Abstract
Crystal (Cry) proteins derived from Bacillus thuringiensis (Bt) have been widely used in transgenic crops due to their toxicity against insect pests. However, the distribution and metabolism of these toxins in insect tissues and organs have remained obscure because the target insects do not ingest much toxin. In this study, several Cry1Ac-resistant strains of Helicoverpa armigera, fed artificial diets containing high doses of Cry1Ac toxin, were used to investigate the distribution and metabolism of Cry1Ac in their bodies. Cry1Ac was only detected in larvae, not in pupae or adults. Also, Cry1Ac passed through the midgut into other tissues, such as the hemolymph and fat body, but did not reach the larval integument. Metabolic tests revealed that Cry1Ac degraded most rapidly in the fat body, followed by the hemolymph, peritrophic membrane and its contents. The toxin was metabolized slowly in the midgut, but was degraded in all locations within 48 h. These findings will improve understanding of the functional mechanism of Bt toxins in target insects and the biotransfer and the bioaccumulation of Bt toxins in arthropod food webs in the Bt crop ecosystem.
Highlights
Bacillus thuringiensis (Bt) is a gram-positive, entomopathogenic, sporulating bacterium, which produces crystal (Cry) proteins or inclusion bodies that have selective insecticidal toxicity against different groups of insect pests [1,2,3]
Cry1Ac toxin was only detected in H. armigera larvae, not in pupae and adults when fed on artificial diets containing Cry1Ac (Figure 1)
The absence of Cry1Ac in pupae and adults suggested that Bt toxins are not transferred to natural enemies if they feed on or parasitize pupae, adults and eggs of H. armigera
Summary
Bacillus thuringiensis (Bt) is a gram-positive, entomopathogenic, sporulating bacterium, which produces crystal (Cry) proteins or inclusion bodies that have selective insecticidal toxicity against different groups of insect pests [1,2,3]. Insecticidal crystal proteins (ICPs) encoded by the Bt gene have been introduced into many crops, making them resistant to lepidopteran or coleopteran pests [4,5,6]. In China, Bt cotton has been commercialized to control Helicoverpa armigera since 1997, and the pest population has greatly declined [7,8,9]. H. armigera is the most serious insect pest of cotton [10]. Several models have been proposed to explain the functional mechanism of the Cry protein against target insects. The most widely accepted model proposed that, the proteolytically activated toxins bind to specific receptors in the midgut epithelial cells, subsequently creating pores in the cell membrane and eventually killing the Toxins 2016, 8, 212; doi:10.3390/toxins8070212 www.mdpi.com/journal/toxins
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