A qPCR assay that specifically quantifies Tricholoma matsutake biomass in natural soil

Tricholoma matsutake is an ectomycorrhizal basidiomycete that produces prized, yet uncultivable, "matsutake" mushrooms along densely developed mycelia, called "shiro," in the rhizosphere of coniferous forests. Pinus densiflora is a major host of this fungus in Japan. Measuring T. matsutake biomass in soil allows us to determine the kinetics of fungal growth before and after fruiting, which is useful for analyzing the conditions of the shiro and its surrounding mycorrhizosphere, predicting fruiting timing, and managing forests to obtain better crop yields. Here, we document a novel method to quantify T. matsutake mycelia in soil by quantifying a single-copy DNA element that is uniquely conserved within T. matsutake but is absent from other fungal species, including close relatives and a wide range of ectomycorrhizal associates of P. densiflora. The targeted DNA region was amplified quantitatively in cultured mycelia that were mixed with other fungal species and soil, as well as in an in vitro co-culture system with P. densiflora seedlings. Using this method, we quantified T. matsutake mycelia not only from shiro in natural environments but also from the surrounding soil in which T. matsutake mycelia could not be observed by visual examination or distinguished by other means. It was demonstrated that the core of the shiro and its underlying area in the B horizon are predominantly composed of fungal mycelia. The fungal mass in the A or A₀ horizon was much lower, although many white mycelia were observed at the A horizon. Additionally, the rhizospheric fungal biomass peaked during the fruiting season.