Sodium sulphite enhances RNA isolation and sensitivity of Cucumber mosaic virus detection by RT-PCR in black pepper

Specific detection of Malus- and Pyrus-infecting viroids by real-time reverse-transcription quantitative PCR assays

Viroids present a number of issues for their detection and diagnosis because of the absence of symptom expression in many hosts and their low titers in infected plants. However, quarantine programs rely on symptom observations and routine diagnostic testing to reduce the risk of spreading viroid-infected materials to situations where they might affect crop health and production. Sensitive, accurate, and specific assays for viroid detection from both asymptomatic and symptomatic hosts are necessary for managing viroids in post-entry quarantine and certification schemes. The aim of this study was to develop and optimize superior assays based on the reverse-transcription quantitative polymerase chain reaction (RT-qPCR) for the specific detection of apple hammerhead viroid (AHVd), apple scar skin viroid (ASSVd) and pear blister canker viroid (PBCVd). The real-time RT-qPCR assays thus developed detected a greater range of viroid isolates and with greater sensitivity than the current endpoint RT-PCR assays, down to 10¹ copies per reaction without any amplification of the non-target viroid or virus sequences tested.

Symptoms of piper yellow mottle virus in black pepper as influenced by temperature and relative humidity

Masking of symptoms in winter and their re-appearance in black pepper (Piper nigrum L.) infected with piper yellow mottle virus (PYMoV) in summer is common, especially on new flushes that appear after pre-monsoon showers. Plants of nineteen cultivars of black pepper infected with PYMoV but without any visible symptoms were grown in a polyhouse under natural conditions and in a greenhouse under controlled conditions from January 2019 to January 2020. The number of plants expressing symptoms in the polyhouse increased gradually from 1% during the 3rd standard meteorological week (SMW) (16 January) to 41% during the 21st SMW (22 May), when the afternoon temperature was 30-40 °C and relative humidity (RH) was 75-93%, but began declining thereafter until the 53rd SMW (1 January), when the afternoon temperature was 30-36 °C and RH was 65-86%. The proportion of plants expressing symptoms varied with the cultivar. However, in the greenhouse, in which temperature and RH were maintained at approximately 26 °C and 80%, respectively, not more than 2% of the plants expressed symptoms. The number of symptomatic plants was positively correlated to maximum temperature (T Max) and maximum relative humidity (RH Max) in the afternoon. Based on this observation, a model for predicting the percentage of symptomatic plants was developed using stepwise regression analysis. Plants at the two sites did not differ significantly in the concentration of virus (virus titre) but differed significantly in the content of total carbohydrates, lipid peroxidase, and phenols.

Supplementary Information

The online version contains supplementary material available at 10.1007/s13337-021-00686-3.

Development of RT-qPCR assays for the detection and quantification of three carlaviruses infecting hop

American hop latent virus (AHLV), hop latent virus (HLV) and hop mosaic virus (HMV) infect members of the Humulus genus worldwide, but very little is known of the biology and etiology of these viruses. A better understanding of these viruses from the molecular level to their economic impact relies on efficient diagnostic assays. Therefore, in this study we developed reverse transcription quantitative polymerase chain reaction (RT-qPCR) assays for the detection of AHLV, HLV, and HMV through an alignment of representative sequences from the National Center for Biotechnology Information (NCBI) database. These assays demonstrated unambiguously their high sensitivity by detecting the respective targets from as low as 10² copies of transcripts per reaction without any amplification from non-targets.

Development of reverse transcription loop-mediated isothermal amplification (RT-LAMP) and reverse transcription recombinase polymerase amplification (RT-RPA) assays for the detection of two novel viruses infecting ginger

Our recent studies have shown the association of two novel viruses namely, ginger chlorotic fleck-associated virus 1 (GCFaV-1) and ginger chlorotic fleck-associated virus 2 (GCFaV-2) with chlorotic fleck disease of ginger. As ginger is propagated through vegetative means, the development of diagnostics would aid in the identification of virus-free plants. In the present study, reverse transcription loop-mediated isothermal amplification (RT-LAMP) and reverse transcription recombinase polymerase amplification (RT-RPA) assays were developed and validated for the quick detection of GCFaV-1 and GCFaV-2. The detection limits of viruses by these assays, when compared with conventional and real-time RT-PCR, showed that RT-LAMP was up to 1000 times more sensitive than conventional RT-PCR and one-hundredth that of real-time RT-PCR for both the viruses. The detection limit of RT-RPA for GCFaV-1 was up to 100 times more than that of RT-PCR and one-thousandth that of real-time RT-PCR. On the other hand, for detecting GCFaV-2, RT-RPA was found up to 1000 times more sensitive than conventional RT-PCR and one hundredth that of real-time RT-PCR. Based on the cost-effectiveness and duration, RT-LAMP and RT-RPA assays can be suggested for the rapid detection of both viruses.

Recombinase polymerase amplification assay for the detection of piper yellow mottle virus infecting black pepper

Recombinase polymerase amplification (RPA) is a quick, specific, sensitive molecular tool carried out at a constant temperature for pathogen detection. In the present study, RPA and reverse transcription (RT) RPA assays were optimized for the detection of piper yellow mottle virus (PYMoV) infecting black pepper. Out of the eight primer pairs targeted to amplify open reading frames (ORFs) 2 and 3 of the virus, the primer pair targeted to ORF2 gave specific amplification only with DNA isolated from infected plant but not with healthy plant. A magnesium acetate concentration of 18 mM, 40 min of incubation time and a temperature of 37-42 °C was found optimum for detection of the virus in RPA assay. Comparison of sensitivity of detection revealed that RPA could detect the virus up to 10^-5 dilution of the total DNA while PCR could detect the virus up to 10^-4 dilution indicating that RPA is 10 times more sensitive than PCR. RPA was further simplified using crude extract as template which could detect the virus up to 10^-3 dilution. RT-RPA was optimized for the detection of PYMoV using total RNA isolated from infected plants as the template. Both RT-RPA and RPA assays were validated using field samples of black pepper representing different varieties and geographical regions by using CTAB isolated DNA, crude DNA extract and cDNA. Our study showed that RPA and RT-RPA can be successfully adopted as a substitute to PCR for detection of PYMoV infecting black pepper.

Detection and quantification of four viruses in Prunus pollen: Implications for biosecurity

Pollen transmitted viruses require accurate detection and identification to minimize the risk of spread through the global import and export of pollen. Therefore in this study we developed RT-qPCR assays for the detection of Cherry leaf roll virus (CLRV), Prune dwarf virus (PDV), Prunus necrotic ringspot virus (PNRSV), and Cherry virus A (CVA), four viruses that infect pollen of Prunus species. Assays were designed against alignments of extant sequences, optimized, and specificity was tested against known positive, negative, and non-target controls. An examination of assay sensitivity showed that detection of virus at concentrations as low as 10¹ copies was possible, although 10² copies was more consistent. Furthermore, comparison against extant assays showed that in both pollen and plant samples, the newly developed RT-qPCR assays were more sensitive and could detect a greater range of isolates than extant endpoint RT-PCR and ELISA assays. Use of updated assays will improve biosecurity protocols as well as the study of viruses infecting pollen.

Complete genome sequencing of cucumber mosaic virus from black pepper revealed rare deletion in the methyltransferase domain of 1a gene

The complete genome of cucumber mosaic virus (CMV) from black pepper was sequenced and compared with CMV isolates from subgroups I and II reported worldwide. Percent identity and phylogenetic analyses clearly indicated that the CMV isolate from black pepper (BP) belongs to subgroup IB. Sequence analyses also showed the presence of a rare deletion of nine nucleotides in the putative methyltransferase domain of 1a gene which was observed only in two more isolates of CMV among one hundred 1a gene sequences of CMV for which sequence information is available in the database. Interestingly this deletion is not present in the black pepper isolate of CMV from China (WN1) and from Indian long pepper that is closely related to black pepper. Percent identity analyses showed that the 3'untranslated region (UTR) of the three RNAs of the BP isolate were conserved with 91% identity whereas the 5'UTR of three RNAs showed 52-80% identity. The level of gene conservation among the subgroups was highest in coat protein and lowest in 2b. The values of nucleotide diversity studies were further consistent with the above data. The ratio of non-synonymous to the synonymous substitution of the five genes of three RNAs was in the order 1a > 2a > 2b > 3a > 3b and less than one for all the genes, indicating purifying selection. These clearly reflect that the protein encoded by RNA1 is highly tolerant to amino acid changes followed by that of RNA2 and, RNA3 is the least tolerant correlating to its functional importance.

High-quality RNA extraction from small cardamom tissues rich in polysaccharides and polyphenols

Due to the presence of a diverse array of metabolites, no standard method of RNA isolation is available for plants. We noted that polysaccharide and polyphenol contents of cardamom tissues critically hinder the RNA extraction procedure. Hence, we attempted several methods for obtaining intact mRNA and small RNA from various cardamom tissues. It was found that protocols involving a combination of commercial kits and conventional CTAB (cetyl trimethylammonium bromide) methods yielded RNA with good purity, higher yield, and good integrity. The total RNA isolated through this approach was found to be amenable for transcriptome and small RNA analysis through next-generation sequencing platforms.

Detection of Cardamom mosaic virus and Banana bract mosaic virus in cardamom using SYBR Green based reverse transcription-quantitative PCR

Cardamom being perennial, propagated vegetatively, detecting viruses in planting material is important to check the spread of viruses through infected material. Thus development of effective and sensitive assay for detection of viruses is need of the time. In this view, assay for the detection of Cardamom mosaic virus (CdMV) and Banana bract mosaic virus (BBrMV), infecting cardamom was developed using SYBR Green one step reverse transcription-quantitative PCR (RT-qPCR). The RT-qPCR assay amplified all isolates of CdMV and BBrMV tested but no amplification was obtained with RNA of healthy plants. Recombinant plasmids carrying target virus regions corresponding to both viruses were quantified, serially diluted and used as standards in qPCR to develop standard curve to enable quantification. When tenfold serial dilutions of the total RNAs from infected plants were tested through RT-qPCR, the detection limit of the assay was estimated to be 16 copies for CdMV and 10 copies for BBrMV, which was approximately 1,000-fold higher than the conventional RT-PCR. The RT-qPCR assay was validated by testing field samples collected from different cardamom growing regions of India. This is the first report of RT-qPCR assay for the detection of CdMV and BBrMV in cardamom.

Rapid detection of Piper yellow mottle virus and Cucumber mosaic virus infecting black pepper (Piper nigrum) by loop-mediated isothermal amplification (LAMP)

The loop-mediated isothermal amplification (LAMP) assay for Piper yellow mottle virus and the reverse transcription (RT) LAMP assay for Cucumber mosaic virus each consisted of a set of five primers designed against the conserved sequences in the viral genome. Both RNA and DNA isolated from black pepper were used as a template for the assay. The results were assessed visually by checking turbidity, green fluorescence and pellet formation in the reaction tube and also by gel electrophoresis. The assay successfully detected both viruses in infected plants whereas no cross-reactions were recorded with healthy plants. Optimum conditions for successful amplification were determined in terms of the concentrations of magnesium sulphate and betaine, temperature, and duration. The detection limit for both LAMP and RT-LAMP was up to 100 times that for conventional PCR and up to one-hundredth of that for real-time PCR. The optimal conditions arrived at were validated by testing field samples of infected vines of three species from different regions.

Removal of real-time reverse transcription polymerase chain reaction (RT-PCR) inhibitors associated with cloacal swab samples and tissues for improved diagnosis of Avian influenza virus by RT-PCR

Real-time reverse transcription polymerase chain reaction (real-time RT-PCR) is routinely used for the rapid detection of Avian influenza virus (AIV) in clinical samples, but inhibitory substances present in some clinical specimens can reduce or block PCR amplification. Most commercial RNA extraction kits have limited capacity to remove inhibitors from clinical samples, but using a modified commercial protocol (Ambion MagMAX, Applied Biosystems, Foster City, CA) with an added high-salt wash of 2 M NaCl and 2 mM ethylenediamine tetra-acetic acid was shown to improve the ability of the kit to remove inhibitors from cloacal swabs and some tissues. Real-time RT-PCR was carried out in the presence of an internal positive control to detect inhibitors present in the purified RNA. Cloacal swabs from wild birds were analyzed by real-time RT-PCR comparing RNA extracted with the standard (MagMAX-S) and modified (MagMAX-M) protocols. Using the standard protocol on 2,668 samples, 18.4% of the samples had evidence of inhibitor(s) in the samples, but the modified protocol removed inhibitors from all but 21 (4.8%) of the problem samples. The modified protocol was also tested for RNA extraction from tissues using a TRIzol-MagMAX-M hybrid protocol. Tissues from chickens and ducks experimentally infected with high-pathogenicity Asian H5N1 AIV were analyzed by real-time RT-PCR, and the limit of detection of the virus was improved by 0.5-3.0 threshold cycle units with the RNA extracted by the MagMAX-M protocol. The MagMAX-M protocol reported in the present study can be useful in extracting high-quality RNA for accurate detection of AIV from cloacal swabs and tissues by real-time RT-PCR.

Methods for isolation of total RNA to recover miRNAs and other small RNAs from diverse species

For the experimental analysis of miRNAs and other small RNAs in the 20-25 nucleotide (nt) size range, the first and most important step is the isolation of high-quality total RNA. Because RNA degradation products can mask or dilute the presence of true miRNAs, it is important when choosing a method that it efficiently extracts RNA from tissues in a manner that prevents degradation of RNA of both high and low molecular weight. In addition, the presence of polyphenols, polysaccharides, and secondary metabolites may render nucleic acids insoluble, and hinder the recovery of the miRNAs. Finally, and most importantly, the method chosen must be capable of retaining the small RNA component. In this chapter, we will present a set of total RNA isolation methods that can be used to maximize the recovery of high-quality RNA to be used in miRNA analysis for a large number of plant species and tissue types.