Rescue of Citrus sudden death-associated virus in Nicotiana benthamiana plants from cloned cDNA: insights into mechanisms of expression of the three capsid proteins

A Breakthrough in Kitavirids: Genetic Variability, Reverse Genetics, Koch's Postulates, and Transmission of Hibiscus Green Spot Virus 2

Hibiscus green spot virus 2 (HGSV-2), a member of the genus Higrevirus (family Kitaviridae), is a positive-stranded RNA virus associated with leprosis-like symptoms in citrus and green spots on leaves in hibiscus. HGSV-2 has only been reported in Hawaii, and while it is speculated that mites in the genus Brevipalpus might be responsible for its transmission, proper transmission assays have yet to be conducted. This study characterizes additional citrus and hibiscus isolates of HGSV-2 collected from two Hawaiian Islands. We constructed an infectious cDNA clone from a hibiscus isolate of HGSV-2 collected on Oahu and demonstrated its ability to infect several experimental hosts, including Phaseolus vulgaris, Nicotiana tabacum, and N. benthamiana, as well as natural hosts, Citrus reticulata and Hibiscus arnottianus. Bacilliform virions with varied sizes of 33 to 120 nm (length) and 14 to 70 nm (diameter) were observed in partially purified preparations obtained from agroinoculated leaves. Virus progeny from the infectious cDNA clone was found to be infectious after mechanical transmission to N. benthamiana and to cause local lesions. Finally, an isoline colony of the mite Brevipalpus azores had vector competence to transmit a citrus isolate of HGSV-2 collected from Maui to citrus and hibiscus plants, demonstrating the mite-borne nature of HGSV-2. The infectious cDNA clone developed in this study is the first reverse-genetics system for a kitavirid and will be fundamental to better characterize basic biology of HGSV-2 and its interactions with host plants and mite vectors.

A plant-infecting subviral RNA associated with poleroviruses produces a subgenomic RNA which resists exonuclease XRN1 in vitro

Subviral agents are nucleic acids which lack the features for classification as a virus. Tombusvirus-like associated RNAs (tlaRNAs) are subviral positive-sense, single-stranded RNAs that replicate autonomously, yet depend on a coinfecting virus for encapsidation and transmission. TlaRNAs produce abundant subgenomic RNA (sgRNA) upon infection. Here, we investigate how the well-studied tlaRNA, ST9, produces sgRNA and its function. We found ST9 is a noncoding RNA, due to its lack of protein coding capacity. We used resistance assays with eukaryotic Exoribonuclease-1 (XRN1) to investigate sgRNA production via incomplete degradation of genomic RNA. The ST9 3' untranslated region stalled XRN1 very near the 5' sgRNA end. Thus, the XRN family of enzymes drives sgRNA accumulation in ST9-infected tissue by incomplete degradation of ST9 RNA. This work suggests tlaRNAs are not just parasites of viruses with compatible capsids, but also mutually beneficial partners that influence host cell RNA biology.

Development of a New Tomato Torrado Virus-Based Vector Tagged with GFP for Monitoring Virus Movement in Plants

Green fluorescent protein (GFP)-tagged viruses are basic research tools widely applied in studies concerning molecular determinants of disease during virus infection. Here, we described a new generation of genetically stable infectious clones of tomato torrado virus isolate Kra (ToTV_pJL-Kra) that could infect Nicotiana benthamiana and Solanum lycopersicum. Importantly, a modified variant of the viral RNA2—with inserted sGFP (forming, together with virus RNA1, into ToTV_pJL-Kra_GFP)—was engineered as well. RNA2 of ToTV_pJL-Kra_GFP was modified by introducing an additional open reading frame (ORF) of sGFP flanked with an amino acid-coding sequence corresponding to the putative virus protease recognition site. Our further analysis revealed that sGFP-tagged ToTV-Kra was successfully passaged by mechanical inoculation and spread systemically in plants. Therefore, the clone might be applied in studying the in vivo cellular, tissue, and organ-level localization of ToTV during infection. By performing whole-plant imaging, followed by fluorescence and confocal microscopy, the presence of the ToTV_pJL-Kra_GFP-derived fluorescence signal was confirmed in infected plants. All this information was verified by sGFP-specific immunoprecipitation and western blot analysis. The molecular biology of the torradovirus-plant interaction is still poorly characterized; therefore, the results obtained here opened up new possibilities for further research. The application of sGFP-tagged virus infectious clones and their development method can be used for analyzing plant-virus interactions in a wide context of plant pathology.

Phylogenetic classification of a group of self-replicating RNAs that are common in co-infections with poleroviruses

Tombusvirus-like associated RNAs (tlaRNAs) are positive-sense single-stranded RNAs found in plants co-infected with viruses of the genus Polerovirus. TlaRNAs depend upon capsid proteins supplied in trans by the co-infecting polerovirus vector for transmission and intra-host systemic movement. Here, the full-length genomes of five tlaRNAs were determined using a combination of RT-PCR and next-generation sequencing, and evidence is provided for an additional tlaRNA associated with potato leafroll virus. Phylogenetic analyses based on conserved domains of the RdRp placed tlaRNAs as a monophyletic clade clustering with members of the family Tombusviridae and comprising three different subclades. Full-length clones of tlaRNAs from two of three subclades were confirmed to replicate autonomously, and each produces a subgenomic RNA during infection.