The role of plant-parasitic nematodes in modulating plant symbioses with arbuscular mycorrhizal fungal networks

Arbuscular mycorrhizal (AM) fungi form widespread symbioses with plants, exchanging soil nutrients for carbon (C) fixed through photosynthesis. While this bidirectional exchange is thought to be regulated by both partners, natural interactions involve multiple co-occurring organisms and complex mycorrhizal networks (MNs) linking neighbouring plants, complicating resource distribution.

This thesis investigated the relationship between AM networks, potato plants, and potato cyst nematodes (PCN). First, using multi-plant systems likely connected to the same MN and isotope tracing, I quantified the movement of fungal-acquired phosphorus (P) and plant-derived C under herbivory by PCN. Fungal-acquired P was preferentially allocated to uninfected plants relative to their infected neighbours, while plant-fixed C was consistently redistributed within the MNs away from infected hosts.

To assess the influence of the wider microbial environment, I characterised root and soil communities using metabarcoding. PCN infection reduced fungal diversity, including AM fungal richness, and altered community composition, whereas bacterial communities remained largely unchanged. This suggests fungal communities are sensitive to PCN infection, with associated shifts potentially influencing C-for-P exchange. However, using simplified microcosms stripped of microbial complexity, I found that AM networks redirected C towards non-infected plants, confirming the MNs capacity to regulate allocation independent of other microbes.

Finally, I explored plant metabolic responses. Non-volatile metabolites in leaves were overall unaffected by PCN infection, with only infected plants showing elevated defence-related compounds relative to their uninfected neighbours, indicating limited below-ground signalling. In contrast, volatile metabolites emitted from PCN-free plants were subtly altered by neighbouring PCN infection, suggesting MN-mediated below-ground signalling can influence some above-ground plant responses.

Collectively, these results show that AM networks can, at least partly, regulate both resource exchange and plant volatile metabolite responses, and that these dynamics are modified by herbivory. This highlights the ecological importance of AM networks and their role in shaping plant–microbe–herbivore interactions.