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Screening for Bacillus thuringiensis crystal proteins active against the cabbage looper, Trichoplusia ni
M M Iracheta1, B Pereyra-Alférez, L Galán-Wong
1Departamento de Microbiología e Immunología, Facultad de Ciencias Biológicas/UANL, Monterrey, NL, México.
Abstract:
Toxicity tests were performed to find among Cry1 and Cry2 Bacillus thuringiensis crystal proteins those with high activity against the cabbage looper. Tests were performed with neonate larvae on surface-contaminated artificial diet. The crystal proteins found to be toxic were, from higher to lower toxicity: Cry1Ac, Cry1Ab, Cry1C, Cry2Aa, Cry1J, and Cry1F (LC50 of 1.14.1, 3.4-4.4, 12, 34, 87, and 250 ng/cm2, respectively). Cry1B, Cry1D, and Cry1E can be considered nontoxic (LC50 higher than 2500 ng/cm2). Cry1Aa was moderately toxic to nontoxic, depending on the source (LC50 of 420 ng/cm2 from PGS and 8100 ng/cm2 from Ecogen). In vitro binding assays with trypsin-activated 125I-labeled Cry1Aa, Cry1Ab, and Cry1Ac crystal proteins and brush border membrane vesicles from midgut larvae showed a direct correlation between toxicity and binding affinity. Heterologous competition experiments indicated that Cry1Aa and Cry1F bind, though only at very high concentrations, to the Cry1Ab/Cry1Ac shared high-affinity binding site.
Insights
Bacillus thuringiensis crystal proteins Cry1Ac and Cry1Ab show high toxicity against the cabbage looper. Toxicity correlates with binding affinity, with Cry1Aa and Cry1F binding at high concentrations.
Area of Science:
- Agricultural Entomology
- Molecular Biology
- Biochemistry
Background:
- Bacillus thuringiensis (Bt) produces crystal proteins toxic to insects.
- Understanding the toxicity of different Bt crystal proteins is crucial for pest management.
Purpose of the Study:
- To identify highly active Bacillus thuringiensis crystal proteins against the cabbage looper.
- To correlate protein toxicity with binding affinity to insect midgut receptors.
Main Methods:
- Toxicity assays using neonate cabbage looper larvae on surface-contaminated artificial diet.
- In vitro binding assays with radiolabeled crystal proteins and insect midgut membrane vesicles.
- Heterologous competition experiments to determine binding site interactions.
Main Results:
- Cry1Ac, Cry1Ab, Cry1C, Cry2Aa, Cry1J, and Cry1F demonstrated toxicity, with LC50 values ranging from 1.14 to 250 ng/cm2.
- Cry1B, Cry1D, and Cry1E were found to be non-toxic (LC50 > 2500 ng/cm2).
- A direct correlation was observed between toxicity and binding affinity of Cry1Aa, Cry1Ab, and Cry1Ac.
Conclusions:
- Cry1Ac and Cry1Ab are the most potent Bt crystal proteins against the cabbage looper among those tested.
- Binding affinity to midgut receptors is a key factor determining Bt crystal protein toxicity.
- Cry1Aa and Cry1F exhibit limited binding to the Cry1Ab/Cry1Ac binding site.