Progress is being made in the scientific understanding of Bt toxin's fate in soil ecosystems

Fate and effects of insect-resistant Bt crops in soil ecosystems

Soil Biology and Biochemistry
Volume 40, Issue 3, March 2008, Pages 559-586
Isik Icoza and Guenther Stotzky
Laboratory of Microbial Ecology, Department of Biology, New York University, New York, NY 10003, USA

Abstract

Recent applications of biotechnology, especially genetic engineering, have revolutionized crop improvement and increased the availability of valuable new traits. A current example is the use of the insecticidal Cry proteins from the bacterium, Bacillus thuringiensis (Bt), to improve crops, known as Bt crops, by reducing injury from various crop pests.

The adoption of geneticallynext term modified (GM) crops has increased dramatically in the last 11 years. However, the introduction of GM previous termplants into agricultural ecosystems has raised a number of questions, including the ecological impact of these plants on soil ecosystems.

Crop residues are the primary source of carbon in soil, and root exudates govern which organisms reside in the rhizosphere. Therefore, any change to the quality of crop residues and rhizosphere inputs could modify the dynamics of the composition and activity of organisms in soil.

Insect-resistant Bt crops have the potential to change the microbial dynamics, biodiversity, and essential ecosystem functions in soil, because they usually produce insecticidal Cry proteins through all parts of the previous term plant.

It is crucial that risk assessment studies on the commercial use of Bt crops consider the impacts on organisms in soil. In general, few or no toxic effects of Cry proteins on woodlice, collembolans, mites, earthworms, nematodes, protozoa, and the activity of various enzymes in soil have been reported. Although some effects, ranging from no effect to minor and significant effects, of Bt plants on microbial communities in soil have been reported, using both culturing and molecular techniques, they were mostly the result of differences in geography, temperature, plant variety, and soil type and, in general, were transient and not related to the presence of the Cry proteins.

The respiration (i.e., CO2 evolution) of soils cultivated with Bt maize or amended with biomass of Bt maize and other Bt crops was generally lower than from soils cultivated with or amended with biomass of the respective non-Bt isolines, which may have been a result of differences in chemical composition (e.g., the content of starch, soluble N, proteins, carbohydrates, lignin) between Bt plants and their near-isogenic counterparts.

Laboratory and field studies have shown differences in the persistence of the Cry proteins in soil, which appear to be the result primarily of differences in microbial activity, which, in turn, is dependent on soil type (e.g., pH, clay mineral composition, other physicochemical characteristics), season (e.g., temperature, water tension), crop species (e.g., chemical composition, C:N ratio, plant part), crop management practices (e.g., till vs. no-till), and other environmental factors that vary with location and climate zones.

This review discusses the available data on the effects of Cry proteins on below-ground organisms, the fate of these proteins in soil, the techniques and indicators that are available to study these aspects, and future directions.

Keywords: Bacillus thuringiensis; Bt crops; Biotechnology; Soil microorganisms; Invertebrates; Genetically modified plants; Insect resistance; Nontarget effects; Soil ecosystem functions



Cry3Bb1 protein from Bacillus thuringiensis in root exudates and biomass of transgenic corn does not persist in soil.
Icoz I, Stotzky G.
Transgenic Res. 2007 Sep 13 [Epub ahead of print]

Laboratory of Microbial Ecology, Department of Biology, New York University, New York, NY, 10003, USA, gs5@nyu.edu.

The Cry3Bb1 protein, insecticidal to the corn rootworm complex (Diabrotica spp.), of Bacillus thuringiensis (Bt) subsp. kumamotoensis was released in root exudates of transgenic Bt corn (event MON863) in sterile hydroponic culture (7.5 +/- 1.12 ng/ml after 28 days of growth) and in nonsterile soil throughout growth of the plants (2.2 +/- 0.62 ng/g after 63 days of growth). Kitchawan soil, which contains predominantly kaolinite (K) but not montmorillonite (M), was amended to 3 or 6% (vol./vol.) with K (3K and 6K soils) or M (3M and 6M soils) and with 1, 3, 5, or 10% (wt./wt.) of ground biomass of Bt corn expressing the Cry3Bb1 protein and incubated at 25 +/- 2 degrees C at the -33-kPa water tension for 60 days. Soils were analyzed for the presence of the protein every 7 to 10 days with a western blot assay (ImmunoStrip) and verified by ELISA. Persistence of the protein varied with the type and amount of clay mineral and the pH of the soils and increased as the concentration of K was increased but decreased as the concentration of M was increased. Persistence decreased when the pH of the K-amended soils was increased from ca. 5 to ca. 7 with CaCO(3): the protein was not detected after 14 and 21 days in the pH-adjusted 3K and 6K soils, respectively, whereas it was detected after 40 days in the 3K and 6K soils not adjusted to pH 7. The protein was detected for only 21 days in the 3M soil and for 14 days in the 6M soil, which were not adjusted in pH. These results indicate that the Cry3Bb1 protein does not persist or accumulate in soil and is degraded rapidly.

PMID: 17851773

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