Coping with ephemeral resources and unpredictable hosts : nitrogen allocation in the post-fire annual Nicotiana attenuata, and a plant-specific pheromone used by its seed predator Corimelaena extensa
This thesis explores phenotypic plasticity in the interaction of the post-fire annual tobacco Nicotiana attenuata with two herbivores: the lepidopteran folivore Manduca sexta and the hemipteran seed predator Corimelaena extensa. Due to mass-germination after fires, N. attenuata grows under strong intra-specific competition for soil nitrogen (N). The first part of this thesis analyses growth-defence trade-offs between proteins and N-based defences of N. attenuata after simulated M. sexta herbivory. A new LC-MSE method for protein (PTN) quantitation and 15N incorporation analysis was used, together with transgenic N. attenuata lines independently silenced for jasmonic acid (JA) signalling and for a transcription factor controlling the N-based phenolamides, NaMYB8. Elicitation decreased N flux into foliar PTN and increased it into defences. N flux was JA-independent and regulation of PTN levels after elicitation is related to MYB8 expression. A 15N pulse-chase showed that N for phenolamides comes from recently assimilated N rather than degradation of ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO), the major foliar PTN of N. attenuata. Experiments on a transgenic line silenced for RuBisCO support these results. The fire-chasing life-history of N. attenuata decreases its apparency to foraging insects and the second part of the thesis identifies the aggregation pheromone of C. extensa, (5Z,8Z)-tetradeca-5,8-dienal, used for host finding. Experiments suggest that C. extensa manipulates host apparency by emitting more pheromone when fed on N. attenuata compared to its perennial host, N. obtusifolia. This emission is also related to higher insect fecundity. JA responses and defences of N. attenuata do not affect C. extensa fecundity or host choice, but differences between two N. attenuata accessions suggest that C. extensa fecundity positively correlates with plant capsule number, and that by feeding on seeds this insect avoids JA-mediated plant defences entirely.