Howard, N. O. A. | bioRxiv Science

bioRxiv2025

Common symbiotic signalling pathway not essential for formation of functional mutualisms with endophytic fungi

Most plants form mutualistic symbioses with soil fungi, including arbuscular mycorrhizal (AM) fungi. These fungi usually transfer soil nutrients to plants and assimilate carbon from host plant photosynthesis. Recently, Mucoromycotina fine root endophytes (MFRE) were identified as nutritionally mutualistic and widespread fungal symbionts of plants, establishing MFRE as a new class of mycorrhizal fungi. However, the regulatory mechanisms for MFRE symbioses are completely unknown. Other symbionts, like AM fungi, use the Common Symbiotic Signalling Pathway (CSSP) to establish symbiosis. To explore whether MFRE interactions also involve this pathway, we cultured MFRE with CSSP mutants of Medicago truncatula which show impaired AM symbioses and tracked carbon and nutrient transfers using isotope tracers. Results show no differences in root colonization or nutrient exchange, suggesting MFRE symbioses are regulated by different molecular mechanisms. This finding highlights the unique nature of MFRE symbiosis, broadening our understanding of diverse fungal symbioses and their evolutionary significance.

microbiology↗

bioRxiv2024

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

O_LIMucoromycotina 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. C_LIO_LIUsing 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. C_LIO_LIWe 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. C_LIO_LIWe 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. C_LI

plant biology↗