LettersFine endophytes (Glomus tenue) are related to Mucoromycotina, not GlomeromycotaFine endophytes are arbuscule-producing fungi of unclear phylogenetic placement Fine endophytes (FE), Glomus tenue, are traditionally considered to be arbuscular mycorrhizal fungi (AMF) with distinctive microscopic morphology when stained. FE have fine hyphae (c. 1.5 lm diameter) which branch intra-cellularly in a distinctive fan-like pattern (Gianinazzi-Pearson et al., 1981;Abbott, 1982). The hyphae contain small swellings along their length, sometimes referred to as vesicle-like swellings (Hall, 1977). FE form arbuscules (or arbuscule-like structures) with fine elements in a tapered, conical shape (Greenall, 1963;Merryweather & Fitter, 1998). Spores of FE are very small (< 20 lm) compared to the majority of Glomeromycota, and colourless (Hall, 1977). Morphological variations indicate that FE may consist of multiple species (Thippayarugs et al., 1999), hence we use the term FE to indicate a species group.Within the kingdom Fungi, both morphological and genetic characteristics are used to determine taxonomic classification (St€ urmer, 2012). In 2001, all AMF were placed within the phylum Glomeromycota (Sch€ ußler et al., 2001). In the listing of glomeromycotan species by Sch€ ußler & Walker (2010), some members of the genus Glomus were not revised due to insufficient taxonomic knowledge, and this included FE. A key reason for classifying FE within the Glomeromycota was the presence of arbuscules, considered apomorphic for the phylum (Morton, 1990). However, the morphological features of root colonization by FE are distinct from other, coarse, AMF so their placement within the genus Glomus and the Glomeromycota was questioned (Hall, 1977;Sch€ ußler & Walker, 2010), and their status as mycorrhizal fungi is ambivalent.Accurate determination of FE usually requires magnification ≥ 9100, hence, where assessments of AMF colonization use lower magnifications they may not be identified. Furthermore, FE may be undetected if samples are not processed within 2 d of harvesting (Orchard et al., 2016a). Nevertheless, FE are globally distributed and prolific within many ecosystems, examples include: pastures and native bushland of New Zealand (Crush, 1973) and Australia (Abbott & Robson, 1982;McGee, 1989), Venezuelan cloud forests (Rabatin et al., 1993), riverine and alpine regions of Europe (Read & Haselwandter, 1981;Turnau et al., 1999;Binet et al., 2011) and an old-field in the United States (Hilbig & Allen, 2015). However, the difficulty of isolating and, hence, genetically characterizing FE has hindered the determination of their phylogenetic placement. A novel method to enrich colonization by fine endophytesTo clarify the identity of FE, we targeted the SSU (18S) ribosomal RNA gene using roots from two independent glasshouse experiments where individual pots contained multiple plants. For each pot we used one root system for DNA extraction and one root system to visually assess the percentage of total root length colonized (%TRL; see Sup...

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.

It has long been postulated that symbiotic fungi facilitated plant migrations onto land through enhancing the scavenging of mineral nutrients and exchanging these for photosynthetically fixed organic carbon. Today, land plant-fungal symbioses are both widespread and diverse. Recent discoveries show that a variety of potential fungal associates were likely available to the earliest land plants, and that these early partnerships were probably affected by changing atmospheric CO concentrations. Here, we evaluate current hypotheses and knowledge gaps regarding early plant-fungal partnerships in the context of newly discovered fungal mutualists of early and more recently evolved land plants and the rapidly changing views on the roles of plant-fungal symbioses in the evolution and ecology of the terrestrial biosphere.

Fine root endophytes (FRE) were traditionally considered a morphotype of arbuscular mycorrhizal fungi (AMF), but recent genetic studies demonstrate that FRE belong within the subphylum Mucoromycotina, rather than in the subphylum Glomeromycotina with the AMF.These findings prompt enquiry into the fundamental ecology of FRE and AMF. We sampled FRE and AMF in roots of Trifolium subterraneum from 58 sites across temperate southern Australia. We investigated the environmental drivers of composition, richness, and colonisation of FRE and AMF by using structural equation modelling and canonical correspondence analyses. Root colonisation by FRE increased with increasing temperature and rainfall; but decreased with increasing phosphorus. Root colonisation by AMF increased with increasing soil organic carbon but decreased with increasing phosphorus. Richness of FRE decreased with increasing temperature and soil pH. Richness of AMF increased with increasing temperature and rainfall, but decreased with increasing soil aluminium and pH. Aluminium, soil pH, and rainfall were, in decreasing order, the strongest drivers of community composition of FRE; they were also important drivers of community composition of AMF, along with temperature, and in decreasing order: rainfall, aluminium, temperature, and soil pH. Thus FRE and AMF showed the same responses to some (e.g., soil P, soil pH) and different responses to other (e.g., temperature) key environmental factors. Overall, our data are evidence for niche differentiation among these co-occurring mycorrhizal associates.

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