Resistance-breaking Strains Research Articles

When Does Spatial Diversification Usefully Maximize the Durability of Crop Disease Resistance?

Maximizing the durability of crop disease resistance genes in the face of pathogen evolution is a major challenge in modern agricultural epidemiology. Spatial diversification in the deployment of resistance genes, where susceptible and resistant fields are more closely intermixed, is predicted to drive lower epidemic intensities over evolutionary timescales. This is due to an increase in the strength of dilution effects, caused by pathogen inoculum challenging host tissue to which it is not well-specialized. The factors that interact with and determine the magnitude of this spatial suppressive effect are not currently well understood, however, leading to uncertainty over the pathosystems where such a strategy is most likely to be cost-effective. We model the effect on landscape scale disease dynamics of spatial heterogeneity in the arrangement of fields planted with either susceptible or resistant cultivars, and the way in which this effect depends on the parameters governing the pathosystem of interest. Our multiseason semidiscrete epidemiological model tracks spatial spread of wild-type and resistance-breaking pathogen strains, and incorporates a localized reservoir of inoculum, as well as the effects of within and between field transmission. The pathogen dispersal characteristics, any fitness cost(s) of the resistance-breaking trait, the efficacy of host resistance, and the length of the timeframe of interest all influence the strength of the spatial diversification effect. A key result is that spatial diversification has the strongest beneficial effect at intermediate fitness costs of the resistance-breaking trait, an effect driven by a complex set of nonlinear interactions. On the other hand, however, if the resistance-breaking strain is not fit enough to invade the landscape, then a partially effective resistance gene can result in spatial diversification actually worsening the epidemic. These results allow us to make general predictions of the types of system for which spatial diversification is most likely to be cost-effective, paving the way for potential economic modeling and pathosystem specific evaluation. These results highlight the importance of studying the effect of genetics on landscape scale spatial dynamics within host-pathogen disease systems.[Formula: see text] Copyright © 2020 The Author(s). This is an open access article distributed under the CC BY 4.0 International license.

First Report of Southern Bean Mosaic Virus Infecting Common Bean in Zambia

Southern bean mosaic virus (SBMV, genus Sobemovirus) is one of the causative agents of mosaic and mottle diseases in common bean (Phaseolus vulgaris) (Tremaine and Hamilton 1983). The ∼4.2 kb (+)ssRNA genome of SBMV is encapsidated in isometric virions of about 30 nm in diameter (Tremaine and Hamilton 1983). During surveys conducted in the Eastern Province of Zambia in April 2018, several common bean plants were observed with virus-like symptoms such as vein clearing, leaf rolling, mosaic, and leaf narrowing. Incidence of symptomatic plants across 20 fields ranged from 23 to 100%, averaging 67.5%. A total of 121 symptomatic leaf samples were collected from 20 farmers’ fields and preserved dry on silica gel. Sixteen samples were randomly selected and pooled into one sample (Mse-1) for total nucleic acid (TNA) extraction as described (Djami-Tchatchou and Straker 2012). The TNA was preserved in RNAStable (Biomatrica, Japan) and shipped to Inqaba Biotechnical Industries (Pretoria, South Africa) for high-throughput sequencing (HTS). HTS was achieved via cDNA synthesis and library construction with the Illumina TruSeq RNA Library Prep Kit (Illumina, San Diego, CA). The cDNA library was sequenced on the Illumina MiSeq platform, generating 4.1 million single raw reads of ∼300 nucleotides (nt) each. De novo assembly of 188,160 virus-aligned reads and their subsequent use to query GenBank with BLASTn resulted in the detection of SBMV, bean common mosaic necrosis virus, bean common mosaic virus, cowpea aphid-borne mosaic virus, and cucumber mosaic virus in the sample. To confirm SBMV occurrence and also validate the HTS results, a pair of virus-specific primers (SBMV-1F, 5′-AGCTGGATTTCCTACCTTTGTG; SBMV-1R, 5′-GGCGTCATCTCCGTTTATCTT) was designed to amplify a 873-bp fragment encompassing 32 bp of the 5′ untranslated region, a 492-bp movement protein (ORF1), and a 353-bp partial P2a gene (ORF2) of SBMV by reverse transcription PCR. Each of the 121 samples was screened for SBMV along with the other four viruses (Ha et al. 2008; Niimi et al. 2003). Twenty-five of the 121 samples (20.7%) tested positive only for SBMV, whereas the remaining 96 samples had different combinations of the other four viruses (data not shown). The SBMV-positive samples had mosaic and vein clearing symptoms. The purified PCR products from three samples (EP-88, Mse-13, and Mse-17) were directly Sanger sequenced and deposited in GenBank (MN653952 to MN653954). Pairwise distance analysis revealed that the three Sanger-derived sequences shared 81 to 100% nt identities with each other. The 4,100-bp-long near-complete genome of SBMV was independently Sanger sequenced from sample Mse-13 (MN326873) and determined to encode all the four predicted ORFs of the virus. In pairwise comparisons, the Mse-13 sequence shared the highest (97.2%) and lowest (89.3%) nt sequence identities with the resistance-breaking strain SBMV-B(ARK) (AF055887) from Arkansas and isolate TZ (MG344643) from Tanzania, respectively. SBMV has been previously reported in common bean in many countries across several continents (Verhoeven et al. 2003). However, to the best of our knowledge, this is the first report of SBMV infecting common bean in Zambia. The result adds to the repertoire of viruses circulating in farmers’ fields in Zambia and further confirms the complexity of probable viruses contributing to suboptimal common bean yields in the country.

Grafting alters tomato transcriptome and enhances tolerance to an airborne virus infection

Grafting of commercial tomato varieties and hybrids on the tomato ecotype Manduria resulted in high levels of tolerance to the infection of Sw5 resistance-breaking strains of tomato spotted wilt virus and of severe cucumber mosaic virus strains supporting hypervirulent satellite RNAs that co-determine stunting and necrotic phenotypes in tomato. To decipher the basis of such tolerance, here we used a RNAseq analysis to study the transcriptome profiles of the Manduria ecotype and of the susceptible variety UC82, and of their graft combinations, exposed or not to infection of the potato virus Y recombinant strain PVYC-to. The analysis identified graft- and virus-responsive mRNAs differentially expressed in UC82 and Manduria, which led to an overall suitable level of tolerance to viral infection confirmed by the appearance of a recovery phenotype in Manduria and in all graft combinations. The transcriptome analysis suggested that graft wounding and viral infection had diverging effects on tomato transcriptome and that the Manduria ecotype was less responsive than the UC82 to both graft wounding and potyviral infection. We propose that the differential response to the two types of stress could account for the tolerance to viral infection observed in the Manduria ecotype as well as in the susceptible tomato variety UC82 self-grafted or grafted on the Manduria ecotype.

Comprehensive transcriptome analysis and functional characterization of PR-5 for its involvement in tomato Sw-7 resistance to tomato spotted wilt tospovirus

Tomato spotted wilt tospovirus (TSWV), one of the most important plant viruses, causes yield losses to many crops including tomato. The current disease management for TSWV is based mainly on breeding tomato cultivars containing the Sw-5 locus. Unfortunately, several Sw-5 resistance-breaking strains of TSWV have been identified. Sw-7 is an alternative locus conferring resistance to a broad range of TSWV strains. In an effort to uncover gene networks that are associated with the Sw-7 resistance, we performed a comparative transcriptome profiling and gene expression analysis between a nearly-isogenic Sw-7 line and its susceptible recurrent parent (Fla. 8059) upon infection by TSWV. A total of 1,244 differentially expressed genes were identified throughout a disease progression process involving networks of host resistance genes, RNA silencing/antiviral defense genes, and crucial transcriptional and translational regulators. Notable induced genes in Sw-7 include those involved in callose accumulation, lignin deposition, proteolysis process, transcriptional activation/repression, and phosphorylation. Finally, we investigated potential involvement of PR-5 in the Sw-7 resistance. Interestingly, PR-5 overexpressed plants conferred enhanced resistance, resulting in delay in virus accumulation and symptom expression. These findings will facilitate breeding and genetic engineering efforts to incorporate this new source of resistance in tomato for protection against TSWV.

First Report of a Resistance-Breaking Strain of Tomato Spotted Wilt Orthotospovirus Infecting Sweet Pepper with the Tsw Resistance Gene in California, U.S.A.

First Report of a Resistance-Breaking Strain of Tomato Spotted Wilt Orthotospovirus Infecting Sweet Pepper with the <i>Tsw</i> Resistance Gene in California, U.S.A.

Identification and Characterization of Resistance-Breaking (RB) Isolates of Citrus tristeza virus.

Resistance-breaking (RB) strains constitute a clade of biological and genetically distinct isolates of Citrus tristeza virus (CTV) that replicate and move systemically in Poncirus trifoliata (trifoliate orange), resistant to other known strains of CTV. Molecular markers have been developed by comparative genome analysis to allow quick identification of potential RB isolates. Here, methods are described to identify and characterize RB strains by reverse transcription-polymerase chain reaction (RT-PCR), quantitative real-time RT-PCR (RT-qPCR), full-length genome sequencing, and biological indexing.

Mutation of a Nicotiana tabacum L. eukaryotic translation-initiation factor gene reduces susceptibility to a resistance-breaking strain of Potato virus Y.

Eukaryotic translation-initiation factors eIF4E and eIF(iso)4E in plants play key roles in infection by potyviruses and other plant RNA viruses. Mutations in the genes encoding these factors reduce susceptibility to the viruses, and are the basis of several recessive virus resistance genes widely used in plant breeding. Because virus variants occasionally break such resistance, the molecular basis for this process must be elucidated. Although deletion mutants of eIF4E1-S of tobacco (Nicotiana tabacum L.) resist Potato virus Y (PVY; the type member of the genus Potyvirus), resistance-breaking strains of PVY threaten tobacco production worldwide. Here, we used RNA interference technology to knock down tobacco eIF4E2-S and eIF4E2-T genes or eIF(iso)4E-S and eIF(iso)4E-T genes. Transgenic plants with reduced transcript levels of both eIF(iso)4E-S and eIF(iso)4E-T showed reduced susceptibility to a resistance-breaking PVY strain with a K105E mutation in the viral genome-associated protein (VPg). By screening a population of chemically induced mutants of eIF(iso)4E-S and eIF(iso)4E-T, we showed that plants with a nonsense mutation in eIF(iso)4E-T, but not eIF(iso)4E-S, showed reduced susceptibility to the resistance-breaking PVY strain. In a yeast two-hybrid assay, VPg of the resistance-breaking strain, but not wild-type PVY, physically interacted with the eIF(iso)4E-T protein. Thus, eIF4E1-S is required for infection by PVY, but eIF(iso)4E-T is required for infection by the resistance-breaking strain. Our study provides the first evidence for the involvement of a host eukaryotic translation-initiation factor in the infection cycle of a resistance-breaking virus strain. The eIF(iso)4E-T mutants will be useful in tobacco breeding to introduce resistance against resistance-breaking PVY strains.

First report of the Citrus tristeza virus resistance-breaking strain in Morocco

First report of the Citrus tristeza virus resistance-breaking strain in Morocco

Closing the Yield Gap of Sugar Beet in the Netherlands-A Joint Effort.

The reform of the European Union's sugar regime caused potential decreasing beet prices. Therefore, the Speeding Up Sugar Yield (SUSY) project was initiated. At the start, a 3 × 15 target was formulated: in 2015 the national average sugar yield in the Netherlands equals 15 t/ha (60% of the sugar beet potential) and the total variable costs 15 euro/t sugar beet, aspiring a saving on total variable costs and a strong increase in sugar yield. Based on their average sugar yield in 2000-2004, 26 pairs of “type top” (high yielding) and “type average” (average yielding) growers were selected from all sugar beet growing regions in the Netherlands. On the fields of those farmers, all measures of sugar beet cultivation were investigated, including cost calculation and recording phytopathological, agronomical and soil characteristics in 2006 and 2007. Although there was no significant difference in total variable costs, the “type top” growers yielded significantly 20% more sugar in each year compared to the “type average” growers. Therefore, the most profitable strategy for the growers is maximizing sugar yield and optimizing costs. The difference in sugar yield between growers could be explained by pests and diseases (50%), weed control (30%), soil structure (25%) and sowing date (14%), all interacting with each other. The SUSY-project revealed the effect of the grower's management on sugar yield. As a follow up for the SUSY-project, a growers' guide “Suikerbietsignalen” was published, Best Practice study groups of growers were formed and trainings and workshops were given and field days organized. Further, the benchmarking and feedback on the crop management recordings and the extension on variety choice, sowing performance, foliar fungi control and harvest losses were intensified. On the research part, a resistance breaking strain of the Beet Necrotic Yellow Vein Virus (BNYVV) and a new foliar fungus, Stemphylium beticola, were identified and options for control were tested, and implemented in growers practices. The joint efforts of sugar industry, sugar beet research and growers resulted in a raise in sugar yield from 10.6 t/ha in 2002-2006 to 13.8 t/ha in 2012-2016.

Engineering resistance to a resistance-breaking strain of Cucumber mosaic virus in plants utilizing viral dsRNA

Cucumber mosaic virus Ca-P1 strain, a P0 resistance-breaking virus, was isolated from the leaves of virus-infected Capsicum annuum ‘Manidda’ (a P0 resistant cultivar) in South Korea. Since CMV-Ca-P1 was first reported in 2006, this virus causing damage on pepper production has been constantly detected in South Korea. We constructed three CMV RNAi vectors based on the post-transcriptional gene silencing of defense system against virus infection. These independent vectors (hpCMV1, hpCMV2, and hpCMV2 + 1 RNAi vectors) were designed to produce dsRNAs containing each hpCMV1 (1270–1629 nt), hpCMV2 (1–300 nt), and hpCMV2 + 1 (fused hpCMV2 and hpCMV1) fragments, in RNA-1 (replicase gene) of CMV-Ca-P1, which were then confirmed by Agrobacterium-mediated transformation transient assay. Among these, dsRNAs expressed from the hpCMV2 + 1 vector showed resistance to both CMV-Ca-P1 and CMV-Fny (ordinary strain). To obtain high level of resistance to both CMV-Ca-P1 and CMV-Fny, transgenic Nicotiana benthamiana plants containing hpCMV2 + 1 vector were developed and conferred resistance to both CMV-Ca-P1 and CMV-Fny. This study contributes to the effective selection of target sequences that may inhibit CMV infection.

Genetics and Genomics of Cotton Leaf Curl Disease, Its Viral Causal Agents and Whitefly Vector: A Way Forward to Sustain Cotton Fiber Security.

Cotton leaf curl disease (CLCuD) after its first epidemic in 1912 in Nigeria, has spread to different cotton growing countries including United States, Pakistan, India, and China. The disease is of viral origin—transmitted by the whitefly Bemisia tabaci, which is difficult to control because of the prevalence of multiple virulent viral strains or related species. The problem is further complicated as the CLCuD causing virus complex has a higher recombination rate. The availability of alternate host crops like tomato, okra, etc., and practicing mixed type farming system have further exaggerated the situation by adding synergy to the evolution of new viral strains and vectors. Efforts to control this disease using host plant resistance remained successful using two gene based-resistance that was broken by the evolution of new resistance breaking strain called Burewala virus. Development of transgenic cotton using both pathogen and non-pathogenic derived approaches are in progress. In future, screening for new forms of host resistance, use of DNA markers for the rapid incorporation of resistance into adapted cultivars overlaid with transgenics and using genome editing by CRISPR/Cas system would be instrumental in adding multiple layers of defense to control the disease—thus cotton fiber production will be sustained.

A single point mutation in Tomato spotted wilt virus NSs protein is sufficient to overcome Tsw-gene-mediated resistance in pepper.

The nonstructural protein (NSs) of Tomato spotted wilt virus (TSWV) was previously identified as an avirulence determinant for Tsw-based resistance on pepper. The NSs of wild-type (WT) and resistance-breaking (RB) TSWV strains isolated in Hungary had only two amino acid substitutions (104, 461). We have analysed the ability of the NSs and their point mutant variants to trigger Tsw-mediated hypersensitive responses and RNA silencing suppressor (RSS) activity in patch assays. We identified a single amino acid change at position 104 (T-A) that was responsible for the necrosis induction or loss, while a significant difference was not detected in the RSS activity of the two parental strains. We have successfully complemented the infection of the WT strain on resistant pepper cultivar with the infectious S RNA transcript of the RB strain and the WT-T104A point mutant. Our work provides direct evidence that a single amino acid change can induce an RB phenotype.

First Report of a Resistance-Breaking Strain of Tomato spotted wilt virus Infecting Tomatoes With the Sw-5 Tospovirus-Resistance Gene in California

First Report of a Resistance-Breaking Strain of <i>Tomato spotted wilt virus</i> Infecting Tomatoes With the <i>Sw-</i>5 Tospovirus-Resistance Gene in California

Biological and Molecular Properties of a Turnip mosaic virus (TuMV) Strain that Breaks TuMV Resistances in Brassica napus.

A new resistance-breaking strain of Turnip mosaic virus (TuMV) overcomes TuMV resistance genes that currently suppress spread of this virus in Brassica napus crops in the Liverpool Plains region of eastern Australia. Isolates 12.1 and 12.5 of this strain and three other isolates in TuMV pathotypes 1 (NSW-2), 7 (NSW-1), and 8 (WA-Ap1) were inoculated to plants of 19 B. napus cultivars and one breeding line. All plants of these cultivars and the breeding line proved susceptible to 12.1 and 12.5 but developed only resistance phenotypes with WA-Ap1 or mostly resistance phenotypes with NSW-1 and NSW-2. Five different TuMV resistance phenotypes occurred either alone or segregating in different combinations. When these five isolates were inoculated to plants of nine other crop or wild Brassicaceae spp. and four indicator hosts in other families, 12.1 and 12.5 resembled the other three in inducing TuMV resistance phenotypes in some Brassicaceae spp. but not others, and by inducing extreme resistance phenotypes in all inoculated plants of B. oleracea var. botrytis and Raphanus sativus. Therefore, the overall resistance-breaking properties of 12.1 and 12.5 were restricted to B. napus. When isolates 12.1, 12.5, and WA-Ap1 and additional Australian isolate WA-EP1 were sequenced and complete genomes of each compared, 12.1 and 12.5 grouped separately from the other 2 and from all 23 Australian isolates with complete genomes sequenced previously. In addition, there was evidence for at least six separate TuMV introductions to Australia. Spread of this B. napus resistance-breaking strain poses a significant threat to the B. napus oilseed industry. Breeding B. napus cultivars with resistance to this strain constitutes a critical priority for B. napus breeding programs in Australia and elsewhere.

Pathogenic Variation and Occurrence of Multiple Resistance-Breaking &amp;lt;i&amp;gt;Rice yellow mottle virus&amp;lt;/i&amp;gt; Strains in Tanzania

Rice yellow mottle virus (RYMV) is a major biotic constraint for rice production in Africa. The resistance-breaking ability of Tanzanian RYMV strains and phylotypes (S4lm (Tz526), S4lv (Tz516), S4ug (Tz508), S5 (Tz429, Tz445), S6c (Tz486) and S6w (Tz539)) were tested by inoculating rice cultivars with RYMV1 resistant alleles (Gigante (rymv1-2), Tog12387 (rymv1-3), Tog5681 (rymv1-3), Tog5438 (rymv1-4), Tog5672 (rymv1-4+rymv2) and Tog5674 (rymv 1-5)) in a screen house. The results revealed multiple resistance-breaking strains and phylotypes on resistant cultivars Gigante, Tog12387, Tog5438 and Tog5681. However, the resistance breakdown was highly variable depending on the strain used, and disease severity ranged from 11% - 75.3%. The virulence potential of RYMV phylotype S4lm (Tz526) was similar to phylotype S6w (Tz539). The impact of strains and phylotypes on yield and its components in rice cultivars revealed highly significant differences (P ≤ 0.001). The lowest percent plant height reduction (2.8%), number of tillers per plant (2.5%), 1000 grain weight (2.7%), spikelet sterility (3.5%) and yield (5%) was recorded in rice cultivar Gigante inoculated with RYMV phylotype S6c (Tz486). Phylotype S6c (Tz486) despite being less virulent compared to other strains, its virus titer in rice cultivar Gigante (1.833) was higher than S5 (Tz429, Tz445) inoculated on Tog5674 (0.171, 0.207) and S6w (Tz539) inoculated on Tog5681 (0.283). The resistant-breaking strain S5 (Tz445) multiplied in resistant rice cultivar Tog5674 without inducing visible symptoms but showed positive reaction to ELISA with low virus titer. The strain S5 overcame wide range of resistant alleles including rymv1-2, rymv1-3, rymv1-4 and rymv1-5 resistance, with exception of rymv1-4 + rymv2. The current results gave a new perspective for future identification of resistance-breaking mutations through sequencing of the RYMV genome in infected rice cultivars and mutagenesis of an infectious viral clone useful for future RYMV resistant breeding programs.

INVESTIGATION ON TOMATO SPOTTED WILT VIRUS INFECTING PEPPER PLANTS IN HUNGARY

In Hungary resurgence of Tomato spotted wilt virus (TSWV) frequently causesheavy crop losses in pepper production since the mid nineties. Management ofTSWV control was first directed against the thrips (using different insecticides orplastic traps), and against weeds as host plants of the virus and the thrips. Later onTsw resistance gene was introduced from Capsicum chinense PI 152225 and PI159236 into different types of pepper. In 2010 and 2011 sporadically, but in 2012more frequently a resistance breaking (RB) strain of TSWV on resistant peppercultivars was observed in the Szentes region (South-East Hungary). The presenceof a new resistance breaking strain was demonstrated by virological (test-plant,serological and RT-PCR) methods. Previously, the non-structural protein (NSs)encoded by small RNA (S RNA) of TSWV was verified as the avirulence factor forTsw resistance, therefore we analyzed the S RNA of the Hungarian RB and wildtype (WT) isolates and compared to previously analyzed TSWV strains with RBproperties from different geographical origins. Phylogenetic analysis demonstratedthat the different RB strains had the closest relationship with the local WT isolatesand there was no conserved mutation present in all the NSs genes of RB isolatesfrom different geographical origins. According to these results, it is concluded thatthe RB isolates evolved separately in geographic point of view and according to theRB mechanism. In order to find new genetic sources of resistance in Capsicumspecies 89 lines from Capsicum annuum, C. chinense, C. frutescens, C. chacoense,C. baccatum var. baccatum, C. baccatum var. pendulum and C. praetermissumwere tested with the Hungarian TSWV-RB isolate.

First Report of a Resistance-breaking Strain of Raspberry bushy dwarf virus in Red Raspberry (Rubus idaeus) in North America

First Report of a Resistance-breaking Strain of <i>Raspberry bushy dwarf virus</i> in Red Raspberry (<i>Rubus idaeus</i>) in North America

Grafting on a Non-Transgenic Tolerant Tomato Variety Confers Resistance to the Infection of a Sw5-Breaking Strain of Tomato spotted wilt virus via RNA Silencing.

RNA silencing controls endogenous gene expression and drives defensive reactions against invasive nucleic acids like viruses. In plants, it has been demonstrated that RNA silencing can be transmitted through grafting between scions and silenced rootstocks to attenuate virus and viroid accumulation in the scions. This has been obtained mostly using transgenic plants, which may be a drawback in current agriculture. In the present study, we examined the dynamics of infection of a resistance-breaking strain of Tomato spotted wilt virus (RB-TSWV) through the graft between an old Apulian (southern Italy) tomato variety, denoted Sl-Ma, used as a rootstock and commercial tomato varieties used as scions. In tests with non-grafted plants, Sl-Ma showed resistance to the RB-TSWV infection as viral RNA accumulated at low levels and plants recovered from disease symptoms by 21 days post inoculation. The resistance trait was transmitted to the otherwise highly susceptible tomato genotypes grafted onto Sl-Ma. The results from the analysis of small RNAs hallmark genes involved in RNA silencing and virus-induced gene silencing suggest that RNA silencing is involved in the resistance showed by Sl-Ma against RB-TSWV and in scions grafted on this rootstock. The results from self-grafted susceptible tomato varieties suggest also that RNA silencing is enhanced by the graft itself. We can foresee interesting practical implications of the approach described in this paper.

Differential abundance of proteins in response to Beet necrotic yellow vein virus during compatible and incompatible interactions in sugar beet containing Rz1 or Rz2

Rhizomania, caused by Beet necrotic yellow vein virus (BNYVV), is an important disease affecting sugar beet. Control is achieved through planting of resistant varieties; however, following the introduction of Rz1, new pathotypes that overcome resistance have appeared. To understand how BNYVV overcomes resistance, we examined quantitative protein differences during compatible and incompatible interactions when sugar beet is infected with either a traditional A-type strain or with an Rz1 resistance breaking strain. Proteomic data suggest distinct biochemical pathways are induced during compatible and incompatible sugar beet interactions with BNYVV. Pathways including the detoxification of reactive oxygen species, UB/proteasome, and photosynthesis should be studied in more depth to characterize roles in symptom development.

Evidence of a tomato spotted wilt virus resistance-breaking strain originated through natural reassortment between two evolutionary-distinct isolates

A Tsw resistance-breaking tomato spotted wilt virus field isolate (TSWV-p331) found in northern Italy originated via reassortment from two evolutionary distinct TSWV strains, as revealed by recombination and phylogenetic analysis. Compared to the closest isolate present in the database, p331 NSs protein carries an unusually high number of amino acid substitutions, but no differences in the nucleocapsid protein. Despite these substitutions, p331 NSs is a potent silencing suppressor. As shown by phylogenetic analyses of TSWV nucleocapsid sequences collected over fifteen years, one likely p331 parental lineage has never been detected in northern Italy, allowing speculations on the origin of TSWV-p331.