The complete nucleotide sequence and synthesis of infectious RNA of genomic and defective interfering RNAs of TBSV-P

Defective RNA Particles of Plant Viruses-Origin, Structure and Role in Pathogenesis

The genomes of RNA viruses may be monopartite or multipartite, and sub-genomic particles such as defective RNAs (D RNAs) or satellite RNAs (satRNAs) can be associated with some of them. D RNAs are small, deletion mutants of a virus that have lost essential functions for independent replication, encapsidation and/or movement. D RNAs are common elements associated with human and animal viruses, and they have been described for numerous plant viruses so far. Over 30 years of studies on D RNAs allow for some general conclusions to be drawn. First, the essential condition for D RNA formation is prolonged passaging of the virus at a high cellular multiplicity of infection (MOI) in one host. Second, recombination plays crucial roles in D RNA formation. Moreover, during virus propagation, D RNAs evolve, and the composition of the particle depends on, e.g., host plant, virus isolate or number of passages. Defective RNAs are often engaged in transient interactions with full-length viruses-they can modulate accumulation, infection dynamics and virulence, and are widely used, i.e., as a tool for research on cis-acting elements crucial for viral replication. Nevertheless, many questions regarding the generation and role of D RNAs in pathogenesis remain open. In this review, we summarise the knowledge about D RNAs of plant viruses obtained so far.

Ecotype-specific blockage of tasiARF production by two different RNA viruses in Arabidopsis

Arabidopsis thaliana is one of the most studied model organisms of plant biology with hundreds of geographical variants called ecotypes. One might expect that this enormous genetic variety could result in differential response to pathogens. Indeed, we observed previously that the Bur ecotype develops much more severe symptoms (upward curling leaves and wavy leaf margins) upon infection with two positive-strand RNA viruses of different families (turnip vein-clearing virus, TVCV, and turnip mosaic virus, TuMV). To find the genes potentially responsible for the ecotype-specific response, we performed a differential expression analysis of the mRNA and sRNA pools of TVCV and TuMV-infected Bur and Col plants along with the corresponding mock controls. We focused on the genes and sRNAs that showed an induced or reduced expression selectively in the Bur virus samples in both virus series. We found that the two ecotypes respond to the viral infection differently, yet both viruses selectively block the production of the TAS3-derived small RNA specimen called tasiARF only in the virus-infected Bur plants. The tasiARF normally forms a gradient through the adaxial and abaxial parts of the leaf (being more abundant in the adaxial part) and post-transcriptionally regulates ARF4, a major leaf polarity determinant in plants. The lack of tasiARF-mediated silencing could lead to an ectopically expressed ARF4 in the adaxial part of the leaf where the misregulation of auxin-dependent signaling would result in an irregular growth of the leaf blade manifesting as upward curling leaf and wavy leaf margin. QTL mapping using Recombinant Inbred Lines (RILs) suggests that the observed symptoms are the result of a multigenic interaction that allows the symptoms to develop only in the Bur ecotype. The particular nature of genetic differences leading to the ecotype-specific symptoms remains obscure and needs further study.

Suppression of NB-LRR genes by miRNAs promotes nitrogen-fixing nodule development in Medicago truncatula

Plant genomes contain two major classes of innate immune receptors to recognize different pathogens. The pattern recognition receptors perceive conserved pathogen-associated molecular patterns and the resistance genes with nucleotide-binding (NB) and leucine-rich repeat (LRR) domains recognize specific pathogen effectors. The precise regulation of resistance genes is important since the unregulated expression of NB-LRR genes can inhibit growth and may result in autoimmunity in the absence of pathogen infection. It was shown that a subset of miRNAs could target NB-LRR genes and act as an important regulator of plant immunity in the absence of pathogens. Plants not only interact with pathogens, but they can also establish symbiotic interactions with microbes. Nitrogen-fixing symbiotic interaction and nodule formation of legumes may also require the suppression of host defence to prevent immune responses. We found that upon symbiotic interactions, miRNAs repressing NB-LRR expression are upregulated in the developing nodules of Medicago truncatula. Furthermore, we show that the suppression of the activity of the NB-LRR genes targeted by these miRNAs is important during nodule development. Our results suggest that the downregulation of NB-LRR resistance genes in the developing nodule produces a suitable niche that facilitates bacterial colonization and the development of an N-fixing nodule.

Molecular characterization and In Vitro synthesis of infectious RNA of a Turnip vein-clearing virus isolated from Alliaria petiolata in Hungary

A tobamovirus was isolated from leaves of Alliaria petiolata plants, showing vein-clearing, interveinal chlorosis, and moderate deformation. Host range experiments revealed a high similarity of isolate ApH both to ribgrass mosaic viruses and turnip vein-clearing viruses. The complete nucleotide sequence of the viral genome was determined. The genomic RNA is composed of 6312 nucleotides and contains four open reading frames (ORF). ORF1 is 3324 nt-long and encodes a polypeptide of about 125.3 kDa. The ORF1 encoded putative replication protein contains an Alphavirus-like methyltransferase domain. ORF2 is 4806 nt-long and encodes a polypeptide of about 182 kDa. The ORF2 encoded putative replication protein contains an RNA-dependent RNA polymerase, catalytic domain. ORF3 encodes the putative cell-to-cell movement protein with a molecular weight of 30.1 kDa. ORF4 overlaps with ORF3 and encodes the coat protein with a size of 17.5 kDa. Sequence comparisons revealed that the ApH isolate has the highest similarity to turnip vein-clearing viruses and should be considered an isolate of Turnip vein-clearing virus (TVCV). This is the first report on the occurrence of TVCV in Hungary. In vitro transcripts prepared from the full-length cDNA clone of TVCV-ApH were highly infectious and induced typical symptoms characteristic to the original isolate of the virus. Since infectious clones of TVCV-ApH and crTMV (another isolate of TVCV) markedly differed in respect to recovery phenotype in Arabidopsis thaliana, it is feasible to carry out gene exchange or mutational studies to determine viral factors responsible for the symptom recovery phenotype.

Molecular characterization of a beet ringspot nepovirus isolated from Begonia ricinifolia in Hungary

A nepovirus was isolated from Begonia ricinifolia showing chlorotic ringspot and line pattern symptoms. The purified virus had spherical particles of ca. 30 nm and contained a single coat protein subunit of ca. 56 kDa. The complete nucleotide sequence of the bipartite viral genome was determined. RNA 1 is 7394 nucleotides long, flanked by 5' and 3' untranslated regions (UTR), and followed by a 3' poly-A tail. It contains a single 6810 nt long open reading frame (ORF), which is translated into a 255 kDa polyprotein composed of 2269 amino acids. The 4684 nt long RNA 2 has a 4053 nt long ORF which encodes a single polyprotein of 1350 amino acids with a molecular weight of 149 kDa. Sequence comparisons revealed that the virus isolated from B. ricinifolia has the highest sequence similarity to beet ringspot virus and should be considered as a strain of BRSV. This is the first report on the occurrence of BRSV in B. ricinifolia and the presence of this virus outside Scotland.

A biodistribution study of two differently shaped plant virus nanoparticles reveals new peculiar traits

Self-assembling plant virus nanoparticles (pVNPs) have started to be explored as nanometre-sized objects for biomedical applications, such as vaccine or drug delivery and imaging. Plant VNPs may be ideal tools in terms of biocompatibility and biodegradability endowed with a wide diversity of symmetries and dimensions, easy chemical/biological engineering, and rapid production in plants. Recently, we defined that icosahedral Tomato bushy stunt virus (TBSV) and filamentous Potato virus X (PVX) are neither toxic nor teratogenic. We report here the results of an interdisciplinary study aimed to define for the first time the biodistribution of unlabelled, unpegylated, underivatized TBSV and PVX by proved detecting antibodies. These data add new insights on the in vivo behaviour of these nano-objects and demonstrate that the pVNPs under scrutiny are each intrinsically endowed with peculiar properties foreshadowing different applications in molecular medicine.

Identification of Nicotiana benthamiana microRNAs and their targets using high throughput sequencing and degradome analysis

BACKGROUND

Nicotiana benthamiana is a widely used model plant species for research on plant-pathogen interactions as well as other areas of plant science. It can be easily transformed or agroinfiltrated, therefore it is commonly used in studies requiring protein localization, interaction, or plant-based systems for protein expression and purification. To discover and characterize the miRNAs and their cleaved target mRNAs in N. benthamiana, we sequenced small RNA transcriptomes and degradomes of two N. benthamiana accessions and validated them by Northern blots.

RESULTS

We used a comprehensive molecular approach to detect and to experimentally validate N. benthamiana miRNAs and their target mRNAs from various tissues. We identified 40 conserved miRNA families and 18 novel microRNA candidates and validated their target mRNAs with a genomic scale approach. The accumulation of thirteen novel miRNAs was confirmed by Northern blot analysis. The conserved and novel miRNA targets were found to be involved in various biological processes including transcription, RNA binding, DNA modification, signal transduction, stress response and metabolic process. Among the novel miRNA targets we found the mRNA of REPRESSOR OF SILENCING (ROS1). Regulation of ROS1 by a miRNA provides a new regulatory layer to reinforce transcriptional gene silencing by a post-transcriptional repression of ROS1 activity.

CONCLUSIONS

The identified conserved and novel miRNAs along with their target mRNAs also provides a tissue specific atlas of known and new miRNA expression and their cleaved target mRNAs of N. benthamiana. Thus this study will serve as a valuable resource to the plant research community that will be beneficial well into the future.

A plant derived multifunctional tool for nanobiotechnology based on Tomato bushy stunt virus

Structure, size, physicochemical properties and production strategies make many plant viruses ideal protein based nanoscaffolds, nanocontainers and nano-building blocks expected to deliver a multitude of applications in different fields such as biomedicine, pharmaceutical chemistry, separation science, catalytic chemistry, crop pest control and biomaterials science. Functionalization of viral nanoparticles through modification by design of their external and internal surfaces is essential to fully exploit the potentiality of these objects. In the present paper we describe the development of a plant derived multifunctional tool for nanobiotechnology based on Tomato bushy stunt virus. We demonstrate the ability of this system to remarkably sustain genetic modifications and in vitro chemical derivatizations of its outer surface, which resulted in the successful display of large chimeric peptides fusions and small chemical molecules, respectively. Moreover, we have defined physicochemical conditions for viral swelling and reversible viral pore gating that we have successfully employed for foreign molecules loading and retention in the inner cavity of this plant virus nanoparticles system. Finally, a production and purification strategy from Nicotiana benthamiana plants has been addressed and optimized.

Infectious cDNA transcripts of Maize necrotic streak virus: infectivity and translational characteristics

Maize necrotic streak virus (MNeSV) is a unique member of the family Tombusviridae that is not infectious by leaf rub inoculation and has a coat protein lacking the protruding domain of aureusviruses, carmoviruses, and tombusviruses (Louie et al., Plant Dis. 84, 1133-1139, 2000). Completion of the MNeSV sequence indicated a genome of 4094 nt. RNA blot and primer extension analysis identified subgenomic RNAs of 1607 and 781 nt. RNA and protein sequence comparisons and RNA secondary structure predictions support the classification of MNeSV as the first monocot-infecting tombusvirus, the smallest tombusvirus yet reported. Uncapped transcripts from cDNAs were infectious in maize (Zea mays L.) protoplasts and plants. Translation of genomic and subgenomic RNA transcripts in wheat germ extracts indicated that MNeSV has a 3' cap-independent translational enhancer (3'CITE) located within the 3' 156 nt. The sequence, predicted structure, and the ability to function in vitro differentiate the MNeSV 3'CITE from that of Tomato bushy stunt virus.

Identification of sequence elements of tombusvirus-associated defective interfering RNAs required for symptom modulation

Defective interfering (DI) RNAs of tombusviruses are short, non-coding, symptom-modulating RNAs originating from the viral genome. The presence of homologous DI RNA in virus infection attenuates the otherwise lethal viral symptoms. Nicotiana benthamiana plants infected with tomato bushy stunt tombusvirus pepper isolate (TBSV-P) show severe symptoms, which culminate in the death of the plant. In contrast, plants co-inoculated with TBSV-P and TBSV-P-derived DI RNA display attenuated symptoms. However, co-inoculation of TBSV-P with heterologous DI RNA, originating from Carnation Italian ringspot tombusvirus results in development of apical necrotic symptoms. To localize the symptom-determining factors on DI RNA genome, chimeras of protective and non-protective DI RNAs have been constructed. All chimeras were biologically active and accumulated to a high level in the presence of helper virus. We identified a 5' proximal sequence element of the DI RNA as the most important symptom determinant region. However, our results demonstrated that the symptom modulating ability of this region is also influenced by the sequence composition of whole DI RNAs.

Defective interfering RNA hinders the activity of a tombusvirus-encoded posttranscriptional gene silencing suppressor

Defective interfering (DI) RNAs are subviral replicons originating from the viral genome and are associated with many plant RNA viruses and nearly all animal RNA viruses. The presence of DI RNAs in tombusvirus-infected plants reduces the accumulation of helper virus RNA and results in the development of attenuated symptoms similar to those caused by tombusviruses defective in p19, the posttranscriptional gene silencing (PTGS) suppressor. In situ analysis of infected plants containing DI RNAs revealed that the extent of virus infection was spatially restricted as was found for p19-defective tombusvirus. Previously, p19 was shown to suppress PTGS by sequestering the small interfering RNAs (siRNAs), which act as the specificity determinant for PTGS. Our results demonstrate that DI RNAs dramatically elevate the level of virus-specific siRNAs in viral infections, resulting in the saturation of p19 and the accumulation of unbound siRNAs. Moreover, we showed that, at low temperature, where PTGS is inhibited, DI RNAs are not able to efficiently interfere with virus accumulation and protect the plants. These data show that the activation of PTGS plays a pivotal role in DI RNA-mediated interference. Our data also support a role for 21-nucleotide siRNAs in PTGS signaling.

Host-specific generation and maintenance of Tomato bushy stunt virus defective interfering RNAs

The accumulation of Tomato bushy stunt virus (TBSV) defective interfering RNAs (DIs) has been observed in several species of plants, but the involvement of host-specific processes and the functional role of DIs are still poorly understood. In this study, the accumulation of DIs was compared after several passages of TBSV through Nicotiana benthamiana and pepper (Capsicum annuum). As anticipated, passages of wild-type TBSV through N. benthamiana resulted in the accumulation of significant levels of TBSV DIs, which caused symptom attenuation and prevented the plants from lethal necrosis. On the contrary, TBSV infection of pepper plants caused severe local and systemic chlorosis, but continuous virus passages did not result in detectable levels of DIs accumulation. In addition, the inoculation of pepper plants with a mixture of helper virus and DI either from in vitro generated transcripts or from infected N. benthamiana did not yield DI in upper pepper leaves. Our cumulative results suggest that complex host-specific determinants play an important role in TBSV DI generation and their subsequent maintenance and accumulation.

Yeast as a model host to study replication and recombination of defective interfering RNA of Tomato bushy stunt virus

Defective interfering (DI) RNA associated with Tomato bushy stunt virus (TBSV), which is a plus-strand RNA virus, requires p33 and p92 proteins of TBSV or the related Cucumber necrosis virus (CNV), for replication in plants. To test if DI RNA can replicate in a model host, we coexpressed TBSV DI RNA and p33/p92 of CNV in yeast. We show evidence for replication of DI RNA in yeast, including (i) dependence on p33 and p92 for DI replication; (ii) presence of active CNV RNA-dependent RNA polymerase in isolated membrane-containing preparations; (iii) increasing amount of DI RNA(+) over time; (iv) accumulation of (-)stranded DI RNA; (v) presence of correct 5' and 3' ends in DI RNA; (vi) inhibition of replication by mutations in the replication enhancer; and (vii) evolution of DI RNA over time, as shown by sequence heterogeneity. We also produced evidence supporting the occurrence of DI RNA recombinants in yeast. In summary, development of yeast as a host for replication of TBSV DI RNA will facilitate studies on the roles of viral and host proteins in replication/recombination.