Genomic Characterization and Population Structure of a Badnavirus Infecting Blackberry

Phyllocoptes parviflori is a distinct species and a vector of the pervasive blackberry leaf mottle associated virus

Several viruses are transmitted by eriophyid mites (Acariformes: Eriophyoidea) including blackberry leaf mottle-associated emaravirus (BLMaV) (Emaravirus rubi). BLMaV is transmitted by an unidentified eriophyid species and is involved in blackberry yellow vein, a devastating disease in the southeastern United States. In this study, we assessed the eriophyid mite Phylocoptes parviflori as a vector of BLMaV and clarified its taxonomic status as it was previously synonymized with Phyllocoptes gracilis. P. parviflori can efficiently transmit BLMaV. The virus was found to cause yellow vein disease symptoms on 'Ouachita' blackberry marking a paradigm shift as disease symptoms have always been associated with multiple virus infections. Therefore, we propose renaming the virus to blackberry leaf mottle virus. The occurrence of P. parviflori on wild and cultivated blackberries, as well as its ability to colonize other Rubus species, enhances its importance as a major contributor to the spread of yellow vein disease.

Development of Rapid and Affordable Virus-Mimicking Artificial Positive Controls

A major bottleneck in the development of detection assays is the availability of positive controls. Their acquisition can be problematic, their maintenance is expensive, and without them, assays cannot be validated. Herein, we present a novel strategy for the development of virus-mimicking artificial positive controls (ViMAPCs). The time between design and application is less than 5 days, unlike alternatives which normally take several weeks to obtain and implement. The ViMAPCs provide a realistic representation of natural infection unlike alternatives and allow for an effortless recognition of laboratory-based contamination. The feasibility and adaptability of the strategy was evaluated using several RNA and DNA plant viruses. ViMAPCs can be used in diagnostics laboratories but also in the monitoring of pathogen outbreaks where rapid response is of utmost importance.

Emerging and Re-Emerging Diseases Caused by Badnaviruses

New and emerging plant diseases are caused by different pathogens including viruses that often cause significant crop losses. Badnaviruses are pararetroviruses that contain a single molecule of ds DNA genome of 7 to 9 kb in size and infect a large number of economically important crops such as banana and plantains, black pepper, cacao, citrus, grapevine, pineapple, sugarcane, sweet potato, taro, and yam, causing significant yield losses. Many of the species in the genus have a restricted host range and several of them are known to infect a single crop. Combined infections of different virus species and strains offer conditions that favor the development of new strains via recombination, especially in vegetatively propagated crops. The primary spread of badnaviruses is through vegetative propagating materials while for the secondary spread, they depend on insects such as mealybugs and aphids. Disease emerges as a consequence of the interactions between host and pathogens under favorable environmental conditions. The viral genome of the pararetroviruses is known to be integrated into the chromosome of the host and a few plants with integrants when subjected to different kinds of abiotic stress will give rise to episomal forms of the virus and cause disease. Attempts have been made to develop management strategies for badnaviruses both conventionally and using precision breeding techniques such as genome editing. Until 2016 only 32 badnavirus species infecting different crops were known, but in a span of six years, this number has gone up to 68. The current review highlights the emerging disease problems and management options for badnaviruses infecting economically important crops.

Identification and distribution of novel badnaviral sequences integrated in the genome of cacao (Theobroma cacao)

Theobroma cacao is one of the most economically important tropical trees, being the source of chocolate. As part of an ongoing study to understand the diversity of the badnavirus complex, responsible for the cacao swollen shoot virus disease in West Africa, evidence was found recently of virus-like sequences in asymptomatic cacao plants. The present study exploited the wealth of genomic resources in this crop, and combined bioinformatic, molecular, and genetic approaches to report for the first time the presence of integrated badnaviral sequences in most of the cacao genetic groups. These sequences, which we propose to name eTcBV for endogenous T. cacao bacilliform virus, varied in type with each predominating in a specific genetic group. A diagnostic multiplex PCR method was developed to identify the homozygous or hemizygous condition of one specific insert, which was inherited as a single Mendelian trait. These data suggest that these integration events occurred before or during the species diversification in Central and South America, and prior to its cultivation in other regions. Such evidence of integrated sequences is relevant to the management of cacao quarantine facilities and may also aid novel methods to reduce the impact of such viruses in this crop.

Next-Generation Sequencing and the CRISPR-Cas Nexus: A Molecular Plant Virology Perspective

In recent years, next-generation sequencing (NGS) and contemporary Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR)-CRISPR-associated (Cas) technologies have revolutionized the life sciences and the field of plant virology. Both these technologies offer an unparalleled platform for sequencing and deciphering viral metagenomes promptly. Over the past two decades, NGS technologies have improved enormously and have impacted plant virology. NGS has enabled the detection of plant viruses that were previously undetectable by conventional approaches, such as quarantine and archeological plant samples, and has helped to track the evolutionary footprints of viral pathogens. The CRISPR-Cas-based genome editing (GE) and detection techniques have enabled the development of effective approaches to virus resistance. Different versions of CRISPR-Cas have been employed to successfully confer resistance against diverse plant viruses by directly targeting the virus genome or indirectly editing certain host susceptibility factors. Applications of CRISPR-Cas systems include targeted insertion and/or deletion, site-directed mutagenesis, induction/expression/repression of the gene(s), epigenome re-modeling, and SNPs detection. The CRISPR-Cas toolbox has been equipped with precision GE tools to engineer the target genome with and without double-stranded (ds) breaks or donor templates. This technique has also enabled the generation of transgene-free genetically engineered plants, DNA repair, base substitution, prime editing, detection of small molecules, and biosensing in plant virology. This review discusses the utilities, advantages, applications, bottlenecks of NGS, and CRISPR-Cas in plant virology.

Pest categorisation of non-EU viruses of Rubus L

The Panel on Plant Health of EFSA conducted a pest categorisation of 17 viruses of Rubus L. that were previously classified as either non-EU or of undetermined standing in a previous opinion. These infectious agents belong to different genera and are heterogeneous in their biology. Blackberry virus X, blackberry virus Z and wineberry latent virus were not categorised because of lack of information while grapevine red blotch virus was excluded because it does not infect Rubus. All 17 viruses are efficiently transmitted by vegetative propagation, with plants for planting representing the major pathway for entry and spread. For some viruses, additional pathway(s) are Rubus seeds, pollen and/or vector(s). Most of the viruses categorised here infect only one or few plant genera, but some of them have a wide host range, thus extending the possible entry pathways. Cherry rasp leaf virus, raspberry latent virus, raspberry leaf curl virus, strawberry necrotic shock virus, tobacco ringspot virus and tomato ringspot virus meet all the criteria to qualify as potential Union quarantine pests (QPs). With the exception of impact in the EU territory, on which the Panel was unable to conclude, blackberry chlorotic ringspot virus, blackberry leaf mottle-associated virus, blackberry vein banding-associated virus, blackberry virus E, blackberry virus F, blackberry virus S, blackberry virus Y and blackberry yellow vein-associated virus satisfy all the other criteria to be considered as potential QPs. Black raspberry cryptic virus, blackberry calico virus and Rubus canadensis virus 1 do not meet the criterion of having a potential negative impact in the EU. For several viruses, the categorisation is associated with high uncertainties, mainly because of the absence of data on biology, distribution and impact. Since the opinion addresses non-EU viruses, they do not meet the criteria to qualify as potential Union regulated non-quarantine pests.

Molecular and biological characterization of Chilli leaf curl virus and associated Tomato leaf curl betasatellite infecting tobacco in Oman

BACKGROUND

In Oman tobacco (Nicotiana tabacum; family Solanaceae) is a minor crop, which is produced only for local consumption. In 2015, tobacco plants exhibiting severe downward leaf curling, leaf thickening, vein swelling, yellowing and stunting were identified in fields of tobacco in Suhar Al-Batina region, Oman. These symptoms are suggestive of begomovirus (genus Begomovirus, family Geminiviridae) infection.

METHODS

Circular DNA molecules were amplified from total DNA extracted from tobacco plants by rolling circle amplification (RCA). Viral genomes were cloned from RCA products by restriction digestion and betasatellites were cloned by PCR amplification from RCA product, using universal primers. The sequences of full-length clones were obtained by Sanger sequencing and primer walking. Constructs for the infectivity of virus and betasatellite were produced and introduced into plants by Agrobacterium-mediated inoculation.

RESULTS

The full-length sequences of 3 begomovirus and 3 betasatellite clones, isolated from 3 plants, were obtained. Analysis of the full-length sequences determined showed the virus to be a variant of Chilli leaf curl virus (ChiLCV) and the betasatellite to be a variant of Tomato leaf curl betasatellite (ToLCB). Both the virus and the betasatellite isolated from tobacco show the greatest levels of sequence identity to isolates of ChiLCV and ToLCB identified in other hosts in Oman. Additionally clones of ChiLCV and ToLCB were shown, by Agrobacterium-mediated inoculation, to be infectious to 3 Nicotiana species, including N. tabacum. In N. benthamiana the betasatellite was shown to change the upward leaf rolling symptoms to a severe downward leaf curl, as is typical for many monopartite begomoviruses with betasatellites.

CONCLUSIONS

The leaf curl disease of tobacco in Oman was shown to be caused by ChiLCV and ToLCB. This is the first identification of ChiLCV with ToLCB infecting tobacco. The study shows that, despite the low diversity of begomoviruses and betasatellites in Oman, the extant viruses/betasatellites are able to fill the niches that present themselves.

List of non-EU viruses and viroids of Cydonia Mill., Fragaria L., Malus Mill., Prunus L., Pyrus L., Ribes L., Rubus L. and Vitis L

The Panel on Plant Health performed a listing of non-EU viruses and viroids (reported hereinafter as viruses) of Cydonia Mill., Fragaria L., Malus Mill., Prunus L., Pyrus L., Ribes L., Rubus L. and Vitis L. A systematic literature review identified 197 viruses infecting one or more of the host genera under consideration. Viruses were allocated into three categories (i) 86 non-EU viruses, known to occur only outside the EU or having only limited presence in the EU (i.e. reported in only one or few Member States (MSs), known to have restricted distribution, outbreaks), (ii) 97 viruses excluded at this stage from further categorisation efforts because they have significant presence in the EU (i.e. only reported so far from the EU or known to occur or be widespread in some MSs or frequently reported in the EU), (iii) 14 viruses with undetermined standing for which available information did not readily allow to allocate to one or the other of the two above groups. Comments provided by MSs during consultation phases were integrated in the opinion. The main knowledge gaps and uncertainties of this listing concern (i) the geographic distribution and prevalence of the viruses analysed, in particular when they were recently described; (ii) the taxonomy and biological status of a number of poorly characterised viruses; (iii) the host status of particular plant genera in relation to some viruses. The viruses considered as non-EU and those with undetermined standing will be categorised in the next steps to answer a specific mandate from the Commission to develop pest categorisations for non-EU viruses. This list does not imply a prejudice on future needs for a pest categorisation for other viruses which are excluded from the current categorisation efforts.

High Throughput Sequencing For Plant Virus Detection and Discovery

Over the last decade, virologists have discovered an unprecedented number of viruses using high throughput sequencing (HTS), which led to the advancement of our knowledge on the diversity of viruses in nature, particularly unraveling the virome of many agricultural crops. However, these new virus discoveries have often widened the gaps in our understanding of virus biology; the forefront of which is the actual role of a new virus in disease, if any. Yet, when used critically in etiological studies, HTS is a powerful tool to establish disease causality between the virus and its host. Conversely, with globalization, movement of plant material is increasingly more common and often a point of dispute between countries. HTS could potentially resolve these issues given its capacity to detect and discover. Although many pipelines are available for plant virus discovery, all share a common backbone. A description of the process of plant virus detection and discovery from HTS data are presented, providing a summary of the different pipelines available for scientists' utility in their research.