In a world with a growing population, there is a need for intensive agriculture. Intensive agriculture enables the development of highly specialised insect pests, such as the western corn rootworn on maize, the Colorado potato beetle on potatoes and the spotted wing drosophila on fruit. The rapid acquisition of resistance in insects increases the need for insecticides with new mechanisms of action. Three entomotoxic fungal lectins have been identified through extensive screening tests: Marasmius oreades agglutinin (MOA) against the Colorado potato beetle (Leptinotarsa decemlineata Say) and Sclerotinia sclerotiorum agglutinin (SSA) and Agrocybe aegerita galectin (AAG) against the spotted wing drosophila (Drosophila suzukii). Despite its close evolutionary similarity to the Colorado potato beetle, the western corn rootworm is resistant to the effects of MOA. The most important question after the discovery of the toxic effect of lectins was their molecular mehanism. We used affinity chromatography and mass spectrometry to identify potential molecular targets of the lectins MOA, SSA and AAG in the target organisms and to determine a possible mechanism of resistance in the western corn rootworm. The use of biotinylated lectins to analyse the glycosylation profile of MOA lectin targets proved to be unsuitable. In vivo tests on target organisms with lectins in combination with simple inhibitory sugars and lectin mutants confirmed the importance of specific regions of lectins for toxicity. We were able to show that MOA binds to aminopeptidase in the Colorado potato beetle, whereas in western corn rootworm contact with the target is probably prevented by mucins. For the SSA target we could confirm that it is a glycoprotein, whereas glycan binding is probably not relevant for the toxic effect of AAG. With these results, we present a new model for the toxicity of dietary proteins in the Colorado potato beetle and report for the first time a protective role of mucins against entomotoxic proteins in insects. Knowledge of the molecular mechanisms of action is important for the development of new bioinsecticides as it will facilitate further development and better define their end use in crop production.
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