Root nodule bacteria with Bt transgene tended to displace
non-manipulated bacteria in legume root nodules.
The root nodule bacterium Rhizobium leguminosarum, which colonizes
the roots of legumes and helps the plants fix nitrogen.
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).
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.
Some of the transgenic bacteria strains gave the plants a degree of
protection from the pest (Skøt et al. 1994).
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.
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).
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.
Institute of Biological Sciences, University of Wales, UK.
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