Fine root endophytes under scrutiny: a review of the literature on arbuscule-producing fungi recently suggested to belong to the Mucoromycotina.

Fine root endophytes (FRE) are arbuscule-forming fungi presently considered as a single species-Glomus tenue in the Glomeromycota (Glomeromycotina)-but probably belong within the Mucoromycotina. Thus, FRE are the only known arbuscule-forming fungi not within the arbuscular mycorrhizal fungi (AMF; Glomeromycotina) as currently understood. Phylogenetic differences between FRE and AMF could reflect ecological differences. To synthesize current ecological knowledge, we reviewed the literature on FRE and identified 108 papers that noted the presence of FRE and, in some, the colonization levels for FRE or AMF (or both). We categorized these records by geographic region, host-plant family and environment (agriculture, moderate-natural, low-temperature, high-altitude and other) and determined their influence on the percentage of root length colonized by FRE in a meta-analysis. We found that FRE are globally distributed, with many observations from Poaceae, perhaps due to grasses being widely distributed. In agricultural environments, colonization by FRE often equalled or exceeded that of AMF, particularly in Australasia. In moderate-natural and high-altitude environments, average colonization by FRE (~10%) was lower than that of AMF (~35%), whereas in low-temperature environments, colonization was similar (~20%). Several studies suggested that FRE can enhance host-plant phosphorus uptake and growth, and may be more resilient than AMF to environmental stress in some host plants. Further research is required on the functioning of FRE in relation to the environment, host plant and co-occurring AMF and, in particular, to examine whether FRE are important for plant growth in stressful environments. Targeted molecular primers are urgently needed for further research on FRE.