Induktion von Pflanzendüften durch Eiablage von Insekteneiern auf Ulme (Ulmus camprestis): Aufklärung eines ökologischen Phänomens unter Verwendung molekularer Methoden und genetischer Transformation
Zusammenfassung der Projektergebnisse
Knowledge on the molecular mechanisms regulating indirect plant defences induced by insect oviposition is scarce. In our previous studies we have shown that the European field elm (Ulmus campestris) releases volatiles after egg deposition by elm leaf beetles (Xanthogaleruca luteola). Leaves in the vicinity of the deposited eggs release specific blends of mostly terpenoid volatiles, which are highly attractive to the egg parasitoid Oomyzus gallerucae. By contrast, odour from egg-free leaves with feeding damage of the beetles or odour from undamaged leaves does not attract the parasitoids. This project aimed to analyse (a) conditions of parasitoid attraction and orientation for standardization of further analyses, (b) the relevance of terpenoids for parasitoid attraction, and (c) transcription changes of elm genes in response to elm leaf beetle infestation (egg deposition, feeding). Ad (a). While parasitoids were strongly attracted to odour from egg-laden plants also from a short distance, they did not show directed movements to the attractive odour indicating presence of host eggs when searching for hosts in an egg-laden plant. Thus, oviposition-induced elm volatiles were not used by parasitoids for small scale (< 40 cm) orientation across twigs with and without eggs of a plant. Egg deposition induced attractive elm odour in acropetal, but not in basipetal direction within the plant. An egg mass/leaf ratio of 1/2 to 1/3 was necessary to render elms attractive to egg parasitoids. Ad (b). By generating clonal elm plants and improving sampling and bio-test conditions we reduced the variability in volatile emission between elm individuals. Nevertheless, the volatile pattern did not differ between attractive egg-induced plants with feeding damage (EF-treatment) and nonattractive, egg-free but feeding-induced plants (F-treatment). However, the volatile pattern from EF- and F-treated elm differed clearly from the volatile pattern of undamaged control plants. Small volatile organic compounds not detected by our methods remain to be addressed in future studies. We used the inhibition of terpene biosynthesis by the inhibitors cerivastatin® and fosmidomycin in order to elucidate the relevance of terpenoids for parasitoid attraction. Inhibitor treated, ovipositioninduced elm released significantly less (E)-ß-caryophyllene, (E,E)-α-farnesene and (E)-4,8-dimethyl-1,3,7-nonatriene (DMNT) than non-inhibitor-treated, but oviposition-induced elm. The inhibitor treatment rendered the odour of elms with eggs on it unattractive. Subsequent bioassays revealed that each of these terpenoids per se attracted the parasitoids, even in the field, indicating that elms alert egg parasitoids after elm leaf beetle oviposition by means of one or more terpenoid volatiles. Ad (c). We established an EST platform for the study of insect feeding and oviposition-induced gene expression in European field elms (Ulmus campestris), which can form the basis for future national (and international) cooperations on elm research. RNA was extracted from clonal tress with (i) undamaged leaves (control), (ii) leaves with feeding damage, and (iii) leaves with feeding damage and eggs of X. luteola. Further treatments inducing defence genes included (iv) application of methyl jasmonate and (v) an artificial transfer of X. luteola eggs. A total of 359.064 EST sequences were obtained by 454 pyrosequencing from five elm leaf cDNA libraries and stored in a database with a suitable web interface. The ESTs were clustered in 21.490 contigs. The transcript signature of U. campestris leaves after egg deposition by the elm leaf beetle included the induction of defence and stress-related gene transcripts. The homology based functional annotation especially of genes involved in terpenoid biosynthesis and defence against herbivores will be checked in future studies. Furthermore transcriptional changes of defence genes in response to elm leaf beetle egg deposition will be analysed by qRT-PCR.
