Interactions between an endophytic fungus, aphids and extrafloral nectaries: do endophytes induce extrafloral‐mediated defences in Vicia faba?

Abstract

Summary There is increasing evidence that extrafloral (EF) nectaries, described in approximately 1000 plant species from more than 90 families, have a defensive function. Endophytic fungi are an important group of microorganisms asymptomatically colonizing host plants, and promoting their defences against natural enemies. We aimed at investigating the role of these microorganisms in inducing EF nectary defences in plants against herbivory. We conducted a full factorial experiment to study the effects of a soil‐borne endophytic fungus, Acremonium strictum, alone or in combination with the aphid, Aphis fabae, on the production of EF nectar and nectaries in broad beans. By manipulating the presence/absence of the fungus in the roots of the host plants and by inflicting herbivore damage at a specific time and location, we tested the hypothesis that endophyte inoculation induces EF‐mediated indirect defences. The quantity of EF nectar production and the number of EF nectaries produced were assessed by repeated samplings at fixed intervals. Endophytic inoculation of bean plants induced a significant short‐term increase in total EF nectar production and a significant prompt increase in number of EF nectaries per expanded leaf. On the other hand, aphid infestation resulted in a prolonged increase in total EF nectar production and a delayed induction of EF nectaries. Conversely, when plants were simultaneously inoculated with the endophyte and infested with aphids, both EF traits were significantly reduced. The effect of endophyte inoculation was further examined by recording the life history traits of Aphis fabae. Aphid performance was generally lower on inoculated plants; however, relative fecundity was the only fitness parameter significantly reduced on endophyte inoculated plants. The organism model in the present study serves as a model for investigating how endophytic colonization alters the response of EF nectary traits to herbivory. From a cost/benefit perspective, variable responses in EF‐mediated indirect defences as influenced by endophytes could be explained as trade‐offs in defence. In addition, other possibilities that may have contributed to the EF response patterns reported in this study are discussed.