Strategies to control a devious pathogen
Phytophthora infestans, the pathogen responsible for potato late blight, is one of the most devastating potato diseases of all. The tiny fungus causes annual harvest losses of around 3 billion euros. Attempts to breed resistant varieties have so far failed to bring lasting success, because the pathogen always manages to breach the bred-in resistance. A genetically modified potato containing two resistance genes from wild potatoes is currently being tested at various sites in Europe.
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Up until now, Phytophthora has been controlled almost exclusively with the use of chemical fungicides. Potato crops in Germany are sprayed up to 16 times during a single growing season. In organic farming, environmentally harmful copper compounds are used to control Phytophthora.
Because Phytophthora is so versatile and adaptable, conventional plant breeders throughout the world are currently working on a resistance type which is determined by several genes and is not race specific. Although this does not result in absolute protection against infestation, the protection is long-lasting because it cannot be breached as easily by new forms of the pathogen, due to the large number of genes involved. The genes responsible for the race-nonspecific resistance are crossed from wild potatoes to cultivated varieties. However, the undesirable traits from the wild potatoes, which are transferred at the same time, must then be bred out again, without losing the resistance traits. The complexity of this hereditary transmission makes the process difficult and time-consuming. Transferring resistance genes from Mexican wild potatoes For many years scientists have also been attempting to develop Phytophthora-resistant potatoes using genetic engineering techniques. Here too, the main problem is the extraordinary versatility of the pathogen, which means that it is not enough simply to transfer a single gene responsible for fungal resistance. |
In recent years BASF has developed a promising fungus-resistant potato which contains two genes from Mexican wild potatoes. The BASF scientists adopted exactly the same approach as if they were using traditional breeding methods: they searched wild potato strains for one which had a high natural resistance to the potato late blight pathogen. They found what they were looking for in the Mexican wild potato Solanum bulbocastanum. Then, using molecular-biological techniques, they located the genes responsible for resistance, which they isolated and inserted into the plant by genetic engineering. Since Phytophthora has always succeeded in overcoming resistances after a short time, they inserted two resistance genes at the same time. The "dual" resistance should offer sustained protection against the disease.
The potatoes have been successfully tested for their resistance in greenhouse trials and have been undergoing field trials in Sweden, the Netherlands, Britain, Germany and Ireland since 2006. The aim is to place the potatoes on the market on completion of the field trials, if they prove to be successful.
Further genetic approaches
In addition to the possibility of transferring resistance genes from wild potatoes, research has also been carried out on a number of other strategies:
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Transferring plant or bacterial genes for substances which destroy the fungal cell walls, e.g. chitinase or glucanase .
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Inserting genes for specific proteins which are produced by plants to protect them against fungi.
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Increasing natural resistance using two genes from a soil bacterium.
These approaches have not yet produced any genetically modified fungus-resistant potato varieties that are likely to be brought onto the market.
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