Arbuscular mycorrhizal fungi form important symbioses with plant roots that help capture nutrients such as phosphorus from the soil. Thus it is conceivable that they also have an important impact on plant allocation to defense against herbivores and pathogens. Plants with greater nutrient supply have more resources available for defense, but higher nutrient status often leads to faster growth and a reduced allocation to defense. In this study we investigated the role of an arbuscular mycorrhizal fungus (Glomus intraradices) on the direct and indirect chemical defenses of ribwort plantain (Plantago lanceolata L.). We compared the volatile organic compound (VOC) emission and iridoid glycoside content of inoculated and non inoculated plants after mechanical wounding and caterpillar herbivory to that of non-treated control plants. Dynamic headspace collections with SuperQ filters were performed to sample VOCs in plantain and analysis was done with GC-MS.
Results/Conclusions
In both the mycorrhizal and non-mycorrhizal plants measured, we identified green leaf volatiles, a number of mono- and sesqui-terpenes as well as the homo-terpene DMNT, compounds reported to serve as attractants for herbivore enemies. Although no distinct qualitative differences were detectable in the VOC profiles between these two groups, there were significant quantitative differences between mycorrhizal and non-mycorrhizal plant indivudals. The non-mycorrhizal plants emitted significantly more VOCs than the mycorrhizal plants. Furthermore, there were qualitative and quantitative differences between the VOCs emitted by mechanically wounded and herbivory treated plants in both inoculated and non-inoculated plants. The major iridoid glycosides, namely aucubin, catalpol and geniposidic acid were quantified with HPLC-UV detection. There were no qualitative and quantitative differences in the iridoid glycosides measurable beween mycorrhizal and non-mycorrhizal plants in the three different treatments. Our results indicate that association with arbuscular mycorrhizal fungi can significantly alter the amount of volatiles emitted by plants after insect herbivory. Such changes may have a dramatic effect on the ability to attract herbivore enemies for indirect defense.
