Amplification-free detection of grapevine viruses using an oligonucleotide microarray

High-throughput detection of a large set of viruses and viroids of pome and stone fruit trees by multiplex PCR-based amplicon sequencing

A comprehensive diagnostic method of known plant viruses and viroids is necessary to provide an accurate phytosanitary status of fruit trees. However, most widely used detection methods have a small limit on either the number of targeted viruses/viroids or the number of samples to be evaluated at a time, hampering the ability to rapidly scale up the test capacity. Here we report that by combining the power of high multiplexing PCR (499 primer pairs) of small amplicons (120-135bp), targeting 27 viruses and 7 viroids of fruit trees, followed by a single high-throughput sequencing (HTS) run, we accurately diagnosed the viruses and viroids on as many as 123 pome and stone fruit tree samples. We compared the accuracy, sensitivity, and reproducibility of this approach and contrast it with other detection methods including HTS of total RNA (RNA-Seq) and individual RT-qPCR for every fruit tree virus or viroid under the study. We argue that this robust and high-throughput cost-effective diagnostic tool will enhance the viral/viroid knowledge of fruit trees while increasing the capacity for large scale diagnostics. This approach can also be adopted for the detection of multiple viruses and viroids in other crops.

Recent Advances on the Multiplex Molecular Detection of Plant Viruses and Viroids

Plant viruses are still one of the main contributors to economic losses in agriculture. It has been estimated that plant viruses can cause as much as 50 billion euros loss worldwide, per year. This situation may be worsened by recent climate change events and the associated changes in disease epidemiology. Reliable and early detection methods are still one of the main and most effective actions to develop control strategies for plant viral diseases. During the last years, considerable progress has been made to develop tools with high specificity and low detection limits for use in the detection of these plant pathogens. Time and cost reductions have been some of the main objectives pursued during the last few years as these increase their feasibility for routine use. Among other strategies, these objectives can be achieved by the simultaneous detection and (or) identification of several viruses in a single assay. Nucleic acid-based detection techniques are especially suitable for this purpose. Polyvalent detection has allowed the detection of multiple plant viruses at the genus level. Multiplexing RT polymerase chain reaction (PCR) has been optimized for the simultaneous detection of more than 10 plant viruses/viroids. In this short review, we provide an update on the progress made during the last decade on techniques such as multiplex PCR, polyvalent PCR, non-isotopic molecular hybridization techniques, real-time PCR, and array technologies to allow simultaneous detection of multiple plant viruses. Also, the potential and benefits of the powerful new technique of deep sequencing/next-generation sequencing are described.

A highly effective and versatile technology for the isolation of RNAs from grapevines and other woody perennials for use in virus diagnostics

BACKGROUND

Isolation of pure RNA from woody perennials, especially fruit crops such as grapevine rich in complex secondary metabolites, has remained very challenging. Lack of effective RNA isolation technology has resulted in difficulties in viral diagnosis and discovery as well as studies on many biological processes of these highly important woody plants. It is imperative to develop and refine methodologies with which large amounts of pure nucleic acids can be readily isolated from woody perennials.

METHODS

We compared five commonly used RNA isolation kits in isolating total RNA from twelve species of woody perennials. We made modifications to select RNA isolation systems to simplify and improve their efficiency in RNA isolation. The yield and quality of isolated RNAs were assessed via gel electrophoresis and spectrophotometric measurement. We also performed RT-PCR and RT-qPCR to detect several major viruses from grapevines.

RESULTS

Two of the kits were shown to be the best in both the yield and quality of the isolated RNA from all twelve woody species. Using disposable extraction bags for tissue homogenization not only improved the yield without affecting quality, but also made the RNA isolation technology simpler, less costly, and suitable for adoption by many potential users with facility limitations. This system was successfully applied to a wide range of woody plants, including fruit crops, ornamentals and timber trees. Inclusion of polyvinylpyrrolidone in the extraction buffer drastically improved the performance of the system in isolating total RNA from old grapevine leaves collected later in the season. This modification made our system highly effective in isolating quality RNA from grapevine leaves throughout the entire growing season. We further demonstrated that the resulting nucleic acid preparations are suitable for detection of several major grapevine viruses with RNA or DNA genomes using PCR, RT-PCR and qPCR as well as for assays on plant microRNAs.

CONCLUSIONS

This improved RNA isolation system would have wide applications in viral diagnostics and discovery, studies on gene expression and regulation, transcriptomics, and small RNA biology in grapevines. We believe this system will also be useful in diverse applications pertaining to research on many other woody perennials and recalcitrant plant species.

Fluorescence-based bioassays for the detection and evaluation of food materials

We summarize here the recent progress in fluorescence-based bioassays for the detection and evaluation of food materials by focusing on fluorescent dyes used in bioassays and applications of these assays for food safety, quality and efficacy. Fluorescent dyes have been used in various bioassays, such as biosensing, cell assay, energy transfer-based assay, probing, protein/immunological assay and microarray/biochip assay. Among the arrays used in microarray/biochip assay, fluorescence-based microarrays/biochips, such as antibody/protein microarrays, bead/suspension arrays, capillary/sensor arrays, DNA microarrays/polymerase chain reaction (PCR)-based arrays, glycan/lectin arrays, immunoassay/enzyme-linked immunosorbent assay (ELISA)-based arrays, microfluidic chips and tissue arrays, have been developed and used for the assessment of allergy/poisoning/toxicity, contamination and efficacy/mechanism, and quality control/safety. DNA microarray assays have been used widely for food safety and quality as well as searches for active components. DNA microarray-based gene expression profiling may be useful for such purposes due to its advantages in the evaluation of pathway-based intracellular signaling in response to food materials.

Development of the large-scale oligonucleotide chip for the diagnosis of plant viruses and its practical use

A large-scale oligonucleotide (LSON) chip was developed for the detection of the plant viruses with known genetic information. The LSON chip contains two sets of 3,978 probes for 538 species of targets including plant viruses, satellite RNAs and viroids. A hundred forty thousand probes, consisting of isolate-, species- and genus-specific probes respectively, are designed from 20,000 of independent nucleotide sequence of plant viruses. Based on the economic importance, the amount of genome information, and the number of strains and/or isolates, one to fifty-one probes for each target virus are selected and spotted on the chip. The standard and field samples for the analysis of the LSON chip have been prepared and tested by RT-PCR. The probe's specific and/or nonspecific reaction patterns by LSON chip allow us to diagnose the unidentified viruses. Thus, the LSON chip in this study could be highly useful for the detection of unexpected plant viruses, the monitoring of emerging viruses and the fluctuation of the population of major viruses in each plant.

Optimization and improvement of oligonucleotide microarray-based detection of tomato viruses and pospiviroids

Tomato (Solanum lycopersicum L.) is a vegetable crop which is affected by many viruses and several viroids, causing significant economic loss. Their detection and identification is of critical importance for plant protection and quarantine and certification programs. The potential was examined of an array based on the Combimatrix platform for the detection of 37 viruses belonging to 13 families, one of which is unassigned, together with six pospiviroid species, genus Pospiviroid, family Pospiviroidae. More than 470 oligonucleotide probes (40-mer) were selected for the microarray diagnostic technique developed in this investigation. Most of the virus probes were highly specific and were able to identify tomato viruses. Most pospiviroid probes, however, were non-specific in terms of species, but were specific at the genus level as they hybridized to members of the genus Pospiviroid. Only one probe of the Tomato apical stem viroid was species specific. The repeatability and specificity of the Combimatrix method showed that it can be considered for routine diagnostic use in suspected tomato germplasm since it detected 37 viruses and one pospiviroid at the species level and 5-6 pospiviroids at the genus level. The estimated cost for testing of a single tomato virus is similar to or less than the cost of using ELISA.