Identification of Novel 5' and 3' Translation Enhancers in Umbravirus-Like Coat Protein-Deficient RNA Replicons

Hairpin inserts in viral genomes are stable when they conform to the thermodynamic properties of viral RNA substructures

Virus-induced gene silencing (VIGS) allows for the rapid targeting of gene expression and has been instrumental in characterizing plant genes. However, foreign sequences inserted into VIGS vectors are rarely maintained for unknown reasons. Citrus yellow vein-associated umbravirus-like virus (CY1) with its solved secondary structure was converted into a VIGS vector to determine why simple hairpins inserted into non-functional, single-stranded locations are not maintained. When CY1 contained foreign hairpins with thermodynamic properties (positional entropy and/or ΔG) differing from those of natural CY1 hairpins, deletions arose within a few weeks of infecting Nicotiana benthamiana. In contrast, duplication and insertion of four natural CY1 hairpins (up to 200 nt) into the same locations were retained until plant senescence. Hairpins containing similar conformations and thermodynamic properties as natural hairpins were also retained, as were hairpins that shared thermodynamic properties but were conformationally distinct. By predicting and modulating these thermodynamic properties, a hairpin was retained by CY1 for at least 30 months in citrus. These findings strongly suggest that RNA viruses have evolved to contain substructures with specific thermodynamic properties, and hairpins containing these properties are stable when inserted into non-functional regions of the genome, opening up VIGS for long-lived trees and vines.

IMPORTANCE

Plus-strand RNA plant viruses are used as tools to introduce small interfering RNAs (siRNAs) into laboratory plants to target and silence genes. However, virus-induced gene silencing (VIGS) vectors engineered to contain foreign hairpins or other sequences for siRNA generation are not stable, and the foreign sequences are rapidly lost. We found that foreign sequences are not maintained in an umbravirus-like VIGS vector (CY1) because their physical properties conflict with the innate properties of the CY1 genome's substructures (i.e., hairpins). When natural CY1 hairpins were duplicated and inserted into locations where previous inserts were rapidly lost, the hairpins were now stable as were unrelated hairpins with the same physical properties. By mimicking the physical properties of the viral genome, one insert was stable for over 30 months. These results suggest that RNA viral genomes have evolved to have specific physical properties, and these properties appear to be similar for other plus-strand RNA viruses.

Direct nanopore RNA sequencing of umbra-like virus-infected plants reveals long non-coding RNAs, specific cleavage sites, D-RNAs, foldback RNAs, and temporal- and tissue-specific profiles

The traditional view of plus (+)-strand RNA virus transcriptomes is that infected cells contain a limited variety of viral RNAs, such as full-length (+)-strand genomic RNA(s), (-)-strand replication intermediate(s), 3' co-terminal subgenomic RNA(s), and viral recombinant defective (D)-RNAs. To ascertain the full complement of viral RNAs associated with the simplest plant viruses, long-read direct RNA nanopore sequencing was used to perform transcriptomic analyses of two related umbra-like viruses: citrus yellow vein-associated virus (CY1) from citrus and CY2 from hemp. Analysis of different timepoints/tissues in CY1- and CY2-infected Nicotiana benthamiana plants and CY2-infected hemp revealed: (i) three 5' co-terminal RNAs of 281 nt, 442 nt and 671 nt, each generated by a different mechanism; (ii) D-RNA populations containing the 671 fragment at their 5'ends; (iii) many full-length genomic RNAs and D-RNAs with identical 3'end 61 nt truncations; (iv) virtually all (-)-strand reads missing 3 nt at their 3' termini; (v) (±) foldback RNAs comprising about one-third of all (-)-strand reads and (vi) a higher proportion of full-length gRNAs in roots than in leaves, suggesting that roots may be functioning as a gRNA reservoir. These findings suggest that viral transcriptomes are much more complex than previously thought.

Expanding the Plant Virome: Umbra-Like Viruses Use Host Proteins for Movement

Before the very recent discovery of umbra-like viruses (ULVs), the signature defining feature of all plant RNA viruses was the encoding of specialized RNA-binding movement proteins (MPs) for transiting their RNA genomes through gated plasmodesmata to establish systemic infections. The vast majority of ULVs share umbravirus-like RNA-dependent RNA polymerases and 3'-terminal structures, but they differ by not encoding cell-to-cell and long-distance MPs and by not relying on a helper virus for trans-encapsidation and plant-to-plant transmission. The recent finding that two groups of ULVs do not necessarily encode MPs is expanding our understanding of the minimum requirements for modern plant RNA viruses. ULV CY1 from citrus uses host protein PHLOEM PROTEIN 2 (PP2) for systemic movement, and related ULVs encode a capsid protein, thereby providing an explanation for the lack of helper viruses present in many ULV-infected plants. ULVs thus resemble the first viruses that infected plants, which were likely deposited from feeding organisms and would have similarly required the use of host proteins such as PP2 to exit initially infected cells.

Grapevine Virome of the Don Ampelographic Collection in Russia Has Concealed Five Novel Viruses

In this study, an analysis of the virome of 51 grapevines from the Don ampelographic collection named after Ya. I. Potapenko (Russia) was performed using high-throughput sequencing of total RNA. A total of 20 previously described grapevine viruses and 4 viroids were identified. The most detected were grapevine rupestris stem pitting-associated virus (98%), hop stunt viroid (98%), grapevine Pinot gris virus (96%), grapevine yellow speckle viroid 1 (94%), and grapevine fleck virus (GFkV, 80%). Among the economically significant viruses, the most present were grapevine leafroll-associated virus 3 (37%), grapevine virus A (24%), and grapevine leafroll-associated virus 1 (16%). For the first time in Russia, a grapevine-associated tymo-like virus (78%) was detected. After a bioinformatics analysis, 123 complete or nearly complete viral genomes and 64 complete viroid genomes were assembled. An analysis of the phylogenetic relationships with reported global isolates was performed. We discovered and characterized the genomes of five novel grapevine viruses: bipartite dsRNA grapevine alphapartitivirus (genus Alphapartitivirus, family Partitiviridae), bipartite (+) ssRNA grapevine secovirus (genus Fabavirus, family Secoviridae) and three (+) ssRNA grapevine umbra-like viruses 2, -3, -4 (which phylogenetically occupy an intermediate position between representatives of the genus Umbravirus and umbravirus-like associated RNAs).

-1 Programmed ribosomal frameshifting in Class 2 umbravirus-like RNAs uses multiple long-distance interactions to shift between active and inactive structures and destabilize the frameshift stimulating element

Plus-strand RNA viruses frequently employ -1 programmed ribosomal frameshifting (-1 PRF) to maximize their coding capacity. Ribosomes can frameshift at a slippery sequence if progression is impeded by a frameshift stimulating element (FSE), which is generally a stable, complex, dynamic structure with multiple conformations that contribute to the efficiency of -1 PRF. As FSE are usually analyzed separate from the viral genome, little is known about cis-acting long-distance interactions. Using full-length genomic RNA of umbravirus-like (ula)RNA citrus yellow vein associated virus (CY1) and translation in wheat germ extracts, six tertiary interactions were found associated with the CY1 FSE that span nearly three-quarters of the 2.7 kb genomic RNA. All six tertiary interactions are conserved in other Class 2 ulaRNAs and two are conserved in all ulaRNAs. Two sets of interactions comprise local and distal pseudoknots that involve overlapping FSE nucleotides and thus are structurally incompatible, suggesting that Class 2 FSEs assume multiple conformations. Importantly, two long-distance interactions connect with sequences on opposite sides of the critical FSE central stem, which would unzip the stem and destabilize the FSE. These latter interactions could allow a frameshifting ribosome to translate through a structurally disrupted upstream FSE that no longer blocks ribosome progression.

Molecular characterization of a novel umbra-like virus from Thuja orientalis (arborvitae) in South Korea

A novel umbra-like virus was identified in arborvitae in South Korea using RNA sequencing (RNA-seq). The virus identified was tentatively named "arborvitae umbra-like virus" (AULV) and contained a 4,300-nucleotide genome organized into four non-structural open reading frames (ORFs). Cloning and Sanger sequencing were used to confirm the viral contig sequence and determine the size of the genome. Genome analysis indicated that ORF2 encodes an RNA-dependent RNA polymerase that is probably expressed through ribosomal frameshifting. ORF3 encodes a putative long-distance movement protein, while the functions of ORFs 1 and 4 are unknown. The virus lacks a coat protein gene. The genome of AULV shares 27.3%-48.4% nucleotide sequence identity with closely related umbraviruses. Phylogenetic analysis based on the complete genome sequences and amino acid sequences of the RNA-dependent RNA polymerase revealed that AULV forms a monophyletic lineage with Guiyang paspalum paspaloides tombus-like virus (GPpTV1). We suggest that AULV is a novel umbra-like virus belonging to the family Tombusviridae.

Metagenomic identification of novel viruses of maize and teosinte in North America

Background

Maize-infecting viruses are known to inflict significant agronomic yield loss throughout the world annually. Identification of known or novel causal agents of disease prior to outbreak is imperative to preserve food security via future crop protection efforts. Toward this goal, a large-scale metagenomic approach utilizing high throughput sequencing (HTS) was employed to identify novel viruses with the potential to contribute to yield loss of graminaceous species, particularly maize, in North America.

Results

Here we present four novel viruses discovered by HTS and individually validated by Sanger sequencing. Three of these viruses are RNA viruses belonging to either the Betaflexiviridae or Tombusviridae families. Additionally, a novel DNA virus belonging to the Geminiviridae family was discovered, the first Mastrevirus identified in North American maize.

Conclusions

Metagenomic studies of crop and crop-related species such as this may be useful for the identification and surveillance of known and novel viral pathogens of crops. Monitoring related species may prove useful in identifying viruses capable of infecting crops due to overlapping insect vectors and viral host-range to protect food security.

Two new umbravirus-like associated RNAs (ulaRNAs) discovered in maize and johnsongrass from Ecuador

Two new umbravirus-like associated RNAs (ulaRNAs) were found, respectively, in maize and Johnsongrass samples from Ecuador. The complete sequences consist of 3,053 and 3,025 nucleotides, respectively, and contain four open reading frames (ORFs). Their genome sequences were 58% identical to each other and 28 to 60% identical to the most closely related viruses. Phylogenetic analysis using full genome sequences and amino acid sequence of the RNA-dependent-RNA polymerase (RdRp) placed both sequences in a clade sharing the most recent common ancestor with ulaRNAs from sugarcane and maize, suggesting that they belong to a monophyletic grass-infecting lineage. Their terminal regions exhibit features common to umbraviruses and ulaRNAs.