Previous Fora / 2011


Research Officer, University of Cape Town, South Africa

Dr Dionne Shepherd holds a PhD in Molecular and Cell Biology (awarded 2003) and is currently a research scientist in the Department of Molecular and Cell Biology at the University of Cape Town, South Africa. She has published 28 peer-reviewed papers and holds the patent on the first all-African produced GMO. Her research focuses on the development of transgenic maize genotypes resistant to Maize streak virus (MSV), which causes maize streak disease – a disease that seriously threatens sustainable food production throughout Africa. Towards this end she has developed innovative systems for testing potential resistance genes and was the first to demonstrate that transgenic resistance to MSV is achievable. She is additionally working on the development of alternative transgenic-based strategies for combating maize streak and other viral diseases that affect important African food-crop species. These include gene silencing strategies and strictly virus-inducible resistance genes that will have no impact on food quality and safety. If successful these “next generation” transgenic resistance strategies will be combined within super-immune maize varieties that would be very unlikely to succumb to resistance-breaking mutant viruses. Dr Shepherd is additionally co-convenor of an extensive pan-African network of crop scientists and virologists that is carrying out the world’s largest ongoing plant pathogenic virus diversity studies. The focus of this collaborative group is the comparative analysis of maize streak disease transmission dynamics throughout Africa. With miniscule financial resources the group is successfully carrying out cutting-edge epidemiological research and has identified the predominant MSV genotypes that will confront MSV-resistant transgenics in different parts of the continent, has worked out the rates and pathways of virus spread throughout the continent and defined the specific regions within the continent that appear to be the cradles from which almost all new maize streak disease epidemics emerge. In 2008 Dr Shepherd was the recipient of the prestigious South African Department of Science and Technology’s Best Emerging Young Scientist award.



17:00-19:00 18 November
THEMATIC SESSION III. Brazil: “Sustainable Food Production”

Importance of virus-resistant transgenic maize in Africa
Despite being Africa’s most important staple food crop, average African maize yields are less than a quarter of world averages. A major contributing factor to these low yields is virus infection. Although African maize is infected by several virus species from at least six different families, there are two that stand out as the greatest threat to African food security: these are Maize streak virus [MSV]; family Geminiviridae, a virus species with an extremely small single-stranded DNA genome that was first discovered infecting African maize over a century ago; and Maize rough dwarf virus [MRDV], family Reoviridae, a virus species with a complex double-stranded RNA genome that until recently was thought to occur only in Europe and Asia. In 2010, however, MRDV struck maize in Uganda, East Africa, causing panic among African maize farmers.
Although MRDV would be potentially devastating if it took hold and spread throughout Africa, MSV is without question currently the biggest viral disease problem that African maize farmers face. It is found only in Africa, and causes the devastating maize streak disease (MSD) that can wipe out a farmer’s entire crop. Considering that over 50% of calories in local diets come from maize, yield losses caused by MSV can result in food shortages and famine.
MSV is transmitted amongst maize and other weed host plants by small insects called leafhoppers. The disease is therefore a result of a complex interplay between the virus, annual variations in maize and wild plant species densities, insect movement and population growth-and-decline cycles. Traditional methods used to limit the impact of MSD include insecticides to control leafhopper populations (which are not affordable to most small-scale farmers and can be environmentally damaging), and conventional selective breeding of maize for resistance traits that naturally exist in some varieties. Attempts have been made for over 40 years to produce MSV-resistant maize in this way, but it is a difficult process and successes have been limited.
To circumvent these problems, we are developing MSV-resistant maize by genetic engineering. We have used three variations of a strategy known as pathogen-derived resistance (PDR), whereby fragments of DNA from the pathogen are inserted into the plant genome providing it with resistance against the pathogen. For the first PDR variation, a mutated form of the viral replication associated protein gene (rep), which is essential for viral replication, was used to transform maize. Transgenic plants producing high levels of the mutant Rep protein are resistant to the virus, due to the mutant Rep interfering with the function of the virus’ non-mutated Rep.
In the second PDR variation, plants were engineered to express a mutant Rep only when they became infected with the virus. This has the advantage that the plant will not waste precious resources by producing large quantities of the mutant Rep when it is not needed.
The third PDR approach and the one that we intend using not only for MSV resistance but also for other viruses such as MRDV, is based on something called RNA-mediated gene silencing. Gene silencing is a natural adaptive defense mechanism used by plants against viruses. We aim to develop novel silencing-based virus resistance genes for inclusion within transgenic maize genotypes. In other words, we aim to trigger the natural anti-virus defense system already present in maize, to prime it to attack the virus before the virus even enters the plant.