BREAD: Engineering Broad-spectrum Resistance Against Multiple Geminiviruses and Cassava Brown Streak Viruses Using Synthetic Trans-acting Small Interfering RNA (tasiRNA)

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PI: Vincent Fondong (Delaware State University)Co-PIs: Oumar Doungous (Delaware State University), Kone Mongomake (Delaware State University), Stephan Winter (German Collection of Microorganisms and Cell Cultures Braunschweig, Germany)Cassava is an important staple food crop to more than 500 million people worldwide, most of whom live in sub-Saharan Africa. In tropical and subtropical regions of Africa, the cassava crop is infected by viruses belonging to Geminiviridae and Potyviridae families, which are the two most important plant viruses in the world. Members of the Geminiviridae family cause the cassava mosaic disease while the Potyviridae cause the cassava brown streak disease. So far, seven virus species of the Geminiviridae family and two species of the Potyviridae have been identified across Africa. These viruses are increasingly occurring in mixed infections, causing synergistic interactions that result in severe symptoms and almost total crop loss. Currently, control of these viruses is through the use of resistant plant varieties coupled with various management strategies. However, transfer of resistance using conventional breeding is limited by several factors including lack of useful genes in core cassava germplasm collections, heterozygosity and allopolyploidy, irregular flowering, and low fertility rates. Thus, although classical breeding efforts have proved to be instrumental in developing virus resistant cassava varieties, it is complicated and slow, and is unlikely to provide a lasting solution to the fast spreading virus disease pandemics. This emphasizes the need for the development of new approaches to contain these viruses. The goal of this project is to design synthetic trans-acting small interfering RNAs (tasiRNAs) from conserved regions of cassava viruses with the capability of targeting members of virus families causing cassava diseases across Africa. TasiRNAs have been shown to be efficient in targeting RNA transcripts, including virus transcripts, and is thus a potentially powerful tool for the control of plant viruses. Because synthetic tasiRNAs of greater than 21 nucleotides are sufficient to efficiently silence cognate genes, this technology provides an unprecedented opportunity to target many viruses using small transgene constructs. This approach is more efficient than the current strategy based on long hairpin transgene constructs, which are unstable and are characterized by inefficient spread of the siRNA to appropriate targets. This project will contribute significantly to cassava virus control efforts in Africa. The project will also provide unique training opportunities in molecular biology to two African co-PIs who will use the technologies in their home institutions. Two postdoctoral researchers and six undergraduate students will be trained in the project. Anticipated outcomes include transgene constructs and DNA sequences with the potential to control cassava viruses. Results and materials generated from this research will be accessible to the scientific community through the Delaware State University website (http://www.appliedplantbiotech.com/index.html) and at the Co-PI website in Germany (http://www.dsmz.de/).