Tobacco curly shoot virus DNAP is not necessary for infection but intensifies symptoms in a host-dependent manner

We demonstrated that only 11 isolates were associated with DNA beta among 39 Tobacco curly shoot virus (TbCSV)-infected, field-collected samples. An infectious clone of TbCSV-[Y35], an isolate associated with DNA beta, induced severe upward leaf curling in Nicotiana benthamiana. In the presence of its cognate DNA beta (TbCSV-[Y35] DNA beta), 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 DNAP intensified symptom severity in tobacco plants. In contrast to other begomovirus-DNAP complexes, the satellite had no effect on the accumulation of TbCSV-[Y35] DNA in systemically infected host plants. The beta C1 mutant caused symptoms comparable to those induced by TbCSV-[Y35] in the absence of DNA beta. TbCSV-[Y35] can be transmitted between plants by a whitefly vector, regardless of the presence or absence of DNA beta. For a TbCSV isolate not associated with DNAP (TbCSV-[Y1]), systemic infection of N. benthamiana induced symptoms resembling those of TbCSV-[Y35]. Co-infection of TbCSV[Y1] with TbCSV-[Y35] DNA beta induced symptoms similar to those following infection by TbCSV-[Y35] and its DNA beta. This indicates that TbCSV DNA beta is not necessary for infection but intensifies symptoms in a host-dependent manner. Thus, TbCSV may represent an evolutionary intermediate between the DNA beta 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.