Background
With the explosion of interest (pro and con) in genetically modified plants, there is great concern that foreign genes inserted into agricultural plants might be transferred into wild relatives of transgenic crop lines and thereby upset the "balance of nature."  This paper is one of the first to probe this important topic, which it does with respect to the Bacillus thuringiensis (Bt) gene that causes plants to produce toxins targeted for various insect pests of agricultural crops.

What was done
The authors grew cotton plants (Gossypium hirsutum L.) in twelve Teflon-film chambers in a temperature-controlled greenhouse.  Six of these chambers were maintained at an atmospheric CO2 concentration of 370 ppm, and six were maintained at 900 ppm CO2.  Half of the cotton plants in each chamber were of a transgenic line containing the Bt gene for the production of the Cry1Ac toxin that is mildly toxic for Spodoptera exigua (Hubner), while the other half were of a near isogenic line without the Bt gene.  In addition, half of the plants in each chamber received high levels of nitrogen (N) fertilization, while half received low levels (130 vs. 30 mg N/kg soil/week).

Between 40 and 45 days after emergence, leaves were removed from the plants and fed to the S. exigua larvae, after which a number of larval responses were measured and analyzed, along with various leaf properties.

What was learned
The low-N plants in the elevated CO2 treatment had lower foliar N concentrations than did the low-N plants in the ambient CO2 treatment; and the transgenic plants from the low-N, high CO2 treatment combination produced lower levels of Bt toxin than did the transgenic plants from the low-N, ambient CO2 treatment combination.  In addition, the high level of N fertilization only partially compensated for this latter high-CO2 effect.

In the ambient CO2 treatment there was also a significant increase in days to pupation for insects fed transgenic plants; but this difference was not evident in elevated CO2.  In addition, pupal weight in ambient CO2 was significantly higher in nontransgenic plants; and, again, this difference was not observed in elevated CO2.

What it means
In the words of the authors, "these results support the hypothesis that the lower N content per unit of plant tissue caused by the elevated CO2 will result in lower toxin production by transgenic plants when nitrogen supply to the plants is a limiting factor."  They also note that "elevated CO2 appears to eliminate differences between transgenic and nontransgenic plants for some key insect developmental/fitness variables including length of the larval stage and pupal weight."

These results suggest that, in the case of inadvertent Bt gene transference to wild relatives of transgenic crop lines, elevated levels of atmospheric CO2 will tend to negate certain of the negative effects the wayward genes might otherwise inflict on the natural world.  Hence, the ongoing rise in the air's CO2 content could be said to constitute an "insurance policy" against this potential eventuality.  Indeed, it could even be construed as a natural manifestation of the Precautionary Principle, which could give some comfort to those who are advocates of genetic engineering.

On the other hand, these results also suggest that transgenic crops designed to produce Bt-type toxins may become less effective in carrying out the objectives of their design as the air's CO2 content continues to rise.  Coupling this likelihood with the fact that the foliar application of Bacillus thuringiensis to crops should become even more effective in a higher-CO2 world (see our Journal Review Elevated CO2 Enhances the Effectiveness of Foliar Applications of Bt Pesticides), one could argue that the implantation of toxin-producing genes in crops is not the way to go in the face of the ongoing rise in the air's CO2 content, which reduces that technique's effectiveness at the same time that it increases the effectiveness of direct foliar applications.

Although it is difficult to predict which path society will pursue in regard to the genetic modification of crop plants for pesticidal purposes, it is comforting to know that increasing atmospheric CO2 concentrations will help both nature and agriculture alike, whatever the outcome of the current debate surrounding this topic.  What an amazing atmospheric constituent this trace gas is!