Feb 15, 2010

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Plant breeding

TILLING: The ‘good’ alternative to genetic engineering?

There has been some debate recently about TILLING, a new plant-breeding method used by Bioplant, a company in Ebsdorf, to create an amylose-free potato. This is the same trait found in the genetically modified Amflora potato developed by BASF. Does TILLING make genetic engineering superfluous for plant breeding?

Not entirely, say experts like Dirk Prüfer of the Fraunhofer Institute for Molecular Biology and Applied Ecology. “TILLING can only be used to modify traits based on genes that are already in the plant.” It is not possible to develop insect-resistant plants like Bt maize and Bt cotton using the TILLING method.

TILLING (which stands for Targeting Induced Local Lesions In Genomes) is based on mutagenesis, a method that has been in use for a long time. The plants are treated with chemical substances or x-rays that cause undirected point mutations in the genome. The only thing new with TILLING is that it links mutagenesis to a DNA-analysis method that selects from thousands of mutagenized plants those that display the mutations in the desired gene. The mutations in question can e.g. cause the gene to be deactivated.

In the case of the amylose-free potato, a mutation deactivated a gene that is responsible for producing amylose in the potato cells. In the genetically modified Amflora potato, the same gene was switched off using a molecular biological method (antisense technology).

For plant breeders, TILLING has become an important tool for making further progress in plant breeding. The German Federal Ministry of Research has been funding the development of this method in barley, oilseed rape, potatoes and sugar beet since 2004 through the ‘Gabi-Till’ project. In the USA, there are firms like Arcadia Biosciences that specialize in TILLING, and a number of crops are currently being developed around the world, including salt-resistant tomatoes, drought-resistant soya beans, strawberries with a longer shelf life, gluten-free cereals, fungus-resistant barley and yellow tomatoes.

TILLING – a protracted breeding method

However, the TILLING method has its limitations compared to biotechnology. Treating the plants with chemicals that cause mutations creates a large number of point mutations throughout the genome. In wheat, for instance, this method can cause up to 20,000 such mutations. The result is that many genes are affected and do not function as they should. This means that the treated plants can lose desirable characteristics like their yielding and performance ability.

Plant breeders have to back-cross the plants that display the desired point mutation in a particular gene with high-yielding varieties before they can obtain productive plants again. This process usually takes several years – the TILLING potato took six years to develop.

Another limitation of the TILLING method is that it can modify only those traits governed by the plant’s own genes. It cannot generate the kind of insect resistance found in Bt plants. Some types of plant do not have genes for resistance to fungal diseases or for drought tolerance – two key plant-breeding aims for adapting to climate change. Introducing new metabolic pathways to produce new plant substances, e.g. provitamin A in Golden Rice, is not possible with TILLING.

Genetic engineering – the only breeding method with ‘risky consequences’?

Biotechnology critics like Greenpeace advisor Christoph Then present the TILLING method as an ‘acceptable’ breeding method that stays within the ‘normal range’ of plant characteristics. They claim that, from a technological point of view, there is a ‘huge difference’ between genetic engineering and TILLING.

However, many plant experts believe that concluding that TILLING leads to plants with a lower risk potential than genetic engineering is oversimplifying the situation. In a paper published in 2004, the National Research Council (USA) pointed out that, in principal, all plant-breeding methods can lead to unexpected negative effects. For instance, when potato plants were crossed in the 1960s and 1990s, two varieties were created (Lenape and Magnum Bonum), which displayed the same high, toxic content of alkaloids typically found in wild potatoes. Both potato varieties had to be withdrawn from the market as a result.

In another case, a new type of celery was found to contain high levels of Furanocoumarin and to cause severe skin rashes in farm workers. It is therefore general practice today to test all new varieties for such negative effects before they reach the market.

In Canada they have gone one step further: all new plants with novel traits (PNTs), either as a result of mutation breeding or natural mutation or through genetic modification, have to go through a complex approval process.