Cassava is an important staple crop in Africa and Asia, where it is eaten by more than 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 (CMD) is caused by a DNA virus complex consisting of seven geminivirus species that act synergistically to enhance disease severity. Recently, two satellite DNAs associated with the complex have been shown to break resistance and enhance symptoms.

 

Cassava mosaic geminiviruses (CMGs) induce diverse symptoms in cassava depending on the host genotype, age of infection, amount of virus inoculum, virus strain, vector activity, and environmental and other host factors. Research on CMGs has generated extensive information about viral diversity, genome sequence, replication, transmission, disease epidemiology and disease control. In contrast, there are no reports that describe the changes that occur in cassava leaves at a cellular level in response to CMG infection. The goal of this proposal is to establish cell biology infrastructure and expertise at Mikocheni Agricultural Research Institute (MARI) in Tanzania and to use these resources to characterize CMG infection in cassava. A combination of light and fluorescent microcopy will be used to examine CMD processes at a cellular level in cassava leaves. Using in situ hybridization to detect CMG and satellite DNAs in cassava leaves, these researchers will seek to determine if different CMGs infect different leaf cell types and if the nature and number of the target cells change in mixed infections and/or in the presence of the satellites. The research team will also examine the cellular architecture of infected leaves as a first step toward understanding the physiological basis of the extreme leaf deformation phenotypes correlated with the presence of CMD-associated satellites. The application of cell biology to CMD represents a unique opportunity to study the interactions of different viruses with a common host and with each other and satellite DNAs. The increased knowledge to be gained through the project should contribute to understanding of this important plant virus and to the development of sustainable strategies to control it and thereby limit its economic and nutritional impact.

 

 

From January through March 2013, research assistant Linus Paul of MARI completed three months of training on use of his institute’s expected new microscope for the in situ virus localization study. A vibratome for sectioning plant tissues was purchased and delivered to MARI, while the new Olympus microscope and its associated computer are to be delivered in the spring of 2013. Plans for the coming months are to conduct a training workshop at MARI by June 2013 on use of the microscope and collect samples of cassava infected with known cassava mosaic geminiviruses with or without satellites for in situ analysis using microscopy.