Herbivore-induced indirect defense of Arabidopsis : ecogenomic approach to the role of infochemicals in parasitoid attraction

Plants defend themselves against herbivorous insects with the induced production of volatiles that attract the enemies of the herbivores. In this thesis I used an ecogenomic approach to study the role of signal-transduction pathways that regulate the induction of herbivore-induced plant volatiles (HIPV) and the effects of HIPVs on parasitoid host-finding behavior. To this end, I have combined transcriptomics, metabolite analyses and insect behavioral analyses. <br/> <br/> Nine Arabidopsis thaliana accessions were screened for the emission of HIPVs. The accessions varied in the emission rate of Pieris rapae–induced volatiles after folivory or treatment with the herbivory-mimicking plant hormone jasmonic acid (JA). The relevance of this observed variation in the emission of JA-induced volatiles for host location was tested with Diadegma semiclausum parasitoids. Furthermore, the accessions also varied in transcript levels of genes that are (putatively) involved in the production of some of the recorded HIPV-compounds. <br/> The oxylipin JA is the key plant hormone involved in the induction of the HIPV-blend emitted in response to caterpillar folivory. Mutant plants affected in the oxylipin signal-transduction pathway were studied to assess the effects of JA and its oxylipin intermediates 12-oxo-phytodienoate (OPDA) and dinor-OPDA (dnOPDA) on HIPV emission and attraction of the parasitoid D. semiclausum. In contrast to the effect of JA on the induced production of HIPVs, dnOPDA and OPDA were found to have no and little effect, respectively. The HIPV-compound methyl salicylate was shown to be JA-regulated and its abundance in the headspace varied among accessions. The contribution of methyl salicylate to parasitoid attraction was investigated. Bioassays with P. rapae-infested transgenic plants, lacking MeSA production, showed that MeSA negatively influenced D. semiclausum host-finding behavior. <br/> Mutant plants were also studied to assess whether JA and its intermediates affected the induction of genes potentially involved in defense. The different oxylipins were shown to have distinct roles in induced defense signaling. Jasmonic acid had the strongest effect on transcript levels of defense-related genes from the oxylipin- and shikimate signal-transduction pathway. Minor roles were observed for OPDA and dnOPDA in the induction of one of these genes. <br/> <br/> Utilizing an ecogenomic approach has provided new insight into the mechanisms underlying insect-plant interactions and holds promising opportunities. <br/> <br/>