Summarized plant growth data are presented in Fig. 2. Full data are accessible in ESM Table 2. On ultramafic substrate, ectomycorrhizal treatment with P. albus induced enhancement of plant growth when compared with controls (Fig. 2): in A. spirorbis the biomass was significantly enhanced with an increase of shoot dry weight from +8.7% to +44.3% (except for plants inoculated with P. albus isolate MD09-001) and an increase of root dry weight from +10.8% to 62.3%; in E. globulus the plant shoot dry weight was significantly enhanced from +57.1% to +82.4% and the root dry weight from +47.4% to +56.1%, all when compared with controls. Ergosterol was also measured in roots as indicator of the quantity of mycelium at the root level.