Construction and characterization of an infectious cDNA clone of potato virus S developed from selected populations that survived genetic bottlenecks

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.

Precisely Monomeric Linear RNAs of Viroids Belonging to Pospiviroid and Hostuviroid Genera Are Infectious Regardless of Transcription Initiation Site and 5'-Terminal Structure

Infectious dimeric RNA transcripts are a powerful tool for reverse genetic analyses in viroid studies. However, the construction of dimeric cDNA clones is laborious and time consuming, especially in mutational analyses by in vitro mutagenesis. In this study, we developed a system to synthesize a precisely monomeric linear RNA that could be transcribed in vitro directly from the cDNA clones of four viroid species. The cDNA clones were constructed such that RNA transcription was initiated at the guanine nucleotide of a predicted processing and ligation site in the viroid replication process. Although the transcribed RNAs were considered to possess 5′-triphosphate and 3′-hydroxyl termini, the RNA transcripts were infectious even without in vitro modifications. Additionally, infectivity was detected in the monomeric RNA transcripts, in which transcription was initiated at guanine nucleotides distinct from the predicted processing/ligation site. Moreover, monomeric viroid RNAs bearing 5′-monophosphate, 5′-hydroxyl, or 5′-capped termini were found to be infectious. Northern blot analysis of the pooled total RNA of the plants inoculated with the 5′-terminal modified RNA of potato spindle tuber viroid (PSTVd) indicated that maximum PSTVd accumulation occurred in plants with 5′-monophosphate RNA inoculation, followed by the plants with 5′-triphosphate RNA inoculation. Our system for synthesizing an infectious monomeric linear viroid RNA from a cDNA clone will facilitate mutational analyses by in vitro mutagenesis in viroid research.

Synthesis and Characterization of a Full-Length Infectious cDNA Clone of Tomato Mottle Mosaic Virus

Tomato mottle mosaic virus (ToMMV) is a noteworthy virus which belongs to the Virgaviridae family and causes serious economic losses in tomato. Here, we isolated and cloned the full-length genome of a ToMMV Chinese isolate (ToMMV-LN) from a naturally infected tomato (Solanum lycopersicum L.). Sequence analysis showed that ToMMV-LN contains 6399 nucleotides (nts) and is most closely related to a ToMMV Mexican isolate with a sequence identity of 99.48%. Next, an infectious cDNA clone of ToMMV was constructed by a homologous recombination approach. Both the model host N. benthamiana and the natural hosts tomato and pepper developed severe symptoms upon agroinfiltration with pToMMV, which had a strong infectivity. Electron micrographs indicated that a large number of rigid rod-shaped ToMMV virions were observed from the agroinfiltrated N. benthamiana leaves. Finally, our results also confirmed that tomato plants inoculated with pToMMV led to a high infection rate of 100% in 4–5 weeks post-infiltration (wpi), while pepper plants inoculated with pToMMV led to an infection rate of 40–47% in 4–5 wpi. This is the first report of the development of a full-length infectious cDNA clone of ToMMV. We believe that this infectious clone will enable further studies of ToMMV genes function, pathogenicity and virus–host interaction.

Construction of an infectious full-length cDNA clone of potato aucuba mosaic virus

Potato aucuba mosaic virus (PAMV), a positive single-strand RNA virus, has one of the longest genomes of the viruses in the genus Potexvirus. In 2019, potato samples with mottle and crinkling symptoms from Huzhou, Zhejiang province, China, were identified to be infected with PAMV, potato virus X (PVX), and potato virus Y (PVY) by transcriptome sequencing. To study the effects of single infection by PAMV, the full-length sequence of PAMV from Huzhou (MT193476) was determined and an infectious full-length cDNA clone was constructed. This cDNA clone was infectious by agro-infiltration, leading to systemic symptoms in Nicotiana benthamiana, tomato, pepper, and potato.