PI: Linda K. Hanley-Bowdoin (North Carolina State University)
CoPI: Joseph Ndunguru [Mikocheni Agricultural Research Institute (MARI), Tanzania]
Key Collaborator: Peter Sseruwagi [National Crops Resource Research Institute (NaCCRI-NARO), Uganda]
Cassava is an important staple crop in Africa and Asia, where it is eaten by over 700 million people every day. It is grown by subsistence farmers in the poorest villages and is often the only food source when other crops fail or are destroyed by conflict. Cassava can grow under drought, high temperature and poor soil conditions, but its production is severely limited by viral diseases. Cassava mosaic disease is caused by a DNA virus complex that includes seven geminivirus species. Two satellite DNAs are associated with the complex and can break resistance and enhance disease severity. The satellites do not resemble other geminivirus satellites and their mechanisms of action are not known. The first goal of the project is to examine how the satellites interact with cassava geminiviruses and the host plant to enhance disease processes. The research will characterize the transcription products of the satellites and ask if they are necessary for enhanced symptoms and resistance breaking. This analysis will serve as the basis of studies that determine if the satellites act through a small RNA or a protein and how they function during infection. The two satellite show only limited homology, cause different phenotypes, and may act via different mechanisms that will require different approaches to overcome their activities and restore disease resistance. If the satellites produce small RNAs that target host genes, this information could serve as the basis for developing transgenic cassava that express cleavage-resistant targets. Conversely, a gene silencing strategy could be deployed against the transcripts of a satellite protein coding region. The second goal of the project will determine if the satellites are encapsidated and transmitted by whiteflies like cassava geminiviruses or are released from the cassava genome during infection. If the satellites are released from the host genome, this information will impact future screens for resistant cassava cultivars that do not contain satellite sequences and cannot serve as reservoirs for their release.
Cassava mosaic disease has constrained cassava production in Africa for more than a century, but changes in the nature of the disease have led to losses on an unprecedented scale in the last 20 years. Several factors have contributed to the pandemic, including recombination between cassava geminiviruses resulting in enhanced virulence and the emergence of resistance breaking satellites. In 2005, the total crop losses due to cassava mosaic disease were 4M metric tons/year in Tanzania, Uganda, Rwanda and Burundi. The impact of the satellites on cassava yields has not yet been documented, but it is very likely that they will significantly increase losses and lead to food insecurity in Africa if measures are not taken to contain their effects. The first step in this process is to understand how the satellites enhance disease processes and are transmitted or released in infected cassava fields. The project will contribute important knowledge about the mechanism(s) of action and transmission of the satellites and enable the rational development of strategies to overcome their detrimental effects on efforts to combat cassava mosaic disease. The project will also provide unique training opportunities for a postdoctoral researcher in the US (funded in part through the NSF Office of International Science and Engineering) and a graduate student in Tanzania that include international collaboration as well as bench and field experience. The project will prepare the trainees to become globally engaged, independent scientists who can effectively address future disease problems caused by rapidly evolving plant viruses. The studies will transfer expertise on geminivirus replication and host interactions to facilitate future studies on cassava geminiviruses and their satellites and increase resources for these types of studies in Tanzania. Finally, the results of the research will be accessible through a project website, the MARI homepage (http://cloud2.gdnet.org/cms.php?id=organization_details&organization_id=3273), and published in open source journals and online newsletters such as the African Crop Science Society News (http://www.acss.ws) to ensure access to the public and to scientists in developing countries. Materials created as part of the project will be provided to the scientific research community upon request.