Howard, N. O. A.; Williams, A.; Durant, E.; Pressel, S.; Daniell, T. J.; Field, K. J.

Mucoromycotina \'fine root endophytes\' (MFRE) are an understudied group of plant fungal symbionts that usually co-occur with arbuscular mycorrhizal fungi. The functional significance of MFRE in plant nutrition remains under-explored, particularly their role in plant N assimilation from the variety of sources typically found in soils. Using four 15N-labelled N sources to track N transfer between MFRE and Plantago lanceolata, applied singly and in tandem, we investigated N source discrimination, preference, and transfer to host plants by MFRE. We traced movement of 14C from plants to MFRE to determine the impact of N source type on plant C allocation to MFRE. We found MFRE preferentially transferred N derived from glycine and ammonium to plant hosts over that derived from nitrate and urea, regardless of other N sources present. MFRE mycelium supplied with glycine and ammonium contained more plant-derived carbon than those supplied with other N sources. We show that MFRE directly assimilates and metabolises organic compounds, retaining C to meet its own metabolic requirements and transferring N to plant hosts. Our findings highlight diversity in function of endomycorrhizal associations with potentially profound implications for our understanding of the physiology and ecology of plant-fungal symbioses.