The effects of the root endophytic fungus Acremonium strictum on plant-herbivore interactions

The widespread occurence of endophytic fungi in virtually all plant species has prompted an increasing number of investigations into the ecological significance of these cryptic microorganisms as mediators of plant-herbivore interactions. In my studies, I investigated the role of the fungal endophyte Acremonium strictum Gams, restricted to the roots of the extrafloral (EF) nectary-bearing broad bean plant Vicia faba L., in induction of EF-mediated defences and reduction of herbivory. In the first experiment, I manipulated the presence/absence of A. strictum in plant roots and inflicted Aphis fabae damage at a specific time and location in order to examine whether the endophyte colonization would induce the EF-mediated indirect defences in response to herbivory. Separately, the endophyte colonization and the herbivore infestation induced the production of two EF traits (EF nectar volume and EF nectary number). On the other hand, both EF traits were significantly reduced in plants simultaneously colonized with the endophyte and infested with the herbivore; which was predicted (from a cost/benefit perspective) as a trade-off between EF- and endophyte-mediated defences. In a subsequent experiment, these interactions were examined under variable levels of nutrient availabilty. Following herbivory, the level of variation in EF nectar and nectary in the absence of endophyte infection was only slightly affected by nutrient addition; whereas these EF rewards responded to nutrient addition in a more complex way in endophyte-infected plants depending on herbivore damage. Also, increasing nutrient supply increased the extent of root colonization with A. strictum and alleviated the negative effects of herbivory on plant fitness in both endophyte-infected and endophyte-free plants. Several measured parameters of the insect fitness were improved by nutrient addition on endophyte-free plants, but were less responsive on endophyte-infected plants. Results from this part suggest that plants regulate multiple mutualisms (i.e. EF- and endophyte-mediated mutualisms) in response to variation in resource availability so as to attain a favourable cost/benefit ratio.Finally, experiments were conducted to examine whether endophyte effects on herbivory would depend on the experimental setting used in investigation and whether they would translate into a subsequent generation of the herbivore. A. strictum negative effects on the fitness of Helicoverpa armigera first generation were more evident when the larvae foraged freely on inoculated intact whole plants than when offered leaf discs of inoculated plants, and these endophyte-mediated negative effects were carried over into the herbivore second generation. A loss of volatiles or inhibitory effects of compounds that were stronger in situ might have caused changes in larval feeding and performance on leaf discs as compared to intact plants, regardless of infection status. Furthermore, the reduction in fitness parameters of the herbivore across two generations might have been due to the endophyte-triggered reduction in plant quality. Results from these studies should have far-reaching conceptual and practical implications for future endophyte research and should also set the stage for a better understading of the context under which organisms interact, adapt, and evolve.