Bacterial arsenal to combat chewing insects

With the Bt toxin, as with all proteins, the "blueprint" is encoded in a specific DNA sequence (gene). The gene is expressed and the respective protein produced. This process is basically the same in both bacterial cells and plants. If the Bt toxin gene isolated from bacteria is inserted into a plant’s genetic material, the plant cells will obey the "command" of the foreign gene: the plant then produces its own Bt toxin. Chewing insects sensitive to the toxin strain in question absorb the active ingredient, which is specifically toxic to them, along with the plant parts. If the concept works, insecticides will no longer be needed to control these pests.

Maize: the most important Bt plant

Reports of Bt genes being integrated into plants started emerging as far back as the 1980s, initially with tobacco and tomatoes. Then in 1995 the US authorised the first Bt plant: Bt maize.

The Bt concept was particularly attractive for maize, since it made it possible for the first time to combat the European corn borer caterpillars inside the plant. Bt maize has now been grown on a large scale for over a decade, particularly in the USA. In 2007, insect-resistant Bt maize was grown on 21 per cent of the total maize cultivation area, and Bt maize with a combination of insect and herbicide resistance was grown on a further 28 per cent. Various Bt maize varieties are authorised in the EU as well. In 2007 there was notable cultivation of Bt maize primarily in Spain, where it was grown on around 75,000 hectares.

In recent years there has been increasing focus on another maize pest, the Western corn rootworm, that is spreading all the time – including in Europe. Bt maize which is resistant to the Western corn rootworm has been authorised in the USA since 2003 and has been grown on a large scale since then.

Bt not just for maize

However, maize is not the only plant into which Bt genes have been transferred. Genes for different toxin variants are transferred depending on the pests that a plant needs to be able to fend off:

• European corn borer: Bt toxin cry 1Ab (cry=crystal protein)

• Western corn rootworm: cry 3Bb1

• Cotton bollworm: cry 1Ac

• Colorado potato beetle: cry 3A

In 2006 18% of the genetically modified crops grown on 114 million hectares worldwide were insect-resistant Bt plants. A further 19% were a combination of herbicide- and insect-resistant crops. Herbicide resistance is the main GM trait, at 63%.

Distribution in the plant

Bt plants and varieties differ not just in terms of the particular strain of toxin they produce but also in terms of the amount of Bt toxin produced and its distribution in the plant.

The first commercially grown Bt maize plants contained high levels of Bt toxin in all plant parts - pollen, stems and maize cobs. They contained greater amounts of toxin than was strictly necessary to achieve the desired effect.

More recent Bt maize varieties not only produce smaller quantities of Bt toxin, they produce it only where it is needed. Today, Bt genes are provided with tissue-specific promoters , which activate toxin production only in specific plant parts. The Bt maize developed in recent years to combat the Western corn rootworm, for instance, produces Bt toxin primarily in the roots.

Bt maize: Effect on non-target organisms

Bt varieties – particularly maize and cotton – are now firmly established in some countries. But they still attract controversy.

IIn the USA a furore was created by a study which found evidence to suggest that pollen from Bt maize was harming the much loved monarch butterfly. The excitement has since abated. The study findings were based on laboratory trials which did not reflect natural conditions and were put into perspective by subsequent studies. Nevertheless, fears that the toxin could harm not just the target organism it is designed to combat, but also other living organisms in the maize field, are behind numerous safety research projects looking at the effect of Bt toxin on non-target organisms. No clear impacts that would indicate harmful effects have yet been found.

Current biosafety research projects are looking at the effects of Bt maize that is resistant to a maize pest that is new to Europe: the Western corn rootworm.