Size reversion of a truncated DNAbeta associated with Tobacco curly shoot virus

Our previous results demonstrated that DNAbeta associated with Tobacco curly shoot virus (TbCSV) is not necessary for infection but intensifies symptoms in some hosts. To better understand the function of DNAbeta in virus infection, a betaC1 deleted infectious clone of the TbCSV DNAbeta was constructed. Agroinoculation showed that the truncated DNAbeta (DNADeltaC1beta) was trans-replicated by TbCSV in tobacco and Petunia hybrida plants. However, PCR and Southern blot analysis demonstrated that the truncated DNAbeta reverted to near wild type component size in some Nicotiana benthamiana, N. glutinosa, N. tabacum Samsun and P. hybrida plants co-inoculated with TbCSV and DNADeltaC1beta. Sequence analysis of four DNADeltaC1beta derivatives revealed that the wild type size DNAbeta molecules were recombinants between TbCSV DNAbeta and the pBinPLUS vector in which dimeric constructs were produced for inoculation. The significance of these findings is discussed with respect to the constraints imposed on begomovirus genome size.