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Bt cotton showed decreased ability to kill cotton bollworm larvae during flower development and flowering.

Manipulated Organism: Cotton (Gossypium hirsutum).

Inserted Transgenes and Intended Effect: crylAc gene derived from Bacillus thuringiensis (Bt). This gene gives the plant the ability to produce an insecticidal delta-endotoxin that can kill the larvae of insect pests, including the cotton bollworm (Helicoverpa armigera), which is a major pest in cotton fields. The crylAc gene was fused to the cauliflower mosaic virus (CaMV-35S) promoter so that the toxin would be produced continuously in all parts of the plant.

Goal of This Study: Based on observations that transgenic cotton plants containing the Bt gene ("Bt cotton") showed a decrease in efficacy later in the growing season at the time of peak flowering, the researchers wanted to investigate what might be causing this problem. They carried out field and greenhouse experiments in Australia, using commercial varieties of Bt cotton. They fed the larvae of a laboratory-bred, Bt-sensitive strain of cotton bollworm on diets containing leaves of Bt cotton that had been harvested at different times during the growing season. They also determined the levels of the crylAc gene expression (mRNA transcript levels) and Bt toxin levels in the leaves at different times during the growing season. They carried out similar experiments with field-grown and greenhouse-grown Bt cotton.

Results of This Study and Unintended Effects: The researchers found:
  • Field-grown Bt cotton leaves showed significantly decreased ability to kill cotton bollworm larvae in relation to the developmental stage of the plant. When the plants reached a particular stage of flower development (after "squaring"), leaf toxicity to larvae decreased dramatically and stayed low during flowering. This was also the case in greenhouse-grown Bt cotton.
  • The lowered toxicity of Bt cotton leaves was clearly correlated with a decline in the expression of the crylAc gene and reduced amounts of Bt toxin in the leaves.

Additional Comments: Since the Bt gene is supposed to be expressed continually in all parts of the plant during its complete life cycle, some physiological or environmental factors must be affecting transgene expression during Bt cotton development. Decreased ability to kill the cotton bollworm larvae during flowering increases the likelihood that some larvae will survive and propagate, which in turn increases the risk that Bt-resistant strains of the cotton bollworm will evolve: "Resistance evolves when insects are exposed to a dose that kills susceptible, but not resistant individuals. If toxin levels fluctuate, it increases the probability that at some stage insects will be exposed to a discriminating dose" (p. 1007).

The authors remark: "Thus, any inadvertent lowering of foliar glycoalkaloids in transgenic potato plants could cause an undesired increase in susceptibility to those pests which are sensitive to threshold concentrations of glycoalkaloids for insect deterrence or toxicity, potentially reducing the benefits of expressing anti-insect transgenes in plants. . . . We think that it is as important to monitor unintended changes in the levels of such secondary plant compounds as it is to evaluate the potential risks and benefits of the intended transgene product (anti-insect gene products) in the agro-ecosystem" (p. 148).

Source: Olsen, K. M., J. C. Daly, H. E. Holt, and E. J. Finnegan (2005). Season-Long Variation in Expression of Cry1Ac Gene and Efficacy of Bacillus thuringiensis Toxin in Transgenic Cotton Against Helicoverpa armigera (Lepidoptera: Noctuidae)," Journal of Economic Entomology vol. 93, pp. 1007-17.

Author Affiliations: Australia's national science agency, CSIRO.

Funding: Cotton Research and Development Corporation, Australia; Australian Cotton Cooperative Research Centre.

Product Status: Bt cotton is grown around the world and has been on the market since the late 1990s.

Copyright 2008 The Nature Institute.

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