Nov 21, 2007

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Bt toxin in the soil

“We found no accumulation at any time.”

How much Bt toxin produced by Bt maize plants is released into the soil and what happens to it when it gets there? Scientists involved in biosafety research have been considering these questions for several years. GMO Safety spoke to Christoph Tebbe from the Johann Heinrich von Thünen-Institut, Federal Research Institute for Rural Areas, Forestry and Fisheries (vTI).

GMO Safety: In safety research projects you are studying the effects of Bt maize plants on the soil. What are Bt maize plants exactly?

Christoph Tebbe: Bt maize plants are genetically modified plants which, unlike conventionally bred varieties, are able to produce a substance to control certain insects. This substance is an insecticide which is actually produced by bacteria collectively referred to as Bacillus thuringiensis. Hence the name Bt toxin or Bt. Bacillus thuringiensis occurs naturally in many soil types and Bt toxin is even used as a pesticide in organic farming.

GMO Safety: How can Bt toxin enter the soil from genetically modified Bt maize?

Christoph Tebbe: Once the maize plants have germinated in the field, the plant cells start producing Bt toxin. Most of the Bt toxin in the soil is concentrated in the roots, and a small quantity, one thousandth of the amount found in the roots, can be present in the soil surrounding the roots, an area known as the rhizosphere. Bt toxin can also enter the soil surface via pollen or fallen leaves. However, the largest release of Bt toxin into the soil occurs after harvesting, when the maize stubble is left on the field. The root remains break down through the natural action of soil-dwelling insects and micro-organisms, enabling the Bt toxin inside the cell to come into contact with the soil.

GMO Safety: So what happens to the Bt toxin in the soil after harvesting?

Christoph Tebbe: We have made a detailed study of this in MON810 maize. This variety is now grown throughout the world, including in some European countries. In the first phase, Bt toxins start to break down in the leaf and root remains after harvesting, probably due to residual activity in the plant. Within four to six weeks, less than fifty percent of the original ten to twenty micrograms of Bt toxin per gram of plant material is left. Toxin breakdown is minimal during the winter period, but as temperatures increase and the decomposition of plant remains resumes, the Bt toxin is broken down further. However, a tiny fraction of the original Bt toxin, just one nanogram per gram in our investigations, for example, binds to soil components, particularly clay particles. This fraction breaks down very slowly.

GMO Safety: Can Bt toxin accumulate in the soil if maize is grown on the same field for several years in succession?

Christoph Tebbe: We have carried out a separate research project on this, which is now almost complete. Before the start of the next growing period, in other words around six month after harvesting, we were unable to detect any Bt toxin, even on fields where MON810 Bt maize had been grown for three years in succession. We found no accumulation at any time, despite investigating fields with different soil characteristics. We assume that Bt toxin bound to the soil components is continually exchanged with other natural compounds (proteins from plant remains) until it eventually disappears. The theoretical feasibility of an accumulation can be avoided by making sure that maize is not grown on the same field for several successive years. Crop rotation is used to avoid depleting our soils and to prevent the occurrence of soil micro-organisms which are harmful to our crops.

GMO Safety: Can the amounts of Bt toxin you found have a harmful effect on soil-dwelling organisms?

Christoph Tebbe: Unlike many chemical insecticides, Bt toxins have an astounding specificity for their target organisms (the pests in question) – a plus point which organic farming has also exploited. With an LD50 of around two micrograms per gram of plant material, European corn borer larvae are the most sensitive organisms to the Bt toxin in MON810 maize. In the soil, however, we find no more than two nanograms per gram, in other words 1000 times less. Direct toxic effects on non-target organisms are not anticipated due to natural breakdown, and this has been confirmed by a range of studies. Even earthworms fed continually on MON810 leaf remains showed no reaction.

GMO Safety: Does this mean that, in your view, Bt maize does not constitute an ecological problem for the soil?

Christoph Tebbe: Agriculture is always an ecological problem, because farmers interfere with nature to produce crops. Compared with using conventional chemical insecticides in maize cultivation, we can actually answer ‘yes’where MON810 is concerned, thanks to the extensive data available from all the safety research projects. MON810 appears to be ecologically safer than conventional cultivation using chemical pesticides. However, these findings cannot vouch for the harmlessness of new Bt maize varieties, because there is not just one Bt toxin; there are in fact several, each with a different spectrum of activity and environmental persistence. When we examined a different Bt toxin (Cry3Bb1), we found much higher degradation rates in the soil of one maize field, compared with MON810 (Cry1Ab). The interactions between new Bt varieties and the soil ecosystem need to be assessed on a case-by-case basis. Trial plantings and ecological accompanying research can make a valuable contribution to safety evaluations.

GMO Safety: Thank you for talking to us.