The below ground root symbiont arbuscular mycorrhizal fungi (AMF) can be involved in multitrophic interactions with plant associated pests, such as aphids. AMF can both increase and decrease aphid performance, but the underlying mechanisms are unclear. Mechanisms could include AMF altering 1) plant defence responses and signalling and/ or 2) the quality of the plant as a food source. Moreover, little is known about how aphid associated microbial facultative symbionts (FS) influence the outcome of the interaction, or how aphid herbivory affects the AMF colonising the plant.
To investigate the role of plant quality as a food source, nitrogen (N), a limiting nutrient in the aphid diet, was tracked through an AMF, barley (Hordeum vulgare) and English grain aphid (Sitobion avenae) glasshouse system. AMF increased the N concentration of the aphid’s food source and it was demonstrated that AMF can deliver N to above ground organisms via the plant. However, aphid performance and N concentration remained unchanged. Metagenomic methods were used to investigate the impact of S. avenae on AMF colonising barley. In an agricultural system, the abundance of the AMF family Gigasporaceae tended to increase when aphids were present, suggesting that aphid herbivory could influence plant associated AMF communities.
To investigate the role of plant defence signalling, plant defence gene expression analysis was carried out in a broad bean (Vicia faba)-pea aphid (Acyrthosiphon pisum) system. AMF augmented the expression of a salicylic acid (SA) pathway gene (PR5) and reduced aphid performance. The FS Hamiltonella defensa did not alter the impact of AMF on the aphid.
This thesis contributes to the identification of driving mechanisms in AMF-aphid interactions and provides evidence that plant defence signalling can play a role. Moreover, in certain cases, aphids may be able to overcome changes in the N concentration of plant hosts caused by AMF.
