Characterization of the 5'- and 3'-terminal subgenomic RNAs produced by a capillovirus: Evidence for a CP subgenomic RNA

A novel RAV transcription factor from pear interacts with viral RNA-silencing suppressors to inhibit viral infection

In plants, RNA silencing constitutes a strong defense against viral infection, which viruses counteract with RNA-silencing suppressors (RSSs). Understanding the interactions between viral RSSs and host factors is crucial for elucidating the molecular arms race between viruses and host plants. We report that the helicase motif (Hel) of the replicase encoded by apple stem grooving virus (ASGV)-the main virus affecting pear trees in China-is an RSS that can inhibit both local and systemic RNA silencing, possibly by binding double-stranded (ds) siRNA. The transcription factor related to ABSCISIC ACID INSENSITIVE3/VIVIPAROUS1 from pear (PbRAV1) enters the cytoplasm and binds Hel through its C terminus, thereby attenuating its RSS activity by reducing its binding affinity to 21- and 24-nt ds siRNA, and suppressing ASGV infection. PbRAV1 can also target p24, an RSS encoded by grapevine leafroll-associated virus 2 (GLRaV-2), with similar negative effects on p24's suppressive function and inhibition of GLRaV-2 infection. Moreover, like the positive role of the PbRAV1 homolog from grapevine (VvRAV1) in p24's previously reported RSS activity, ASGV Hel can also hijack VvRAV1 and employ the protein to sequester 21-nt ds siRNA, thereby enhancing its own RSS activity and promoting ASGV infection. Furthermore, PbRAV1 neither interacts with CP, an RSS encoded by grapevine inner necrosis virus, nor has any obvious effect on CP's RSS activity. Our results identify an RSS encoded by ASGV and demonstrate that PbRAV1, representing a novel type of RAV transcription factor, plays a defensive role against viral infection by targeting viral RSSs.

Evolution and biogeography of apple stem grooving virus

BACKGROUND

Apple stem grooving virus (ASGV) has a wide host range, notably including apples, pears, prunes and citrus. It is found worldwide.

METHOD

In this study, two near complete genomes, and seven coat protein (CP) sequences of Iranian isolates from apple were determined. Sequences added from GenBank provided alignments of 120 genomic sequences (54 of which were recombinant), and 276 coat protein genes (none of them recombinant).

RESULT

The non-recombinant genomes gave a well supported phylogeny with isolates from diverse hosts in China forming the base of the phylogeny, and a monophyletic clade of at least seven clusters of isolates from around the world with no host or provenace groupings among them, and all but one including isolates from China. The six regions of the ASGV genome (five in one frame, one - 2 overlapping) gave significantly correlated phylogenies, but individually had less statistical support. The largest cluster of isolates contained those from Iran and had isolates with worldwide provenances, and came from a wide range of mono- and dicotyledonous hosts. Population genetic comparisons of the six regions of the ASGV genome showed that four were under strong negative selection, but two of unknown function were under positive selection.

CONCLUSION

ASGV most likely originated and spread in East Asia in one or more of various plant species, but not in Eurasia; the ASGV population of China had the greatest overall nucleotide diversity and largest number of segregating sites.

Complete genome sequence of a tentative novel capillovirus isolated from Gerbera jamesonii

The currently named gerbera virus A (GeVA) has been shown to be a novel capillovirus with a complete genome of 6929 nucleotides (nt) (GenBank accession no. OM525829.1). GeVA was detected in Gerbera jamesonii using high-throughput RNA sequencing analysis. The GeVA genome is a single linear RNA with two open reading frames (ORF), similar to those of other capilloviruses. The larger ORF encodes a polyprotein containing four domains, while the smaller ORF encodes a movement protein. The complete genome had 41.0-54.9% nt sequence identity to other those of capilloviruses, while the polyprotein and the movement protein had 26.5-36.4% and 13.1-32.2% amino acid (aa) sequence identity, respectively. Two UUAGGU promoters for subgenomic RNA (sgRNA) transcription were also identified in this study. BLAST analysis demonstrated that the GeVA genome shared the highest sequence similarity with rubber tree capillovirus 1 (MN047299.1) (complete nucleotide sequence identity, 68.54%; polyprotein amino acid sequence identity, 44.53%). Phylogenetic analysis based on complete genome and replication protein sequences placed GeVA alongside other members of the genus Capillovirus in the family Betaflexiviridae. These data suggest that GeVA is a new member of the genus Capillovirus.

Construction and Characterization of a Botrytis Virus F Infectious Clone

Botrytis virus F (BVF) is a positive-sense, single-stranded RNA (+ssRNA) virus within the Gammaflexiviridae family of the plant-pathogenic fungus Botrytis cinerea. In this study, the complete sequence of a BVF strain isolated from B. cinerea collected from grapevine fields in Spain was analyzed. This virus, in this work BVF-V448, has a genome of 6827 nt in length, excluding the poly(A) tail, with two open reading frames encoding an RNA dependent RNA polymerase (RdRP) and a coat protein (CP). The 5′- and 3′-terminal regions of the genome were determined by rapid amplification of cDNA ends (RACE). Furthermore, a yet undetected subgenomic RNA species in BVF-V448 was identified, indicating that the CP is expressed via 3′ coterminal subgenomic RNAs (sgRNAs). We also report the successful construction of the first BVF full-length cDNA clone and synthesized in vitro RNA transcripts using the T7 polymerase, which could efficiently transfect two different strains of B. cinerea, B05.10 and Pi258.9. The levels of growth in culture and virulence on plants of BVF-V448 transfected strains were comparable to BVF-free strains. The infectious clones generated in this work provide a useful tool for the future development of an efficient BVF foreign gene expression vector and a virus-induced gene silencing (VIGS) vector as a biological agent for the control of B. cinerea.

Complete nucleotide sequence of loquat virus A, a member of the family Betaflexiviridae with a novel genome organization

Analysis of a loquat tree with leaf curl symptoms by deep sequencing revealed a novel virus with a single-stranded RNA genome, for which the name "loquat virus A" (LoVA) was proposed. The complete genome sequence comprised 7553 nucleotides (nt) and an additional poly(A) tail at the 3' terminus. Sequence comparisons of LoVA showed moderate similarity to cherry virus A (CVA), currant virus A (CuVA), and mume virus A (MuVA), which are members of the genus Capillovirus in the family Betaflexiviridae. Phylogenetic analysis of the full-genome nt sequence and replicase-like protein supported the placement of LoVA within the genus Capillovirus. However, it has a distinct genome organization, differing from recognized capilloviruses, as it contains three open reading frames (ORFs), with the coat protein (CP) expressed separately from the replication-associated protein (RP) rather than being encoded in the same ORF. This indicates that LoVA is a novel member of the genus Capillovirus in the family Betaflexiviridae with a distinct genomic organization.

Differential use of 3'CITEs by the subgenomic RNA of Pea enation mosaic virus 2

The genomic RNA (gRNA) of Pea enation mosaic virus 2 (PEMV2) is the template for p33 and -1 frameshift product p94. The PEMV2 subgenomic RNA (sgRNA) encodes two overlapping ORFs, p26 and p27, which are required for movement and stability of the gRNA. Efficient translation of p33 requires two of three 3' proximal cap-independent translation enhancers (3'CITEs): the kl-TSS, which binds ribosomes and engages in a long-distance interaction with the 5'end; and the adjacent eIF4E-binding PTE. Unlike the gRNA, all three 3'CITEs were required for efficient translation of the sgRNA, which included the ribosome-binding 3'TSS. A hairpin in the 5' proximal coding region of p26/p27 supported translation by the 3'CITEs by engaging in a long-distance RNA:RNA interaction with the kl-TSS. These results strongly suggest that the 5' ends of PEMV2 gRNA and sgRNA connect with the 3'UTR through similar long-distance interactions while having different requirements for 3'CITEs.

Characterization of virus-derived small interfering RNAs in Apple stem grooving virus-infected in vitro-cultured Pyrus pyrifolia shoot tips in response to high temperature treatment

Background

Heat treatment (known as thermotherapy) together with in vitro culture of shoot meristem tips is a commonly used technology to obtain virus-free germplasm for the effective control of virus diseases in fruit trees. RNA silencing as an antiviral defense mechanism has been implicated in this process. To understand if high temperature-mediated acceleration of the host antiviral gene silencing system in the meristem tip facilitates virus-derived small interfering RNAs (vsiRNA) accumulation to reduce the viral RNA titer in the fruit tree meristem tip cells, we used the Apple stem grooving virus (ASGV)–Pyrus pyrifolia pathosystem to explore the possible roles of vsiRNA in thermotherapy.

Results

At first we determined the full-length genome sequence of the ASGV-Js2 isolate and then profiled vsiRNAs in the meristem tip of in vitro-grown pear (cv. ‘Jinshui no. 2’) shoots infected by ASGV-Js2 and cultured at 24 and 37 °C. A total of 7,495 and 7,949 small RNA reads were obtained from the tips of pear shoots cultured at 24 and 37 °C, respectively. Mapping of the vsiRNAs to the ASGV-Js2 genome revealed that they were unevenly distributed along the ASGV-Js2 genome, and that 21- and 22-nt vsiRNAs preferentially accumulated at both temperatures. The 5′-terminal nucleotides of ASGV-specific siRNAs in the tips cultured under different temperatures had a similar distribution pattern, and the nucleotide U was the most frequent. RT-qPCR analyses suggested that viral genome accumulation was drastically compromised at 37 °C compared to 24 °C, which was accompanied with the elevated levels of vsiRNAs at 37 °C. As plant Dicer-like proteins (DCLs), Argonaute proteins (AGOs), and RNA-dependent RNA polymerases (RDRs) are implicated in vsiRNA biogenesis, we also cloned the partial sequences of PpDCL2,4, PpAGO1,2,4 and PpRDR1 genes, and found their expression levels were up-regulated in the ASGV-infected pear shoots at 37 °C.

Conclusions

Collectively, these results showed that upon high temperature treatment, the ASGV-infected meristem shoot tips up-regulated the expression of key genes in the RNA silencing pathway, induced the biogenesis of vsiRNAs and inhibited viral RNA accumulation. This study represents the first report on the characterization of the vsiRNA population in pear plants infected by ASGV-Js2, in response to high temperature treatment.

Electronic supplementary material

The online version of this article (doi:10.1186/s12985-016-0625-0) contains supplementary material, which is available to authorized users.

Molecular characterization of a novel capillovirus from red currant

The complete nucleotide sequence of a novel virus from red currant, provisionally named currant virus A (CuVA), was determined. The genome is 7925 nucleotides long and has a 3'-poly(A) tail. The genome organization with two overlapping open reading frames is similar to that of capilloviruses, but the CuVA genome is about 600 nucleotides longer than that of the longest known capillovirus, cherry virus A. The RNA is predicted to encode a polyprotein with domains of methyltransferase, 2OG-Fe(II) oxygenase, papain-like protease, RNA helicase, RdRp, and capsid protein. Phylogenetic analysis confirms that CuVA is a new and distinct member of the genus Capillovirus.

Mapping overlapping functional elements embedded within the protein-coding regions of RNA viruses

Identification of the full complement of genes and other functional elements in any virus is crucial to fully understand its molecular biology and guide the development of effective control strategies. RNA viruses have compact multifunctional genomes that frequently contain overlapping genes and non-coding functional elements embedded within protein-coding sequences. Overlapping features often escape detection because it can be difficult to disentangle the multiple roles of the constituent nucleotides via mutational analyses, while high-throughput experimental techniques are often unable to distinguish functional elements from incidental features. However, RNA viruses evolve very rapidly so that, even within a single species, substitutions rapidly accumulate at neutral or near-neutral sites providing great potential for comparative genomics to distinguish the signature of purifying selection. Computationally identified features can then be efficiently targeted for experimental analysis. Here we analyze alignments of protein-coding virus sequences to identify regions where there is a statistically significant reduction in the degree of variability at synonymous sites, a characteristic signature of overlapping functional elements. Having previously tested this technique by experimental verification of discoveries in selected viruses, we now analyze sequence alignments for ∼700 RNA virus species to identify hundreds of such regions, many of which have not been previously described.

High-throughput sequencing reveals small RNAs involved in ASGV infection

Background

Plant small RNAs (sRNAs) associated with virulent virus infections have been reported by previous studies, while the involvement of sRNAs in latent virus infection remains largely uncharacterised. Apple trees show a high degree of resistance and tolerance to viral infections. We analysed two sRNA deep sequencing datasets, prepared from different RNA size fractions, to identify sRNAs involved in Apple stem grooving virus (ASGV) infection.

Results

sRNA analysis revealed virus-derived siRNAs (vsiRNAs) originating from two ASGV genetic variants. A vsiRNA profile for one of the ASGV variants was also generated showing an increase in siRNA production towards the 3′ end of the virus genome. Virus-derived sRNAs longer than those previously analysed were also observed in the sequencing data. Additionally, tRNA-derived sRNAs were identified and characterised. These sRNAs covered a broad size-range and originated from both ends of the mature tRNAs as well as from their central regions. Several tRNA-derived sRNAs showed differential regulation due to ASGV infection. No changes in microRNA, natural-antisense transcript siRNA, phased-siRNA and repeat-associated siRNA levels were observed.

Conclusions

This study is the first report on the apple sRNA-response to virus infection. The results revealed the vsiRNAs profile of an ASGV variant, as well as the alteration of the tRNA-derived sRNA profile in response to latent virus infection. It also highlights the importance of library preparation in the interpretation of high-throughput sequencing data.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2164-15-568) contains supplementary material, which is available to authorized users.

Infection of apple by apple stem grooving virus leads to extensive alterations in gene expression patterns but no disease symptoms

To understand the molecular basis of viral diseases, transcriptome profiling has been widely used to correlate host gene expression change patterns with disease symptoms during viral infection in many plant hosts. We used infection of apple by Apple stem grooving virus (ASGV), which produces no disease symptoms, to assess the significance of host gene expression changes in disease development. We specifically asked the question of whether such asymptomatic infection is attributed to limited changes in host gene expression. Using RNA-seq, we identified a total of 184 up-regulated and 136 down-regulated genes in apple shoot cultures permanently infected by ASGV in comparison with virus-free shoot cultures. As in most plant hosts showing disease symptoms during viral infection, these differentially expressed genes encode known or putative proteins involved in cell cycle, cell wall biogenesis, response to biotic and abiotic stress, development and fruit ripening, phytohormone function, metabolism, signal transduction, transcription regulation, translation, transport, and photosynthesis. Thus, global host gene expression changes do not necessarily lead to virus disease symptoms. Our data suggest that the general approaches to correlate host gene expression changes under viral infection conditions to specific disease symptom, based on the interpretation of transcription profiling data and altered individual gene functions, may have limitations depending on particular experimental systems.

Molecular evolution of the genomic RNA of Apple stem grooving capillovirus

The complete genome of the German isolate AC of Apple stem grooving virus (ASGV) was sequenced. It encodes two overlapping open reading frames (ORFs), similarly to previously described ASGV isolates. Two regions of high variability were detected between the ASGV isolates, variable region 1 (V1, from amino acids (aa) 532 to 570), and variable region 2 (V2, from aa 1,583 to 1,868). The phylogenetic analysis of the V1 and V2 regions suggested that the ASGV diversity was structured by host plant species rather than geographical origin. The dN/dS ratio between nonsynonymous and synonymous nucleotide substitution rates varied greatly along the ASGV genome. Most of ORF1 showed predominant negative selection except for the two regions V1 and V2. V1 showed an elevated dN and an average dS when compared to the ORF1 background but no significant positive selection was detected. The V2 region of ORF1 showed an elevated dN and a low dS when compared to the ORF1 background with an average dN/dS ≈ 3.0 indicative of positive selection. However, the V2 area includes overlapping ORFs, making the dN/dS estimate biased. Joint estimates of the selection intensity in the different ORFs by a recent method indicated that this region of ORF1 was in fact evolving close to neutrality. This was convergent with previous results showing that introduction of stop codons in this region of ORF1 did not impair plant infection. These data suggest that the elimination of a stop codon caused the overprinting of a novel coding region over the ancestral ORF.

Complete sequence of an Apple stem grooving virus (ASGV) isolate from China

The complete genome sequence of a Chinese isolate of Apple stem grooving virus (ASGV) was determined to be 6,495 nucleotides long, single-stranded, plus-sense RNA. The viral RNA has two overlapping open reading frames (ORFs): ORF1 and ORF2. Compared with the genome sequences of ASGV isolates available in GenBank, the nucleotide identities ranged from 80.1 to 86.3 %. The amino acid identities of proteins encoded by ORF1 and ORF2 ranged from 79.5 to 86.1 % and 82.0 to 85.9 %, respectively.

Subgenomic messenger RNAs: mastering regulation of (+)-strand RNA virus life cycle

Many (+)-strand RNA viruses use subgenomic (SG) RNAs as messengers for protein expression, or to regulate their viral life cycle. Three different mechanisms have been described for the synthesis of SG RNAs. The first mechanism involves internal initiation on a (−)-strand RNA template and requires an internal SGP promoter. The second mechanism makes a prematurely terminated (−)-strand RNA which is used as template to make the SG RNA. The third mechanism uses discontinuous RNA synthesis while making the (−)-strand RNA templates. Most SG RNAs are translated into structural proteins or proteins related to pathogenesis: however other SG RNAs regulate the transition between translation and replication, function as riboregulators of replication or translation, or support RNA–RNA recombination. In this review we discuss these functions of SG RNAs and how they influence viral replication, translation and recombination.

Complete nucleotide sequence of cherry virus A (CVA) infecting sweet cherry in India

Cherry virus A (CVA) is a graft-transmissible member of the genus Capillovirus that infects different stone fruits. Sweet cherry (Prunus avium L; family Rosaceae) is an important deciduous temperate fruit crop in the Western Himalayan region of India. In order to determine the health status of cherry plantations and the incidence of the virus in India, cherry orchards in the states of Jammu and Kashmir (J&K) and Himachal Pradesh (H.P.) were surveyed during the months of May and September 2009. The incidence of CVA was found to be 28 and 13% from J&K and H.P., respectively, by RT-PCR. In order to characterize the virus at the molecular level, the complete genome was amplified by RT-PCR using specific primers. The amplicon of about 7.4 kb was sequenced and was found to be 7,379 bp long, with sequence specificity to CVA. The genome organization was similar to that of isolates characterized earlier, coding for two ORFs, in which ORF 2 is nested in ORF1. The complete sequence was 81 and 84% similar to that of the type isolate at the nucleotide and amino acid level, respectively, with 5' and 3' UTRs of 54 and 299 nucleotides, respectively. This is the first report of the complete nucleotide sequence of cherry virus A infecting sweet cherry in India.

3'-coterminal subgenomic RNAs and putative cis-acting elements of Grapevine leafroll-associated virus 3 reveals 'unique' features of gene expression strategy in the genus Ampelovirus

BACKGROUND

The family Closteroviridae comprises genera with monopartite genomes, Closterovirus and Ampelovirus, and with bipartite and tripartite genomes, Crinivirus. By contrast to closteroviruses in the genera Closterovirus and Crinivirus, much less is known about the molecular biology of viruses in the genus Ampelovirus, although they cause serious diseases in agriculturally important perennial crops like grapevines, pineapple, cherries and plums.

RESULTS

The gene expression and cis-acting elements of Grapevine leafroll-associated virus 3 (GLRaV-3; genus Ampelovirus) was examined and compared to that of other members of the family Closteroviridae. Six putative 3'-coterminal subgenomic (sg) RNAs were abundantly present in grapevine (Vitis vinifera) infected with GLRaV-3. The sgRNAs for coat protein (CP), p21, p20A and p20B were confirmed using gene-specific riboprobes in Northern blot analysis. The 5'-termini of sgRNAs specific to CP, p21, p20A and p20B were mapped in the 18,498 nucleotide (nt) virus genome and their leader sequences determined to be 48, 23, 95 and 125 nt, respectively. No conserved motifs were found around the transcription start site or in the leader sequence of these sgRNAs. The predicted secondary structure analysis of sequences around the start site failed to reveal any conserved motifs among the four sgRNAs. The GLRaV-3 isolate from Washington had a 737 nt long 5' nontranslated region (NTR) with a tandem repeat of 65 nt sequence and differed in sequence and predicted secondary structure with a South Africa isolate. Comparison of the dissimilar sequences of the 5'NTRs did not reveal any common predicted structures. The 3'NTR was shorter and more conserved. The lack of similarity among the cis-acting elements of the diverse viruses in the family Closteroviridae is another measure of the complexity of their evolution.

CONCLUSIONS

The results indicate that transcription regulation of GLRaV-3 sgRNAs appears to be different from members of the genus Closterovirus. An analysis of the genome sequence confirmed that GLRaV-3 has an unusually long 5'NTR of 737 nt compared to other monopartite members of the family Closteroviridae, with distinct differences in the sequence and predicted secondary structure when compared to the corresponding region of the GLRaV-3 isolate from South Africa.