Quantitative real-time PCR (qPCR) is slowly becoming established as a tool to quantify abundance of different arbuscular mycorrhizal fungal (AMF) taxa in roots and in soil. Here, we describe the development and field validation of qPCR markers (i.e. primers with associated hydrolysis probes), targeting taxon-specific motifs in the nuclear large ribosomal subunit RNA genes. Design of such markers is complicated by the multinuclear and multigenomic cellular organization of these fungi and the high DNA sequence diversity within the smallest biologically relevant units (i.e. single-spore isolates). These limitations are further compounded by inefficient biomass production of these fungi, resulting in limited availability of pure genomic DNA (gDNA) of well-defined isolates for cross-specificity testing of the markers. Here we demonstrate, using a number of AMF isolates, the possibility to establish stringent qPCR running conditions allowing quantification of phylogenetically disjunctive AMF taxa. Further, we show that these markers can more generally be used to quantify abundance (i.e. number of target gene copies or amount of gDNA) of what is usually considered the level of AMF species, regardless of the isolate identities. We also illustrate the range of variation within qPCR signal strength across different AMF taxa with respect to the detected number of gene copies per unit amount of gDNA. This information is paramount for interpretation of the qPCR analyses of field samples. Finally, the field validation of these markers confirmed their potential to assess composition of field AMF communities and monitor the changes owing to agricultural practices such as soil tillage.

Diversity in phosphorus (P) acquisition strategies was assessed among three species of arbuscular mycorrhizal fungi (AMF) isolated from a single field in Switzerland. Medicago truncatula was used as a test plant. It was grown in a compartmented system with root and root-free zones separated by a fine mesh. Dual radioisotope labeling ( 32 P and 33 P) was employed in the root-free zone as follows: 33 P labeling determined hyphal P uptake from different distances from roots over the entire growth period, whereas 32 P labeling investigated hyphal P uptake close to the roots over the 48 hours immediately prior to harvest. Glomus intraradices, Glomus claroideum and Gigaspora margarita were able to take up and deliver P to the plants from maximal distances of 10, 6 and 1 cm from the roots, respectively. Glomus intraradices most rapidly colonized the available substrate and transported significant amounts of P towards the roots, but provided the same growth benefit as compared to Glomus claroideum, whose Plant Soil (2011) 339:231-245 mycelium was less efficient in soil exploration and in P uptake and delivery to the roots. These differences are probably related to different carbon requirements by these different Glomus species. Gigaspora margarita provided low P benefits to the plants and formed dense mycelium networks close to the roots where P was probably transiently immobilized. Numerical modeling identified possible mechanisms underlying the observed differences in patterns of mycelium growth. High external hyphal production at the rootfungus interface together with rapid hyphal turnover were pointed out as important factors governing hyphal network development by Gigaspora, whereas nonlinearity in apical branching and hyphal anastomoses were key features for G. intraradices and G. claroideum, respectively.

Many studies have scrutinized the nutritional benefits of arbuscular mycorrhizal associations to their host plants, while the carbon (C) balance of the symbiosis has often been neglected. Here, we present quantification of both the C costs and the phosphorus (P) uptake benefits of mycorrhizal association between barrel medic (Medicago truncatula) and three arbuscular mycorrhizal fungal species, namely Glomus intraradices, Glomus claroideum, and Gigaspora margarita. Plant growth, P uptake and C allocation were assessed 7 weeks after sowing by comparing inoculated plants with their non-mycorrhizal counterparts, supplemented with different amounts of P. Isotope tracing ³³P and ¹³C) was used to quantify both the mycorrhizal benefits and the costs, respectively. G. intraradices supported greatest plant P acquisition and incurred high C costs, which lead to similar plant growth benefits as inoculation with G. claroideum, which was less efficient in supporting plant P acquisition, but also required less C. G. margarita imposed large C requirement on the host plant and provided negligible P uptake benefits. However, it did not significantly reduce plant growth due to sink strength stimulation of plant photosynthesis. A simple experimental system such as the one established here should allow quantification of mycorrhizal costs and benefits routinely on a large number of experimental units. This is necessary for rapid progress in assessment of C fluxes between the plants and different mycorrhizal fungi or fungal communities, and for understanding the dynamics between mutualism and parasitism in mycorrhizal symbioses.