Priming of pine defence against insect infestation

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Induced defences of plants against herbivorous insect are known to be affected by exposure to environmental cues preceding the infestation. Response to infestation-indicating cues can prime and improve plant defences. This project addresses molecular-ecological questions on priming defence of Scots pine (Pinus sylvestris) against infestation by the sawfly Diprion pini. Scots pine is known to show improved defence against herbivorous D. pini larvae when it has responded to D. pini egg depositions on its needles prior to larval feeding. Furthermore, Scots pine enhances its direct defence against sawfly eggs when it has previously been exposed to D. pini sex pheromones. So far, little is known about the mechanisms of priming defence of P. sylvestris against D. pini. In contrast, for angiosperms, it is well studied that egg-mediated priming of defence against larval feeding relies on significant changes in the plant transcriptome and concentrations of phytohormones and secondary plant metabolites. Therefore, we will investigate mechanisms of egg-mediated priming of Scots pine defence against D. pini larvae. We will test the hypothesis that angiosperms and gymnosperms (here, P. sylvestris) show some common, conserved transcriptomic and phytohormonal responses in this priming process. Furthermore, we will study whether egg-mediated, feeding-induced changes in concentrations of secondary pine metabolites (here, phenylpropanoid derivatives and phytosterols) can offer an explanation for the worse performance of D. pini on previously egg-laden pine when compared to egg-free pine. We will also investigate mechanisms of pheromone-mediated priming of P. sylvestris defence against D. pini eggs and study the transcriptomic and phytohormonal responses in this priming process. Thereby, we also want to examine whether these responses show parallels to egg-mediated priming of defence against larvae. This would provide evidence that stimuli that indicate insect infestation also elicit general "alarm” responses. In addition to the mechanisms of pheromone-mediated defence against eggs, we will further investigate the ecological effects of pine exposure to sawfly sex pheromones. We want to test whether not only direct, but also indirect defence responses to eggs (by attracting egg parasitoids) are amplified after pheromone exposure, whether direct defence against larvae is also affected, and whether egg-laying D. pini females show counter-adaptations to pine defence against their offspring.