Viral suppression of systemic silencing

Pathogen-derived resistance in potato to Potato virus Y—aspects of stability and biosafety under field conditions

Plants of three different potato cultivars/lines were transformed via Agrobacterium tumefaciens with a truncated NIb gene of a necrotic strain of Potato virus Y (PVY(N)) which had been C-terminally fused to enhanced blue-fluorescing protein. Resistance of the resulting transgenic clones was evaluated under glass house conditions using an NTN-strain of PVY. Four clones with the highest levels of resistance were chosen for further experiments. Their type of resistance was either recovery or extreme resistance. These clones and their resistance types were also characterised at the molecular level. Mechanisms other than post-transcriptional gene silencing seemed to be involved in the resistance which was not dependent on sequence homology between transgene and challenging virus. Stability of resistance was tested under field conditions. The plants usually became infected with PVY. Tubers of the clone with extreme resistance did not recover from infection whereas those from clones with the recovery type did. No influence of transgenic potatoes was apparent on aphid population numbers in test plots. Recombination events could not be detected at the RNA level between transgene and challenging virus.

Coevolution of viruses with hosts and vectors and possible paleontology

The coevolution of viruses with their hosts and vectors depends on the evolution of the hosts and vectors coupled with factors involved in virus evolution. The long-term perspective involves the origin of life forms, the evolution of host and vector (especially arthropods) kingdoms and families, and changes in biological diversity induced mainly by the last five great extinctions. In the medium term, the diversification of hosts and vectors is important, and in the short term, recent events, especially humans, have had a great impact on virus coevolution. As there are few, if any, examples of conventional fossils of viruses, evidence for their evolution related to host and vector evolution is being found from other sources, especially virus-induced cellular structures and recent developments in molecular biology. Recognizing these other sources is becoming important for paleontologists gaining an understanding of the influence that viruses have had on the development of higher organisms.

Field Evaluation of Tomato spotted wilt virus Resistance in Transgenic Peanut (Arachis hypogaea)

Spotted wilt, caused by Tomato spotted wilt virus (TSWV), is a devastating disease of many crops including peanut (Arachis hypogaea). Because the virus has a broad host range and is spread by ubiquitous thrips, disease management by traditional means is difficult. Developing new peanut cultivars with resistance to TSWV presents a significant challenge since existing genetic resistance in peanut germ plasm is limited. A genetic engineering approach appears to have great potential for resistance enhancement to TSWV. Transgenic peanut progenies that expressed the nucleocapsid protein of TSWV were subjected to natural infection of the virus under field conditions during the growing seasons of 1999 and 2000 in Tifton, GA, and in three locations (Tifton, GA, Marianna, FL, and Headland, AL) in 2001. Significantly lower incidence of spotted wilt was observed for the transgenic progeny in comparison to the nontransgenic checks in the field (in multiple years and locations) as well as during challenge inoculation under controlled environmental conditions. This transgenic event could potentially be used in a traditional breeding program to enhance host resistance.

Molecular mechanism of RNA silencing suppression mediated by p19 protein of tombusviruses

RNA silencing is an evolutionarily conserved surveillance system that occurs in a broad range of eukaryotic organisms. In plants, RNA silencing acts as an antiviral system; thus, successful virus infection requires suppression of gene silencing. A number of viral suppressors have been identified so far; however, the molecular bases of silencing suppression are still poorly understood. Here we show that p19 of Cymbidium ringspot virus (CymRSV) inhibits RNA silencing via its small RNA-binding activity in vivo. Small RNAs bound by p19 in planta are bona fide double-stranded siRNAs and they are silencing competent in the in vitro RNA-silencing system. p19 also suppresses RNA silencing in the heterologous Drosophila in vitro system by preventing siRNA incorporation into RISC. During CymRSV infection, p19 markedly diminishes the amount of free siRNA in cells by forming p19-siRNA complexes, thus making siRNAs inaccessible for effector complexes of RNA-silencing machinery. Furthermore, the obtained results also suggest that the p19-mediated sequestration of siRNAs in virus-infected cells blocks the spread of the mobile, systemic signal of RNA silencing.

Epigenetic control of plant development: new layers of complexity

Important aspects of plant development are under epigenetic control, that is, under the control of heritable changes in gene expression that are not associated with alterations in DNA sequence. It is becoming clear that RNA molecules play a key role in epigenetic gene regulation by providing sequence specificity for the targeting of developmentally important genes. RNA-based control of gene expression can be exerted posttranscriptionally by interfering with transcript stability or translation. Moreover, RNA molecules also appear to direct developmentally relevant gene regulation at the transcriptional level by modifying chromatin structure and/or DNA methylation.

Size selective recognition of siRNA by an RNA silencing suppressor

RNA silencing in plants likely exists as a defense mechanism against molecular parasites such as RNA viruses, retrotransposons, and transgenes. As a result, many plant viruses have adapted mechanisms to evade and suppress gene silencing. Tombusviruses express a 19 kDa protein (p19), which has been shown to suppress RNA silencing in vivo and bind silencing-generated and synthetic small interfering RNAs (siRNAs) in vitro. Here we report the 2.5 A crystal structure of p19 from the Carnation Italian ringspot virus (CIRV) bound to a 21 nt siRNA and demonstrate in biochemical and in vivo assays that CIRV p19 protein acts as a molecular caliper to specifically select siRNAs based on the length of the duplex region of the RNA.

Suppressor activity of potyviral and cucumoviral infections in potyvirus-induced transgene silencing

The process known as 'recovery' by which virus-infected plants become resistant to the infection is an interesting phenomenon where both RNA silencing and virus resistance fully converge. In a previous study, we showed that transgenic Nicotiana benthamiana NIbV3 plants, transformed with a mutated NIb coding sequence from Plum pox virus (PPV), showed a delayed, very specific, resistance phenotype, which was induced by the initial infection. This recovery was the consequence of the activation of an RNA silencing mechanism in the PPV-infected plant, which took place even though PPV encodes a silencing suppressor (HCPro). Making use of plants regenerated from the recovered tissue, which maintained the transgene silencing/virus resistance phenotype, we have demonstrated that both Cucumber mosaic virus (CMV) and Tobacco vein mottling virus (TVMV), expressing the silencing suppressor 2b and HCPro, respectively, were able to reactivate transgene expression. Surprisingly, only the silencing suppression caused by CMV, but not that originating from TVMV, was able to revert the recovered NIbV3 plants to a PPV-susceptible phenotype.

Fitness of a turnip crinkle virus satellite RNA correlates with a sequence-nonspecific hairpin and flanking sequences that enhance replication and repress the accumulation of virions

satC, a satellite RNA associated with Turnip crinkle virus (TCV), enhances the ability of the virus to colonize plants by interfering with stable virion accumulation (F. Zhang and A. E. Simon, unpublished data). Previous results suggested that the motif1-hairpin (M1H), a replication enhancer on minus strands, forms a plus-strand hairpin flanked by CA-rich sequence that may be involved in enhancing systemic infection (G. Zhang and A. E. Simon, J. Mol. Biol. 326:35-48, 2003). In this study, sequence and structural requirements of the M1H were further assayed by replacing the 28-base M1H with 10 random bases and then subjecting the pool of satellite RNA to functional selection in plants. Unlike previous results with 28-base replacement sequences (G. Zhang and A. E. Simon, J. Mol. Biol. 326:35-48, 2003), only a few of the 10-base SELEX (systematic evolution of ligands by exponential enrichment) assay winners contained short motifs in their minus-sense orientation that were similar to TCV replication elements. However, all second- and third-round winning replacement sequences folded into hairpins flanked by CA-rich sequence predicted to be more stable on plus strands than minus strands. Plus strands of several of the most fit satellite RNAs contained insertions of CA-rich sequence at the base of their hairpins whose presence correlated with enhanced replication and reduced detection of virions. Deletion of the M1H resulted in no detectable virions despite very low satellite accumulation. These results support the hypothesis that a sequence-nonspecific plus-strand hairpin brings together flanking CA-rich sequences in the M1H region that confers fitness to satC by reducing the accumulation of stable virions.

Triple gene block: modular design of a multifunctional machine for plant virus movement

Many plant virus genera encode a 'triple gene block' (TGB), a specialized evolutionarily conserved gene module involved in the cell-to-cell and long-distance movement of viruses. The TGB-based transport system exploits the co-ordinated action of three polypeptides to deliver viral genomes to plasmodesmata and to accomplish virus entry into neighbouring cells. Although data obtained on both the TGB and well-studied single protein transport systems clearly demonstrate that plant viruses employ host cell pathways for intra- and intercellular trafficking of genomic nucleic acids and proteins, there is no integral picture of the details of molecular events during TGB-mediated virus movement. Undoubtedly, understanding the molecular basis of the concerted action of TGB-encoded proteins in transporting viral genomes from cell to cell should provide new insights into the general principles of movement protein function. This review describes the structure, phylogeny and expression of TGB proteins, their roles in virus cell-to-cell movement and potential influence on host antiviral defences.

In situ characterization of Cymbidium Ringspot Tombusvirus infection-induced posttranscriptional gene silencing in Nicotiana benthamiana

In plants, posttranscriptional gene silencing (PTGS) is an ancient and effective defense mechanism against viral infection. A number of viruses encode proteins that suppress virus-activated PTGS. The p19 protein of tombusviruses is a potent PTGS suppressor which interferes with the onset of PTGS-generated systemic signaling and is not required for viral replication or for viral movement in Nicotiana benthamiana. This unique feature of p19 suppressor allowed us to analyze the mechanism of PTGS-based host defense and its viral suppression without interfering with other viral functions. In contrast to the necrotic symptoms caused by wild-type tombusvirus, the infection of p19-defective mutant virus results in the development of a typical PTGS-associated recovery phenotype in N. benthamiana. In this report we show the effect of PTGS on the viral infection process for N. benthamiana infected with either wild-type Cymbidium Ringspot Tombusvirus (CymRSV) or a p19-defective mutant (Cym19stop). In situ analyses of different virus-derived products revealed that PTGS is not able to reduce accumulation of virus in primary infected cells regardless of the presence of p19 PTGS suppressor. We also showed that both CymRSV and Cym19stop viruses move systemically in the vasculature, with similar efficiencies. However, in contrast to the uniform accumulation of CymRSV throughout systemically infected leaves, the presence of Cym19stop virus was confined to and around the vascular bundles. These results suggest that the role of p19 is to prevent the onset of mobile signal-induced systemic PTGS ahead of the viral infection front, leading to generalized infection.

Replication of human hepatitis delta virus: recent developments

In a natural setting, hepatitis delta virus (HDV) is only found in patients that are also infected with hepatitis B virus (HBV). In hepatocytes infected with these two viruses, HDV RNA genomes are assembled using the envelope proteins of HBV. Since 1986, we have known that HDV has a small single-stranded RNA genome with a unique circular conformation that is replicated using a host RNA polymerase. These and other features make HDV and its replication unique, at least among agents that infect animals. This mini-review focuses on advances gained over the last 2-3 years, together with an evaluation of HDV questions that are either unsolved or not yet solved satisfactorily.

Beet yellows virus: the importance of being different

SUMMARY Taxonomic relationship: Type member of the genus Closterovirus, family Closteroviridae. A member of the alphavirus-like supergroup of positive-strand RNA viruses. Physical properties: Virions are flexuous filaments of approximately 1300 nm in length and approximately 12 nm in diameter that are made up of a approximately 15.5 kb RNA and five proteins. The major capsid protein forms virion body of helical symmetry that constitutes approximately 95% of the virion length. The short virion tail is assembled by the minor capsid protein, Hsp70-homologue, approximately 64-kDa protein, and approximately 20-kDa protein. Viral proteins: The 5'-most ORFs 1a and 1b encode leader proteinase and RNA replicase. The remaining ORFs 2-8 are expressed by subgenomic mRNAs that encode 6-kDa membrane protein, Hsp70 homologue, approximately 64-kDa protein, minor and major capsid proteins, approximately 20-kDa protein, and approximately 21-kDa protein, respectively. Hosts: The principal crop plants affected by Beet yellows virus (BYV) are sugar beet (Beta vulgaris) and spinach (Spinacea oleracea). In addition, BYV was reported to infect approximately 120 species in 15 families. Most suitable propagation species are Nicotiana benthamiana, Tetragonia expansa, and Claytonia perfoliata.