Direct nitrogen, phosphorus and carbon exchanges between Mucoromycotina ‘fine root endophyte’ fungi and a flowering plant in novel monoxenic cultures

Most plants form mycorrhizal associations with mutualistic soil fungi. Through these partnerships, resources are exchanged including photosynthetically-fixed carbon for fungal-acquired nutrients. Recently, it was shown that the diversity of associated fungi is greater than previously assumed, extending to Mucoromycotina fungi. These Mucoromycotina ‘fine root endophytes’ (MFRE) are widespread and generally co-colonise plant roots together with Glomeromycotina ‘coarse’ arbuscular mycorrhizal fungi (AMF). Until now, this co-occurrence has hindered determination of the direct function of MFRE symbiosis.

To overcome this major barrier, we developed new techniques for fungal isolation and culture and established the first monoxenic in vitro cultures of MFRE colonising a flowering plant, clover. Using radio- and stable-isotope tracers in these in vitro systems, we measured transfer of 33P, 15N and 14C between MFRE hyphae and the host plant.

Our results provide the first unequivocal evidence that MFRE fungi are nutritional mutualists with a flowering plant by showing that clover gained both 15N and 33P tracers directly from the fungus in exchange for plant-fixed C in the absence of other microorganisms.

Our findings and methods pave the way for a new era in mycorrhizal research, firmly establishing Mucoromycotina ‘fine root endophytes’ as both mycorrhizal and functionally important in terrestrial ecosystems.