Functional significance of Mucoromycotina 'fine root endophyte' fungi in the nitrogen nutrition of host plants

Mycorrhizal associations between fungi and plant roots have globally significant impacts on terrestrial nutrient cycling. Mucoromycotina ‘fine root endophytes’ (MFRE) are a distinct group of mycorrhiza-forming fungi that associate with the roots of a range of host plant species. Previous misidentification and assignment as arbuscular mycorrhizal (AM) fungi of the Glomeromycotina – sister subphylum to the Mucoromycotina – has resulted in systematic under investigation of these fungi. Therefore, it is now important to establish how MFRE-plant symbioses function.

In this thesis I develop novel monoxenic microcosms, and non-sterile mesocosms that are used in experiments throughout my project. Monoxenic microcosms are first used to investigate the nutritional function of MFRE in the absence of a soil microbiome. I then use non-sterile mesocosms to investigate the effects of a soil microbiome on the nutrient dynamics between MFRE and Plantago lanceolata.

From these experiments I establish that in the absence of other soil microbes, MFRE preferentially assimilates nitrogen from glycine and ammonium chloride for transfer to Plantago. This occurs regardless of whether these sources of nitrogen are applied singly or in equal mixtures with other sources of nitrogen. I also determine that the availability of nitrogen to MFRE and host plants affects the amount of nitrogen and carbon exchanged between MFRE and Plantago. Under reduced inorganic nitrogen conditions, MFRE transports proportionately more nitrogen to Plantago hosts than when inorganic nitrogen is more available. This variation in nitrogen transfer from MFRE occurs concurrently with no alteration in the photosynthetic carbon transfer from Plantago to MFRE and a significant reduction in colonisation by MFRE of Plantago roots.

My research adds new nuance to our knowledge of MFRE symbiotic functions with plants. I present clear evidence that MFRE are functionally distinct from AM fungi. I show that MFRE have a significant capacity to assimilate organic compounds and utilise their carbon and nitrogen components differently, while altering its relative benefit to host plants.