Preferential nitrogen and carbon exchange dynamics in Mucoromycotina “fine root endophyte”-plant symbiosis

Mucoromycotina “fine root endophyte” (MFRE) fungi are an understudied group of plant symbionts that regularly co-occur with arbuscular mycorrhizal fungi. The functional significance of MFRE in plant nutrition remains underexplored, particularly their role in plant nitrogen (N) assimilation from the variety of sources typically found in soils. Using four 15 N-labeled 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 carbon (C) allocation to MFRE. We found that 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 the MFRE directly assimilates and metabolizes organic compounds, retaining C to meet its own metabolic requirements and transferring N to plant hosts. Our findings highlight diversity in the function of endomycorrhizal associations, with potentially profound implications for our understanding of the physiology and ecology of plant-fungal symbioses. • MFRE preferentially transfer 15 N from glycine and ammonium to plants • MFRE supplied with glycine and ammonium received most plant carbon • MFRE fungi use organic compounds, retaining C and transferring N to plants • Plants benefits from MFRE symbiosis depend on substrate nutrient availability Mucoromycotina fine root endophytes (MFRE) are symbiotic fungi that transfer soil nutrients to plants, gaining carbon (C). Howard et al. show that MFRE prefer nitrogen (N) from glycine and ammonium over nitrate and urea, transferring N to the plant and retaining C; N availability influences this, highlighting the importance of MFRE in plant nutrition.