Fungal symbiont diversity drives growth of Holcus lanatus depending on soil nutrient availability

Arbuscular mycorrhizal (AM) fungi frequently colonise plant roots and can affect plant morphology and physiology through their contribution to plant nutrition. However, the functional role of AM fungi in the presence of other microbial symbionts, including widespread Mucoromycotina ‘fine root endophytes’ (MFRE) fungi, remains largely unknown.
While both AM fungi and MFRE transfer nutrients, including nitrogen, from inorganic and organic sources to host plants, their combined effects on co-colonised plants have only been investigated in liverworts. Here, we compare the morphology and physiology of the grass Holcus lanatus grown with an AM fungal community versus a more diverse symbiotic fungal community containing both AM fungi and MFRE. 

Holcus lanatus plants were grown in the presence of either a diverse MFRE+AM fungi soil inoculum or a multi-species AM fungal inoculum. Plant traits associated with growth were quantified, along with fungal transfer of 15N tracer to plants from a variety of sources (ammonium chloride, alanine, glycine, algal necromass). 

Holcus lanatus grown with the AM fungal community had greater root and shoot growth during early development and prior to the addition of 15N-labelled sources, compared to plants grown with the more diverse symbiotic fungal community. When nitrogen sources were made available to the fungal symbionts in the pot microcosms, plants growing with the MFRE+AM fungi soil inoculum had a faster growth rate than plants growing with the AM fungal community. At harvest, H. lanatus grown with the AM fungal community had a larger biomass and there were no differences in 15N tracer assimilation in plants across the two fungal community treatments.
Our results demonstrate that the diversity of fungal inocula in conjunction with soil nutrient availability determines the benefits derived by plants from diverse fungal symbionts. Our research contributes to understanding host plant outcomes in diverse multi-symbiont scenarios.

Methods
The dataset was collected from a single pot experiment using measurements taken by hand/through observation or using specialist equipment. A detailed account is provided: 

Fungal colonisation data – harvested root material was mounted on microscope slides and fungal structures were quantified manually under a compound microscope through observation.
Hyphal length data – isolated fungal hyphae from soil were mounted on microscope slides and fungal structures were quantified manually under a compound microscope through observation.
Biomasses – harvested material was weighed on electronic scales.
Number of Tillers – plant tillers were quantified weekly through counting by hand.
Number of Leaves – plant leaves were quantified weekly through counting by hand.
Maximum Canopy Height & RGR – the height of the tallest leaf was measured weekly using hand-held callipers. From this data, the relative growth rate per pot was determined (see equations within Excel sheet).
Chlorophyll content (SPAD) – relative chlorophyll content from the tallest leaf was measured using a hand-held Minolta SPAD meter.

15N and N in shoots ­– shoot material was analysed using isotope ratio mass spectrometry (IRMS) to produce values for %N, 15N Atom % and 15N Delta Air, which were then processed with the recorded biomass data to give absolute and concentration data for nitrogen. All steps are detailed in the Excel sheet.

15N and N in roots ­– root material was analysed using isotope ratio mass spectrometry (IRMS) to produce values for %N, 15N Atom % and 15N Delta Air, which were then processed with the recorded biomass data to give absolute and concentration data for nitrogen. All steps are detailed in the Excel sheet.