Nucleocapsid Gene-Mediated Transgenic Resistance Provides Protection Against Tomato spotted wilt virus Epidemics in the Field

Natural Resources Resistance to Tomato Spotted Wilt Virus (TSWV) in Tomato (Solanum lycopersicum)

Tomato spotted wilt virus (TSWV) is one of the most destructive diseases affecting tomato (Solanum lycopersicum) cultivation and production worldwide. As defenses against TSWV, natural resistance genes have been identified in tomato, including Sw-1a, Sw-1b, sw-2, sw-3, sw-4, Sw-5, Sw-6, and Sw-7. However, only Sw-5 exhibits a high level of resistance to the TSWV. Thus, it has been cloned and widely used in the breeding of tomato with resistance to the disease. Due to the global spread of TSWV, resistance induced by Sw-5 decreases over time and can be overcome or broken by a high concentration of TSWV. How to utilize other resistance genes and identify novel resistance resources are key approaches for breeding tomato with resistance to TSWV. In this review, the characteristics of natural resistance genes, natural resistance resources, molecular markers for assisted selection, and methods for evaluating resistance to TSWV are summarized. The aim is to provide a theoretical basis for identifying, utilizing resistance genes, and developing tomato varieties that are resistant to TSWV.

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.

The Induction of an Effective dsRNA-Mediated Resistance Against Tomato Spotted Wilt Virus by Exogenous Application of Double-Stranded RNA Largely Depends on the Selection of the Viral RNA Target Region

Tomato spotted wilt virus (TSWV) is a devastating plant pathogen, causing huge crop losses worldwide. Unfortunately, due to its wide host range and emergence of resistance breaking strains, its management is challenging. Up to now, resistance to TSWV infection based on RNA interference (RNAi) has been achieved only in transgenic plants expressing parts of the viral genome or artificial microRNAs targeting it. Exogenous application of double-stranded RNAs (dsRNAs) for inducing virus resistance in plants, namely RNAi-based vaccination, represents an attractive and promising alternative, already shown to be effective against different positive-sense RNA viruses and viroids. In the present study, the protection efficacy of exogenous application of dsRNAs targeting the nucleocapsid (N) or the movement protein (NSm) coding genes of the negative-sense RNA virus TSWV was evaluated in Nicotiana benthamiana as model plant and in tomato as economically important crop. Most of the plants treated with N-targeting dsRNAs, but not with NSm-targeting dsRNAs, remained asymptomatic until 40 (N. benthamiana) and 63 (tomato) dpi, while the remaining ones showed a significant delay in systemic symptoms appearance. The different efficacy of N- and NSm-targeting dsRNAs in protecting plants is discussed in the light of their processing, mobility and biological role. These results indicate that the RNAi-based vaccination is effective also against negative-sense RNA viruses but emphasize that the choice of the target viral sequence in designing RNAi-based vaccines is crucial for its success.

Enhanced virus resistance in transgenic maize expressing a dsRNA-specific endoribonuclease gene from E. coli

Maize rough dwarf disease (MRDD), caused by several Fijiviruses in the family Reoviridae, is a global disease that is responsible for substantial yield losses in maize. Although some maize germplasm have low levels of polygenic resistance to MRDD, highly resistant cultivated varieties are not available for agronomic field production in China. In this work, we have generated transgenic maize lines that constitutively express rnc70, a mutant E. coli dsRNA-specific endoribonuclease gene. Transgenic lines were propagated and screened under field conditions for 12 generations. During three years of evaluations, two transgenic lines and their progeny were challenged with Rice black-streaked dwarf virus (RBSDV), the causal agent of MRDD in China, and these plants exhibited reduced levels of disease severity. In two normal years of MRDD abundance, both lines were more resistant than non-transgenic plants. Even in the most serious MRDD year, six out of seven progeny from one line were resistant, whereas non-transgenic plants were highly susceptible. Molecular approaches in the T12 generation revealed that the rnc70 transgene was integrated and expressed stably in transgenic lines. Under artificial conditions permitting heavy virus inoculation, the T12 progeny of two highly resistant lines had a reduced incidence of MRDD and accumulation of RBSDV in infected plants. In addition, we confirmed that the RNC70 protein could bind directly to RBSDV dsRNA in vitro. Overall, our data show that RNC70-mediated resistance in transgenic maize can provide efficient protection against dsRNA virus infection.

Tospoviruses in the Mediterranean area

Tospoviruses are among the most serious threats to vegetable crops in the Mediterranean basin. Tospovirus introduction, spread, and the diseases these viruses cause have been traced by epidemiological case studies. Recent research has centered on the close relationship between tospoviruses and their arthropod vectors (species of the Thripidae family). Here, we review several specific features of tospovirus-thrips associations in the Mediterranean. Since the introduction of Frankliniella occidentalis in Europe, Tomato spotted wilt virus (TSWV) has become one of the limiting factors for vegetable crops such as tomato, pepper, and lettuce. An increasing problem is the emergence of TSWV resistance-breaking strains that overcome the resistance genes in pepper and tomato. F. occidentalis is also a vector of Impatiens necrotic spot virus, which was first observed in the Mediterranean basin in the 1980s. Its importance as a cause of vegetable crop diseases is limited to occasional incidence in pepper and tomato fields. A recent introduction is Iris yellow spot virus, transmitted by the onion thrips Thrips tabaci, in onion and leek crops. Control measures in vegetable crops specific to Mediterranean conditions were examined in the context of their epidemiological features and tospovirus species which could pose a future potential risk for vegetable crops in the Mediterranean were discussed.

Phylogenetic evidence for rapid rates of molecular evolution in the single-stranded DNA begomovirus tomato yellow leaf curl virus

Geminiviruses are devastating viruses of plants that possess single-stranded DNA (ssDNA) DNA genomes. Despite the importance of this class of phytopathogen, there have been no estimates of the rate of nucleotide substitution in the geminiviruses. We report here the evolutionary rate of the tomato yellow leaf curl disease-causing viruses, an intensively studied group of monopartite begomoviruses. Sequences from GenBank, isolated from diseased plants between 1988 and 2006, were analyzed using Bayesian coalescent methods. The mean genomic substitution rate was estimated to be 2.88 x 10(-4) nucleotide substitutions per site per year (subs/site/year), although this rate could be confounded by frequent recombination within Tomato yellow leaf curl virus genomes. A recombinant-free data set comprising the coat protein (V1) gene in isolation yielded a similar mean rate (4.63 x 10(-4) subs/site/year), validating the order of magnitude of genomic substitution rate for protein-coding regions. The intergenic region, which is known to be more variable, was found to evolve even more rapidly, with a mean substitution rate of approximately 1.56 x 10(-3) subs/site/year. Notably, these substitution rates, the first reported for a plant DNA virus, are in line with those estimated previously for mammalian ssDNA viruses and RNA viruses. Our results therefore suggest that the high evolutionary rate of the geminiviruses is not primarily due to frequent recombination and may explain their ability to emerge in novel hosts.

Matrix attachment regions increase the efficiency and stability of RNA-mediated resistance to tomato spotted wilt virus in transgenic tobacco

Matrix attachment regions (MARs) are DNA elements that can increase and stabilize transgene expression. We investigated the effect of the RB7 MAR on transgenic virus resistance. Constructs for resistance to tomato spotted wilt virus (TSWV) with and without flanking RB7 MARs were used to transform tobacco and produce homozygous lines. The population with the MAR construct had a significantly higher percentage of TSWV resistant plants in the R1 generation than the nonMAR population. Each resistant line was advanced to the R4 generation, and significantly fewer MAR lines lost resistance over generations compared to the nonMAR population. Lines with TSWV resistance in growth chamber tests were also resistant in field trials. Two lines that were resistant in the R1 generation and susceptible in the R4 were examined in more detail in order to determine if transcriptional silencing of the transgene was occurring in the later generation. Short interfering 21-25 nt RNAs from the transgene that are characteristic of post-transcriptional gene silencing (PTGS) were present in the resistant R1 plants, but not the susceptible R4 plants, indicating that virus resistance was associated with PTGS of the transgene. Loss of resistance was accompanied by an increase in promoter methylation in both lines. In line M41, the transgene was fully silenced at the transcriptional level in the R4 as shown by nuclear run-on assays. In line NM13, transgene transcription and RNA accumulation was still present in the R4 generation, but the level of transcription was not sufficient to trigger PTGS, suggesting that this line may have partial transcriptional silencing. These results are consistent with the concept that MARs may prevent transcriptional silencing.

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.

Overcoming host- and pathogen-mediated resistance in tomato and tobacco maps to the M RNA of Tomato spotted wilt virus

A viral genetic system was used to map the determinants of the ability of Tomato spotted wilt virus (TSWV) to overcome the R gene (Sw-5) in tomato and the resistance conferred by the nucleocapsid gene of TSWV (N gene) in tobacco. A complete set of reassortant genotypes was generated from TSWV isolates A and D. TSWV-A was able to overcome the Sw-5 gene in tomato and the TSWV N gene in tobacco, whereas TSWV-D was repressed by both forms of resistance. The ability to overcome both forms of resistance was associated with the M RNA segment of TSWV-A (M(A)). Overcoming the Sw-5 gene was linked solely to the presence of M(A), and the ability of M(A) to overcome the TSWV N gene was modified by the L RNA and the S RNA of TSWV-A, which is consistent with previous reports that suggest that the nucleocapsid gene is not the primary determinant for overcoming the nucleocapsid-mediated resistance. Sequence analysis of the M RNA segment of TSWV-A, -D, and the type isolate BR-01 revealed multiple differences in the coding and noncoding regions, which prevented identification of the resistance-breaking nucleotide sequences.

Tomato spotted wilt virus-positive steps towards negative success

Abstract Taxonomy: Tomato spotted wilt virus (TSWV) is the type member of the plant-infecting Tospovirus genus in the family Bunyaviridae, a large group of predominantly vertebrate- and insect-infecting RNA viruses. Physical properties: Virions are 80-120-nm pleomorphic particles with surface projections composed of two viral glycoproteins, G1 and G2 (Fig. 1). Virion composition is 5% nucleic acid, 70% protein, 5% carbohydrate and 20% lipid. The genome consists of three negative or ambisense ssRNAs designated S (2.9 kb), M (4.8 kb) and L (8.9 kb), with partially complementary terminal sequences that allow the RNA to adopt a pseudocircular or panhandle conformation. Each genomic RNA is encapsidated by multiple copies of the viral nucleocapsid (N) protein to form ribonucleoprotein structures also known as nucleocapsids. The nucleocapsids are enclosed in a host-derived membrane bilayer along with an estimated 10-20 copies of the L protein, the putative RNA-dependent RNA polymerase. Hosts: Over 800 plant species, both dicots and monocots, in more than 80 plant families are susceptible to TSWV (Goldbach and Peters, 1994). The Solanaceae and Compositae families contain the largest numbers of susceptible plant species (Prins and Kormelink, 1998). TSWV also replicates in its insect vector, thrips (Thysanoptera: Thripidae) (Ullman et al., 1993; Wijkamp et al., 1993). Useful web site: http://www4.ncbi.nlm.nih.gov/ICTVdb/ICTVdB/11050003.htm.