The role of the kauralexin biosynthetic pathway in maize defences against the fall armyworm, Spodoptera frugiperda (J.E. Smith) (Lepidoptera, Noctuidae)

Abstract

Maize is a staple food source for millions of people across the African continent. A threat to maize production is therefore a threat to food security. The increased spread of alien insect pests threatens agriculture in Africa, one such pest is the fall armyworm, Spodoptera frugiperda. Kauralexins are a class of maize diterpenoids which when applied externally to maize plants, have resulted in significantly lower herbivory by European corn borer larvae. Studying kauralexins, their role in maize defences and their effect on fall armyworm larvae may aid in developing new control strategies, preventing outbreaks and major yield losses. In this study a review is provided of maize chemical defences, terpenoids, kauralexins, as well as the fall armyworm, its biology, the different strains, recent introduction into Africa and its impact on maize production. The importance of accurate fall armyworm strain identification, the pitfalls of defining strain identity using either COI and Tpi markers in isolation and suggestions on how to improve strain determination is discussed. The kauralexin biosynthesis pathway, especially the An2 protein which is critical in this pathway, was investigated using a bioinformatics approach. The An2 protein was compared across multiple maize inbred lines through amino acid sequence comparison, protein modelling and ligand docking. Differences in binding affinity of the An2 proteins to the ligand correlated to total kauralexin synthesis data present in published literature. Specific amino acid differences between the An2 proteins were identified as the potential source of differences in binding affinities. The role of the kauralexin biosynthesis pathway in the maize defences against fall armyworm larvae was investigated using non-choice and choice herbivory assays. These assays determined that kauralexins do not play a significant role in maize defences against this insect species.