Cutting edge

Plants are being genetically engineered to tolerate drought and resist crop virus to boost food output in sub-Saharan Africa

A recent paper on world food trends estimates that sub-Saharan Africa will have a food shortfall of 88.7 million tonnes by 2025.

Based on a linear projection from current yields, the yield in 2025 is calculated to be 1,536 tonnes per hectare. This is deemed insuficient and increases in food output will have to come from an expansion of the cropland area. One way to achieve this in this water-poor region is by improving drought tolerance of crops. Another way would be to boost the yield above the projected linear increase.

Both of these approaches are being studied in the department of microbiology at the University of Cape Town. We are using genetic engineering to transform plants to become tolerant towards drought and resistant to a major crop virus, maize streak virus.

South Africa has a number of so-called "resurrection plants", which are able to tolerate desiccation down to 5 per cent relative water content. We have chosen Xerophyta viscosa and prepared a gene bank from genes expressed in desiccated plants.

We determined the DNA sequence of a number of them to find out the possible function of the gene's product. To our delight they all "make sense" as having some role to play in drought tolerance.

One is an aldose reductase, which converts glucose into sorbitol. Sugars such as sorbitol are involved in osmotic stabilisation during desiccation. Another is a cold-regulated protein. A plant's response to cold often involves some of the same biological pathways as its response to drought. A third is a dehydrin, a protein involved in dehydration. A further one is a calcium-binding protein. Intracellular calcium is known to be important in signal transduction leading to changes in gene expression that allow the plant to respond to desiccation. A fifth is a late embryogenesis abundant protein, which is involved in dehydration tolerance in seeds.

These genes are being introduced into plants to investigate their effects on drought tolerance. "For many developing countries, even slight improvements in stress tolerances would significantly increase yields," said David Hoisington et al's 1999 paper on what plant genetic resources can contribute toward increased crop productivity.

This work created a stir at a recent Nato conference in Poland. The meeting addressed the role of genetic engineering in abiotic stress tolerance in agricultural crops, and our work was considered highly significant.

Maize is the staple food of most Africans but the crop is susceptible to maize streak virus (MSV), which is endemic in Africa. It is spread by the leafhopper, Cicadulina mbila. Losses due to MSV run into many millions of pounds, especially in east, central and west Africa. Increasing resistance to it could significantly increase maize yields.

Unlike most plant viruses, which use RNA as their genetic material, MSV has a single-stranded DNA genome. As a result we cannot use the standard method of rendering plants resistant to a virus, in which the plant makes lots of the virus's "coat" so that when the virus infects the plant it cannot "uncoat" itself. As soon as it tries to do so the plant simply re-coats it. Instead we are preventing the virus from replicating itself. It is therefore unable to spread and cause disease. We are working using an MSV-sensitive grass and have obtained our first MSV-resistant plants.

The applications for our work are obvious and the potential benefit is enormous. Unfortunately government agencies do not appear to agree, as our funds were cut drastically for 1999. But we are now receiving funding from industry and a charitable trust, allowing us to improve agricultural outputs in the years ahead.

Jennifer Thomson Head of the department of microbiology at the University of Cape Town