Faba bean necrotic yellows virus (FBYNV), Milk vetch dwarf virus (MDV), and Subterranean clover stunt virus (SCSV) are nanoviruses that infect leguminous plants. From MDV- and SCSV-infected tissue we identified viral DNAs that encode a replication initiator protein (Rep), essential for replication of the multiple circular single-stranded DNAs of these viruses. These previously undescribed Rep proteins of MDV and SCSV are strikingly similar in sequence and functionally equivalent to the master Rep protein of FBYNV. Moreover, we demonstrated that the master Rep proteins of the three viruses are able to trigger replication of heterologous nanovirus DNAs. Such cross-species replication may reflect a considerable potential for genetic reassortment among nanoviruses in nature and be of significance for their evolution.
Tomato yellow leaf curl virus (TYLCV) is a whitefly-transmitted geminivirus with a monopartite genome. We have investigated the functions of the V1, V2, and C2 ORFs by mutational analysis. We analyzed the ability of TYLCV mutants containing disrupted ORFs V1, V2, or C2 to replicate, spread, and cause symptoms in Nicotiana benthamiana and tomato plants. All the mutants retained the capability of autonomous replication in protoplast-derived cells of tomato and leaf discs of N. benthamiana, although both V1 and V2 gene products appeared to play a role in the accumulation of viral single-stranded DNA. In contrast, none of the mutants was able to systemically infect tomato plants, demonstrating that the V1, V2, and C2 gene products are all required for a successful infection process in this host. The effect of the mutation in ORF C2 appeared to be host-specific, since N. benthamiana plants were systemically infected, although symptom development was attenuated.
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