Development and mechanisms of resistance to Bacillus thuringiensis endotoxin Cry1Ac in the American bollworm, Helicoverpa armigera (Hübner) (original) (raw)
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Biochemical Journal, 2009
The bacterium Bacillus thuringiensis produces ICPs (insecticidal crystal proteins) that are deposited in their spore mother cells. When susceptible lepidopteran larvae ingest these spore mother cells, the ICPs get solubilized in the alkaline gut environment. Of approx. 140 insecticidal proteins described thus far, insecticidal protein Cry1Ac has been applied extensively as the main ingredient of spray formulation as well as the principal ICP introduced into crops as transgene for agricultural crop protection. The 135 kDa Cry1Ac protein, upon ingestion by the insect, is processed successively at the N- and C-terminus by the insect midgut proteases to generate a 65 kDa bioactive core protein. The activated core protein interacts with specific receptors located at the midgut epithilium resulting in the lysis of cells and eventual death of the larvae. A laboratory-reared population of Helicoverpa armigera displayed 72-fold resistance to the B. thuringiensis insecticidal protein Cry1Ac. ...
Different Mechanisms of Resistance to Bacillus thuringiensis Toxins in the Indianmeal Moth
Applied and Environmental Microbiology, 2001
Susceptibility to protoxin and toxin forms of Cry1Ab and the binding of 125 I-labeled Cry1Ab and Cry1Ac has been examined in three Plodia interpunctella colonies, one susceptible (688 s ) and two resistant (198 r and Dpl r ) to Bacillus thuringiensis. Toxicological studies showed that the 198 r colony was 11-fold more resistant to Cry1Ab protoxin than to Cry1Ab activated toxin, whereas the Dpl r colony was 4-fold more resistant to protoxin versus toxin. Binding results with 125 I-labeled toxins indicated the occurrence of two different binding sites for Cry1Ab in the susceptible insects, one of them shared with Cry1Ac. Cry1Ab binding was found to be altered in insects from both resistant colonies, though in different ways. Compared with the susceptible colony, insects from the Dpl r colony showed a drastic reduction in binding affinity (60-fold higher K d ), although they had similar concentrations of binding sites. Insects from the 198 r colony showed a slight reduction in both binding affinity and binding site concentration (five-fold-higher K d and ca. three-fold-lower R t compared with the 688 s colony). No major difference in Cry1Ac binding was found among the three colonies. The fact that the 198 r colony also has a protease-mediated mechanism of resistance (B. Oppert, R. Hammel, J. E. Throne, and K. J. Kramer, J. Biol. Chem. 272:23473-23476, 1997) is in agreement with our toxicological data in which this colony has a different susceptibility to the protoxin and toxin forms of Cry1Ab. It is noteworthy that the three colonies used in this work derived originally from ca. 100 insects, which reflects the high variability and high frequency of B. thuringiensis resistance genes occurring in natural populations.
Continuous evolution of Bacillus thuringiensis toxins overcomes insect resistance
2016
The expression of insecticidal proteins from B. thuringiensis (Bt toxins) in crops has proved to be a valuable strategy for agricultural pest management1. Bt-toxin-producing crops have been widely adopted in agriculture with substantial economic and environmental benefits2, and have increased global agricultural productivity by an estimated US$78 billion from 1996 to 2013 (ref. 3). Unfortunately, Bt toxin resistance has evolved among insect pests and threatens the continued success of this strategy for pest control4. While resistance management strategies have been developed, including the use of multiple Bt toxins and preserving susceptible alleles in insect populations, the evolution of insect resistance to Bt toxins remains the most serious current threat to sustaining the gains offered by transgenic crops4. Bt toxins interact with protein receptors on the surface of insect midgut cells, leading to pore formation in the cell membrane and cell death5. Bt toxin resistance is common...
Resistance: A Threat to the Insecticidal Crystal Proteins of Bacillus thuringiensis
The Florida Entomologist, 1995
Insecticidal crystal proteins (also known as δ-endotoxins) synthesized by the bacterium Bacillus thuringiensis Berliner (Bt) are the active ingredient of various environmentally friendly insecticides that are 1) highly compatible with natural enemies and other nontarget organisms due to narrow host specificity, 2) harmless to vertebrates, 3) biodegradable in the environment, and 4) highly amenable to genetic engineering. The use of transgenic plants expressing Bt δ-endotoxins has the potential to greatly reduce the environmental and health costs associated with the use of conventional insecticides. The complex mode of action of Bt is the subject of intensive research. When eaten by a susceptible insect δ-endotoxin crystals are solubilized in the midgut; proteases then cleave protoxin molecules into activated toxin which binds to receptors on the midgut brush border membrane. Part of the toxin molecule inserts into the membrane causing the midgut cells to leak, swell, and lyse; death results from bacterial septicemia. Insecticides formulated with Bt account for less than 1% of the total insecticides used each year worldwide because of high cost, narrow host range, and comparatively low efficacy. Environmental contamination, food safety concerns, and pest resistance to conventional insecticides have caused a steady increase in demand for Bt-based insecticides. The recent escalation of commercial interest in Bt has resulted in more persistent and efficacious formulations. For example, improved Bt-based insecticides have allowed management of the diamondback moth, Plutella xylostella (L.). Unfortunately this has resulted in the evolution of resistance to δ-endotoxins in P. xylostella populations worldwide. The recent appearance of Bt resistance in the field, corroborated by the results of laboratory selection experiments, demonstrates genetically-based resistance in several species of Lepidoptera, Diptera, and Coleoptera. The genetic capacity to evolve resistance to these toxins is probably Florida Entomologist 78(3) September, 1995 ducir la presión de seleccción, minimizando la exposición a Bt e incrementando otros factores de mortalidad, para disminuir la velocidad de adaptación de la plaga a Bt. The bacterium Bacillus thuringiensis Berliner (Bt) is a complex of subspecies characterized by their ability to synthesize crystalline inclusions during sporulation. These crystalline inclusions are comprised of relatively high quantities of one or more glycoproteins known as δ-endotoxins or Cry toxins (Table 1). The toxins produced by Bt play a vital role in the pathogenicity of this bacterium to insects and other invertebrates. The Cry toxins have enormous commercial value as safe, biodegradable pesticides. The specificity of Bt toxicity is highly desirable in integrated pest management (IPM) programs, particularly in sensitive aquatic and forest ecosystems where other life forms, including many beneficial and nontarget insects, must be conserved (May 1993). The selective toxicity, rapid environmental degradation, and vertebrate safety of Bt-based insecticides provide growers and the public with environmentally friendly and effective alternatives to conventional insecticides (Meadows 1993). Advances in biotechnology and genetic engineering, as well as the proteinaceous nature of the Cry toxins, led to the selection of the cry genes as the primary insect-resistance genes transferred into, and expressed in, plants and microbes (
Insect Biochemistry and Molecular Biology, 2005
Bacillus thuringiensis endotoxins (Bt-toxins) are the most important biopesticides used in controlling insect pests and vectors of diseases. The emergence of widespread resistance to Bt in some insect species is a serious threat to agricultural production. Analysis of Bt-resistant and susceptible laboratory strains of Helicoverpa armigera revealed elevated immune responses involving increased melanization and the presence of a soluble toxin-binding glycoprotein in the hemolymph and gut lumen of the resistant strain. We propose a resistance mechanism against toxins based on a systemic immune-induction that can be transmitted to the next generation by a maternal effect. r
Applied and Environmental Microbiology, 2008
Laboratory-selected Bacillus thuringiensis-resistant colonies are important tools for elucidating B. thuringiensis resistance mechanisms. However, cotton bollworm, Helicoverpa zea, a target pest of transgenic corn and cotton expressing B. thuringiensis Cry1Ac (Bt corn and cotton), has proven difficult to select for stable resistance. Two populations of H. zea (AR and MR), resistant to the B. thuringiensis protein found in all commercial Bt cotton varieties (Cry1Ac), were established by selection with Cry1Ac activated toxin (AR) or MVP II (MR). Cry1Ac toxin reflects the form ingested by H. zea when feeding on Bt cotton, whereas MVP II is a Cry1Ac formulation used for resistance selection and monitoring. The resistance ratio (RR) for AR exceeded 100-fold after 11 generations and has been maintained at this level for nine generations. This is the first report of stable Cry1Ac resistance in H. zea. MR crashed after 11 generations, reaching only an RR of 12. AR was only partially cross-resistant to MVP II, suggesting that MVP II does not have the same Cry1Ac selection pressure as Cry1Ac toxin against H. zea and that proteases may be involved with resistance. AR was highly cross-resistant to Cry1Ab toxin but only slightly cross-resistant to Cry1Ab expressing corn leaf powder. AR was not cross-resistant to Cry2Aa2, Cry2Ab2-expressing corn leaf powder, Vip3A, and cypermethrin. Toxin-binding assays showed no significant differences, indicating that resistance was not linked to a reduction in binding. These results aid in understanding why this pest has not evolved B. thuringiensis resistance, and highlight the need to choose carefully the form of B. thuringiensis protein used in experiments.
Insect Biochemistry and Molecular Biology, 2003
Evolution of resistance by pests could cut short the success of transgenic plants producing toxins from Bacillus thuringiensis, such as Bt cotton. The most common mechanism of insect resistance to B. thuringiensis is reduced binding of toxins to target sites in the brush border membrane of the larval midgut. We compared toxin binding in resistant and susceptible strains of Pectinophora gossypiella, a major pest of cotton worldwide. Using Cry1Ab and Cry1Ac labeled with 125 I and brush border membrane vesicles (BBMV), competition experiments were performed with unlabeled Cry1Aa, Cry1Ab, Cry1Ac, Cry1Ba, Cry1Ca, Cry1Ja, Cry2Aa, and Cry9Ca. In the susceptible strain, Cry1Aa, Cry1Ab, Cry1Ac, and Cry1Ja bound to a common binding site that was not shared by the other toxins tested. Reciprocal competition experiments with Cry1Ab, Cry1Ac, and Cry1Ja showed that these toxins do not bind to any additional binding sites. In the resistant strain, binding of 125 I-Cry1Ac was not significantly affected; however, 125 I-Cry1Ab did not bind to the BBMV. This result, along with previous data from this strain, shows that the resistance fits the "mode 1" pattern of resistance described previously in Plutella xylostella, Plodia interpunctella, and Heliothis virescens.
BMC Microbiology, 2014
Background The cotton bollworm, Helicoverpa armigera is one of the most important crop pests worldwide. It has developed high levels of resistance to synthetic insecticides, and hence, Bacillus thuringiensis (Bt) formulations are used as a safer pesticide and the Bt genes have been deployed in transgenic crops for controlling this pest. There is an apprehension that H. armigera might develop resistance to transgenic crops in future. Therefore, we studied the role of gut microbes by eliminating them with antibiotics in H. armigera larvae on the toxicity of Bt toxins against this pest. Results Commercial formulation of Bt (Biolep®) and the pure Cry1Ab and Cry1Ac toxin proteins were evaluated at ED50, LC50, and LC90 dosages against the H. armigera larvae with and without antibiotics (which removed the gut microbes). Lowest H. armigera larval mortality due to Bt formulation and the Bt toxins Cry1Ab and Cry1Ac was recorded in insects reared on diets with 250 and 500 μg ml−1 diet of each ...
Applied and Environmental Microbiology, 2001
We tested toxins of Bacillus thuringiensis against larvae from susceptible, Cry1C-resistant, and Cry1A-resistant strains of diamondback moth ( Plutella xylostella ). The Cry1C-resistant strain, which was derived from a field population that had evolved resistance to B. thuringiensis subsp. kurstaki and B. thuringiensis subsp. aizawai , was selected repeatedly with Cry1C in the laboratory. The Cry1C-resistant strain had strong cross-resistance to Cry1Ab, Cry1Ac, and Cry1F, low to moderate cross-resistance to Cry1Aa and Cry9Ca, and no cross-resistance to Cry1Bb, Cry1Ja, and Cry2A. Resistance to Cry1C declined when selection was relaxed. Together with previously reported data, the new data on the cross-resistance of a Cry1C-resistant strain reported here suggest that resistance to Cry1A and Cry1C toxins confers little or no cross-resistance to Cry1Bb, Cry2Aa, or Cry9Ca. Therefore, these toxins might be useful in rotations or combinations with Cry1A and Cry1C toxins. Cry9Ca was much mor...
Genetics of pink bollworm resistance to Bacillus thuringiensis toxin Cry1Ac
Journal of economic …, 2001
Laboratory selection increased resistance of pink bollworm (Pectinophora gossypiella) to the Bacillus thuringiensis toxin Cry1Ac. Three selections with Cry1Ac in artiÞcial diet increased resistance from a low level to Ͼ100-fold relative to a susceptible strain. We used artiÞcial diet bioassays to test F 1 hybrid progeny from reciprocal crosses between resistant and susceptible strains. The similarity between F 1 progeny from the two reciprocal crosses indicates autosomal inheritance of resistance. The dominance of resistance to Cry1Ac depended on the concentration. Resistance was codominant at a low concentration of Cry1Ac, partially recessive at an intermediate concentration, and completely recessive at a high concentration. Comparison of the artiÞcial diet results with previously reported results from greenhouse bioassays shows that the high concentration of Cry1Ac in bolls of transgenic cotton is essential for achieving functionally recessive inheritance of resistance.