Development of High-Throughput Sequencing (HTS), also known as next generation sequencing, revolutionized diagnostic research of plant viruses. HTS outperforms bioassays and molecular diagnostic assays that are used to screen domestic and quarantine grapevine materials in data throughput, cost, scalability, and detection of novel and highly variant virus species. However, before HTS-based assays can be routinely used for plant virus diagnostics, performance specifications need to be developed and assessed. In this study, we selected 18 virus-infected grapevines as a test panel for measuring performance characteristics of an HTS-based diagnostic assay. Total nucleic acid (TNA) was extracted from petioles and dormant canes of individual samples and constructed libraries were run on Illumina NextSeq 500 instrument using a 75-bp single-end read platform. Sensitivity was 98% measured over 264 distinct virus and viroid infections with a false discovery rate (FDR) of approximately 1 in 5 positives. The results also showed that combining a spring petiole test with a fall cane test increased sensitivity to 100% for this TNA HTS assay. To evaluate extraction methodology, these results were compared to parallel dsRNA extractions. In addition, in a more detailed dilution study, the TNA HTS assay described here consistently performed well down to a dilution of 5%. In that range, sensitivity was 98% with a corresponding FDR of approximately 1 in 5. Repeatability and reproducibility were assessed at 99% and 93%, respectively. The protocol, criteria, and performance levels described here may help to standardize HTS for quality assurance and accreditation purposes in plant quarantine or certification programs.

Two novel negative-stranded (ns)RNA viruses were identified by high throughput sequencing in grapevine. The genomes of both viruses, named grapevine Muscat rose virus (GMRV) and grapevine Garan dmak virus (GGDV), comprise three segments with each containing a unique gene. Based on sequence identity and presence of typical domains/motifs, the proteins encoded by the two viruses were predicted to be: RNA-dependent RNA polymerase (RdRp), nucleocapsid protein (NP), and putative movement protein (MP). These proteins showed the highest identities with orthologs in the recently discovered apple rubbery wood viruses 1 and 2, members of a tentative genus (Rubodvirus) within the family Phenuiviridae. The three segments of GMRV and GGDV share almost identical sequences at their 5′ and 3′ termini, which are also complementary to each other and may form a panhandle structure. Phylogenetics based on RdRp, NP and MP placed GMRV and GGDV in the same cluster with rubodviruses. Grapevine collections were screened for the presence of both novel viruses via RT-PCR, identifying infected plants. GMRV and GGDV were successfully graft-transmitted, thus, they are the first nsRNA viruses identified and transmitted in grapevine. Lastly, different evolutionary scenarios of nsRNA viruses are discussed.

Viruses are considered of major importance in strawberry (Fragaria × ananassa Duchesne) production given their negative impact on plant vigor and growth. Strawberry accessions from the National Clonal Germplasm Repository were screened for viruses using high throughput sequencing (HTS). Analyses of sequence information from 45 plants identified multiple variants of 14 known viruses, comprising strawberry mottle virus (SMoV), beet pseudo yellows virus (BPYV), strawberry pallidosis-associated virus (SPaV), tomato ringspot virus (ToRSV), strawberry mild yellow edge virus (SMYEV), strawberry vein banding virus (SVBV), strawberry crinkle virus (SCV), strawberry polerovirus 1 (SPV-1), apple mosaic virus (ApMV), strawberry chlorotic fleck virus (SCFaV), strawberry crinivirus 4 (SCrV-4), strawberry crinivirus 3 (SCrV-3), Fragaria chiloensis latent virus (FClLV) and Fragaria chiloensis cryptic virus (FCCV). Genetic diversity of sequenced virus isolates was investigated via sequence homology analysis, and partial-genome sequences were deposited into GenBank. To confirm the HTS results and expand the detection of strawberry viruses, new reverse transcription quantitative PCR (RT-qPCR) assays were designed for the above-listed viruses. Further in silico and in vitro validation of the new diagnostic assays indicated high efficiency and reliability. Thus, the occurrence of different viruses, including divergent variants, among the strawberries was verified. This is the first viral metagenomic survey in strawberry, additionally, this study describes the design and validation of multiple RT-qPCR assays for strawberry viruses, which represent important detection tools for clean plant programs.

Quarantine and certification programs exist to prevent the entry or spread of harmful pests and pathogens into agricultural systems. Their common objective is to identify pathogen-free source material through the application of validated testing methods for subsequent release for propagation. Tests must be accurate, efficient, and cost-effective. In recent decades, the best tests have been biological assays in conjunction with PCR testing. High-throughput sequencing (HTS) has now become a reliable and cost-effective diagnostic method having greater accuracy and efficiency than biological assays. In this article, we review the role of clean plant centers in quarantine and certification programs, as well as the process by which HTS was evaluated as a testing method to replace biological assays for screening source material. The data from this evaluation included a side-by-side comparison of HTS and biological assays for cultivars of grapevine, Prunus, and rose and intra- and interlaboratory validations of an HTS protocol. Based on the results of these evaluations, in 2021, the U.S. Department of Agriculture Animal and Plant Health Inspection Service and several state regulatory agencies accepted the use of HTS and quantitative PCR to test new introductions of source material, replacing biological indexing. This new protocol requires testing at two timepoints within at least a 6-month interval and a dormancy separating the two tests. Under ideal conditions, testing can be completed in 18 to 24 months with subsequent release from quarantine of plant material that has tested negative for regulated pathogens. This new testing protocol has a profound impact on quarantine and certification programs, facilitating quicker access of stakeholders to clean materials for propagation and increasing the number of pathogens that are detected, and even discovered, with reduced cost, effort, and time.

In 2012, dormant canes of a proprietary wine grape (Vitis vinifera L.) accession were included in the collection of the University of California–Davis Foundation Plant Services. No virus-like symptoms were elicited when bud chips from propagated own-rooted canes of the accession were graft-inoculated onto a panel of biological indicators. However, chlorotic ringspot symptoms were observed on sap-inoculated Chenopodium amaranticolor Coste & A. Rein and C. quinoa Willd. plants, indicating the presence of a mechanically transmissible virus. Transmission electron microscopy of virus preparations from symptomatic C. quinoa revealed spherical, nonenveloped virions about 27 nm in diameter. Nepovirus-like haplotypes of sequence contigs were detected in both the source grape accession and symptomatic C. quinoa plants via high-throughput sequencing. A novel bipartite nepovirus-like genome was assembled from these contigs, and the termini of each RNA segment were verified by rapid amplification of complementary DNA ends assays. The RNA1 (7,186-nt) of the virus encodes a large polyprotein 1 of 231.1 kDa, and the RNA2 (4,460-nt) encodes a large polyprotein 2 of 148.9 kDa. Each of the polyadenylated RNA segments is flanked by 5′- (RNA1 = 156-nt; RNA2 = 170-nt) and 3′- (RNA1 = 834-nt; RNA2 = 261-nt) untranslated region sequences with >90% identities. Maximum likelihood phylogenetic analyses of the conserved Pro-Pol amino acid sequences revealed the clustering of the new virus within the genus Nepovirus of the family Secoviridae. Considering its biological and molecular characteristics, and based on current taxonomic criteria, we propose that the novel virus, named grapevine nepovirus A, be assigned to the genus Nepovirus.

Background: Using plant viruses to produce desirable proteins in plants allows for using non-transgenic plant hosts and if necessary, the ability to make rapid changes in the virus construct for increased or modified protein product yields. The objective of this work was the development of advanced CMV-based protein production systems to produce Acidothermus cellulolyticus endo-1, 4-β-glucanase (E1) in non-transgenic plants. Results: We used two new Cucumber mosaic virus (CMV)-based vector systems for producing the green fluorescent protein (GFP) and more importantly, the Acidothermus cellulolyticus endo-1, 4-β-glucanase (E1) in non-transgenic Nicotiana benthamiana plants. These are the inducible CMVin (CMV-based inducible) and the autonomously replicating CMVar (CMV-based advanced replicating) systems. We modified a binary plasmid containing the complete CMV RNA 3 cDNA to facilitate insertion of desired sequences, and to give modifications of the subgenomic mRNA 4 leader sequence yielding several variants. Quantitative RT-PCR and immunoblot analysis showed good levels of CMV RNA and coat protein accumulation for some variants of both CMVin and CMVar. When genes for E1 or GFP were inserted in place of the CMV coat protein, both were produced in plants as shown by fluorescence (GFP) and immunoblot analysis. Enzymatic activity assays showed that active E1 was produced in plants with yields up to ~ 11 μg/g fresh weight (FW) for specific variant constructs. We also compared in vitro CMV genomic RNA reassortants, and CMV RNA 3 mutants which lacked the C’ terminal 33 amino acids of the 3A movement protein in attempts to further increase E1 yield. Taken together specific variant constructs yielded up to ~21 μg/g FW of E1 in non-transgenic plants. Conclusions: Intact, active E1 was rapidly produced in non-transgenic plants by using agroinfiltration with the CMV-based systems. This reduces the time and cost compared to that required to generate transgenic plants and still gives the comparable yields of active E1. Our modifications described here, including manipulating cloning sites for foreign gene introduction, enhance the ease of use. Also, N. benthamiana, which is particularly suitable for agroinfiltration, is a very good plant for transient protein production.