Monoclonal antibody-based serological methods for detection of Cucumber green mottle mosaic virus

Preparation and Application of Polyclonal Antibodies for the Rapid Detection of Actinidia Chlorotic Ringspot-Associated Virus

Actinidia chlorotic ringspot-associated virus (AcCRaV, Emaravirus actinidiae) is prevalent in Chinese kiwifruit, leading to substantial yield reduction. The intricate nature of symptoms presents diagnostic challenges, underscoring the necessity for a rapid and accurate detection method that facilitates effective control. In this investigation, AcCRaV isolates from key kiwi-producing regions in Sichuan province were collected and analyzed, with representative strains chosen as experimental materials. Primers targeting the nucleoprotein gene of AcCRaV were designed, and their codon usage was optimized to enhance performance. Various serological methods utilizing polyclonal antibodies were developed, including ELISA, dot immunobinding assay, and AcCRaV-specific gold immunochromatographic bands (AcCRaV-GICS). Field samples exhibited high specificity and sensitivity when tested using these methods. Furthermore, the results obtained from a large number of field samples are consistent with those derived from RT-PCR analysis, further validating the applicability of our approach. A detection method capable of handling a large volume of field samples infected with AcCRaV is currently lacking; thus, our system construction provides an important reference for addressing this gap.

Visual and Super-Sensitive Detection of Maize Chlorotic Mottle Virus by Dot-ELISA and Au Nanoparticle-Based Immunochromatographic Test Strip

Maize chlorotic mottle virus (MCMV) is the only species in the Mahromovirus genus and is often co-infected with one or several viruses of the Potyvirus genus, posing a great threat to the global maize industry. Effective viral integrated management measures are dependent on the timely and proper detection of the causal agent of the disease. In this work, six super-sensitive and specific monoclonal antibodies (mAbs) against MCMV were first prepared using purified MCMV virions as the immunogen. Then, the Dot enzyme-linked immunosorbent assay (Dot-ELISA) was established based on the obtained mAbs, and it can detect MCMV in infected maize leaf crude extracts diluted up to 1:10,240-fold (w/v, g/mL). Furthermore, a rapid and user-friendly Au nanoparticle-based immunochromatographic test strip (AuNP-ICTS) based on paired mAbs 7B12 and 17C4 was created for monitoring MCMV in point-of-care tests, and it can detect the virus in a 25,600-fold dilution (w/v, g/mL) of MCMV-infected maize leaf crude extracts. The whole test process for ICTS was completed in 10 min. Compared with conventional reverse transcription-polymerase chain reaction (RT-PCR), the detection endpoint of both serological methods is higher than that of RT-PCR, especially the Dot-ELISA, which is 12.1 times more sensitive than that of RT-PCR. In addition, the detection results of 20 blinded maize samples by the two serological assays were consistent with those of RT-PCR. Therefore, the newly created Dot-ELISA and AuNP-ICTS exhibit favorable application potential for the detection of MCMV in plant samples.

Suppression of Cucumber Green Mottle Mosaic Virus Infection by Boron Application: From the Perspective of Nutrient Elements and Carbohydrates

Cucumber green mottle mosaic virus (CGMMV) infection causes "blood flesh" symptoms in watermelon fruits, which severely reduces yield and edibleness. However, the growth of watermelon fruits is strongly associated with boron (B), a trace element for improving fruit quality. In this study, B-gradient hydroponic experiments (B concentration: 0, 2.86, and 5.72 mg·L^-1 H₃BO₃) and foliar-spray experiments (B concentration: 30 and 300 mg·L^-1 H₃BO₃) were performed. We found that the B-supplement could inhibit CGMMV infection and especially relieve "blood flesh" symptoms in watermelon fruits. The nutrient element, soluble sugar, and cell wall polysaccharide contents and their metabolism- and transport-related gene expressions were determined in leaves and fruits of the watermelons in B-gradient hydroponic and foliar-spray experiments. We found that the accumulation and metabolism of nutrients and carbohydrates in cells were disrupted by CGMMV infection; however, the B-supplement could restore and maintain their homeostasis. Additionally, we uncovered that NIP5;1 and SWEET4, induced by B-application with CGMMV infection, could majorly contribute to the resistance to CGMMV infection by regulating nutrient elements and carbohydrate homeostasis. These results provided a novel insight into the molecular mechanism of B-mediated CGMMV suppression and an efficient method of B-application for the improvement of watermelon quality after CGMMV infection.

Rapid Quantification of Infectious Cucumber green mottle mosaic virus in Watermelon Tissues by PMA Coupled with RT-qPCR

Cucumber green mottle mosaic virus (CGMMV) belongs to the Tobamovirus genus and is an important quarantine virus of cucurbit crops. Seedborne transmission is one of the principal modes for CGMMV spread, and effective early detection is helpful to prevent the occurrence of the disease. Quantitative real-time reverse-transcription PCR (RT-qPCR) is a sensitive and rapid method for detecting CGMMV nucleic acids, but it cannot distinguish between infectious and noninfectious viruses. In the present work, a propidium monoazide (PMA) assisted RT-qPCR method (PMA-RT-qPCR) was developed to rapidly distinguish infectious and inactive CGMMV. PMA is a photoactive dye that can selectively react with viral RNA released or inside inactive CGMMV virions but not viral RNA inside active virions. The formation of PMA-RNA conjugates prevents PCR amplification, leaving only infectious virions to be amplified. The primer pair cp3-1F/cp3-1R was designed based on the coat protein (cp) gene for specific amplification of CGMMV RNA by RT-qPCR. The detection limit of the RT-qPCR assay was 1.57 × 10² copies·μL⁻¹. PMA at 120 μmol·L⁻¹ was suitable for the selective quantification of infectious CGMMV virions. Under optimal conditions, RT-qPCR detection of heat-inactivated CGMMV resulted in Ct value differences larger than 16 between PMA-treated and non-PMA-treated groups, while Ct differences less than 0.23 were observed in the detection of infectious CGMMV. For naturally contaminated watermelon leaf, fruit and seedlot samples, infectious CGMMV were quantified in 13 out of the 22 samples, with infestation levels of 10²~10⁵ copies·g⁻¹. Application of this assay enabled the selective detection of infectious CGMMV and facilitated the monitoring of the viral pathogen in watermelon seeds and tissues, which could be useful for avoiding the potential risks of primary inoculum sources.

A novel strategy for improving watermelon resistance to cucumber green mottle mosaic virus by exogenous boron application

The molecular mode controlling cucumber green mottle mosaic virus (CGMMV)-induced watermelon blood flesh disease (WBFD) is largely unknown. In this study, we have found that application of exogenous boron suppressed CGMMV infection in watermelon fruit and alleviated WBFD symptoms. Our transcriptome analysis showed that the most up-regulated differentially expressed genes (DEGs) were associated with polyamine and auxin biosynthesis, abscisic acid catabolism, defence-related pathways, cell wall modification, and energy and secondary metabolism, while the down-regulated DEGs were mostly involved in ethylene biosynthesis, cell wall catabolism, and plasma membrane functions. Our virus-induced gene silencing results showed that silencing of SPDS expression in watermelon resulted in a higher putrescine content and an inhibited CGMMV infection correlating with no WBFD symptoms. SBT and TUBB1 were also required for CGMMV infection. In contrast, silencing of XTH23 and PE/PEI7 (low-level lignin, cellulose and pectin) and ATPS1 (low-level glutathione) promoted CGMMV accumulation. Furthermore, RAP2-3, MYB6, WRKY12, H2A, and DnaJ11 are likely to participate in host antiviral resistance. In addition, a higher (spermidine + spermine):putrescine ratio, malondialdehyde content, and lactic acid content were responsible for fruit decay and acidification. Our results provide new knowledge on the roles of boron in watermelon resistance to CGMMV-induced WBFD. This new knowledge can be used to design better control methods for CGMMV in the field and to breed CGMMV resistant watermelon and other cucurbit crops.

Genome and Phylogenetic Analysis of Cucumber Green Mottle Mosaic Virus Global Isolates and Validation of a Highly Sensitive RT-qPCR Assay

The last two decades have seen exponential growth in the international movement of seeds for annual food crops, from a gross U.S. import value of $349 million in 1999 to $1.05 billion in 2019. This has led to the proportionate growth of seedborne pathogens dispersed with seed stocks. One such viral pathogen is cucumber green mottle mosaic virus (CGMMV), a tobamovirus that infects cucurbit crops such as melon, watermelon, cucumber, pumpkin, and squash. The first CGMMV introduction to California occurred in 2013, with subsequent annual outbreaks or detections since then. Here, we describe the use of next-generation sequencing to characterize the full genomes of 25 CGMMV isolates collected between 2013 and 2020 in California, either from CGMMV field detections or seed lots identified as CGMMV positive. We sequenced an additional 31 CGMMV isolates collected in Europe, Israel, and southeast Asia that were provided by industry collaborators. We also performed an in silico nucleotide database search in GenBank for full genome CGMMV sequences to include in all in silico analyses. Based on conserved regions within the coat protein gene, we then developed a quantitative reverse-transcription PCR assay for the sensitive and specific detection of CGMMV in seed and plant samples. Finally, based on our sequence and phylogenetic analysis, our data support that CGMMV has been introduced multiple times into California.

Detection of Tomato Spotted Wilt Virus (TSWV) Infection in Plants Using DAS-ELISA and Dot-ELISA

Plant viruses cause severe damages to crop productions each year worldwide. To prevent the losses caused by plant viruses, it is necessary to develop specific and efficient diagnostic tools to detect viruses. Among the current virus detection techniques, serological detection methods are considered to be rapid, simple, sensitive, and high throughput. Therefore, serological detection methods such as double antibody sandwich enzyme-linked immunosorbent assay (DAS-ELISA), triple antibody sandwich ELISA (TAS-ELISA), antigen coated plate-ELISA (ACP-ELISA), Dot-ELISA and tissue print-ELISA as well as colloidal gold immunochromatographic strip are now wildly used to detect viruses in plants. In this chapter, we describe the DAS-ELISA and Dot-ELISA methods, and their applications in the detection of Tomato spotted wilt virus (TSWV) infection in plants. These two methods can be easily adapted for diagnosis of other plant viruses.

Detection of Cucumber green mottle mosaic virus (CGMMV) in Cucurbitaceous Crop Seeds by RT-PCR

Plant virus infections cause damages to all kinds of crops, and result in enormous economic and yield losses worldwide. Numerous reports indicated that many plant viruses are vertically transmitted through seeds, and cause severe disease symptoms on seedlings that again serve as secondary transmission sources in fields. Therefore, it is necessary to develop efficient methods to detect the seeds infected by viruses. Here, we describe a RT-PCR protocol for detection of Cucumber green mottle mosaic virus (CGMMV), a tobamovirus in cucurbitaceous crop seeds. This method can be easily adapted for diagnosis of other plant viruses such as Tomato brown rugose fruit virus in seeds.

A one-step reverse-transcription loop-mediated isothermal amplification assay optimized for the direct detection of cucumber green mottle mosaic virus in cucurbit seeds

Cucumber green mottle mosaic virus (CGMMV) is a seed-borne virus that causes significant economic losses in farms cultivating cucurbit plants. With the increase in global trade of cucurbit seeds, it is essential to develop a rapid, reliable, and convenient diagnostic method for the direct detection of CGMMV in these seeds for prevention and management of the disease. Here, we developed a one-step reverse-transcription loop-mediated isothermal amplification (RT-LAMP) assay for the direct detection of CGMMV in cucurbit seeds. To improve the efficiency of the one-step RT-LAMP assay, six primers were designed to target the most conserved regions of the gene encoding the movement protein of CGMMV. Our one-step RT-LAMP assay was optimized to improve specificity and sensitivity for CGMMV detection in individual seeds. A comparison of the detection sensitivity revealed that our one-step RT-LAMP assay was 100-fold more sensitive than the current reverse transcription-polymerase chain reaction assay used for CGMMV quarantine in Korea. Collectively, the one-step RT-LAMP assay developed in the present study is appropriate for the direct detection of CGMMV in individual cucurbit seeds.

Three Highly Sensitive and High-Throughput Serological Approaches for Detecting Dickeya dadantii in Sweet Potato

Sweet potato stem and root rot is an important bacterial disease and often causes serious economic losses to sweet potato. Development of rapid and sensitive detection methods is crucial for diagnosis and management of this disease in field. Here, we report the production of four hybridoma cell lines (25C4, 16C10, 9B1, and 9H10) using Dickeya dadantii strain FY1710 as an immunogen. Monoclonal antibodies (MAbs) produced by these four hybridoma cell lines were highly specific and sensitive for D. dadantii detection. Indirect enzyme-linked immunosorbent assay (indirect-ELISA) results showed that the four MAbs 25C4, 16C10, 9B1, and 9H10 could detect D. dadantii in suspensions diluted to 4.89 × 10⁴, 4.89 × 10⁴, 9.78 × 10⁴, and 9.78 × 10⁴ CFU/ml, respectively. Furthermore, all four MAbs can react strongly and specifically with all four D. dadantii strains used in this study, not with the other seven tested bacterial strains. Using these four MAbs, three different serological approaches, triple-antibody sandwich enzyme-linked immunosorbent assay (TAS-ELISA), dot-ELISA, and tissue-print-ELISA, were developed for detection of D. dadantii in crude extracts prepared from field-collected sweet potato plants. Among these three methods, TAS-ELISA and dot-ELISA were used to detect D. dadantii in suspensions diluted up to 1.23 × 10⁴ and 1.17 × 10⁶ CFU/ml, respectively, or in sweet potato crude extracts diluted up to 1:3,840 and 1:1,920 (wt/vol, grams per milliliter), respectively. Surprisingly, both TAS-ELISA and dot-ELISA serological approaches were more sensitive than the conventional PCR. Analyses using field-collected sweet potato samples showed that the newly developed TAS-ELISA, dot-ELISA, or tissue-print-ELISA were reliable in detecting D. dadantii in sweet potato tissues. Thus, the three serological approaches were highly valuable for diagnosis of stem and root rot in sweet potato production.

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.

Development and Application of an Immunocapture PCR Diagnostic Assay Based on the Monoclonal Antibody for the Detection of Shigella

Background:

Shigella is among the most important human pathogenic microorganisms, infecting both humans and nonhuman and causing clinically severe diarrhea. Shigella must be enriched before detection, which is time-consuming.

Objectives:

To develop a sensitive, rapid, and specific method for Shigella detection.

Materials and Methods:

Shigella was used as an antigen to generate monoclonal antibodies (mAbs). mAbs were screened via indirect enzyme-linked immunosorbent assay (ELISA) and western blot, and two mAbs were selected. The mAb A3 showed high affinity and specificity and was used to develop immune magnetic beads (IMBs) for Shigella enrichment. An immunocapture (IC)-PCR primer was designed from the ipaH gene, and IC-PCR was developed based on the IMBs and PCR.

Results:

This system shortened the Shigella detection time to 70 min. The sensitivity of the IC-PCR was 9 colony-forming units.mL^-1 in artificial milk. The accuracy of the IC-PCR was confirmed using 46 clinical samples collected from monkeys. The IC-PCR results were consistent with the serological and biochemical assays.

Conclusion:

The IC-PCR described herein accurately detected Shigella from milk samples, monkeys and can thus be used to complement classical detection methods.

Monoclonal Antibody-Based Serological Detection of Rice Stripe Mosaic Virus Infection in Rice Plants or Leafhoppers

Rice stripe mosaic virus (RSMV) is a rhabdovirus recently found in southern part of China and can cause severe reduction in rice production. To establish serological methods for RSMV epidemiological studies and to establish a control strategy for this virus, we first purified RSMV virions from infected rice plants and then used them as an immunogen to produce four RSMV-specific monoclonal antibodies (MAbs) (i.e.,1D4, 4A8, 8E4 and 11F11). With these MAbs, we have developed a highly specific and sensitive antigen-coated plate enzyme-linked immunosorbent assay (ACP-ELISA), a Dot-ELISA and a Tissue print-ELISA for rapid detections of RSMV infection in rice plants or in leafhoppers. Our results showed that RSMV can be readily detected in RSMV-infected rice plant tissue crude extracts diluted at 1:20,971,520 (w/v, g/mL) through ACP-ELISA or diluted at 1:327,680 (w/v, g/mL) through Dot-ELISA. Both ACP-ELISA and Dot-ELISA can also be used to detect RSMV infection in individual RSMV viruliferous leafhopper (Recilia dorsalis) homogenate diluted at 1:307,200 and 1:163,840 (individual leafhopper/µL), respectively. Detection of RSMV infection in field-collected rice samples or in RSMV viruliferous leafhoppers indicated that the three serological methods can produce same results with that produced by RT-PCR (19 of the 33 rice samples and 5 of the 16 leafhoppers were RSMV-positive). We consider that the four MAbs produced in this study are very specific and sensitive, and the three new serological methods are very useful for detections of RSMV infection in rice plants or in leafhoppers and the establishment of the disease control strategies.

Development of a multiplex RT-PCR assay for simultaneous detection of Cucumber green mottle mosaic virus and Acidovorax citrulli in watermelon

Watermelon (Citrullus lanatus Thunb.) is considered as a popular and nutritious fruit crop worldwide. Watermelon blood flesh disease caused by Cucumber green mottle mosaic virus (CGMMV) and bacterial fruit blotch caused by Acidovorax citrulli, are two major quarantine diseases of watermelon and result in considerable losses to global watermelon production. In this study, a multiplex reverse-transcription polymerase chain reaction (RT-PCR) method was developed for simultaneous detection of CGMMV and A. citrulli in both watermelon leaves and seeds. Two pairs of specific primers were designed based on the conserved sequences of the genomic RNA of CGMMV and the internal transcribed spacer of A. citrulli, respectively. Transcriptional elongation factor-1α from watermelon was added as an internal reference gene to prevent false negatives. No cross-reactivity was detected with other viral or bacterial pathogens infecting watermelon. Moreover, the multiplex RT-PCR showed high sensitivity and could simultaneously detect CGMMV and A. citrulli as little as 10² copies of plasmid DNA. This method was successfully applied to test field-collected watermelon leaves and stored seeds of cucurbitaceous crops. These results suggested that the developed multiplex RT-PCR technique is a rapid, efficient, and sensitive method for simultaneous detection of CGMMV and A. citrulli, providing technical support for monitoring, predicting, and preventing these two quarantine diseases. To our knowledge, this is the first report on simultaneous detection of a virus and a bacterium by multiplex RT-PCR in watermelon.

Rapid detection of Cucumber green mottle mosaic virus in watermelon through a recombinase polymerase amplification assay

Cucumber green mottle mosaic virus (CGMMV), a member of the genus Tobamovirus, is an important quarantine plant virus worldwide, and often causes seriously damages to productions of watermelon, melon, cucumber and other cucurbit crops. In this study, we developed a novel isothermal recombinase polymerase amplification (RPA) technique for detection of CGMMV in watermelon samples. A pair of CGMMV specific RPA primers was prepared based on the conserved CGMMV coat protein gene sequences. The result showed that this RPA detection method can be performed at 38 °C and completed in about 30 min, and there was no cross-reactivity with other common cucurbit viruses. Sensitivity assay showed that this RPA method was more sensitive compared with the regular RT-PCR. Using field-collected watermelon tissue samples, we have demonstrated that this newly developed method is rapid, easy to use and reliable for CGMMV detection, especially in resource-limited laboratories or on-site facilities.

Visual Detection of Cucumber Green Mottle Mosaic Virus Based on Terminal Deoxynucleotidyl Transferase Coupled with DNAzymes Amplification

A simple, rapid, and sensitive visual detection method for observing cucumber green mottle mosaic virus was reported based on the template-independent polymerization activity of terminal deoxynucleotidyl transferase (TdT), coupled with the cascade amplification of Mg²⁺-dependent DNAzyme and hemin/G-quadruplex DNAzyme. Briefly, the hybridized dsDNA of T1/P1 was cut into two parts at its position of 5′-AA↓CG↑TT-3′ by the restricted enzyme AcII. The longer, newborn fragment originating from P1 was tailed at its 3’-end by oligo dG, and an intact enzymatic sequence of Mg²⁺-dependent DNAzyme was generated. The substrate sequence in the loop segment of the hairpin probe (HP) hybridized with the newborn enzymatic sequence and was cleaved into two parts in the presence of Mg²⁺. The locked G-quadruplex sequence in the stem segment of the HP was released, which catalyzed the oxidation of ABTS^2- in the presence of H₂O₂, and the resulting solution turned green. A correlation between the absorbance and concentration of T1 was obtained in a range from 0.1 pM to 2 nM, with a detection limit of 0.1 pM. In addition to promoting a lower detection limit and shorter monitoring time, this method also demonstrated an excellent selectivity to single or double nucleotide changes. Therefore, the designed strategy provided a rapid and efficient platform for viral inspection and plant protection.

Monoclonal antibody-based serological assays for detection of Potato virus S in potato plants

Potato virus S (PVS) often causes significant losses in potato production in potato-growing countries. In this study, the ordinary strain of PVS (PVS^O) was purified from PVS-infected potato plants and used as the immunogen to produce hybridomas secreting monoclonal antibodies (MAbs). Five highly specific and sensitive murine MAbs (1A3, 16C10, 18A9, 20B12, and 22H4) against PVS were prepared using conventional hybridoma technology. Using these MAbs, tissue print-enzyme-linked immunosorbent assay (ELISA), dot-ELISA, and double-antibody sandwich (DAS)-ELISA were developed for sensitive and specific detection of PVS infection in potato plants. The results of sensitivity assays revealed that PVS could be reliably detected in PVS-infected leaf crude extracts diluted at 1:10 240 and 1:163 840 (w/v, g/ml) in phosphate buffer saline (PBS) by dot-ELISA and DAS-ELISA, respectively. Twenty-two samples collected from potato fields in Yunnan Province, China were tested for PVS infection using the serological assays we had developed, and 14 of them were found to be positive. This indicates that PVS is now prevalent in potato fields in Yunnan Province.

Monoclonal Antibody-Based Serological Detection Methods for Wheat Dwarf Virus

Wheat dwarf disease caused by wheat dwarf virus (WDV) is currently present in wheat growing regions in China and causes serious losses in wheat yield. To develop reliable and effective serological detection methods for WDV, the coat protein (CP) gene of WDV was cloned and expressed in Escherichia coli. The purified recombinant CP protein was immunized to BALB/c mice, and four hybridoma cell lines (i.e. 18G10, 9G4, 23F4 and 22A10) secreting anti-WDV monoclonal antibodies (MAbs) were obtained through the hybridoma technique. Using the prepared MAbs, an antigen-coated-plate enzyme-linked immunosorbent assay (ACP-ELISA) and a dot-ELISA were established for detecting WDV in wheat samples. The most sensitive ACP-ELISA based on MAb 23F4 or 22A10 was able to detect WDV in 1:163,840 (w/v, g/mL) diluted WDV-infected wheat plant crude extracts. The dot-ELISA based on MAb 23F4 was the most sensitive and able to detect the virus in 1:5,120 (w/v, g/mL) diluted wheat plant crude extracts. A total of 128 wheat samples were collected from wheat growing regions in the Shaanxi and Qinghai provinces, China, and were screened for the presence of WDV using two developed serological assays. Results from the survey showed that approximately 62% of the samples were infected with WDV. PCR followed by DNA sequencing and sequence alignment validated the results from the two serological assays. Therefore, we consider that these two serological detection methods can be significantly useful for the control of WDV in China.

Raman Hyperspectral Imaging for Detection of Watermelon Seeds Infected with Acidovorax citrulli

The bacterial infection of seeds is one of the most important quality factors affecting yield. Conventional detection methods for bacteria-infected seeds, such as biological, serological, and molecular tests, are not feasible since they require expensive equipment, and furthermore, the testing processes are also time-consuming. In this study, we use the Raman hyperspectral imaging technique to distinguish bacteria-infected seeds from healthy seeds as a rapid, accurate, and non-destructive detection tool. We utilize Raman hyperspectral imaging data in the spectral range of 400–1800 cm⁻¹ to determine the optimal band-ratio for the discrimination of watermelon seeds infected by the bacteria Acidovorax citrulli using ANOVA. Two bands at 1076.8 cm⁻¹ and 437 cm⁻¹ are selected as the optimal Raman peaks for the detection of bacteria-infected seeds. The results demonstrate that the Raman hyperspectral imaging technique has a good potential for the detection of bacteria-infected watermelon seeds and that it could form a suitable alternative to conventional methods.

Non-destructive evaluation of bacteria-infected watermelon seeds using visible/near-infrared hyperspectral imaging

BACKGROUND

There is a need to minimize economic damage by sorting infected seeds from healthy seeds before seeding. However, current methods of detecting infected seeds, such as seedling grow-out, enzyme-linked immunosorbent assays, the polymerase chain reaction (PCR) and the real-time PCR have a critical drawbacks in that they are time-consuming, labor-intensive and destructive procedures. The present study aimed to evaluate the potential of visible/near-infrared (Vis/NIR) hyperspectral imaging system for detecting bacteria-infected watermelon seeds.

RESULTS

A hyperspectral Vis/NIR reflectance imaging system (spectral region of 400-1000 nm) was constructed to obtain hyperspectral reflectance images for 336 bacteria-infected watermelon seeds, which were then subjected to partial least square discriminant analysis (PLS-DA) and a least-squares support vector machine (LS-SVM) to classify bacteria-infected watermelon seeds from healthy watermelon seeds. The developed system detected bacteria-infected watermelon seeds with an accuracy > 90% (PLS-DA: 91.7%, LS-SVM: 90.5%), suggesting that the Vis/NIR hyperspectral imaging system is effective for quarantining bacteria-infected watermelon seeds.

CONCLUSION

The results of the present study show that it is possible to use the Vis/NIR hyperspectral imaging system for detecting bacteria-infected watermelon seeds. © 2016 Society of Chemical Industry.

Production of mono- and polyclonal antibodies to Citrus leprosis virus C2 and their application in triple antibody sandwich ELISA and immunocapture RT-PCR diagnostic assays

The newly discovered Citrus leprosis virus cytoplasmic type 2 (CiLV-C2) is one of the causal virus of citrus leprosis disease complex; which leads to substantial loss of citrus production in the states of Meta and Casanare of Colombia. Specific and sensitive detection methods are needed to monitor the dissemination of CiLV-C2 in Colombia, and to prevent introduction of CiLV-C2 to other citrus growing countries. Toward this end, putative coat protein gene (CPG) of CiLV-C2 was amplified from CiLV-C2 infected citrus tissues. The CPG was cloned, expressed and purified a recombinant coat protein of ∼31kDa which used to generate monoclonal antibodies and polyclonal antisera. Four monoclonal antibodies and two polyclonal antisera were selected as being specific following Western blotting. The monoclonal antibody MAb E5 and polyclonal antiserum PAb UF715 were selected testing with an extract of CiLV-C2 infected leaves using triple antibody sandwich enzyme-linked immunosorbent assay (TAS-ELISA). In addition, an immunocapture RT-PCR was standardized using MAb E5 for specific and sensitive detection of CiLV-C2. The standardized TAS-ELISA and IC-RT-PCR were able to detect CiLV-C2 in the extracts of symptomatic citrus leprosis tissues up to the dilutions of 1:160 and 1:2580, respectively. Result demonstrated that CiLV-C2 is present in citrus orchards in Meta and Casanare citrus growing areas of Colombia. TAS-ELISA could be used for routine detection of CiLV-C2, epidemiological studies, and for border inspections for quarantine purposes. IC-RT-PCR could be valuable for CiLV-C2 validation and viral genome analysis.

Monoclonal antibody-based serological methods for detecting Citrus tristeza virus in citrus groves

Citrus tristeza virus (CTV) is one of the most economically important citrus viruses and harms the citrus industry worldwide. To develop reliable and effective serological detection assays of CTV, the major capsid protein (CP) gene of CTV was expressed in Escherichia coli BL21 (DE3) using the expression vector pET-28a and purified through Ni+-NTA affinity chromatography. The recombinant protein was used to immunize BALB/c mice. Four hybridoma cell lines (14B10, 14H11, 20D5, and 20G12) secreting monoclonal antibodies (MAbs) against CTV were obtained through conventional hybridoma technology. The titers of MAb-containing ascitic fluids secreted by the four hybridoma lines ranged from 10(-6) to 10(-7) in indirect enzyme-linked immunosorbent assay (ELISA). Western blots showed that all four MAbs could specifically react with CTV CP. Using the prepared MAbs, dot-ELISA, Tissue print-ELISA, and triple antibody sandwich (TAS)-ELISA were developed to detect CTV in tree nurseries and epidemiological studies. The developed dot-ELISA and TAS-ELISA methods could detect CTV in crude extracts of infected citrus leaves with dilutions of 1:2560 and 1:10, 240 (w/v, g/mL), respectively. Tissue print-ELISA was particularly useful for large-scale field sample detection, mainly owing to its simplicity and lack of sample preparation requirements. The field survey revealed that CTV is prevalent on citrus trees in the Chongqing Municipality, Jiangxi Province, and Zhejiang Province of China. The coincidence rate of serological and RT-PCR test results reached more than 99.5%. The prepared MAbs against CTV and established sensitive and specific serological assays have a significant role in the detection and prevention and control of CTV in our country.

Optimized Expression, Purification of Herpes B Virus gD Protein in Escherichia coli, and Production of Its Monoclonal Antibodies

BACKGROUND

Herpes B virus (BV) is a zoonotic disease caused by double-stranded enveloped DNA virus with cercopithecidae as its natural host. The mortality rate of infected people could be up to 70% with fatal encephalitis and encephalomyelitis. Up to now, there are no effective treatments for BV infection. Among the various proteins encoded by monkey B virus, gD, a conserved structural protein, harbors important application value for serological diagnosis of frequent variations of the monkey B virus.

OBJECTIVES

This study aimed to expressed the gD protein of BV in Escherichia coli by a recombinant vector, and prepare specific monoclonal antibodies against gD of BV to pave the way for effective and quick diagnosis reagent research.

MATERIALS AND METHODS

The gD gene of BV was optimized by OptimWiz to improve codon usage bias and synthesis, and the recombinant plasmid, pET32a/gD, was constructed and expressed in E. coli Rosetta (DE3). The expressed fusion protein, His-gD, was purified and the BALB/c mice were immunized by this protein. Spleen cells from the immunized mice and SP2/0 myeloma cells were fused together, and the monoclonal cell strains were obtained by indirect enzyme-linked immunosorbent assay (ELISA) screening, followed by preparation of monoclonal antibody ascetic fluid.

RESULTS

The optimized gD protein was highly expressed in E. coli and successfully purified. Five monoclonal antibodies (mAbs) against BV were obtained and named as 4E3, 3F8, 3E7, 1H3 and 4B6, and with ascetic fluid titers of 2 × 10(6), 2 × 10(5), 2 × 10(5), 2 × 10(3) and 2 × 10(2), respectively. The 1H3 and 4E3 belonged to the IgG2b subclass, while 3E7, 3F8 and 4B6 belonged to the IgG1 subclass.

CONCLUSIONS

The cell lines obtained in this work secreted potent, stable and specific anti-BV mAbs, which were suitable for the development of herpes B virus diagnosis reagents.

RepA Protein Encoded by Oat dwarf virus Elicits a Temperature-Sensitive Hypersensitive Response-Type Cell Death That Involves Jasmonic Acid-Dependent Signaling

The hypersensitive response (HR) is a component of disease resistance that is often induced by pathogen infection, but essentially no information is available for members of the destructive mastreviruses. We have investigated an HR-type response elicited in Nicotiana species by Oat dwarf virus (ODV) and have found that expression of the ODV RepA protein but not other ODV-encoded proteins elicits the HR-type cell death associated with a burst of H2O2. Deletion mutagenesis indicates that the first nine amino acids (aa) at the N terminus of RepA and the two regions located between aa residues 173 and 195 and between aa residues 241 and 260 near the C terminus are essential for HR-type cell-death elicitation. Confocal and electron microscopy showed that the RepA protein is localized in the nuclei of plant cells and might contain bipartite nuclear localization signals. The HR-like lesions mediated by RepA were inhibited by temperatures above 30°C and involvement of jasmonic acid (JA) in HR was identified by gain- and loss-of-function experiments. To our knowledge, this is the first report of an elicitor of HR-type cell death from mastreviruses.

Development of monoclonal antibodies and serological assays specific for Barley yellow dwarf virus GAV strain

BACKGROUND

Barley yellow dwarf virus (BYDV) is one of the most devastating plant viruses and belongs to a ubiquitous plant virus group. In China, four BYDV strains (GPV, GAV, PAV and RMV) have been identified based on their specific aphid vectors and serological properties. Among the four identified strains, the GAV is the most common BYDV strain in China. To diagnose, forecast of BYDV GAV, two reliable serological assays for BYDV GAV detection were established.

METHODS

We purified virion from a confirmed BYDV GAV source and used it as the immunogen to produce monoclonal antibodies against the virus. Using the hybridoma technology, three highly specific murine monoclonal antibodies were produced and two serological assays [antigen-coated-plate enzyme-linked immunosorbent assay (ACP-ELISA) and dot enzyme-linked immunosorbent assay (dot-ELISA)] were established for the BYDV GAV detection.

RESULTS

All three monoclonal antibodies reacted strongly and specifically with the BYDV GAV strain in crude leaf extracts. Titers of the monoclonal antibodies in ascitic fluids were up to 10(-7) by indirect-ELISA. These three monoclonal antibodies (18A1, 18A9 and 12A11) all belonged to the isotype IgG1, kappa light chain. The highest dilution points for the three antibodies during the ACP-ELISA using infected crude leaf extracts were 1:163,840, 1:81,920 and 1:81,920 (w/v, g · mL(-1)), respectively. Result of dot-ELISA showed a successful detection of BYDV GAV strain in 1:5,120 (w/v, g · mL(-1)) diluted wheat leaf crude extracts. Analysis of 22 field wheat leaf samples and 33 aphid samples from the Shaanxi Province in China, using the two newly developed assays confirmed the presence of BYDV GAV in about 80 % of the wheat samples and 18 % of the aphid samples.

CONCLUSIONS

All three monoclonal antibodies are highly sensitive and specific to the BYDV GAV. The two newly developed serological assays are simple and effective. These two assays, particularly the dot-ELISA, are useful for high throughput detection of BYDV GAV in host plants and aphid vectors.

Monoclonal antibody-based serological methods for maize chlorotic mottle virus detection in China

Maize chlorotic mottle virus (MCMV) infects maize plants and causes significant losses in corn production worldwide. In this study, purified MCMV particles were used as the immunogen to produce monoclonal antibodies (MAbs) and polyclonal antibodies (PAbs). Four murine MAbs (4B8, 8C11, 6F4, and 9G1) against MCMV were obtained through the hybridoma technology. The triple antibody sandwich enzyme-linked immunosorbent assay (TAS-ELISA), dot-immunobinding assay (DIBA), and immunocapture reverse transcription-polymerase chain reaction (IC-RT-PCR) using the MAb 4B8 were then developed for sensitive, specific, and rapid detection of MCMV in fields. MCMV could be detected in infected leaf crude extracts at dilutions of 1:327680, 1:64000, and 1:3276800 (w/v, g/ml) by TAS-ELISA, DIBA, and IC-RT-PCR, respectively. One hundred and sixty-one maize field samples showing virus-like symptoms and sixty-nine symptomless maize field samples from ten different provinces of China were collected and screened for the presence of MCMV using the established serological methods. A phylogenetic tree was constructed based on the full length CP genes and Chinese MCMV isolates formed one branch with Thailand isolates. The detection results demonstrated that MCMV is one of most prevalent viruses infecting maize in the Yunnan and Sichuan provinces of China.

Highly sensitive serological methods for detecting tomato yellow leaf curl virus in tomato plants and whiteflies

BACKGROUND

Tomato yellow leaf curl virus (TYLCV) is a member of the genus Begomovirus in the family Geminiviridae, which causes severe losses in tomato production in tropic and subtropic regions.

METHODS

The purified TYLCV virions were used as the immunogen to produce monoclonal antibodies (MAbs) using the hybridoma technology. MAb-based dot enzyme-linked immunosorbent assay (dot-ELISA) and direct tissue blot immunoassay (DTBIA) were developed for sensitive, simple, and rapid detection of TYLCV in field tomato and whitefly (Bemisia tabaci) samples collected from TYLCV prevalent provinces in China.

RESULTS

Using the hybridoma technology, six murine MAbs (1C4, 8D10, 6E3, 2F2, 3F4 and 4G3) against TYLCV were prepared. Using the MAb 1C4, dot-ELISA and DTBIA were then established for detecting TYLCV in field tomato and whitefly samples collected from TYLCV prevalent provinces in China. The dot-ELISA could detect TYLCV in infected tissue crude extract diluted at 1:5,120 (w/v, g mL-1), and in viruliferous whitefly homogenate diluted at 1:128 (individual whitefly/μL), respectively. Field tomato samples (n=487) and whitefly samples (n=110) from TYLCV prevalent districts in China were screened for the presence of TYLCV using the two developed methods, and the results were further confirmed by PCR and nucleotide sequencing. The survey revealed that TYLCV is widespread on tomato plants in Zhejiang, Shandong and Henan provinces in China.

CONCLUSIONS

The developed dot-ELISA is very suitable for the routine detection of TYLCV in field tomato and whitefly samples, and the DTBIA is more suitable for the routine detection of TYLCV in large-scale tomato plant samples collected from TYLCV prevalent areas.

Development and use of three monoclonal antibodies for the detection of rice black-streaked dwarf virus in field plants and planthopper vectors

BACKGROUND

Rice black-streaked dwarf virus (RBSDV) causes great losses in rice, maize and wheat production in Asian countries. The use of serological methods for RBSDV detection depends on the availability of antibodies. In this study, three highly sensitive and specific murine monoclonal antibodies (MAbs) against RBSDV antigens were produced using crude extracts from tumors of RBSDV-infected maize as the immunogen, and two serological assays, antigen-coated-plate enzyme-linked immunosorbent assay (ACP-ELISA) and dot enzyme-linked immunosorbent assay (dot-ELISA) were developed for RBSDV detection.

RESULTS

All three MAbs reacted strongly and specifically with the crude extracts from RBSDV-infected plant and planthopper tissues. The detection endpoints of three MAbs (12E10, 18F10 and 5G5) in ACP-ELISA were respectively 1:40,960, 1:40,960, 1:81,920 (w/v, g mL-1) with the crude extract of infected maize, 1:10,240, 1:20,480, 1:20,480 (w/v, g mL-1) with the crude extract of infected rice, 1:5,120, 1:10,240, 1:10,240 (w/v, g mL-1) with the crude extract of infected wheat, 1:9,600, 1:9,600, 19,200 (individual planthopper/μL) with the crude extract of infected planthopper. The newly developed ACP-ELISA could detect the virus in the infected maize, wheat, rice tissue crude extracts diluted at 1:81,920, 1:20,480, 1:10,240 (w/v, g mL-1), respectively, and in individual viruliferous planthopper extract diluted at 1:19200 (individual planthopper/μL). The dot-ELISA was proved to detect the virus in the infected maize, wheat and rice tissue crude extracts diluted at 1:320 (w/v, g mL-1), and in individual viruliferous planthopper extract diluted at 1:1,600 (individual planthopper/μL), respectively. Field plants (915) and planthopper samples (594) from five provinces of China were screened for the presence of RBSDV using the two developed serological assays. The results indicated that 338 of the 915 plant samples and 19 of the 594 planthopper samples were infected by RBSDV.

CONCLUSIONS

The newly developed ACP-ELISA and dot-ELISA were highly sensitive and specific to detect RBSDV in field plant and planthopper samples. The field survey demonstrated that RBSDV is widespread in rice, maize and wheat crops in Jiangsu, Zhejiang, Shandong provinces of China.