The potential role of Mucoromycotina ‘fine root endophytes’ in plant nitrogen nutrition

Abstract

Mycorrhizal associations between fungi and plant roots have globally significant impacts on nutrient cycling. Mucoromycotina ‘fine root endophytes’ (MFRE) are a distinct and recently characterised group of mycorrhiza‐forming fungi that associate with the roots of a range of host plant species. Given their previous misidentification and assignment as arbuscular mycorrhizal fungi (AMF) of the Glomeromycotina, it is now important to untangle the specific form and function of MFRE symbioses. In particular, relatively little is known about the nature of MFRE colonisation and its role in N uptake and transfer to host plants. Even less is known about the mechanisms by which MFRE access and assimilate N, and how this N is processed and subsequently exchanged with host plants for photosynthates. Here, we summarise and contrast the structures formed by MFRE and arbuscular mycorrhizal fungi in host plants as well as compare the N source preference of each mycorrhizal fungal group with what is currently known for MFRE N uptake. We compare the mechanisms of N assimilation and transfer to host plants utilised by the main groups of mycorrhizal fungi and hypothesise potential mechanisms for MFRE N assimilation and transfer, outlining directions for future research.

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

“…Other Mucoromycotina fungi closely related to symbiotic Endogonales are saprotrophic, indicating that MFRE may well share at least some of the molecular machinery required for a facultative saprotrophic lifestyle (Spatafora et al, 2016). The putative saprotrophic capabilities of MFRE and their assumed ability to access and transfer N from organic sources to plants-albeit only tested so far with non-vascular plants (e.g., liverworts; Field et al, 2019)-support the hypothesis that MFRE have a complementary role in plant nutrition alongside AM fungi, with potentially important ecological implications for plant community structure and nutrient cycling, especially with regard to N (Field et al, 2019;Howard et al, 2022;Hoysted et al, 2019Hoysted et al, , 2023.…”

“…However, unlike strictly biotrophic AM fungi, MFRE can be isolated and grown in vitro in the absence of a plant host (Field, Rimington, et al., 2015; Hoysted et al., 2023). This suggests that MFRE may persist in the soil in a free‐living state, probably due to being equipped with transporters and enzymatic pathways necessary to obtain nutrients from complex organic nutrient sources (Howard et al., 2022). Other Mucoromycotina fungi closely related to symbiotic Endogonales are saprotrophic, indicating that MFRE may well share at least some of the molecular machinery required for a facultative saprotrophic lifestyle (Spatafora et al., 2016).…”

“…In plants, N is a key component of the most abundant proteins and pigments, such as RuBisCO and chlorophyll; in fungi, it is needed for hyphal proliferation and the production of nutrient transporters in membranes (Howard et al., 2022). Nitrogen is present in soils in concentrations that can range from 0.39 to 32 g kg −1 depending on edaphic factors (Bronson, 2008).…”

“…Associations with MFRE that have saprotrophic capabilities could offer host plants enhanced access to organic soil N sources beyond that afforded by associations with AM fungi alone (Field et al, 2019;Howard et al, 2022). Given recent reports that these two groups of fungi proliferate under different edaphic conditions (Albornoz et al, 2021(Albornoz et al, , 2022Mansfield et al, 2023), it is important that we learn more about their respective and combined roles in plant N acquisition as this will allow us to better predict how land management may affect plant community reliance on fungal acquired N in both managed and natural habitats.…”

Update from underground: Special issue on root responses on abiotic and biotic stresses

“…However, when Azospirillum was co‐inoculated, these changes were essentially reversed back to normal. In addition, Howard et al (2022) discuss the role of the recently recognized Mucoromycotina Fine Root Endophytes for their growth‐promotive role and Goode and Mitchum (2022) review the current understanding of the role of nematode‐induced molecular patterns in root–nematode interaction.…”