RESISTANCE OF CASSAVA EXPRESSING dsRNAs AGAINST CODING SEQUENCES OF GEMINIVIRUSES

Cassava Mosaic Disease (CMD) is a major constraint to cassava production in Africa and in the Indian sub-continent. CMD is caused by cassava geminiviruses (CGMs), which are bipartite ssDNA viruses. At least 9 CGM species are infecting cassava and CGM diversity is particularly high on the African continent. In order to engineer resistance against African cassava mosaic virus (ACMV), we developed transgenic cassava expressing dsRNA against the ACMV Rep (AC1) sequences that are conserved amongst CGMs. Transgenic dsAC1 cassava lines were selected based on the accumulation of transgene-derived small RNAs. We established a rapid and high-throughput agroinoculation method in order to assess virus resistance in cassava. Selected transgenic cassava lines displayed robust resistance against increasing loads of ACMV delivered by gene gun and by agroinoculation. We also observed a fast recovery phenotype in transgenic lines following graft inoculation of ACMV. Best performing dsAC1 cassava lines were also inoculated with South African cassava mosaic virus (SACMV) and East African cassava mosaic virus (EACMV). Recovery from SACMV and EACMV infections in transgenic cassava lines was delayed as compared to the recovery phenotype observed with ACMV infection. Collectively, our results suggest that high similarity between transgene-derived dsRNAs and target virus sequences is required to engineer transgenic plants rapidly recovering from virus infection. Selected transgenic dsAC1 cassava lines also showed delayed development of CMD symptoms in the field. Molecular analysis of CGM species in the field is being performed in order to better characterize the level of resistance of dsAC1 lines against multiple CGM species.