Detection of Pepino mosaic virus isolates from tomato by one-step reverse transcription loop-mediated isothermal amplification

Low-High-Low Rotationally Pulse-Actuated Serial Dissolvable Film Valves Applied to Solid Phase Extraction and LAMP Isothermal Amplification for Plant Pathogen Detection on a Lab-on-a-Disc

The ability of the centrifugal Lab-on-a-Disc (LoaD) platform to closely mimic the "on bench" liquid handling steps (laboratory unit operations (LUOs)) such as metering, mixing, and aliquoting supports on-disc automation of bioassay without the need for extensive biological optimization. Thus, well-established bioassays, normally conducted manually using pipettes or using liquid handling robots, can be relatively easily automated in self-contained microfluidic chips suitable for use in point-of-care or point-of-use settings. The LoaD's ease of automation is largely dependent on valves that can control liquid movement on the rotating disc. The optimum valving strategy for a true low-cost and portable device is rotationally actuated valves, which are actuated by changes in the disc spin-speed. However, due to tolerances in disc manufacturing and variations in reagent properties, most of these valving technologies have inherent variation in their actuation spin-speed. Most valves are actuated through stepped increases in disc spin-speed until the motor reaches its maximum speed (rarely more than 6000 rpm). These manufacturing tolerances combined with this "analogue" mechanism of valve actuation limits the number of LUOs that can be placed on-disc. In this work, we present a novel valving mechanism called low-high-low serial dissolvable film (DF) valves. In these valves, a DF membrane is placed in a dead-end pneumatic chamber. Below an actuation spin-speed, the trapped air prevents liquid wetting and dissolving the membrane. Above this spin-speed, the liquid will enter and wet the DF and open the valve. However, as DFs take ∼40 s to dissolve, the membrane can be wetted, and the disc spin-speed reduced before the film opens. Thus, by placing valves in a series, we can govern on which "digital pulse" in spin-speeding a reagent is released; a reservoir with one serial valve will open on the first pulse, a reservoir with two serial valves on the second, and so on. This "digital" flow control mechanism allows the automation of complex assays with high reliability. In this work, we first describe the operation of the valves, outline the theoretical basis for their operation, and support this analysis with an experiment. Next, we demonstrate how these valves can be used to automate the solid-phase extraction of DNA on on-disc LAMP amplification for applications in plant pathogen detection. The disc was successfully used to extract and detect, from a sample lysed off-disc, DNA indicating the presence of thermally inactivated Clavibacter michiganensis ssp. michiganensis (Cmm), a bacterial pathogen on tomato leaf samples.

Field-Portable, Rapid, and Low-Cost RT-LAMP Assay for the Detection of Tomato Chlorotic Spot Virus

Tomato chlorotic spot virus (TCSV) is a highly destructive, thrips-transmitted, emerging orthotospovirus in various vegetable and ornamental crops. It is important to reduce the risk of spreading this virus by limiting the movement of infected plant materials to other geographic areas by utilizing point-of-care diagnostics. Current diagnostic assays for TCSV require costly lab equipment, skilled personnel, and electricity. Here, we report the development of a simple rechargeable battery-operated handwarmer-assisted reverse transcription-loop-mediated isothermal amplification (RT-LAMP) assay and demonstrate a step-by-step protocol to achieve in-field detection of TCSV. Under field conditions, handwarmer-assisted RT-LAMP can detect as little as 0.9 pg/μl of total RNA from TCSV-infected tomato plants in <35 min. When fully charged, the field-portable device can be used in six consecutive RT-LAMP detection assays, yielding test results for 96 individual samples. Dye-based colorimetric methods, including pH and metal ion indicators, were evaluated to eliminate laboratory-dependent LAMP visualization. Phenol red combined with hydroxynaphthol blue was adopted in the handwarmer-assisted RT-LAMP detection method to obtain a more robust color difference distinguishable by the naked eye. Overall, handwarmer-assisted RT-LAMP is a rapid, highly sensitive, and cost-effective diagnostic technique that can be used by nonspecialist personnel in the field, particularly in rural production areas lacking access to a diagnostic lab or constant electricity. [Formula: see text] Copyright © 2023 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Plant pathogen detection on a lab-on-a-disc using solid-phase extraction and isothermal nucleic acid amplification enabled by digital pulse-actuated dissolvable film valves

By virtue of its ruggedness, portability, rapid processing times, and ease-of-use, academic and commercial interest in centrifugal microfluidic systems has soared over the last decade. A key advantage of the LoaD platform is the ability to automate laboratory unit operations (LUOs) (mixing, metering, washing etc.) to support direct translation of 'on-bench' assays to 'on-chip'. Additionally, the LoaD requires just a low-cost spindle motor rather than specialized and expensive microfluidic pumps. Furthermore, when flow control (valves) is implemented through purely rotational changes in this same spindle motor (rather than using additional support instrumentation), the LoaD offers the potential to be a truly portable, low-cost and accessible platform. Current rotationally controlled valves are typically opened by sequentially increasing the disc spin-rate to a specific opening frequency. However, due lack of manufacturing fidelity these specific opening frequencies are better described as spin frequency 'bands'. With low-cost motors typically having a maximum spin-rate of 6000 rpm (100 Hz), using this 'analogue' approach places a limitation on the number of valves, which can be serially actuated thus limiting the number of LUOs that can be automated. In this work, a novel flow control scheme is presented where the sequence of valve actuation is determined by architecture of the disc while its timing is governed by freely programmable 'digital' pulses in its spin profile. This paradigm shift to 'digital' flow control enables automation of multi-step assays with high reliability, with full temporal control, and with the number of LUOs theoretically only limited by available space on the disc. We first describe the operational principle of these valves followed by a demonstration of the capability of these valves to automate complex assays by screening tomato leaf samples against plant pathogens. Reagents and lysed sample are loaded on-disc and then, in a fully autonomous fashion using only spindle-motor control, the complete assay is automated. Amplification and fluorescent acquisition take place on a custom spin-stand enabling the generation of real-time LAMP amplification curves using custom software. To prevent environmental contamination, the entire discs are sealed from atmosphere following loading with internal venting channels permitting easy movement of liquids about the disc. The disc was successfully used to detect the presence of thermally inactivated Clavibacter michiganensis. Michiganensis (CMM) bacterial pathogen on tomato leaf samples.

Occurrence, Genetic Variability of Tomato Yellow Ring Orthotospovirus Population and the Development of Reverse Transcription Loop-Mediated Isothermal Amplification Assay for Its Rapid Detection

Tomato-infecting viruses have been considered as a serious threat to tomato crops in Poland. Therefore, during 2014–2021, 234 tomato samples delivered directly by greenhouse tomato growers to Plant Disease Clinic of IPP-NRI were tested. Eight virus species: pepino mosaic virus (PepMV), tomato yellow ring orthotospovirus (TYRV), tomato spotted wilt orthotospovirus (TSWV), potato virus Y (PVY), cucumber mosaic virus (CMV), tomato black ring virus (TBRV) and tomato mosaic virus (ToMV) were detected in single or mixed infection in 89 samples. The presence of TYRV was established for the first time in Poland in 2014. Since then, its presence has been observed in single and mixed infection with TSWV and CMV. Here, we analysed the genetic variability of TYRV population based on complete nucleocapsid (N) protein gene sequence of 55 TYRV isolates. Maximum-likelihood reconstruction revealed the presence of three distinct, well-supported phylogroups. Moreover, the effect of host species on virus diversity was confirmed. Therefore, RT-LAMP assay was developed for the rapid and efficient detection of TYRV isolates that can be implemented in field and greenhouse conditions.

Immunocapture-Reverse Transcriptase Loop-Mediated Isothermal Amplification Assay for Detection of Plant RNA Viruses

Loop-mediated isothermal amplification (LAMP) is a sensitive method that can rapidly amplify a specific nucleic acid target with high specificity. The LAMP reaction process has no denaturation step, instead DNA amplification occurs by strand displacement activity of the Bacillus stearothermophilus (Bst) DNA polymerase under isothermal conditions. It utilizes three sets of forward and reverse oligonucleotide primers specific to six distinct sequences on the target gene. These primers are used to generate amplification products that contain single-stranded loops, thereby allowing primers to bind to these sequences without the need for repeated cycles of thermal denaturation. For diagnosis of pathogens with RNA genome, LAMP has been merged with reverse transcription (RT) step to create RT-LAMP. To further reduce the cost of diagnosis and increase the throughput, immunocapture (IC) step was added to develop IC-RT-LAMP assay. Hence, this chapter focuses on utilizing IC-RT-LAMP assay to specifically identify severe strain of a plant virus from field samples.

Development of loop-mediated isothermal amplification assay for rapid detection of genetically different wheat dwarf virus isolates

Wheat dwarf virus (WDV) is considered as one of the most common viruses on cereal crops. Recently, severe outbreaks of WDV have been observed especially on winter wheat in southwestern part of Poland. Moreover, the presence of genetically different WDV-barley-specific and WDV-wheat-specific forms (WDV-B and WDV-W, respectively) was confirmed. In this study, a loop-mediated isothermal amplification assay (LAMP) was developed for the first time for efficient and rapid detection of WDV-B and WDV-W in infected plants. The reaction was performed using a set of three primer pairs: WDVF3/WDVB3, WDVFIB/WDVBIP and WDVLoopF/WDVLoopB specific for coat protein coding sequence. The amplified products were analyzed by direct staining of DNA, gel electrophoresis and real-time monitoring of the amplification curves. The sensitivity of optimized reaction was tenfold higher in comparison with conventional PCR. LAMP assay developed here is a useful and practical method for the rapid detection of different WDV isolates and can be implemented by phytosanitary services.

Highly sensitive serological approaches for Pepino mosaic virus detection

Pepino mosaic virus (PepMV) causes severe disease in tomato and other Solanaceous crops around globe. To effectively study and manage this viral disease, researchers need new, sensitive, and high-throughput approaches for viral detection. In this study, we purified PepMV particles from the infected Nicotiana benthamiana plants and used virions to immunize BALB/c mice to prepare hybridomas secreting anti-PepMV monoclonal antibodies (mAbs). A panel of highly specific and sensitive murine mAbs (15B2, 8H6, 23D11, 20D9, 3A6, and 8E3) could be produced through cell fusion, antibody selection, and cell cloning. Using the mAbs as the detection antibodies, we established double antibody sandwich enzyme-linked immunosorbent assay (DAS-ELISA), Dot-ELISA, and Tissue print-ELISA for detecting PepMV infection in tomato plants. Resulting data on sensitivity analysis assays showed that both DAS-ELISA and Dot-ELISA can efficiently monitor the virus in PepMV-infected tissue crude extracts when diluted at 1:1 310 720 and 1:20 480 (weight/volume ratio (w/v), g/mL), respectively. Among the three methods developed, the Tissue print-ELISA was found to be the most practical detection technique. Survey results from field samples by the established serological approaches were verified by reverse transcription polymerase chain reaction (RT-PCR) and DNA sequencing, demonstrating all three serological methods are reliable and effective for monitoring PepMV. Anti-PepMV mAbs and the newly developed DAS-ELISA, Dot-ELISA, and Tissue print-ELISA can benefit PepMV detection and field epidemiological study, and management of this viral disease, which is already widespread in tomato plants in Yunnan Province of China.

High-Throughput Sequencing Facilitates Discovery of New Plant Viruses in Poland

Viruses cause epidemics on all major crops of agronomic importance, and a timely and accurate identification is essential for control. High throughput sequencing (HTS) is a technology that allows the identification of all viruses without prior knowledge on the targeted pathogens. In this paper, we used HTS technique for the detection and identification of different viral species occurring in single and mixed infections in plants in Poland. We analysed various host plants representing different families. Within the 20 tested samples, we identified a total of 13 different virus species, including those whose presence has not been reported in Poland before: clover yellow mosaic virus (ClYMV) and melandrium yellow fleck virus (MYFV). Due to this new finding, the obtained sequences were compared with others retrieved from GenBank. In addition, cucurbit aphid-borne yellows virus (CABYV) was also detected, and due to the recent occurrence of this virus in Poland, a phylogenetic analysis of these new isolates was performed. The analysis revealed that CABYV population is highly diverse and the Polish isolates of CABYV belong to two different phylogenetic groups. Our results showed that HTS-based technology is a valuable diagnostic tool for the identification of different virus species originating from variable hosts, and can provide rapid information about the spectrum of plant viruses previously not detected in a region.

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.

A rapid detection tool for VT isolates of Citrus tristeza virus by immunocapture-reverse transcriptase loop-mediated isothermal amplification assay

Severe strains of Citrus tristeza virus (CTV) cause quick decline and stem pitting resulting in significant economic losses in citrus production. A immunocapture reverse-transcriptase loop-mediated amplification (IC-RT-LAMP) assay was developed in this study to detect the severe VT strains that are typically associated with severe CTV symptoms. The sensitivity of RT-LAMP assay was determined by ten-fold serial dilutions of CA-VT-AT39 RNA, in comparison to one-step RT-droplet digital (dd) PCR. RT-LAMP detected up to 0.002 ng RNA with an amplification time of 10:35 (min:sec.), equivalent to 11.3 copies as determined by one step RT-ddPCR. The RT-LAMP assay specifically detected CA-VT-AT39 RNA and did not cross react with other CTV genotypes tested (T36, T30, RB, S1 and T68). To facilitate rapid on-site detection, the RT-LAMP assay was improved by first capturing the CTV virions from citrus crude leaf sap using CTV-IgG (IC-RT-LAMP), thereby eliminating nucleic acid extraction steps. IC-RT-LAMP assay was optimized with two-fold dilutions of CTV-IgG ranging from 1:500 to 1:16,000. The IC-RT-LAMP assay detected the CA-VT-AT39 virions in all dilutions tested. The minimum amplification time was 6:45 (min:sec) with 1:500 and 1:1000 of CTV-IgG dilutions. The limit of detection of IC-RT-LAMP assay with crude leaf sap of CA-VT-AT39 was 1:320 with a maximum amplification time of 9:08 (min:sec). The IC-RT-LAMP assay was validated for VT genotype by comparing to IC-RT-qPCR using the CTV from 40 field tree samples. A 100% agreement was observed between tests, regardless of single or mixed infections of CTV VT with other genotypes. Therefore, the IC-RT-LAMP assay can serve as a useful tool in the management of potentially severe strains of CTV.

Rapid detection of genetically diverse tomato black ring virus isolates using reverse transcription loop-mediated isothermal amplification

A reverse transcription loop-mediated isothermal amplification assay (RT-LAMP) has been developed for detection of tomato black ring virus (TBRV) isolates collected from different hosts. One-step RT-LAMP was performed with a set of four primers, the design of which was based on the coat protein gene. Results of RT-LAMP were visualized by direct staining of products with fluorescent dyes, agarose gel electrophoresis, and analysis of amplification curves. The sensitivity of RT-LAMP was 100-fold greater than that of RT-PCR. The RT-LAMP assay developed here is a useful and practical method for diagnosis of TBRV.

Variability of Potato virus Y in Tomato Crops in Poland and Development of a Reverse-Transcription Loop-Mediated Isothermal Amplification Method for Virus Detection

A collection of 147 Potato virus Y (PVY) isolates from tomato, originating from several commercial fields and greenhouses in different regions of Poland, was tested for the presence of PVY by reverse-transcription polymerase chain reaction. However, in some cases, the results obtained were ambiguous. Therefore, a sensitive reverse-transcription loop-mediated isothermal amplification method was developed for rapid detection of PVY isolates. Phylogenetic and recombination analyses were performed based on sequences of the coat protein gene. In comparison with results obtained in 2008, the presence of other strains besides PVY(N)Wi-P was confirmed. A novel recombinant between PVY(NTN) and PVY(N)Wi-P strains was detected. Our results indicate an increasing distribution and variability of the PVY population on tomato in Poland.