Drought rearranges preferential carbon allocation to arbuscular mycorrhizal community members co-inhabiting roots of Medicago truncatula

Arbuscular mycorrhizal (AM) fungi establish symbiosis with majority of plant species, supporting their abiotic and biotic stress tolerances, and receiving reduced carbon in return. However, how and why plants control the composition of their associated mycorrhizal communities remains largely unknown. Our aim was to analyze the consequences of abrupt changes in environmental conditions such as light intensity or water supply on carbon allocation from plant (Medicago truncatula) to different AM fungal species coexisting in plant roots, employing 13C labeling and tracing. Significant differences were detected in the composition of synthetic communities of AM fungi just ten days after the environmental change induction. Under simulated drought, plants preferentially allocated their carbon to Funneliformis mosseae to the detriment of Claroideoglomus claroideum. Compared to drought, shading did not lead to a significant rearrangement of carbon fluxes from plants to the different AM fungi. Our observations strongly suggest that plants actively promote, through preferential allocation of their carbon, specific AM fungal symbionts in their roots depending on environmental conditions. Yet, it still needs to be elucidated which fungal traits are playing a role in this process, how are the different symbionts recognized, and which molecular mechanisms are involved in such preferential carbon routing.