Pepino mosaic virus genotype shift in North America and development of a loop-mediated isothermal amplification for rapid genotype identification

Co-Infection of Tomato Brown Rugose Fruit Virus and Pepino Mosaic Virus in Grocery Tomatoes in South Florida: Prevalence and Genomic Diversity

Tomato brown rugose fruit virus (ToBRFV) is an economically important seed and mechanically transmitted pathogen of significant importance to tomato production around the globe. Synergistic interaction with pepino mosaic virus (PepMV), another seed and mechanically transmitted virus, and long-distance dissemination of these two viruses via contaminated tomato fruits through global marketing were previously suggested. In 2019, we detected both viruses in several grocery store-purchased tomatoes in South Florida, USA. In this study, to identify potential sources of inoculum, co-infection status, prevalence, and genomic diversity of these viruses, we surveyed symptomatic and asymptomatic imported tomatoes sold in ten different groceries in four cities in South Florida. According to the product labels, all collected tomatoes originated from Canada, Mexico, or repacking houses in the United States. With high prevalence levels, 86.5% of the collected samples were infected with ToBRFV, 90% with PepMV alone, and 73% were mixed-infected. The phylogenetic study showed no significant correlations between ToBRFV genomic diversity and the tomato label origin. Phylogenetic analysis of PepMV isolates revealed the prevalence of the PepMV strains, Chilean (CH2) and recombinant (US2). The results of this study highlight the continual presence of PepMV and ToBRFV in imported tomatoes in Florida grocery stores.

Transcriptome analyses unveiled differential regulation of AGO and DCL genes by pepino mosaic virus strains

Pepino mosaic virus (PepMV) is a single-stranded (ss), positive-sense (+) RNA potexvirus that affects tomato crops worldwide. We have described an in planta antagonistic interaction between PepMV isolates of two strains in which the EU isolate represses the accumulation of the CH2 isolate during mixed infections. Reports describing transcriptomic responses to mixed infections are scant. We carried out transcriptomic analyses of tomato plants singly and mixed-infected with two PepMV isolates of both strains. Comparison of the transcriptomes of singly infected plants showed that deeper transcriptomic alterations occurred at early infection times, and also that each of the viral strains modulated the host transcriptome differentially. Mixed infections caused transcriptomic alterations similar to those for the sum of single infections at early infection times, but clearly differing at later times postinfection. We next tested the hypothesis that PepMV-EU, in either single or mixed infections, deregulates host gene expression differentially so that virus accumulation of both strains gets repressed. That seemed to be the case for the genes AGO1a, DCL2d, AGO2a, and DCL2b, which are involved in the antiviral silencing pathway and were upregulated by PepMV-EU but not by PepMV-CH2 at early times postinfection. The pattern of AGO2a expression was validated by reverse transcription-quantitative PCR in tomato and Nicotiana benthamiana plants. Using an N. benthamiana ago2 mutant line, we showed that AGO2 indeed plays an important role in the antiviral defence against PepMV, but it is not the primary determinant of the outcome of the antagonistic interaction between the two PepMV strains.

Development and optimization of a pepino mosaic virus-based vector for rapid expression of heterologous proteins in plants

Plant-virus-derived vectors are versatile tools with multiple applications in agricultural and medical biotechnology. In this study, we developed pepino mosaic virus (PepMV) (family Alphaflexiviridae; genus Potexvirus) into a vector for heterologous protein expression in plants. PepMV was initially cloned in a step-wise manner, fully sequenced and the full-length infectious clone was tested for infectivity in Nicotiana benthamiana. Initial infectious clones resulted in poor replication of PepMV and lack of systemic movement. Mutations in the viral sequence affected systemic infection. Two suspected mutations were altered to restore systemic infectivity. PepMV infection was apparent as early as 4 days post agroinfiltration (dpa) inoculation in N. benthamiana. A multiple cloning site was inserted into the PepMV genome for introduction and expression of foreign genes. Several modifications to the wild-type vector were made, such as a replacing the native subgenomic promoter (SGP) with a heterologous SGP, and introduction of translational enhancers and terminators, to improve heterologous expression of the foreign gene-of-interest. GFP was used as a reporter for monitoring virus infection and protein production. Strong GFP expression was observed as early as 4 dpa with a translational enhancer. The PepMV-based vector produces rapid expression of the foreign gene in comparison to two other potexvirus-based vectors. GFP production was monitored over time and optimal protein production was recorded between 5 and 7 dpa. GFP protein levels reached up to 4% and decreased to 0.5% total soluble protein at 7 and 14 dpa, respectively. Future studies will evaluate this virus-based vector for large-scale production of pharmaceutical compounds. KEY POINTS: • A pepino mosaic virus isolate was developed into a plant-based expression vector. • Expression levels of the heterologous protein were comparable or exceeded previously developed viral vectors. • Protein levels in plants were highest between 5 and 7 days and decreased gradually.

Loop Mediated Isothermal Amplification: Principles and Applications in Plant Virology

In the last decades, the evolution of molecular diagnosis methods has generated different advanced tools, like loop-mediated isothermal amplification (LAMP). Currently, it is a well-established technique, applied in different fields, such as the medicine, agriculture, and food industries, owing to its simplicity, specificity, rapidity, and low-cost efforts. LAMP is a nucleic acid amplification under isothermal conditions, which is highly compatible with point-of-care (POC) analysis and has the potential to improve the diagnosis in plant protection. The great advantages of LAMP have led to several upgrades in order to implement the technique. In this review, the authors provide an overview reporting in detail the different LAMP steps, focusing on designing and main characteristics of the primer set, different methods of result visualization, evolution and different application fields, reporting in detail LAMP application in plant virology, and the main advantages of the use of this technique.

One-Step Reverse-Transcription Digital PCR for Reliable Quantification of Different Pepino Mosaic Virus Genotypes

In recent years, pepino mosaic virus (PepMV) has rapidly evolved from an emerging virus to an endemic pathogen, as it causes significant loses to tomato crops worldwide. At present, the main control strategy for prevention of PepMV disease in tomato production remains based on strict hygiene measures. To prevent damage caused by PepMV, cross-protection is used in some countries. Reliable characterisation, detection and quantification of the pathogen are vital for disease control. At present, reverse-transcription real-time quantitative polymerase chain reaction (RT-qPCR) is generally used for this purpose. However, quantitative use of RT-qPCR is linked to standardised reference materials, which are not available for PepMV. In addition, many factors can influence RT-qPCR efficiencies and lead to lower accuracy of the quantification. In this study, well-characterised PepMV-genotype-specific RT-qPCR assays were transferred to two digital PCR (dPCR) platforms. dPCR-based assays allow absolute quantification without the need for standard curves, and due to the binary nature of the reaction, dPCR also overcomes many of the other drawbacks of RT-qPCR. We have shown that these newly developed and validated PepMV-genotype-specific dPCR assays are suitable candidates for higher-order methods for quantification of PepMV RNA, as they show lower measurement variability, with sensitivity and specificity comparable to RT-qPCR.

Genetic Characterization of Pepino Mosaic Virus Isolates from Morocco

Introduction:

Throughout the past few years, Pepino Mosaic Virus (PepMV) has rapidly evolved from an emerging virus to endemic pathogen that causes significant losses in tomato crops worldwide. Reliable detection and molecular characterization are very important tools to support disease control. Cross-protection can also be an effective strategy, but the efficacy depends strongly on the genotype. The genetic composition of the PepMV population in Morocco has not yet been determined.

Aims:

The current study aims to genetically characterize twelve PepMV isolates (PepMV-MA), all from different Moroccan tomato production areas, by analyzing nucleotide sequences of a part of the RNA-dependent RNA polymerase (RdRp), Triple Gene Block (TGB) and Coat Protein (CP) genes.

Results:

The sequence analysis of the twelve PepMV-MA isolates shows minor nucleotide differences between them, which implies a homogenous population. The phylogenetic analysis, based on the comparison with the major genotypes, showed that Moroccan PepMV populations share a very high sequence identity, 98%, with the Chilean strain (CH2), while the shared identity with the European strains (EU) is only 85%. Interestingly, Moroccan isolates reveal specific single nucleotide polymorphisms, some of which lead to amino acids changes. These mutations have never been described before, suggesting distinct variants that may enhance aggressiveness and symptomatology.

Conclusion:

Our careful sequence analysis and genotype determination, which placing homogenous Moroccan PepMV strains into CH2 genotype, would be a prerequisite for deploying effective cross-protection strategies for controlling the pathogen in the field.

Reverse transcription loop-mediated isothermal amplification for species-specific detection of tomato chlorotic spot orthotospovirus

Tomato chlorotic spot orthotospovirus (TCSV) is an emerging orthotospovirus that can cause severe disease on tomato plants. There are at least four orthotospoviruses infecting tomato, and mixed infection of two or more orthotospoviruses in a single tomato plant is quite common in the field. With similarity in the symptomatology and cross serological reactivity among tomato-infecting orthotospoviruses, especially between TCSV and groundnut ringspot orthotospovirus (GRSV), the current serological tests could not achieve definite and accurate species-specific determination in disease diagnosis. Here, a one-step reverse transcription loop-mediated isothermal amplification (RT-LAMP) assay was developed for TCSV. Under optimum conditions, the virus was detected in as little as 0.2 ng of total RNA or in 1:10,000 dilution of a simple diluted tissue extract, which was ten times more sensitive than a conventional RT-PCR assay. The RT-LAMP assay was highly specific for TCSV, with no cross reaction with the other two orthotospoviruses: GRSV and tomato spotted wilt orthotospovirus (TSWV). These results demonstrate that this simple and sensitive RT-LAMP could be used to achieve species-specific detection for TCSV under field conditions.

Strain-dependent mutational effects for Pepino mosaic virus in a natural host

Background

Pepino mosaic virus (PepMV) is an emerging plant pathogen that infects tomatoes worldwide. Understanding the factors that influence its evolutionary success is essential for developing new control strategies that may be more robust against the evolution of new viral strains. One of these evolutionary factors is the distribution of mutational fitness effect (DMFE), that is, the fraction of mutations that are lethal, deleterious, neutral, and beneficial on a given viral strain and host species. The goal of this study was to characterize the DMFE of introduced nonsynonymous mutations on a mild isolate of PepMV from the Chilean 2 strain (PepMV-P22). Additionally, we also explored whether the fitness effect of a given mutation depends on the gene where it appears or on epistatic interactions with the genetic background. To address this latter possibility, a subset of mutations were also introduced in a mild isolate of the European strain (PepMV-P11) and the fitness of the resulting clones measured.

Results

A collection of 25 PepMV clones each containing a single nucleotide nonsynonymous substitution was created by site-directed mutagenesis and the fitness of each mutant was determined. PepMV-P22 genome showed a high degree of robustness against point mutations, with 80% of mutations being either neutral or even beneficial and only 20% being deleterious or lethal. We found that the effect of mutations strongly depended on the gene in which they were introduced. Mutations with the largest average beneficial effects were those affecting the RdRp gene, in contrast to mutations affecting TGB1 and CP genes, for which the average effects were deleterious. Moreover, significant epistatic interactions were observed between nonsynonymous mutations and the genetic background, meaning that the effect of a given nucleotide substitution on a particular genomic context cannot be predicted by knowing its effect in a different one.

Conclusions

Our results indicated that PepMV genome has a surprisingly high robustness against mutations. We also found that fitness consequences of a given mutation differ between the two strains analyzed. This discovery suggests that the strength of selection, and thus the rates of evolution, vary among PepMV strains.

Ultrastructural insights into tomato infections caused by three different pathotypes of Pepino mosaic virus and immunolocalization of viral coat proteins

This paper presents studies on an ultrastructural analysis of plant tissue infected with different pathotypes of Pepino mosaic virus (PepMV) and the immunolocalization of viral coat proteins. Because the PepMV virus replicates with a high mutation rate and exhibits significant genetic diversity, therefore, isolates of PepMV display a wide range of symptoms on infected plants. In this work, tomato plants of the Beta Lux cultivar were inoculated mechanically with three pathotypes representing the Chilean 2 (CH2) genotype: mild (PepMV-P22), necrotic (PepMV-P19) and yellowing (PepMV-P5-IY). The presence of viral particles in all infected plants in the different compartments of various cell types (i.e. spongy and palisade mesophyll, sieve elements and xylem vessels) was revealed via ultrastructural analyses. For the first time, it was possible to demonstrate the presence of crystalline inclusions, composed of virus-like particles. In the later stage of PepMV infection (14 dpi) various pathotype-dependent changes in the structure of the individual organelles (i.e. mitochondria, chloroplasts) were found. The strongest immunogold labeling of the viral coat proteins was also observed in plants infected by necrotic isolates. A large number of viral coat proteins were marked in the plant conductive elements, both xylem and phloem.

Evaluation of disinfectants to prevent mechanical transmission of viruses and a viroid in greenhouse tomato production

BACKGROUND

In recent years, a number of serious disease outbreaks caused by viruses and viroids on greenhouse tomatoes in North America have resulted in significant economic losses to growers. The objectives of this study were to evaluate the effectiveness of commercial disinfectants against mechanical transmission of these pathogens, and to select disinfectants with broad spectrum reactivity to control general virus and viroid diseases in greenhouse tomato production.

METHODS

A total of 16 disinfectants were evaluated against Pepino mosaic virus (PepMV), Potato spindle tuber viroid (PSTVd), Tomato mosaic virus (ToMV), and Tobacco mosaic virus (TMV). The efficacy of each disinfectant to deactivate the pathogen's infectivity was evaluated in replicate experiments from at least three independent experiments. Any infectivity that remained in the treated solutions was assessed through bioassays on susceptible tomato plants through mechanical inoculation using inocula that had been exposed with the individual disinfectant for three short time periods (0-10 sec, 30 sec and 60 sec). A positive infection on the inoculated plant was determined through symptom observation and confirmed with enzyme-linked immunosorbent assay (PepMV, ToMV, and TMV) and real-time reverse transcription-PCR (PSTVd). Experimental data were analyzed using Logistic regression and the Bayesian methodology.

RESULTS

Statistical analyses using logistic regression and the Bayesian methodology indicated that two disinfectants (2% Virkon S and 10% Clorox regular bleach) were the most effective to prevent transmission of PepMV, PSTVd, ToMV, and TMV from mechanical inoculation. Lysol all-purpose cleaner (50%) and nonfat dry milk (20%) were also effective against ToMV and TMV, but with only partial effects for PepMV and PSTVd.

CONCLUSION

With the broad spectrum efficacy against three common viruses and a viroid, several disinfectants, including 2% Virkon S, 10% Clorox regular bleach and 20% nonfat dry milk, are recommend to greenhouse facilities for consideration to prevent general virus and viroid infection on tomato plants.

Multifaceted capsid proteins: multiple interactions suggest multiple roles for Pepino mosaic virus capsid protein

Pepino mosaic virus (PepMV) (family Alphaflexiviridae, genus Potexvirus) is a mechanically transmitted tomato pathogen that, over the last decade, has evolved from emerging to endemic worldwide. Here, two heat-shock cognate (Hsc70) isoforms were identified as part of the coat protein (CP)/Hsc70 complex in vivo, following full-length PepMV and CP agroinoculation. PepMV accumulation was severely reduced in Hsp70 virus-induced gene silenced and in quercetin-treated Nicotiana benthamiana plants. Similarly, in vitro-transcribed as well as virion RNA input levels were reduced in quercetin-treated protoplasts, suggesting an essential role for Hsp70 in PepMV replication. As for Potato virus X, the PepMV CP and triple gene-block protein 1 (TGBp1) self-associate and interact with each other in vitro but, unlike in the prototype, both PepMV proteins represent suppressors of transgene-induced RNA silencing with different modes of action; CP is a more efficient suppressor of RNA silencing, sequesters the silencing signal by preventing its spread to neighboring cells and its systemic movement. Here, we provide evidence for additional roles of the PepMV CP and host-encoded Hsp70 in viral infection, the first as a truly multifunctional protein able to specifically bind to a host chaperone and to counterattack an RNA-based defense mechanism, and the latter as an essential factor for PepMV infection.

Ecological and genetic determinants of Pepino Mosaic Virus emergence

Virus emergence is a complex phenomenon, which generally involves spread to a new host from a wild host, followed by adaptation to the new host. Although viruses account for the largest fraction of emerging crop pathogens, knowledge about their emergence is incomplete. We address here the question of whether Pepino Mosaic Virus (PepMV) emergence as a major tomato pathogen worldwide could have involved spread from wild to cultivated plant species and host adaptation. For this, we surveyed natural populations of wild tomatoes in southern Peru for PepMV infection. PepMV incidence, genetic variation, population structure, and accumulation in various hosts were analyzed. PepMV incidence in wild tomatoes was high, and a strain not yet reported in domestic tomato was characterized. This strain had a wide host range within the Solanaceae, multiplying efficiently in most assayed Solanum species and being adapted to wild tomato hosts. Conversely, PepMV isolates from tomato crops showed evidence of adaptation to domestic tomato, possibly traded against adaptation to wild tomatoes. Phylogenetic reconstructions indicated that the most probable ancestral sequence came from a wild Solanum species. A high incidence of PepMV in wild tomato relatives would favor virus spread to crops and its efficient multiplication in different Solanum species, including tomato, allowing its establishment as an epidemic pathogen. Later, adaptation to tomato, traded off against adaptation to other Solanum species, would isolate tomato populations from those in other hosts.

IMPORTANCE

Virus emergence is a complex phenomenon involving multiple ecological and genetic factors and is considered to involve three phases: virus encounter with the new host, virus adaptation to the new host, and changes in the epidemiological dynamics. We analyze here if this was the case in the recent emergence of Pepino Mosaic Virus (PepMV) in tomato crops worldwide. We characterized a new strain of PepMV infecting wild tomato populations in Peru. Comparison of this strain with PepMV isolates from tomato crops, plus phylogenetic reconstructions, supports a scenario in which PepMV would have spread to crops from wild tomato relatives, followed by adaptation to the new host and eventually leading to population isolation. Our data, which derive from the analysis of field isolates rather than from experimental evolution approaches, significantly contribute to understanding of plant virus emergence, which is necessary for its anticipation and prevention.