Rescue of Tomato spotted wilt tospovirus entirely from cDNA clones, establishment of the first reverse genetics system for a segmented (-)RNA plant virus

The group of negative strand RNA viruses (NSVs) includes not only dangerous pathogens of medical importance but also serious plant pathogens of agronomical importance. Tomato spotted wilt tospovirus (TSWV) is one of those plant NSVs that cause severe diseases on agronomic crops and pose major threats to global food security. Its negative-strand segmented RNA genome has, however, always posed a major obstacle to molecular genetic manipulation. In this study, we report the complete recovery of infectious TSWV entirely from cDNA clones, the first reverse genetics (RG) system for a segmented plant NSV. First, a replication and transcription competent mini-genome replication system was established based on 35S-driven constructs of the S(-)-genomic (g) or S(+)-antigenomic (ag) RNA template, flanked by a 5 Hammerhead and 3 Ribozyme sequence of Hepatitis Delta virus, a nucleocapsid (N) protein gene and codon-optimized viral RNA dependent RNA polymerase (RdRp) gene. Next, a movement competent mini-genome replication system was developed based on M(-)-gRNA, which was able to complement cell-to-cell and systemic movement of reconstituted ribonucleoprotein complexes (RNPs) of S RNA replicon. After further optimization, infectious TSWV and derivatives carrying eGFP reporters were successfully rescued in planta via simultaneous expression of full-length cDNA constructs coding for S(+)-agRNA, M(-)-gRNA and L(+)-agRNA. Viral rescue occurred in the additional presence of various viral suppressors of RNAi, but TSWV NSs interfered with the rescue of genomic RNA. The establishment of a RG system for TSWV now allows detailed molecular genetic analysis of all aspects of tospovirus life cycle and their pathogenicity.\n\nSignificanceFor many different animal-infecting segmented negative-strand viruses (NSVs), a reverse genetics system has been established that allows the generation of mutant viruses to study disease pathology and the role of cis- and trans-acting elements in the virus life cycle. In contrast to the relative ease to establish RG systems for animal-infecting NSVs, establishment of such system for the plant-infecting NSVs with a segmented RNA genome so far has not been successful. Here we report the first reverse genetics system for a segmented plant NSV, the Tomato spotted wilt tospovirus, a virus with a tripartite RNA genome. The establishment of this RG system now provides us with a new and powerful platform to study their disease pathology during a natural infection.