Infectivity and complete nucleotide sequence of cucumber fruit mottle mosaic virus isolate Cm cDNA

Virus-induced gene silencing for in planta validation of gene function in cucurbits

Virus-induced gene silencing (VIGS) is a powerful tool for high-throughput analysis of gene function. Here, we developed the VIGS vector pCF93, from which expression of the cucumber fruit mottle mosaic virus genome is driven by the cauliflower mosaic virus 35S promoter to produce viral transcripts in inoculated plants. To test the utility of the pCF93 vector, we identified candidate genes related to male sterility (MS) in watermelon (Citrullus lanatus), which is recalcitrant to genetic transformation. Specifically, we exploited previously reported reference-based and de novo transcriptome data to define 38 differentially expressed genes between a male-sterile line and its fertile near-isogenic line in the watermelon cultivar DAH. We amplified 200- to 300-bp fragments of these genes, cloned them into pCF93, and inoculated DAH with the resulting VIGS clones. The small watermelon cultivar DAH enabled high-throughput screening using a small cultivation area. We simultaneously characterized the phenotypes associated with each of the 38 candidate genes in plants grown in a greenhouse. Silencing of 8 of the 38 candidate genes produced male-sterile flowers with abnormal stamens and no pollen. We confirmed the extent of gene silencing in inoculated flowers using reverse transcription-qPCR. Histological analysis of stamens from male-fertile and male-sterile floral buds and mature flowers revealed developmental defects and shrunken pollen sacs. Based on these findings, we propose that the pCF93 vector and our VIGS system will facilitate high-throughput analysis for the study of gene function in watermelons.

Detection and complete genome sequence of a divergent isolate of cucumber fruit mottle mosaic virus on Coccinia grandis in Sudan

Cucurbit-infecting tobamoviruses known so far belong to six acknowledged or tentative species. Except for cucumber green mottle mosaic virus (CGMMV), which is present worldwide, they are geographically restricted, mostly to Asia, and have not been observed in Africa so far. A tobamovirus isolate infecting a wild Coccinia grandis plant was collected in central Sudan in 2012. Its host range appeared to be mostly limited to cucurbits. Its full-length genome sequence was determined and found to be 85% identical to those of isolates of cucumber fruit mottle mosaic virus (CFMMV) described in Israel and Korea, whereas the aa sequence identity to CFMMV isolates was 92 to 95%, depending on the protein. Based on its biological and molecular properties, we suggest that the Sudanese isolate should be considered a divergent isolate of CFMMV. This is the first description of CFMMV in Africa. Its high divergence from isolates from Israel and Korea suggests a lack of recent exchanges between CFMMV from Sudan and the other known populations.

Exploring Different Mutations at a Single Amino Acid Position of Cucumber green mottle mosaic virus Replicase to Attain Stable Symptom Attenuation

Cucumber green mottle mosaic virus (CGMMV) is a member of the genus Tobamovirus (family Virgaviridae) that causes serious economic losses in cucurbit crops. A possibility for CGMMV control is the use of cross-protection, for which stable attenuated isolates are required. In this study, an infectious clone was constructed for the hn isolate of CGMMV. Unexpectedly, this clone carried a nonconserved mutation involving a single nucleotide change resulting in the replacement of Arg by Cys at residue 284 of the replicase protein; this mutation correlated with delayed symptom induction and RNA accumulation, as shown in time-course experiments. Sequencing of the viral progeny showed that restoration of wild-type symptoms and increased RNA accumulation correlated with reversion of the mutation to the wild-type sequence, a phenomenon that occurred at approximately 7 to 10 days postinoculation. Thus, Arg284 seems to be crucial but not strictly necessary for virus infection. Subsequently, four other mutants in the triplet encoding Arg284 were constructed and assayed. Results showed that symptoms and their timing were diverse for the different mutants, with enhanced pathogenicity and RNA accumulation always correlating with reversion to Arg284. Therefore, the nature of the mutation strongly influenced the genetic stability of the mutant. At least two mutants were identified for which reversion did not occur by 30 days postinoculation, and these were defined as good candidates to attain stable symptom attenuation that could be useful in cross-protection.

Identification of the subgenomic promoter of the coat protein gene of cucumber fruit mottle mosaic virus and development of a heterologous expression vector

Heterologous gene expression using plant virus vectors enables research on host-virus interactions and the production of useful proteins, but the host range of plant viruses limits the practical applications of such vectors. Here, we aimed to develop a viral vector based on cucumber fruit mottle mosaic virus (CFMMV), a member of the genus Tobamovirus, whose members infect cucurbits. The subgenomic promoter (SGP) in the coat protein (CP) gene, which was used to drive heterologous expression, was mapped by analyzing deletion mutants from a CaMV 35S promoter-driven infectious CFMMV clone. The region from nucleotides (nt) -55 to +160 relative to the start codon of the open reading frame (ORF) of CP was found to be a fully active promoter, and the region from nt -55 to +100 was identified as the active core promoter. Based on these SGPs, we constructed a cloning site in the CFMMV vector and successfully expressed enhanced green fluorescent protein (EGFP) in Nicotiana benthamiana and watermelon (Citrullus lanatus). Co-inoculation with the P19 suppressor increased EGFP expression and viral replication by blocking degradation of the viral genome. Our CFMMV vector will be useful as an expression vector in cucurbits.

Construction of infectious clones of tomato torrado virus and their delivery by agroinfiltration

The first biologically active infectious clones of tomato torrado virus (ToTV) were generated and delivered into Nicotiana benthamiana and Solanum lycopersicum plants via Agrobacterium tumefaciens. The engineered constructs consisted of PCR-amplified complementary DNAs derived from the ToTV RNA1 and RNA2 components, individually inserted into an engineered pGreen binary vector between the CaMV 35S promoter and nopaline synthase terminator. These constructs were introduced into the plant hosts by means of A. tumefaciens-mediated infiltration. In the presence of the progeny virus, typical symptoms of ToTV infection developed in N. benthamiana and S. lycopersicum. Moreover, the virus was sap-transmissible when isolated from agroinfiltrated plants and induced symptoms similar to those caused by the wild-type virus. The presence of viral particles and viral genetic material was confirmed by electron microscopy and re-inoculation to S. lycopersicum and N. benthamiana, as well as by reverse transcription polymerase chain reaction and high-resolution melt analysis.