Effects of exogenous glucose on mycorrhizal colonization in vitro by early-stage and late-stage ectomycorrhizal fungi

Abstract

Under aseptic conditions, seedlings of 12 common tree species found in eastern Canada (Alnus rugosa, Betula papyrifera, Betula alleghaniensis, Abies balsamea, Tsuga Canadensis, Pinus strobes, Pinus resinosa, Pinus banksiana, Larix laricina, Picea glauca, Picea mariana, and Picea rubens) were inoculated with 10 species of ectomycorrhizal fungi (Piloderma bicolour, Lactarius thyinos, Lactarius subpurpureus, Lactarius torminosus, Hebeloma longicaudum, Cenococcum geophilum, Suillus sinuspaulianus, Suillus tomentosus, Leccinum holopus, and Boletinus paluster) in the absence or presence of exogenous glucose (2 g/L). Early-stage ectomycorrhizal colonizers with a broad host range (e.g., H. longicaudum) appeared to be less dependent upon the exogenous carbohydrate supply for successful formation of ectomycorrhizae than were host-specific late-stage ectomycorrhizal colonizers (e.g., Lactarius subpurpureus). Further investigations revealed, however, that while levels of exogenous glucose (1.0 and 10.0 g/L) increased mycelial growth of late-stage ectomycorrhizal colonizers, a detrimental effect on the growth of the seedlings took place in the presence of these fungi, rather than a concurrent increase in colonization and infection of the host roots. It is suggested that secondary fungal metabolites toxic to the plants are released as a consequence of increased mycelial growth in response to an increase in glucose concentrations. Thus, when dealing with late-stage ectomycorrhizal colonizers and host plants in mycorrhizal synthesis experiments, the exogenous glucose concentration is critical. Key words: early-stage fungi, late-stage fungi, ectomycorrhizae, glucose, root colonization, fungal metabolites.