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 Target 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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