Plants evolved elaborate strategies to cope with the ubiquitous threat posed by herbivorous insects. Beside constitutive or induced defence mechanisms, priming of plant defences describes a strategy by which the plants improve their defence response upon perceiving a previous stimulus indicative of the stress. For instance, insect egg deposition can serve as stimuli for the plant which indicate future herbivory, as numerous herbivorous insects deposit their eggs directly on the future host plant of their larvae. Indeed, various plant species improve their defence against herbivorous larvae when they previously perceived insect egg deposition as a priming signal. However, knowledge of the signalling and mechanisms which facilitate such an enhanced anti-herbivore defence in oviposited plants is scarce. Therefore, this dissertation aims to further investigate the temporal dynamics of the activation of different signalling pathways as well as the fitness consequences of oviposition-mediated priming of anti-herbivore defences in two solanaceous plant species in interaction with generalist and specialist lepidopteran herbivores. Within the first part, responses of the bittersweet nightshade (Solanum dulcamara) to oviposition and / or larval feeding by the beet armyworm Spodoptera exigua and the leaf mining specialist Acrolepia autumnitella were examined to investigate the largely unknown temporal dynamics of the primed state. Firstly, the accumulation of phytohormones and associated transcripts of defence related genes in oviposited (primed) leaves were compared to those of untreated control leaves at different time points within and after the natural egg incubation time, as well as a time point matching the time of larval hatching. The so far undescribed phytohormonal and transcriptional responses of S. dulcamara to oviposition by A. autumnitella were largely similar to those of S. exigua oviposition. The induction of salicylic acid (SA) by oviposition was restricted to the period of egg exposure, while a differentially transcriptional induction in oviposited leaves was detectable for at least ten days after oviposition, i.e. six days after egg removal. Interestingly, jasmonic acid (JA) and jasmonic acid-isoleucine (JA-Ile), highly important phytohormones for the induction of defence mechanisms against chewing herbivores, were induced in small quantities within and consistently after period of egg exposure, which could indicate for a preparation of the defence response against the hatching larvae. Consequently, the next aim was to investigate if oviposition-mediated defence priming alters responses to the feeding larvae during the beginning of the larval attack. Therefore, full-factorial experiments with oviposition by S. exigua (priming stimulus) and a short phase of herbivory or simulated herbivory (triggering stimulus) were conducted, while both stimuli were applied on different but vascular fully-connected leaves. During the onset of the response, oviposition and natural/simulated herbivory caused an additive effect on the accumulation of abscisic acid (ABA). Moreover, the primed jasmonate induction in oviposited plants in response to natural or simulated herbivory suggests an earlier or faster induction which could entail a more effective defence against the larvae. Furthermore, transcriptional results suggest an involvement of cytokinins in oviposition-mediated priming, which could further point to an important role of the phytohormonal interplay for a primed defence induction. Defence priming is postulated as adaptive strategy, however, actual knowledge regarding the effect of oviposition priming on the plant fitness is largely missing. Consequently, the aim of the second part of this thesis was to examine the fitness consequences of oviposition priming for the annual plant Nicotiana attenuata in interaction with the generalist herbivore S. exigua and the tobacco specialist Manduca sexta. Therefore, full factorial priming experiments with both herbivores were conducted which assessed the growth (stalk length) and fitness (flowering, number of capsules and seed weight) of oviposited and non-oviposited plants, induced and noninduced by natural or simulated herbivory. Larval feeding by both herbivores and associated induced defence caused a clearly diminished growth and fitness of N. attenuata. Oviposition by both herbivores without subsequent larval feeding had no effect on growth or plant fitness, indicating that fitness incurs as a consequence of the onset and maintenance of the primed state are minimal in occasions when herbivory does not occur. Consistent with the diminished larval performance of S. exigua on prior oviposited plants, the fitness loss due to larval feeding by S. exigua was slightly smaller for oviposited plants in terms of capsule and seed production. Benefits of oviposition priming likely lie in the decline of fitness losses due to herbivory. In contrast, the fitness loss due to herbivory by M. sexta was not affected by prior oviposition, but in this interaction also the larval performance is not impaired. The induced defence, triggered by simulated herbivory without leaf tissue loss, had only an effect on flowering but not on the reproductive output. Oviposition in combination with simulated herbivory, i.e. a higher primed defence induction, had no further negative effect on fitness of N. attenuata. Probably fit-ness consequences of oviposition priming are mainly influenced by the effect of the primed defence induction on the herbivore and associated lower leaf tissue loss than due to physiological costs caused by a higher defence induction. Interestingly, M. sexta herbivory on N. attenuata is also known to induce tolerance responses, such as transient carbon allocation to the roots that could enhance the ability to regrow after the herbivore threat is gone. To assess if such induced tolerance mechanisms are enhanced by oviposition-mediated priming, experiments including oviposition and / or larval feeding by M. sexta followed by a removal of all aboveground plant parts were conducted to observe the fitness of regrown plants. Interestingly, the fitness of regrown plants was enhanced if plants were exposed to oviposition in combination with subsequent larval feeding before defoliation, which suggests that oviposition priming affects tolerance responses. As the physiological state of a plant changes during plant development, which is assumed to affect the inducibility of defence and tolerance responses such as carbon reallocation, further experiments with plants in different developmental stages were conducted. Young rosette and matured flowering plants were exposed to the similar experimental setup as early elongating plants in the previous experiments. Varying effects of larval feeding and prior oviposition on the plant fitness of regrown plants in the distinctive developmental stages, suggest that the ability to regrow and the enhancing effect of prior oviposition follow a developmental pattern. Overall, this doctoral thesis highlights the involvement of different phytohormones in the context of oviposition-mediated defence priming. Within an elaborate phytohormonal interplay involving salicylic acid, absicic acid, jasmonates facilitate an earlier or faster response to larval feeding, which may enable the oviposited plant to mount a more effective defence. Furthermore, this thesis indicates that oviposition-mediated defence priming may not just be beneficial for plant fitness if the defence is effective against the herbivore but additionally by increasing tolerance responses to larval feeding.
