SlBIGPETAL1 was up-regulated in flowers. Up-regulation of SlBIGPETAL1 and down-regulation of SlOVA6 were positively correlated with symptoms such as reduced petal size and flower abortion. Expression analysis of additional tomato genes and a prediction of a global network association of genes involved in flower and fruit development and impacted by viroid infection may further elucidate the pathways underlying viroid pathogenicity.

Allelic RNA Motifs in Regulating Systemic Trafficking of Potato Spindle Tuber Viroid

Intercellular RNA trafficking has been shown as a widely-existing phenomenon that has significant functions in many aspects of biology. Viroids, circular noncoding RNAs that cause plant diseases, have been a model to dissect the role of RNA structural motifs in regulating intercellular RNA trafficking in plants. Recent studies on potato spindle tuber viroid (PSTVd) showed that the RNA motif loop 19 is important for PSTVd to spread from palisade to spongy mesophyll in infected leaves. Here, we performed saturated mutational analysis to uncover all possible functional variants of loop 19 and exploit this data to pinpoint to a three-dimensional structural model of this motif. Interestingly, we found that two distinct structural motifs can replace loop 19 and retain the systemic trafficking capacity. One of the alternative structures rapidly emerged from the inoculation using a loop 19 abolished mutant that is not capable of systemic trafficking. Our observation indicates the flexibility of multiple structural arrangements interchangeably exerting similar function at a particular RNA locus. Taken together, this study deepens the understanding of RNA structural motifs-regulated viroid RNA trafficking, which has broad implications for studying RNA intercellular trafficking as well.

Plant Responses to Pathogen Attack: Small RNAs in Focus

Small RNAs (sRNA) are a significant group of gene expression regulators for multiple biological processes in eukaryotes. In plants, many sRNA silencing pathways produce extensive array of sRNAs with specialized roles. The evidence on record advocates for the functions of sRNAs during plant microbe interactions. Host sRNAs are reckoned as mandatory elements of plant defense. sRNAs involved in plant defense processes via different pathways include both short interfering RNA (siRNA) and microRNA (miRNA) that actively regulate immunity in response to pathogenic attack via tackling pathogen-associated molecular patterns (PAMPs) and other effectors. In response to pathogen attack, plants protect themselves with the help of sRNA-dependent immune systems. That sRNA-mediated plant defense responses play a role during infections is an established fact. However, the regulations of several sRNAs still need extensive research. In this review, we discussed the topical advancements and findings relevant to pathogen attack and plant defense mediated by sRNAs. We attempted to point out diverse sRNAs as key defenders in plant systems. It is hoped that sRNAs would be exploited as a mainstream player to achieve food security by tackling different plant diseases.

Engineering resistance against viroids

Viroids, the smallest infectious agents endowed with autonomous replication, are tiny single-stranded circular RNAs (∼250 to 400nt) without protein-coding ability that, despite their simplicity, infect and often cause disease in herbaceous and woody plants of economic relevance. To mitigate the resulting losses, several strategies have been developed, the most effective of which include: firstly, search for naturally resistant cultivars and breeding for resistance, secondly, induced resistance by pre-infection with mild strains, thirdly, ribonucleases targeting double-stranded RNAs and catalytic antibodies endowed with intrinsic ribonuclease activity, fourthly, antisense, and sense, RNAs, fifthly, catalytic antisense RNAs derived from hammerhead ribozymes, and sixthly, hairpin RNAs and artificial small RNAs for RNA interference. The mechanisms underpinning these strategies, most of which have been implemented via genetic transformation, together with their present results and future potential, are the subject of this review.

Comprehensive Transcriptome Analyses Reveal that Potato Spindle Tuber Viroid Triggers Genome-Wide Changes in Alternative Splicing, Inducible trans-Acting Activity of Phased Secondary Small Interfering RNAs, and Immune Responses

Many pathogens express noncoding RNAs (ncRNAs) during infection processes. In the most extreme case, pathogenic ncRNAs alone (such as viroids) can infect eukaryotic organisms, leading to diseases. While a few pathogenic ncRNAs have been implicated in regulating gene expression, the functions of most pathogenic ncRNAs in host-pathogen interactions remain unclear. Here, we employ potato spindle tuber viroid (PSTVd) infecting tomato as a system to dissect host interactions with pathogenic ncRNAs, using comprehensive transcriptome analyses. We uncover various new activities in regulating gene expression during PSTVd infection, such as genome-wide alteration in alternative splicing of host protein-coding genes, enhanced guided-cleavage activities of a host microRNA, and induction of the trans-acting function of phased secondary small interfering RNAs. Furthermore, we reveal that PSTVd infection massively activates genes involved in plant immune responses, mainly those in the calcium-dependent protein kinase and mitogen-activated protein kinase cascades, as well as prominent genes involved in hypersensitive responses, cell wall fortification, and hormone signaling. Intriguingly, our data support a notion that plant immune systems can respond to pathogenic ncRNAs, which has broad implications for providing new opportunities for understanding the complexity of immune systems in differentiating "self" and "nonself," as well as lay the foundation for resolving the long-standing question regarding the pathogenesis mechanisms of viroids and perhaps other infectious RNAs.IMPORTANCE Numerous pathogens, including viruses, express pathogenic noncoding transcripts during infection. In the most extreme case, pathogenic noncoding RNAs alone (i.e., viroids) can cause disease in plants. While some work has demonstrated that pathogenic noncoding RNAs interact with host factors for function, the biological significance of pathogenic noncoding RNAs in host-pathogen interactions remains largely unclear. Here, we apply comprehensive genome-wide analyses of plant-viroid interactions and discover several novel molecular activities underlying nuclear-replicating viroid infection processes in plants, including effects on the expression and function of host noncoding transcripts, as well as the alternative splicing of host protein-coding genes. Importantly, we show that plant immunity is activated upon infection of a nuclear-replicating viroid, which is a new concept that helps to understand viroid-based pathogenesis. Our finding has broad implications for understanding the complexity of host immune systems and the diverse functions of noncoding RNAs.

Host-Pathogen interactions modulated by small RNAs

Biological processes such as defense mechanisms and microbial offense strategies are regulated through RNA induced interference in eukaryotes. Genetic mutations are modulated through biogenesis of small RNAs which directly impacts upon host development. Plant defense mechanisms are regulated and supported by a diversified group of small RNAs which are involved in streamlining several RNA interference pathways leading toward the initiation of pathogen gene silencing mechanisms. In the similar context, pathogens also utilize the support of small RNAs to launch their offensive attacks. Also there are strong evidences about the active involvement of these RNAs in symbiotic associations. Interestingly, small RNAs are not limited to the individuals in whom they are produced; they also show cross kingdom influences through variable interactions with other species thus leading toward the inter-organismic gene silencing. The phenomenon is understandable in the microbes which utilize these mechanisms to overcome host defense line. Understanding the mechanism of triggering host defense strategies can be a valuable step toward the generation of disease resistant host plants. We think that the cross kingdom trafficking of small RNA is an interesting insight that is needed to be explored for its vitality.

Improved Pathogenicity of a Beet Black Scorch Virus Variant by Low Temperature and Co-infection with Its Satellite RNA

Co-infection of none-coding satellite RNAs (sat-RNAs) usually inhibits replication and attenuates disease symptoms of helper viruses. However, we find that the sat-RNA of Beet black scorch virus (BBSV) and low temperature (18°C) additively enhance the systemic infection of BBSV in Nicotiana benthamiana. Northern blotting hybridization revealed a relatively reduced accumulation of BBSV-derived small interfering RNAs (siRNAs) in presence of sat-RNA as compared to that of BBSV alone. Cloning and sequencing of total small RNAs showed that more than 50% of the total small RNAs sequenced from BBSV-infected plants were BBSV-siRNAs, whereas the abundance of sat-siRNAs were higher than BBSV-siRNAs in the sat-RNA co-infected plants, indicating that the sat-RNA occupies most of the silencing components and possibly relieves the RNA silencing-mediated defense against BBSV. Interestingly, the 5' termini of siRNAs derived from BBSV and sat-RNA were dominated by Uridines (U) and Adenines (A), respectively. Besides, the infection of BBSV alone and with sat-RNA induce down-regulation of miR168 and miR403, respectively, which leads to high accumulation of their targets, Argonaute 1 (AGO1) and AGO2. Our work reveals the profiles of siRNAs of BBSV and sat-RNA and provides an additional clue to investigate the complicated interaction between the helper virus and sat-RNA.

Replication of a pathogenic non-coding RNA increases DNA methylation in plants associated with a bromodomain-containing viroid-binding protein

Viroids are plant-pathogenic molecules made up of single-stranded circular non-coding RNAs. How replicating viroids interfere with host silencing remains largely unknown. In this study, we investigated the effects of a nuclear-replicating Potato spindle tuber viroid (PSTVd) on interference with plant RNA silencing. Using transient induction of silencing in GFP transgenic Nicotiana benthamiana plants (line 16c), we found that PSTVd replication accelerated GFP silencing and increased Virp1 mRNA, which encodes bromodomain-containing viroid-binding protein 1 and is required for PSTVd replication. DNA methylation was increased in the GFP transgene promoter of PSTVd-replicating plants, indicating involvement of transcriptional gene silencing. Consistently, accelerated GFP silencing and increased DNA methylation in the of GFP transgene promoter were detected in plants transiently expressing Virp1. Virp1 mRNA was also increased upon PSTVd infection in natural host potato plants. Reduced transcript levels of certain endogenous genes were also consistent with increases in DNA methylation in related gene promoters in PSTVd-infected potato plants. Together, our data demonstrate that PSTVd replication interferes with the nuclear silencing pathway in that host plant, and this is at least partially attributable to Virp1. This study provides new insights into the plant-viroid interaction on viroid pathogenicity by subverting the plant cell silencing machinery.

A nuclear-replicating viroid antagonizes infectivity and accumulation of a geminivirus by upregulating methylation-related genes and inducing hypermethylation of viral DNA

DNA methylation and post-transcriptional gene silencing play critical roles in controlling infection of single-stranded (ss) DNA geminiviruses and ssRNA viroids, respectively, but both pathogens can counteract these host defense mechanisms and promote their infectivity. Moreover, a specific role of DNA methylation in viroid-host interactions is not yet confirmed. Here, using an experimental system where two nuclear-replicating agents, the geminivirus tomato yellow leaf curl Sardinia virus (TYLCSV) and potato spindle tuber viroid (PSTVd), co-infect their common host tomato, we observed that PSTVd severely interferes with TYLCSV infectivity and accumulation, most likely as a consequence of strong activation of host DNA methylation pathways. In fact, PSTVd alone or in co-infection with TYLCSV significantly upregulates the expression of key genes governing DNA methylation in plants. Using methylation-sensitive restriction and bisulfite conversion assays, we further showed that PSTVd infection promotes a strong hypermethylation of TYLCSV DNA, thus supporting a mechanistic link with the antagonism of the viroid on the virus in co-infected tomato plants. These results describe the interaction between two nuclear-replicating pathogens and show that they differentially interfere with DNA methylation pathways.

RNA Silencing in Plants: Mechanisms, Technologies and Applications in Horticultural Crops

Understanding the fundamental nature of a molecular process or a biological pathway is often a catalyst for the development of new technologies in biology. Indeed, studies from late 1990s to early 2000s have uncovered multiple overlapping but functionally distinct RNA silencing pathways in plants, including the posttranscriptional microRNA and small interfering RNA pathways and the transcriptional RNA-directed DNA methylation pathway. These findings have in turn been exploited for developing artificial RNA silencing technologies such as hairpin RNA, artificial microRNA, intrinsic direct repeat, 3' UTR inverted repeat, artificial trans-acting siRNA, and virus-induced gene silencing technologies. Some of these RNA silencing technologies, such as the hairpin RNA technology, have already been widely used for genetic improvement of crop plants in agriculture. For horticultural plants, RNA silencing technologies have been used to increase disease and pest resistance, alter plant architecture and flowering time, improve commercial traits of fruits and flowers, enhance nutritional values, remove toxic compounds and allergens, and develop high-value industrial products. In this article we aim to provide an overview of the RNA silencing pathways in plants, summarize the existing RNA silencing technologies, and review the current progress in applying these technologies for the improvement of agricultural crops particularly horticultural crops.

Combined Activity of DCL2 and DCL3 Is Crucial in the Defense against Potato Spindle Tuber Viroid

Viroids are self replicating non-coding RNAs capable of infecting a wide range of plant hosts. They do not encode any proteins, thus the mechanism by which they escape plant defenses remains unclear. RNAi silencing is a major defense mechanism against virus infections, with the four DCL proteins being principal components of the pathway. We have used Nicotiana benthamiana as a model to study Potato spindle tuber viroid infection. This viroid is a member of the Pospiviroidae family and replicates in the nucleus via an asymmetric rolling circle mechanism. We have created knock-down plants for all four DCL genes and their combinations. Previously, we showed that DCL4 has a positive effect on PSTVd infectivity since viroid levels drop when DCL4 is suppressed. Here, we show that PSTVd levels remain decreased throughout infection in DCL4 knockdown plants, and that simultaneous knockdown of DCL1, DCL2 or DCL3 together with DCL4 cannot reverse this effect. Through infection of plants suppressed for multiple DCLs we further show that a combined suppression of DCL2 and DCL3 has a major effect in succumbing plant antiviral defense. Based on our results, we further suggest that Pospoviroids may have evolved to be primarily processed by DCL4 as it seems to be a DCL protein with less detrimental effects on viroid infectivity. These findings pave the way to delineate the complexity of the relationship between viroids and plant RNA silencing response.

Active Shiga-Like Toxin Produced by Some Aeromonas spp., Isolated in Mexico City

RNA silencing is a conserved mechanism that utilizes small RNAs (sRNAs) to direct the regulation of gene expression at the transcriptional or post-transcriptional level. Plants utilizing RNA silencing machinery to defend pathogen infection was first identified in plant–virus interaction and later was observed in distinct plant–pathogen interactions. RNA silencing is not only responsible for suppressing RNA accumulation and movement of virus and viroid, but also facilitates plant immune responses against bacterial, oomycete, and fungal infection. Interestingly, even the same plant sRNA can perform different roles when encounters with different pathogens. On the other side, pathogens counteract by generating sRNAs that directly regulate pathogen gene expression to increase virulence or target host genes to facilitate pathogen infection. Here, we summarize the current knowledge of the characterization and biogenesis of host- and pathogen-derived sRNAs, as well as the different RNA silencing machineries that plants utilize to defend against different pathogens. The functions of these sRNAs in defense and counter-defense and their mechanisms for regulation during different plant–pathogen interactions are also discussed.

Effects of potato spindle tuber viroid infection on tomato metabolic profile

Viroids are the smallest plant pathogens consisting of a single stranded circular RNA molecule with a strong secondary structure, lacking a coat protein or any other proteins. The mechanism of viroid pathogenicity has remained unclear. Recent advances in instrumentation and data mining have made it possible to study the effects of various stresses on primary and secondary metabolisms. Here, we have utilized metabolic profiling approach to show how PSTVd infection alters tomato metabolic profile and the related pathways. Three terminal leaflets of third true leaf of 20-day-old tolerant tomato cultivar 'Moneymaker' were mechanically inoculated by PSTVd intermediate variant cDNAs and samples were taken from eighth leaf, 19days post-inoculation. Metabolites were extracted and analyzed by gas chromatography/mass spectrometry (GC/MS) and subjected to statistical data analysis. Affected pathways were identified by Pathway Tools program and were compared with microarray data previously reported. The study showed that 79 metabolites changed significantly and 23 pathways were identified in relation to these metabolites. Fourteen of these pathways were similar to those reported in other works. The altered pathways in PSTVd infected tomato leaves included, eight cutin and wax biosynthesis, seven pathways that produce defense related compounds, two energy generator pathways, three hormone biosynthesis pathways, two signal transduction pathways, and one nucleotide biosynthesis pathway. Our data on up/down-regulation of pathways supported the data produced on their corresponding gene(s) up/down-regulation.

RNA-directed DNA methylation efficiency depends on trigger and target sequence identity

RNA-directed DNA methylation (RdDM) in plants has been extensively studied, but the RNA molecules guiding the RdDM machinery to their targets are still to be characterized. It is unclear whether these molecules require full complementarity with their target. In this study, we have generated Nicotiana tabacum (Nt) plants carrying an infectious tomato apical stunt viroid (TASVd) transgene (Nt-TASVd) and a non-infectious potato spindle tuber viroid (PSTVd) transgene (Nt-SB2). The two viroid sequences exhibit 81% sequence identity. Nt-TASVd and Nt-SB2 plants were genetically crossed. In the progeny plants (Nt-SB2/TASVd), deep sequencing of small RNAs (sRNAs) showed that TASVd infection was associated with the accumulation of abundant small interfering RNAs (siRNAs) that mapped along the entire TASVd but only partially matched the SB2 transgene. TASVd siRNAs efficiently targeted SB2 RNA for degradation, but no transitivity was detectable. Bisulfite sequencing in the Nt-SB2/TASVd plants revealed that the TASVd transgene was targeted for dense cis-RdDM along its entire sequence. In the same plants, the SB2 transgene was targeted for trans-RdDM. The SB2 methylation pattern, however, was weak and heterogeneous, pointing to a positive correlation between trigger-target sequence identity and RdDM efficiency. Importantly, trans-RdDM on SB2 was also detected at sites where no homologous siRNAs were detected. Our data indicate that RdDM efficiency depends on the trigger-target sequence identity, and is not restricted to siRNA occupancy. These findings support recent data suggesting that RNAs with sizes longer than 24 nt (>24-nt RNAs) trigger RdDM.

A Land Plant-Specific Transcription Factor Directly Enhances Transcription of a Pathogenic Noncoding RNA Template by DNA-Dependent RNA Polymerase II

Some DNA-dependent RNA polymerases (DdRPs) possess RNA-dependent RNA polymerase activity, as was first discovered in the replication of Potato spindle tuber viroid (PSTVd) RNA genome in tomato (Solanum lycopersicum). Recent studies revealed that this activity in bacteria and mammals is important for transcriptional and posttranscriptional regulatory mechanisms. Here, we used PSTVd as a model to uncover auxiliary factors essential for RNA-templated transcription by DdRP PSTVd replication in the nucleoplasm generates (-)-PSTVd intermediates and (+)-PSTVd copies. We found that the Nicotiana benthamiana canonical 9-zinc finger (ZF) Transcription Factor IIIA (TFIIIA-9ZF) as well as its variant TFIIIA-7ZF interacted with (+)-PSTVd, but only TFIIIA-7ZF interacted with (-)-PSTVd. Suppression of TFIIIA-7ZF reduced PSTVd replication, and overexpression of TFIIIA-7ZF enhanced PSTVd replication in planta. Consistent with the locale of PSTVd replication, TFIIIA-7ZF was found in the nucleoplasm and nucleolus, in contrast to the strictly nucleolar localization of TFIIIA-9ZF. Footprinting assays revealed that only TFIIIA-7ZF bound to a region of PSTVd critical for initiating transcription. Furthermore, TFIIIA-7ZF strongly enhanced the in vitro transcription of circular (+)-PSTVd by partially purified Pol II. Together, our results identify TFIIIA-7ZF as a dedicated cellular transcription factor that acts in DdRP-catalyzed RNA-templated transcription, highlighting both the extraordinary evolutionary adaptation of viroids and the potential of DdRPs for a broader role in cellular processes.

Pathogenesis by subviral agents: viroids and hepatitis delta virus

The viroids of plants are the simplest known infectious genetic elements. They have RNA genomes of up to 400 nucleotides in length and no protein encoding capacity. Hepatitis delta virus (HDV), an infectious agent found only in humans co-infected with hepatitis B virus (HBV), is just slightly more complex, with an RNA genome of about 1700 nucleotides, and the ability to express just one small protein. Viroid and HDV RNAs share several features that include circular structure, compact folding, and replication via a rolling-circle mechanism. Both agents were detected because of their obvious pathogenic effects. Their simplicity demands a greater need than conventional RNA or DNA viruses to redirect host components for facilitating their infectious cycle, a need that directly and indirectly incites pathogenic effects. The mechanisms by which these pathogenic effects are produced are the topic of this review. In this context, RNA silencing mediates certain aspects of viroid pathogenesis.