Characterization of Canna yellow mottle virus in a New Host, Alpinia purpurata, in Hawaii

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.

Genome characterization of fig umbra-like virus

The complete genome of a new umbra-like virus from edible fig (Ficus carica) was identified by high-throughput sequencing. Based on its similarity to umbra-like virus genome sequences available in GenBank, the proposed name of this new virus is "fig umbra-like virus" (FULV). The genome of full-length FULV-1 consists of 3049 nucleotides organized into three open reading frames (ORFs). Pairwise comparisons showed that the complete nucleotide sequence of the virus had the highest identity (71.3%) to citrus yellow vein-associated virus (CYVaV). In addition, phylogenetic trees based on whole-genome nucleotide sequences and amino acid sequences of the RNA-dependent RNA polymerase showed that FULV forms a monophyletic lineage with CYVaV and other umbra-like viruses. Based on the demarcation criteria of the genus Umbravirus, and lack of two umbravirus ORFs, we propose that FULV is a putative new member of the umbra-like virus clade within the family Tombusviridae.

Characterization of a highly divergent Sugarcane mosaic virus from Canna indica L. by deep sequencing

Background

Cannas are popular ornamental plants and widely planted for the beautiful foliage and flower. Viral disease is a major threaten to canna horticulture industry. In the city of Beijing, mosaic disease in canna was frequently observed, but the associated causal agent and its biological characterization is still unknown.

Results

After small RNA deep sequencing, 36,776 contigs were assembled and 16 of them shared high sequence identities with the different proteins of Sugarcane mosaic virus (SCMV) of the size ranging from 86 to 1911 nt. The complete genome of SCMV isolate (canna) was reconstructed by sequencing all cDNA clones obtained from RT-PCR and 5′\3′ RACE amplifications. SCMV-canna isolate showed to have a full RNA genome of 9579 nt in length and to share 78% nt and 85% aa sequence identities with SCMV isolates from other hosts. The phylogenetic tree constructed based on the full genome sequence of SCMV isolates allocated separately the canna-isolate in a distinct clade, indicating a new strain. Recombination analyses demonstrated that SCMV-canna isolate was a recombinant originating from a sugarcane-infecting isolate (major parent, acc. no. AJ310103) and a maize-infecting isolate (minor parent, acc. no. AJ297628). Pathogenicity test showed SCMV-canna could cause typical symptoms of mosaic and necrosis in some tested plants with varying levels of severity but was less virulent than the isolate SCMV-BJ. Field survey showed that the virus was widely distributed.

Conclusions

This study identified SCMV as the major agent causing the prevalent mosaic symptom in canna plants in Beijing and its genomic and biological characterizations were further explored. All these data enriched the knowledge of the viruses infecting canna and would be helpful in effective disease management in canna.

Combining Analysis of DNA in a Crude Virion Extraction with the Analysis of RNA from Infected Leaves to Discover New Virus Genomes

This metagenome approach is used to identify plant viruses with circular DNA genomes and their transcripts. Often plant DNA viruses that occur in low titers in their host or cannot be mechanically inoculated to another host are difficult to propagate to achieve a greater titer of infectious material. Infected leaves are ground in a mild buffer with optimal pH and ionic composition recommended for purifying most bacilliform Para retroviruses. Urea is used to break up inclusion bodies that trap virions and to dissolve cellular components. Differential centrifugation provides further separation of virions from plant contaminants. Then proteinase K treatment removes the capsids. Then the viral DNA is concentrated and used for next-generation sequencing (NGS). The NGS data are used to assemble contigs which are submitted to NCBI-BLASTn to identify a subset of virus sequences in the generated dataset. In a parallel pipeline, RNA is isolated from infected leaves using a standard column-based RNA extraction method. Then ribosome depletion is carried out to enrich for a subset of mRNA and virus transcripts. Assembled sequences derived from RNA sequencing (RNA-seq) were submitted to NCBI-BLASTn to identify a subset of virus sequences in this dataset. In our study, we identified two related full-length badnavirus genomes in the two datasets. This method is preferred to another common approach which extracts the aggregate population of small RNA sequences to reconstitute plant virus genomic sequences. This latter metagenomic pipeline recovers virus related sequences that are retro-transcribing elements inserted into the plant genome. This is coupled to biochemical or molecular assays to further discern the actively infectious agents. The approach documented in this study, recovers sequences representative of replicating viruses that likely indicate active virus infection.

Molecular characterization of two badnavirus genomes associated with Canna yellow mottle disease

Members of the genus Badnavirus have a single non-covalently closed circular double-stranded DNA genome of 7.2-9.2kb. The genome encodes three open reading frames (ORFs) on the positive DNA strand. Canna yellow mottle virus (CaYMV) is a badnavirus that has been described as the etiological cause of yellow mottle disease in canna, although only a 565bp fragment of the genome has been previously reported from cannas. In this report, concentrated virions were recovered from infected canna plants and nucleic acids were extracted. Two full-length sequences represent two badnavirus genomes were recovered and were determined to be 6966bp and 7385bp in length. These DNAs represent a virus strain belonging to Canna yellow mottle virus and a novel species tentatively termed Canna yellow mottle associated virus. Phylogenetic analysis indicates that these two viruses are closely related to sugarcane bacilliform GD virus, pineapple bacilliform comosus virus, banana streak MY virus, and cycad leaf necrosis virus. We also showed naturally grown canna plants to be frequently co-infected by these two badnaviruses along with a potyvirus, Canna yellow streak virus.