A DNAbeta associated with Tomato yellow leaf curl China virus is required for symptom induction

We report here that all 25 isolates of Tomato yellow leaf curl China virus (TYLCCNV) collected from tobacco, tomato, or Siegesbeckia orientalis plants in different regions of Yunnan Province, China, were associated with DNAbeta molecules. To investigate the biological role of DNAbeta, full-length infectious clones of viral DNA and DNAbeta of TYLCCNV isolate Y10 (TYLCCNV-Y10) were agroinoculated into Nicotiana benthamiana, Nicotiana glutinosa, Nicotiana. tabacum Samsun (NN or nn), tomato, and petunia plants. We found that TYLCCNV-Y10 alone could systemically infect these plants, but no symptoms were induced. TYLCCNV-Y10 DNAbeta was required, in addition to TYLCCNV-Y10, for induction of leaf curl disease in these hosts. Similar to TYLCCNV-Y10, DNAbeta of TYLCCNV isolate Y64 was also found to be required for induction of typical leaf curl diseases in the hosts tested. When the betaC1 gene of TYLCCNV-Y10 DNAbeta was mutated, the mutants failed to induce leaf curl symptoms in N. benthamiana when coinoculated with TYLCCNV-Y10. However, Southern blot hybridization analyses showed that the mutated DNAbeta molecules were replicated. When N. benthamiana and N. tabacum plants were transformed with a construct containing the betaC1 gene under the control of the Cauliflower mosaic virus 35S promoter, many transgenic plants developed leaf curl symptoms similar to those caused by a virus, the severity of which paralleled the level of betaC1 transcripts, while transgenic plants transformed with the betaC1 gene containing a stop codon after the start codon remained symptomless. Thus, expression of a betaC1 gene is adequate for induction of symptoms of viral infection in the absence of virus.