In vitro evidence of mycoparasitism of the ectomycorrhizal fungus Laccaria laccata against Mucor hiemalis in the rhizosphere of Pinus sylvestris

Interactions between the ectomycorrhizal fungus Laccaria laccata and the soil fungus Mucor hiemalis f. hiemalis in co-culture, and in the rhizosphere of in vitro-grown Pinus sylvestris seedlings were investigated by light- and scanning electron-microscopy. In co-culture, mycelial growth away from the L. laccata colony reduced the number of aerial hyphae at the contact zone and increased the density and compactness of the mycelium-characterized gross morphology of the saprobic fungus. Although the growth of M. hiemalis was suppressed, no penetration of M. hiemalis hyphae after the colony was entered by L. laccata was observed. Instead, dense coiling of L. laccata hyphae around sporangiophores, overpowering them and causing them to disappear, was quite common. On nonmycorrhizal roots, sporangiospores germinated heavily and formed long hyphae for 2 days post inoculation, whereas their germination was totally inhibited on mycorrhizal roots. At 3 days after inoculation, only sporangia were seen with mycelial mats firmly attached to the roots by the mantle hyphae, whereas some remnants of sporangiophores, ruptured sporangial walls and degraded hyphae of M. hiemalis were overgrown by the mantle hyphae. During the next 3 days, the mantle-hyphae-invading sporangia formed short, thin branches that grew directly towards individual spores, tapering off upon contact.