Phytophagy impacts the quality and quantity of plant carbon resources acquired by mutualistic arbuscular mycorrhizal fungi.

Arbuscular mycorrhizal (AM) fungi associate with the roots of many plant species, enhancing their hosts access to soil nutrients whilst obtaining their carbon supply directly as photosynthates. AM fungi often face competition for plant carbon from other organisms. The mechanisms by which plants prioritise carbon allocation to mutualistic AM fungi over parasitic symbionts remain poorly understood. Here, we show that host potato plants (Solanum tuberosum cv. Désirée) selectively allocate carbon resources to tissues interacting with AM fungi rather than those interacting with phytophagous parasites (the nematode Globodera pallida). We found that plants reduce the supply of hexoses but maintain the flow of plant-derived fatty acids to AM fungi when concurrently interacting with parasites. Transcriptomic analysis suggest that plants prioritise carbon transfer to AM fungi by maintaining expression of fatty acid biosynthesis and transportation pathways, whilst decreasing the expression of mycorrhizal-induced hexose transporters. We also report similar findings from a different plant host species (Medicago truncatula) and phytophagous pest (the aphid Myzus persicae). These findings suggest a general mechanism of plant-driven resource allocation in scenarios involving multiple symbionts.