The impact of insect egg deposition on Pinus sylvestris transcriptomic and phytohormonal responses to larval herbivory

Plants activate defensive responses to insect herbivory, but also to the initial step of infestation by many insect species, i.e. the egg deposition. They can even improve their defenses against herbivory by responding to previous insect egg deposition. Molecular analysis of these egg-mediated defenses against herbivory focused so far on angiosperm species. Yet, it is unknown how the transcriptome of a gymnosperm species is responding to insect eggs and subsequent larval feeding. Scots pine (Pinus sylvestris) is known to improve its defenses against herbivorous sawfly larvae when previously having received sawfly eggs. Here, we analyzed the transcriptomic and phytohormonal responses of Scots pine needles to eggs and larvae of the sawfly Diprion pini. Pine showed strong transcriptomic responses to both sawfly eggs and larval feeding. Many of the egg-responsive genes were differentially regulated also in response to feeding damage. Various gene ontology (GO) terms were enriched in both egg-laden and in feeding-damaged pine, among them especially GO terms related to cell wall modification, cell death, and jasmonic acid signaling. In contrast to responses of egg-laden angiosperms to insect feeding, pine laden with sawfly eggs showed an attenuated transcriptomic response to subsequent feeding damage. However, in accordance with angiosperm responses, egg-laden pine responded to feeding damage with high SA levels. Based on the detected considerable overlap of egg- and feeding-responsive pine genes we suggest that the strong egg-induced transcriptomic pine response results in maintained defenses against the feeding larvae, thus allowing pine to "afford" attenuated transcriptomic responses to the feeding damage.