Plant pests influence the movement of plant-fixed carbon and fungal-acquired nutrients through arbuscular mycorrhizal networks

Plants typically interact with multiple, co-occurring symbionts, including arbuscular mycorrhizal (AM) fungi which can form networks, connecting neighbouring plants. A characteristic aspect of the mycorrhizal symbiosis is the bi-directional exchange of nutrients between host plants and fungal partners. Concurrent interactions with competing organisms such as aphids or potato cyst nematodes (PCN) can disrupt the carbon-for-nutrient exchange between plants and AM fungi. However, the role of mycorrhizal networks in mediating these interactions remains unclear.

Using isotope tracing in multi-plant experimental systems, we investigated the movement of plant photosynthates and fungal-acquired soil phosphorus through mycorrhizal networks and the interactive effects of PCN infection on this.

We found evidence of preferential allocation of fungal-acquired phosphorus to plants that were not infected by PCN compared to infected neighbours. Contrary to previous findings using single plants, we did not detect a PCN-induced reduction in the amounts of plant carbon delivered to AM fungi in multi-plant systems. However, the mycorrhizal network(s) moved plant-fixed carbon away from PCN-infected host plants, regardless of the PCN infection status of the neighbouring plant host. 

Our work highlights the responsiveness of mycorrhizal networks to interactions with below-ground organisms. It also strengthens the argument for a more mycocentric view of AM-plant symbioses. Experimental designs of increasing ecological complexity are needed for a more comprehensive understanding of the carbon-for-nutrient dynamics in AM fungi-plant networks. This will, in turn, elucidate the role of AM fungi in terrestrial carbon cycling and their function in agricultural systems.