Maize Streak Virus-Resistant Transgenic Maize: an African solution to an African Problem
7 Aug, 2007 11:58 am
Maize is not native to Africa; however, since being introduced by Portuguese traders in the 16th century, maize has replaced traditional African crops such as sorghum and millet as the continent?s most important staple food crop.
MSV is transmitted to maize by small insects called leafhoppers. The disease is therefore a result of a complex interplay between the plant, the virus and insect. Factors that can influence the severity of the disease include the age at which the plant is infected (the younger the plant, the more severe the infection), the maize variety (some are more susceptible than others), and environmental conditions.
Traditional methods used to limit the impact of MSD include insecticide use to control the leafhopper population (which is not affordable to most small scale farmers and can be environmentally damaging), and conventional breeding, or breeding of maize to select 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 are limited. Firstly, the resistance traits are not present in a single gene, but are dispersed throughout the maize genome. When maize is bred (one inbred crossed with another), sometimes one or more of these genes is not inherited by the offspring, resulting in a breakdown of resistance. Secondly, resistance traits can be linked to undesirable characteristics such as small kernel size or low yield and it is difficult to separate the good genes from the bad. Thirdly, a maize variety that is resistant under one set of climatic conditions for which it has been bred may not be resistant under another set of climatic conditions. Lastly, a maize variety that has been bred to be resistant to a predominant viral strain in one region of Africa may not be resistant to different viral strains that exist in other regions.
To circumvent these problems, our team at the University of Cape Town, South Africa has developed an MSV-resistant maize variety by genetic engineering (GE) (Shepherd et al., 2007b). Since the resistance trait is present in a single gene it is relatively simple to introduce it into a range of maize hybrids suited to different climatic or environmental conditions. We have also shown that our GE maize is resistant to the most prevalent and severe viral strains currently identified throughout sub-Saharan Africa.
To engineer MSV-resistant maize, we used an approach called pathogen-derived resistance (PDR). This means that a gene from the viral pathogen in this case is used to protect the plant from that pathogen. We mutated a viral gene that under normal circumstances produces a protein (called the Rep protein) that is essential for the virus to replicate itself (Shepherd et al., 2007a). We inserted the mutated gene, which produces a defective Rep protein, into the maize plant’s genome, creating genetically modified (GM) maize. When the virus infects the plant, the first thing it does is to produce its own Rep protein so that it can replicate and spread throughout the plant. However, since the defective Rep protein produced by the GM plant was originally derived from the virus, it can potentially bind to both the virus’ native Rep protein and to the virus’ DNA. In doing so, it interferes with the function of the virus’ native Rep protein and inhibits viral replication. The virus can no longer spread throughout the plant and the severity of symptoms it produces is drastically reduced.
We have shown in greenhouse trials that this strategy is extremely effective and that the resistance is passed on through each successive generation. We have also shown that the single-gene resistance trait can be easily transferred to different maize inbreds, hugely simplifying the breeding process. We envisage that the resistant maize inbreds will allow the simple and rapid development of hybrids for commercial use in all climatic regions in Africa and its surrounding Indian Ocean islands.
The next step is to perform field trials, to prove that the resistance holds up over several growing seasons in the field, and to evaluate the safety and environmental impact of the GM crop. Before the crop can be grown commercially, we need to perform a “risk assessment”, to show for example that the gene inserted in the GM maize does not cause allergies, is digestible, is not toxic to animals or humans, and has no negative effects on the environment and other organisms. Only when it has been proven to be safe in all these respects will it be allowed to be sold for food or feed. We will also be performing post-market environmental monitoring to ensure that the GM trait can be traced and has no long term negative environmental impacts.
MSV is endemic to Africa. This, the first GM MSV-resistant maize, was developed by an all-African team. It is a first step towards Africa losing its reliance on foreign biotechnology and multinational seed companies. Our commercial partner, the South African seed company PANNAR (Pty) Ltd, is committed to making the MSV-resistant seed available at an affordable price to small-scale farmers who need it the most. It is an African solution to a uniquely African problem.
Shepherd, D. N., Mangwende, T., Martin, D. P., Bezuidenhout, M., Thomson, J. A. and
Rybicki, E. P. (2007a). Inhibition of maize streak virus (MSV) replication by transient and transgenic expression of MSV replication-associated protein mutants. J. Gen. Virol. 88, 325-336.
Shepherd, D.N., Mangwende, T., Martin, D.P., Bezuidenhout, M., Kloppers, F., Carolissen, C.H., Monjane, A.L., Rybicki, E.P. & Thomson, J.A. (2007b). Maize streak virus-resistant transgenic maize: a first for Africa. Plant Biotech J. in press.