First Complete Genome Sequence of an Isolate of Tomato Mottle Mosaic Virus Infecting Plants of Solanum lycopersicum in South America

A Reverse-Transcription Loop-Mediated Isothermal Amplification Technique to Detect Tomato Mottle Mosaic Virus, an Emerging Tobamovirus

Tomato mottle mosaic virus (ToMMV) is an emerging seed-transmissible tobamovirus that infects tomato and pepper. Since the first report in 2013 in Mexico, ToMMV has spread worldwide, posing a serious threat to the production of both crops. To prevent the spread of this virus, early and accurate detection of infection is required. In this study, we developed a detection method for ToMMV based on reverse-transcription loop-mediated isothermal amplification (RT-LAMP). A LAMP primer set was designed to target the genomic region spanning the movement protein and coat protein genes, which is a highly conserved sequence unique to ToMMV. This RT-LAMP detection method achieved 10-fold higher sensitivity than conventional RT-polymerase chain reaction methods and obtained high specificity without false positives for closely related tobamoviruses or healthy tomato plants. This method can detect ToMMV within 30 min of direct sampling of an infected tomato leaf using a toothpick and therefore does not require RNA purification. Given its high sensitivity, specificity, simplicity, and rapidity, the RT-LAMP method developed in this study is expected to be valuable for point-of-care testing in field surveys and for large-scale testing.

Metagenomic analyses of plant virus sequences in sewage water for plant viruses monitoring

Frequent monitoring of emerging viruses of agricultural crops is one of the most important missions for plant virologists. A fast and precise identification of potential harmful viruses may prevent the occurrence of serious epidemics. Nowadays, high-throughput sequencing (HTS) technologies became an accessible and powerful tool for this purpose. The major discussion of this strategy resides in the process of sample collection, which is usually laborious, costly and nonrepresentative. In this study, we assessed the use of sewage water samples for monitoring the widespread, numerous, and stable plant viruses using HTS analysis and RT-qPCR. Plant viruses belonged to 12 virus families were found, from which Virgaviridae, Solemoviridae, Tymoviridae, Alphaflexiviridae, Betaflexiviridae, Closteroviridae and Secoviridae were the most abundant ones with more than 20 species. Additionally, we detected one quarantine virus in Brazil and a new tobamovirus species. To assess the importance of the processed foods as virus release origins to sewage, we selected two viruses, the tobamovirus pepper mild mottle virus (PMMoV) and the carlavirus garlic common latent virus (GarCLV), to detect in processed food materials by RT-qPCR. PMMoV was detected in large amount in pepper-based processed foods and in sewage samples, while GarCLV was less frequent in dried and fresh garlic samples, and in the sewage samples. This suggested a high correlation of virus abundance in sewage and processed food sources. The potential use of sewage for a virus survey is discussed in this study.

Supplementary Information

The online version contains supplementary material available at 10.1007/s40858-023-00575-8.

Nanopore Technology Applied to Targeted Detection of Tomato Brown Rugose Fruit Virus Allows Sequencing of Related Viruses and the Diagnosis of Mixed Infections

Tomato (Solanum lycopersicum) plants from a commercial glasshouse were identified with symptoms compatible with a tomato brown rugose fruit virus (ToBRFV) infection. Reverse transcription-PCR and quantitative PCR confirmed the presence of ToBRFV. Subsequently, the same RNA sample and a second from tomato plants infected with a similar tobamovirus, tomato mottle mosaic virus (ToMMV), were extracted and processed for high-throughput sequencing with the Oxford Nanopore Technology (ONT). For the targeted detection of ToBRFV, the two libraries were synthesized by using six ToBRFV sequence-specific primers in the reverse transcription step. This innovative target enrichment technology enabled deep coverage sequencing of ToBRFV, with 30% of the total reads mapping to the target virus genome and 57% mapping to the host genome. The same set of primers applied to the ToMMV library generated 5% of the total reads mapping to the latter virus, indicating that sequencing of similar, non-target viral sequences was also allowed. Further, the complete genome of pepino mosaic virus (PepMV) was also sequenced from the ToBRFV library, thus suggesting that, even using multiple sequence-specific primers, a low rate of off-target sequencing can usefully provide additional information on unexpected viral species coinfecting the same samples in an individual assay. These results demonstrate that targeted nanopore sequencing can specifically identify viral agents and has sufficient sensitivity towards non-target organisms to provide evidence of mixed virus infections.

Development and Validation of a One-Step Reverse Transcription Real-Time PCR Assay for Simultaneous Detection and Identification of Tomato Mottle Mosaic Virus and Tomato Brown Rugose Fruit Virus

Tobamovirus species represent a threat to solanaceous crops worldwide, due to their extreme stability and because they are seed borne. In particular, recent outbreaks of tomato brown rugose fruit virus in tomato and pepper crops led to the establishment of prompt control measures, and the need for reliable diagnosis was urged. Another member of the genus, tomato mottle mosaic virus, has recently gained attention due to reports in different continents and its common features with tomato brown rugose fruit virus. In this study, a new real-time RT-PCR detection system was developed for tomato brown rugose fruit virus and tomato mottle mosaic virus on tomato leaves and seeds using TaqMan chemistry. This test was designed to detect tomato mottle mosaic virus by amplifying the movement protein gene in a duplex assay with the tomato brown rugose fruit virus target on the CP-3’NTR region, which was previously validated as a single assay. The performance of this test was evaluated, displaying analytical sensitivity 10⁻⁵–10⁻⁶-fold dilution for seeds and leaves, respectively, and good analytical specificity, repeatability, and reproducibility. Using the newly developed and validated test, tomato brown rugose fruit virus detection was 100% concordant with previously performed analyses on 106 official samples collected in 2021 from different continents.

Synthesis and Characterization of a Full-Length Infectious cDNA Clone of Tomato Mottle Mosaic Virus

Tomato mottle mosaic virus (ToMMV) is a noteworthy virus which belongs to the Virgaviridae family and causes serious economic losses in tomato. Here, we isolated and cloned the full-length genome of a ToMMV Chinese isolate (ToMMV-LN) from a naturally infected tomato (Solanum lycopersicum L.). Sequence analysis showed that ToMMV-LN contains 6399 nucleotides (nts) and is most closely related to a ToMMV Mexican isolate with a sequence identity of 99.48%. Next, an infectious cDNA clone of ToMMV was constructed by a homologous recombination approach. Both the model host N. benthamiana and the natural hosts tomato and pepper developed severe symptoms upon agroinfiltration with pToMMV, which had a strong infectivity. Electron micrographs indicated that a large number of rigid rod-shaped ToMMV virions were observed from the agroinfiltrated N. benthamiana leaves. Finally, our results also confirmed that tomato plants inoculated with pToMMV led to a high infection rate of 100% in 4–5 weeks post-infiltration (wpi), while pepper plants inoculated with pToMMV led to an infection rate of 40–47% in 4–5 wpi. This is the first report of the development of a full-length infectious cDNA clone of ToMMV. We believe that this infectious clone will enable further studies of ToMMV genes function, pathogenicity and virus–host interaction.