GMO COMPASS - Information on genetically modified organisms
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 Breeding Aims

Disease Resistance


Plants can suffer from infections caused by fungi, bacteria, viruses, nematodes, and other pathogens. Various high-tech approaches have been proposed to protect plants from harmful afflictions. To date, most interest has been focused on virus resistant transgenic plants, but using biotechnology to confer resistance to fungi, bacteria, or nematodes has also been gaining attention.

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Late blight of potato is caused by the fungus Phytophtora infestans, the disease responsible for the great Irish potato famine of 1846-47. It led to the deaths of more than one million people.

Fighting fungal infections

Fungi are responsible for a range of serious plant diseases such as blight, grey mould, bunts, powdery mildew, and downy mildew. Crops of all kinds often suffer heavy losses.

Fungal plant diseases are usually managed with applications of chemical fungicides or heavy metals. In some cases, conventional breeding has provided fungus resistantcultivars.

Besides combatting yield losses, preventing fungal infection keeps crops free of toxic compounds produced by some pathogenic fungi. These compounds, often referred to as mycotoxins, can affect affect the immune system and disrupt hormone balances. Some mycotoxins are carcinogenic.

Fungus resistant GM plants

Genetic engineering enables new ways of managing fungal infections. Several approaches have been taken:

  • Introducing genes from other plants or bacteria encoding enzymes like chitinase or glucanase: These enzymes break down chitin or glucan, respectively, which are essential components of fungal cell walls.

  • Introducing plant genes to enhance innate plant defense mechanisms (e.g. activing phytoalexins, proteinase inhibitors, or toxic proteins).

  • Invoking the hypersensitive reaction: Plants varieties that are naturally resistant to specific types of fungal diseases are often programmed to have individual cells quickly die at the site of fungal infection. This response, known as the hypersensitive reaction, effectively stops an infection in its tracks. Genetic engineering can help plant cells 'know' when a fungus is attacking.

No commercial cultivars available today use these approaches.

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Papaya ring spot potyvirus is a serious viral disease of papaya, which inhibits photosynthesis and stunts growth. Virus resistant GM papyas have been developed (see below).

Viral diseases

Viruses cause many economically important plant diseases. For example, the Beet necrotic yellow vein virus (BNYVV) causes sugar beets to have smaller, hairier roots, reducing yields by up to 50 percent. The spread of most viruses is very difficult to control. Once infection sets in, no chemical treatment methods are available. Losses are usually very high and require longer rotation intervals and modified cropping systems. This translates into considerable losses.

Viruses are often transmitted from plant to plant by insects. Insecticides are sometimes used to control viral infections, but success is very limited.

The most effective ways of managing viruses are cultural controls (e.g. removing diseased plants) and using resistant cultivars. Although conventional methods of breeding have been able to provide some virus resistant or tolerant cultivars, they are not available for most corps.

Virus resistant GM plants

In some cases, biotechnology can be used to make virus resistant crops. The most common way of doing this is by giving a plant a viral gene encoding the virus' 'coat protein'. The plant can then produce this viral protein before the virus infects the plant. If the virus arrives, it is not able to reproduce.

The explanation for this is called cosuppression. The plant has ways of knowing that the viral coat protein should not be produced, and it has ways of eventually shutting down the protein's expression. When the virus tries to infect the plant, the production of its essential coat protein is already blocked.

All genetically modified virus resistant plants on the market (e.g. papayas and squash) have coat protein mediated resistance. It may also be possible to confer resistance by taking a resistance gene naturally found in one plant and then transferring it to an important crop.

 

December 11, 2006 [nach oben springen]

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