(340g) Composites Based on Biomimicry of the Prunus Spp. Seed Cyanide Defense System as Alternative to Pesticides

Many crops are ill-protected against insect pests during storage. Globally, there is still a significant loss of stored grains due to pest infestation especially in developing countries.To protect cereal grains from herbivores during storage, pesticides are often applied. While pesticides have an undoubtable functionality, increasing concerns are arising about their application.

Recently, we were able to mimic the principle of cyanogenic defense from plants such as bitter almonds (Prunus dulcis) and apply it onto wheat grains that naturally do not possess any cyanogenic defense.[1] Our motivation was to engineer a protective biomimetic coating material that defends the grain only upon triggering, is cost-effective, and free of conventional pesticides commonly used for grain protection during storage. To achieve an artificial cyanogenic defense mechanism similar to that found in seeds of Prunusspecies, we developed a novel onion-type grain coating consisting of separate layers containing either pure biodegradable polylactic acid (PLA), PLA mixed with the cyanogenic precursor mandelonitrile or the HCN releasing enzyme hydroxynitrile lyase. While the grains showed a cyanogenic potential high enough to theoretically have a effect on herbivores, their germination rate was reduced to approximately 25% compared to untreated grains, which was a clear drawback.

Here we would like to present the results from our recent study where we have significantly improved the cyanogenic grain coating design in terms of its effect on germination efficiency and have tested its actual performance against herbivore insects in laboratory and field experiments.[2] The optimized cyanogenic multi-layer coating consists of biodegradable polylactic acid with individual layers containing either the cyanogenic precursor amygdalin or β-glucosidase which is capable of degrading amygdalin to HCN. Cyanogenesis occurred only when the layers were ruptured e.g. by a herbivore attack. We could show that upon feeding coated cyanogenic wheat grains to the insect pest species Tenebrio molitor (mealworm beetle), Rhizopertha dominica (lesser grain borer) and Plodia interpunctella (Indianmeal moth), their reproduction as well as consumption rate were significantly reduced, whereas germination ability increased compared to non-coated grains. In field experiments, we observed an initial growth delay compared to uncoated grains which became negligible at later growth stages. Our proposed and experimentally tested strategy to artificially mimic and apply a naturally occurring defense mechanisms could be expanded to other crops than wheat and has the potential to replace certain pesticides with the benefit of complete biodegradability and increased safety during storage.

[1] Halter, J. G.; Chen, W. D.; Hild, N.; Mora, C. A.; Stoessel, P. R.; Koehler, F. M.; Grass, R. N.; Stark, W. J. Induced cyanogenesis from hydroxynitrile lyase and mandelonitrile on wheat with polylactic acid multilayer-coating produces self-defending seeds. Journal of Materials Chemistry A 2014, 2, 853-858.

[2] Mora, C. A.; Halter, J. G.; Adler, C.; Hund, A.; Anders, H.; Yu, K.; Stark, W. J. Application of the Prunus spp. cyanide seed defense system onto wheat: Reduced insect feeding and field growth tests. Journal of Agricultural and Food Chemistry 2016 (in print).