Abstract
Tritrophic interactions allow plants to recruit natural enemies for protection against herbivory. Here we investigated genetic variability in induced responses to stemborer egg-laying in maize Zea mays (L.) (Poaceae). We conducted a genome wide association study (GWAS) of 146 maize genotypes comprising of landraces, inbred lines and commercial hybrids. Plants were phenotyped in bioassays measuring parasitic wasp Cotesia sesamiae (Cameron) (Hymenoptera: Braconidae) attraction to volatiles collected from plants exposed to stemborer Chilo partellus (Swinhoe) (Lepidoptera: Crambidae) eggs. Genotyping-by-sequencing was used to generate maize germplasm SNP data for GWAS. The egg-induced parasitoid attraction trait was more common in landraces than in improved inbred lines and hybrids. GWAS identified 101 marker-trait associations (MTAs), some of which were adjacent to genes involved in the JA-defence pathway (opr7, aos1, 2, 3), terpene biosynthesis (fps3, tps2, 3, 4, 5, 7, 9, 10), benzoxazinone synthesis (bx7, 9) and known resistance genes (e.g. maize insect resistance 1, mir1). Intriguingly, there was also association with a transmembrane protein kinase that may function as a receptor for the egg elicitor and other genes implicated in early plant defence signalling. We report maize genomic regions associated with indirect defence and provide a valuable resource for future studies of tritrophic interactions in maize. The markers identified may facilitate selection of indirect defence by maize breeders.
Highlights
Plants, being rooted to the ground and unable to flee from attack, have evolved highly sophisticated ways of defending themselves from insect herbivores, over a 400-million-year period of plant-insect interaction and coevolution[1,2,3]
Evidence in support of this hypothesis was provided when we discovered that three farmer-selected landraces of maize emitted herbivore induced plant volatiles (HIPVs) in response to stemborer, Chilo partellus (Swinhoe) (Lepidoptera: Crambidae), egg laying whereas the two commercial varieties initially tested did n ot[7]
These landraces were of South American origin and further studies provided evidence that the egg induced indirect defence trait exists in some African maize landraces[20] and in wild teosinte s pecies[21]
Summary
Plants, being rooted to the ground and unable to flee from attack, have evolved highly sophisticated ways of defending themselves from insect herbivores, over a 400-million-year period of plant-insect interaction and coevolution[1,2,3]. The HIPVs emitted by the landraces attracted Cotesia sesamiae (Cameron) (Hymenoptera: Braconidae) parasitoid wasps, which are a key natural enemy of the stemborer These landraces were of South American origin and further studies provided evidence that the egg induced indirect defence trait exists in some African maize landraces[20] and in wild teosinte s pecies[21]. Given these promising initial findings, we embarked on a much larger genome wide association study (GWAS), involving 146 maize genotypes, reported here. To the best of our knowledge, this is the first GWAS of parasitoid response to plants induced with insect eggs
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