Characterisation of a Betasatellite Associated With Tomato Yellow Leaf Curl Guangdong Virus and Discovery of an Unusual Modulation of Virus Infection Associated With C4 Protein.

Tomato yellow leaf curl Guangdong virus (TYLCGdV), a monopartite begomovirus first identified in 2004, remains poorly characterised. In this study, we demonstrate that TYLCGdV associates with a betasatellite, TYLCGdB, and the βC1 protein encoded by TYLCGdB is essential for symptom development. We also explore the role of TYLCGdV C4 protein by generating a C4-deficient infectious clone (TYLCGdV), revealing a dynamic role for TYLCGdV C4. Specifically, viral accumulation in TYLCGdV/TYLCGdB-inoculated plants was significantly lower than that in TYLCGdV/TYLCGdB-inoculated plants at 7 and 14 days post-inoculation (dpi), but surpassed that of TYLCGdV/TYLCGdB-inoculated plants by 25 dpi. Furthermore, although C4 proteins in other begomoviruses typically exhibit one or more of the following properties: (i) suppression of post-transcriptional gene silencing (PTGS), (ii) suppression of transcriptional gene silencing (TGS), (iii) enhancement of pathogenicity in potato virus X (PVX) and (iv) symptom induction when transgenically expressed, TYLCGdV C4 did not exhibit any of these properties. However, the dynamic role of TYLCGdV C4 in viral infection appears to result from its effects on viral DNA methylation. At 7 dpi, the cytosine methylation level in the TYLCGdVmC4 genome was notably elevated compared to that of the wild-type virus. However, this trend reversed by 14 dpi, with the wild-type virus exhibiting a higher methylation level. By 25 dpi, the cytosine methylation levels of both TYLCGdVmC4 and TYLCGdV were comparable. These results indicate that TYLCGdV C4 modulates viral infection via an unconventional mechanism. This novel observation highlights the need for further investigation into the diverse roles of C4 proteins in begomoviruses.