Institute of Biotechnology, Zhejiang University, Hangzhou 310029, P.R. China
We demonstrated that only 11 isolates were associated with DNAβ among 39 Tobacco curly shoot virus (TbCSV)-infected, field-collected samples. An infectious clone of TbCSV-[Y35], an isolate associated with DNAβ, induced severe upward leaf curling in Nicotiana benthamiana. In the presence of its cognate DNAβ (TbCSV-[Y35] DNAβ), the symptom changed to a downward leaf curl. Furthermore, TbCSV-[Y35] alone was able to induce severe symptoms in tobacco and tomato plants, although co-infection with DNAβ intensified symptom severity in tobacco plants. In contrast to other begomovirus-DNAβ complexes, the satellite had no effect on the accumulation of TbCSV-[Y35] DNA in systemically infected host plants. The βC1 mutant caused symptoms comparable to those induced by TbCSV-[Y35] in the absence of DNAβ. TbCSV-[Y35] can be transmitted between plants by a whitefly vector, regardless of the presence or absence of DNAβ. For a TbCSV isolate not associated with DNAβ (TbCSV-[Y1]), systemic infection of N. benthamiana induced symptoms resembling those of TbCSV-[Y35]. Co-infection of TbCSV-[Y1] with TbCSV-[Y35] DNAβ induced symptoms similar to those following infection by TbCSV-[Y35] and its DNAβ. This indicates that TbCSV DNAβ is not necessary for infection but intensifies symptoms in a host-dependent manner. Thus, TbCSV may represent an evolutionary intermediate between the DNAβ requiring begomoviruses and the truly monopartite begomoviruses. The relevance of these results to our present understanding of the evolution of begomovirus-satellite disease complexes is discussed.