Characterization of Cassava brown streak virus proteins draw their sides during mixed infections and reveal P1 as a silencing suppressor

Cassava brown streak disease (CBSD) is currently one of the major constraints on cassava production in Africa. CBSD is estimated to cause annual economic losses of over $100 million USD. CBSD is caused by at least two viral species: Cassava brown streak virus (CBSV) and the Uganda cassava brown streak virus (UCBSV). In field, CBSV and UCBSV occur in single and mixed infections with the potential to be found in mixed infections with other viruses. The interactions between CBSV and other viruses are poorly understood and many functions of CBSV genes are not fully characterised. In this study we analysed the interaction of CBSV with non-related viruses, for potential synergistic interactions, namely tobacco mosaic virus (TMV), and potato virus Y (PVY), both very well characterised for their infection and symptomatology in Nicotiana species. These interactions demonstrated to be synergistic with TMV and antagonistic with PVY. Then P1, P3, 6k1, CI, 6k2, VPg, NIa, NIb, Ham1-like and CP from CBSV were analysed separately, to determine which genes from CBSV were responsible for the direction of these interactions. For this analysis, transgenic lines expressing single CBSV genes were used, providing information about the importance of Ham1. Further functional analysis of these CBSV genes was carried out, analysing silencing suppression activity through agroinfiltration assays. This confirmed silencing suppression activity for the CBSV P1 protein and demonstrated that a functional LRRA domain is required for this activity.