A Virus in American Blackcurrant (Ribes americanum) with Distinct Genome Features Reshapes Classification in the Tymovirales

Exploring the tymovirales landscape through metatranscriptomics data

Tymovirales is an order of viruses with positive-sense RNA genomes that mostly infect plants, but also fungi and insects. The number of genome sequences of viruses that could fit this taxon has been growing in the last few years with the extensive use of high-throughput sequencing. Here, we report the discovery of 31 novel viral genome sequences associated with 27 different host plant species, which were hidden in public databases. These viral sequences were identified through homology searches in more than 3,000 plant transcriptomes from the NCBI Sequence Read Archive (SRA) using known tymovirales sequences as queries. Identification, assembly, and curation of raw SRA reads resulted in 29 viral genome sequences with complete coding regions, and two representing partial genomes. Some of the obtained sequences highlight novel genome organizations for members of the order. Phylogenetic analysis showed that six of the novel viruses are related to alphaflexiviruses, 17 to betaflexiviruses, two to deltaflexiviruses, and six to tymovirids. These findings shed new light on the phylogenetic relationships and evolutionary landscape of this group of viruses. Furthermore, this study illustrates the complexity and genome diversity among members of the order and demonstrates that analyzing public SRA data provides an invaluable tool to accelerate virus discovery and refine virus taxonomy.

Graft-Transmissible Diseases of Ribes - Pathogens, Impact, and Control

This article provides an up-to-date review of disease-causing viruses and phytoplasmas of currants including symptoms, transmission, detection, economic impact, and control measures. Currants are widely cultivated in more than 30 countries in the temperate zones of Europe, Asia, South America, Australia, and New Zealand. Ribes spp. can be infected by more than 20 virus species and four Candidatus Phytoplasma species, with more to be described in the future. High-throughput sequencing opened a new era of deciphering virus variants and mixed infections, leading to the characterization of several new species. The use of clean propagation material is the basis for control of Ribes graft-transmissible diseases, but this has become a challenging task given the ever-growing number of newly discovered pathogens.

Small hydrophobic viral proteins involved in intercellular movement of diverse plant virus genomes

Most plant viruses code for movement proteins (MPs) targeting plasmodesmata to enable cell-to-cell and systemic spread in infected plants. Small membrane-embedded MPs have been first identified in two viral transport gene modules, triple gene block (TGB) coding for an RNA-binding helicase TGB1 and two small hydrophobic proteins TGB2 and TGB3 and double gene block (DGB) encoding two small polypeptides representing an RNA-binding protein and a membrane protein. These findings indicated that movement gene modules composed of two or more cistrons may encode the nucleic acid-binding protein and at least one membrane-bound movement protein. The same rule was revealed for small DNA-containing plant viruses, namely, viruses belonging to genus Mastrevirus (family Geminiviridae) and the family Nanoviridae. In multi-component transport modules the nucleic acid-binding MP can be viral capsid protein(s), as in RNA-containing viruses of the families Closteroviridae and Potyviridae. However, membrane proteins are always found among MPs of these multicomponent viral transport systems. Moreover, it was found that small membrane MPs encoded by many viruses can be involved in coupling viral replication and cell-to-cell movement. Currently, the studies of evolutionary origin and functioning of small membrane MPs is regarded as an important pre-requisite for understanding of the evolution of the existing plant virus transport systems. This paper represents the first comprehensive review which describes the whole diversity of small membrane MPs and presents the current views on their role in plant virus movement.

Pest categorisation of non-EU viruses of Ribes L

Following a request from the EU Commission, the Panel on Plant Health addressed the pest categorisation of the viruses of Ribes L. determined as being either non-EU or of undetermined standing in a previous EFSA opinion. These infectious agents belong to different genera and are heterogeneous in their biology. Alaska vitivirus 1 and Ribes virus F were excluded from categorisation because these are very poorly characterised viruses. The pest categorisation was completed for seven viruses with clear identity and for which detection methods are available. All these viruses are efficiently transmitted by vegetative propagation techniques, with plants for planting representing the major pathway for long-distance dispersal and thus considered as the major pathway for entry. Depending on the virus, additional pathway(s) can also be Ribes seeds, pollen and/or vector(s). Most of the viruses categorised here are known to infect only one or few plant genera, but tomato ringspot virus (ToRSV) has a wide host range, thus extending the possible entry pathways. ToRSV meets all the criteria evaluated by EFSA to qualify as potential Union quarantine pest (QP). With the exception of impact in the EU territory, on which the Panel was unable to conclude, Actinidia virus X, blackcurrant leaf chlorosis-associated virus, blackcurrant leafroll-associated virus, black currant-associated rhabdovirus, blackcurrant waikavirus A and Ribes americanum virus A satisfy all the other criteria to be considered as potential Union QPs. For several viruses, especially those recently discovered, the categorisation is associated with high uncertainties mainly because of the absence of data on their biology, distribution and impact. Since this opinion addresses specifically the non-EU viruses, in general these viruses do not meet the criteria assessed by EFSA to qualify as potential Union regulated non-quarantine pests.