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Bt cotton had altered protein metabolism and produced less insecticidal toxin in leaves when subjected to high temperatures during boll formation.

Manipulated Organism: Cotton (Gossypium hirsutum).

Inserted Transgenes and Intended Effect: crylA 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, especially the cotton bollworm (Helicoverpa armigera), which is a major pest in cotton fields. The crylA 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 in China that transgenic Bt cotton has not always shown adequate levels of resistance to the cotton bollworm, the researchers investigated whether a high temperature (37°C/99°F) might affect the plant's insecticidal capacities. Such a high temperature is not untypical during the growing season in China. The researchers grew two different commercial varieties of Bt cotton in greenhouses and subjected them to the high temperature at different stages of development, in periods ranging from 12 to 48 hours. They measured the levels of Bt toxin and other proteins in the leaves at each stage.

Results of This Study: While the high temperature had little effect on Bt toxin and other protein levels in the leaves at developmental stages up through peak flowering, there were significant effects when the plants were forming cotton bolls (peak boll stage) and subjected to the high temperature (37°C/99°F):
  • With 24 hours of high temperature, the levels of Bt toxin dropped by 51% in one variety (Kumina No. 1) and by 30% in the other (Xinyang822).
  • With 48 hours of high temperature, the levels of Bt toxin dropped by 73% in one variety (Kumina No. 1) and by 62% in the other (Xinyang822).
  • Levels of glutamic-pyruvic transaminase (GPT), an important enzyme in plant protein metabolism, dropped markedly.
  • Level of soluble protein in the leaves decreased significantly.
  • Levels of protease, an important enzyme in the breakdown (catabolism) of proteins, rose significantly.
  • Levels of free amino acids (that is, amino acids that are not yet, or no longer, part of a protein) rose sharply, up to a 4-fold increase.

Additional Comments: These findings agree with the observations of commercially grown crops, where reduced efficacy of Bt cotton usually occurred during the boll phase and especially after very warm weather. The authors remark that high temperatures may "increase the probability of gene silencing" (p. 340). Both the levels of Bt toxin (which is a protein) and total levels of soluble proteins in the leaves were strongly reduced at high temperatures in peak boll stage plants, while the levels of protease, which degrades proteins, and of free amino acids rose. This suggests that the high temperature at that particular stage induced overall degradation of proteins, including Bt toxin, in the leaves. So although Bt cotton is designed to produce Bt toxin continuously in adequate amounts, field observations and this experiment indicate that the environment (in this case high temperature) can induce changes in the plant's physiology that decrease the amount of Bt toxin the plant accumulates.

Source: Chen, D., G. Ye, C. Yang, Y. Chen et al. (2005). "The Effects of High Temperature on the Insecticidal Properties of Bt Cotton," Environmental and Experimental Botany vol. 53, pp. 333-42.

Author Affiliations: Agronomic Department, Yangzhou University, China; School of Land and Food Sciences, University of Queensland, Australia.

Funding: Grant from Education Department of Jiangsu Province, China.

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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