Preferential assimilation, metabolism, and transfer of organic nitrogen to host plants by Mucoromycotina ‘fine root endophytes’

Summary 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 15 N-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 14 C 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.