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Unintended Effects of Genetic Manipulation
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Posted: May 2014

Innate Immunity is Affected in Insect-Resistant GM Aspens

Aspen trees have been genetically modified to produce an insecticide, originally derived from the bacterium Bacillus thuriengiensis (Bt). The hope is that Bt aspen trees would thrive better, since they would be protected against pests such as the leaf beetle (Chrysomela trumulae). But there is also concern that the genetic manipulation could have unintended effects on nontarget organisms, and since trees have a long life span and provide long-term structure for the forest ecosystem, any change in relation to nontarget organisms could have significant ripple effects in the forest as a whole.

To see whether the Bt manipulation in any way affects the innate immunity of aspens, Axelsson et al. (2011) carried out a series of investigations. They worked with two different lines of Bt aspens, one of which expressed more Bt toxin than the other, and wildtype aspens. All lines (both the wildtype and Bt aspens) were originally cloned from the same parent plant and were propagated and grown in a greenhouse.

They carried out the following experiments:
  • A phytochemical analysis of young and old leaves from Bt and wildtype plants that resulted in a profile of 25 different compounds, including ones related to pest resistance.
  • Fed leaves to a nontarget organism — a slug (Deroceras spp.) — to discern any feeding preferences.
  • Determined the stem biomass in all lines.
Since all the young trees were derived from the same parent, if the genetic alteration had only resulted in the production of the Bt toxin, one would not expect to find any differences in the above investigations. The GM aspens would behave in the same way as the unmanipulated wildtype aspens. But the researchers found a number of unexpected differences:
  • The phytochemical profile revealed statistically significant differences between the wildtype leaves and the leaves of the two Bt lines; the two Bt lines also differed from each other. Especially in one of the Bt lines (line 17) mature leaves had lower levels of several compounds that are important for plant defense against pests.
  • When given both Bt line 17 and wildtype leaf material to feed on, the slugs preferred older Bt leaves over wildtype leaves; there was no difference in the preference for younger leaves.
  • Slugs that were fed exclusively on either old Bt line 17 leaves or old wildtype leaves, the slugs feeding on the Bt leaves ate more than the slugs feeding on wildtype leaves.
  • The Bt aspens showed reduced growth. Growth was measured by determining the increase of stem biomass over a ten week period. The wildtype aspens had 1.7 and 2.4 times more biomass than the two Bt lines.
Evidently, in some unknown way, the genetic modification process also affected the plant as a whole with respect to the substances it produces in its leaves that are related to innate immunity. As a result, the GM aspens, while resistant to insects, were more likely to be eaten by slugs, another common pest of aspens.

The authors conclude that “our results are of principal importance, as these indicate that genetic modifications can affect innate resistance and thus non-target herbivores in ways that may have commercial and/or environmental consequences. The finding of a modification-ontogeny interaction effect on innate resistance may be especially important in assessments of GM plants with long lifespan such as trees.”

Source: Axelsson, E. P., J. Hjältén, T. G. Whitham et al. (2011). “Leaf Ontogeny Interacts with Bt Modification to Affect Innate Resistance in GM Aspens,” Chemoecology vol. 21, pp. 161-9. doi:10.1007/s00049-011-0080-8

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