Induced defence in wild cabbage : integrating genes, volatiles & insect behaviour

This study integrates three methods for detection of an induced response to aphid feeding. Firstly, it is shown that volatile organic compounds emitted from wild cabbage infested with the cabbage aphid Brevicoryne brassicae attract the aphid parasitoid Diaeretiella rapae and could constitute a reliable and detectable cue during parasitoid host foraging. It is found that there is no difference in attraction to aphid-infested plants when comparing field-collected and lab-reared parasitoids. Secondly, collection and analysis of compounds emitted from the aphid and Plutella xylostella damaged plants reveals several compounds exhibiting significant induction when compared to uninfested plants. Univariate techniques and multivariate canonical discriminant analysis reveal distinct volatile profiles contain several terpenes and 3-butenyl isothiocyanate as induced compounds that could be involved in the attraction of the parasitoid. It is proposed that this multivariate technique may closer resemble the method of signal recognition used by foraging insects. Finally, global gene expression analysis using Arabidopsis thaliana microarrays coupled to a Brassica oleracea specific software filter is used to assay the expression of genes induced by aphid and Plutella feeding. These experiments reveal the aphid- and Plutella-related induction of genes involved in isoprenoid biosynthesis, involvement of the octadecanoid pathway following Plutella feeding and that although the genes induced are different, number of genes induced by the different herbivores is roughly equal. AFLP analysis illustrates the high degree of genotypic variation in wild Brassica populations when compared to crop cultivars and Arabidopsis thaliana. This suggests that the wild populations are segregating with a high level of gene flow between populations.