Evidence for resource transfer via common endophyte networks

Fungal symbionts play essential roles in ecosystems influencing plant development and biodiversity. Mycorrhizal fungi can form common mycorrhizal networks (CMNs) where a fungus connects the roots of at least two plants via continuous extraradical mycelium and transfers resources such as nitrogen and carbon. In addition to mycorrhizal fungi, there is another group of fungal mutualists known as endophytes. They also support plant development and may form common endophyte networks (CENs). Whether endophytes can transfer soil resources like nitrogen, carbon, and water through such networks remains an open question. To test this, we established a CEN experiment in split petri dishes involving Arabidopsis thaliana hosts and three phylogenetically diverse endophytes (Trichoderma viride, Mucor hiemalis, and Fusarium temperatum) to test whether resources like isotopically labelled amino acid ¹⁵nitrogen (N), amino acid ¹³carbon (C), ¹⁵N-ammonium, or deuterated water can be transferred by donor to receiver plants connected via CENs. We show that the tested endophytes can form CENs and transfer growth limiting resources from donor plant soil to receiver plant tissues. F. temperatum boosted plant growth by 38% relative to the uninoculated control, and it enriched plant ¹⁵N content derived from amino acids by 55%. Surprisingly, we also observed amino acid-derived ¹³C transport from donor plant soil to receiver plant tissues by T. viride (+ 2.83% > control). We also demonstrate that soil resource transfer, evaluated as isotope enrichment, by all three endophytes shifted in the presence of two versus a single host plant even when root systems were physically separated to avoid competition, underscoring that endophytic functioning, not just that of plants, also shifts when CENs are formed. Our results demonstrate that non-mycorrhizal fungi, like endophytes, can form networks similar to the idea of CMNs and transfer plant growth relevant resources. Endophytes display a broad array of symbiotic functions with their hosts, and formation of CENs may be a newly discovered component of their symbiotic tool kit.