Induced defence of pine (Pinus sylvestris) against egg deposition by sawflies: Characterisation of the elicitor
Final Report Abstract
Indirect defence of P. sylvestris against eggs of the sawfly D. pini has been known to be inducible by the oviduct secretion which encloses the eggs. The present study revealed that the indirect defence eliciting activity of this secretion is independent of whether the sawfly females are mated or not. Hence, indirect pine defence is induced by oviduct secretion of both mated and unmated females laying fertilized and unfertilized eggs. Since D. pini can reproduce parthenogenetically, larvae will also hatch from unfertilized eggs. Thus, pine does not “waste” costs of indirect defence when investing in indirect defence against both unfertilised eggs and fertilised eggs. Our analyses further showed that the indirect defence-eliciting factor in this oviduct secretion of D. pini is a small protein of about 13 kDa which shows sequence homologies to fatty-acid binding proteins of the lipocalin family. A similar, but not identical protein is also present in the oviduct secretion of G. pallida, although the secretion of this pine sawfly species does not induce indirect defence by pine (no attraction of egg parasitoids). This finding indicates a high sequence specificity of the elicitor. Future studies need to elucidate whether a heterologously expressed protein of the same sequence as the isolated elicitor protein shows the same activity and how it activates defence pathways of pine. Direct defence responses of P. sylvestris to D. pini eggs involve accumulation of H2O2 in needle tissue directly adjacent to the eggs, as was shown in the present study. While the entire egg deposition process can significantly induce activities of pine enzymes involved in H2O2 processing, oviduct secretion per se cannot significantly affect activities of these pine enzymes. However, oviduct secretion shows high catalase activity. Hence, enclosure of eggs by oviduct secretion might be a countermeasure by D. pini females against direct “anti-egg” defence of pine.
