Carbohydrate dynamics in plant–fungal–nematode networks: implications for stress resilience

Plant-parasitic nematodes are among the most destructive agricultural pests, driving substantial yield losses across diverse cropping systems through their exploitation of plant resources. Conversely, root-associated obligate mutualistic fungi, including arbuscular mycorrhizal fungi and endophytic taxa such as Serendipita spp., can reshape plant responses to nematode attack by increasing nutrient uptake, modulating stress responses, and influencing immune signalling. These fungi form intimate symbioses with plant roots, exchanging mineral nutrients for host-derived carbohydrates, placing carbon allocation at the centre of plant-fungal-nematode interactions. This review synthesises current understanding of the tripartite interactions between plants, mutualistic fungi, and plant-parasitic nematodes, with a particular focus on carbohydrate allocation as a shared and contested resource. We discuss how nematodes redirect host carbon flows to establish and maintain feeding sites, and how fungal mutualists alter carbohydrate partitioning, plant defence and stress physiology. Importantly, we highlight that fungal symbioses do not uniformly enhance resistance; in some cases, they may increase host susceptibility to nematodes by altering carbohydrate availability or defence prioritisation. By integrating findings across plant-fungal-nematode systems, this review identifies mechanisms that underpin resistance- versus susceptibility-enhancing outcomes and outlines key knowledge gaps. Together, these insights provide a framework to inform the strategic use of beneficial fungi to strengthen crop resilience against nematode stress.