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Unintended Effects of Genetic Manipulation
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Root nodule bacteria with Bt transgene tended to displace non-manipulated bacteria in legume root nodules.

Manipulated Organism: The root nodule bacterium Rhizobium leguminosarum, which colonizes the roots of legumes and helps the plants fix nitrogen.

Inserted Transgenes: cryIIIAb for the production of delta-endotoxin, derived from Bacillus thuringiensis (Bt). The gene was fused to a promoter that "responds to legume root exudates and gives enhanced levels of toxin expression in the rhizosphere" (Giddings et al. 1997, p. 1063).

Goal: Give legumes, via the root nodule bacterium, the capacity to produce delta-endotoxin. delta-endotoxin is an insecticide and is intended in this case to kill the larvae of the insect Sitona, which feed on legume roots and their root nodules.

Intended Effect: Some of the transgenic bacteria strains gave the plants a degree of protection from the pest (Skøt et al. 1994).

Unintended Effects: The researchers found (Giddings et al. 2000) that when a mixed culture (1:1) of unmanipulated and transgenic nodule bacteria is introduced into the soil to inoculate pea plants, there is not, subsequently, a 1:1 distribution of nodule occupation by the two types of bacteria. Rather, the transgenic bacteria "outcompete" the unmanipulated controls and occupy two-thirds of the nodules. In lab cultures, the growth rates of unmanipulated bacteria and transgenic bacteria were the same.

Additional Comments: The authors summarize: "These results demonstrate that the ecological and agronomic characters of transgenics might be unexpectedly altered by transformation. Such characters might have a bearing on the safety and/or success of transgenics released into the environment" (Giddings et al. 2000).

Source: Giddings, G., L. Mytton, L. Taylor, S. Thomas et al. (2000). "A Secondary Effect of Transformation in Rhizobium leguminosarum for Bacillus thuringiensis subspecies tenebrionis delta-endotoxin (cryIIIA) Genes. Part 2," Theor Appl Genet vol. 100, pp. 820-3.

Giddings, G., L. Mytton, M. Griffiths, A. McCarthy et al. (1997). "A Secondary Effect of Transformation in Rhizobium leguminosarum for Bacillus thuringiensis subspecies tenebrionis delta-endotoxin (cryIIIA) Genes. Part 2," Theor Appl Genet vol. 97, pp. 1062-8.

Skøt, L., E. Timms, L. Mytton (1994). "The Effect of Toxin-Producing Rhizobium Strains on Larvae of Sitona flavescens Feeding on Legume Roots and Nodules," Plant Soil vol. 163, pp. 141-50.

Author Affiliations: Institute of Biological Sciences, University of Wales, UK.

Funding: Not reported.

Product Status: Not on the market as of 2008.

Copyright 2008 The Nature Institute.

This document: http://natureinstitute.org/nontarget/reports/root_nodule_bacteria_001.php

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