Study of the artificial cultivation of the ectomycorrhizal mushroom, Rhizopogon roseolus

Abstract

Rhizopogon roseolus (Corda) Th. M. Fr., known as “shouro” in Japanese, is an edible ectomycorrhizal mushroom. To successfully cultivate ectomycorrhizal mushrooms, inoculation and propagat on methods must be developed and isolates with superior traits must be selected. To advance successful cultivation, a cross-breeding system is desired to improve agronomic traits. The establishment of a cross-breeding system requires basic information on the life cycle of the mushroom. Pinus thunbergii seedlings were inoculated with several strains of R. roseolus using the Petri dish technique and their mycorrhizal abilities were assessed. Superior strains that rapidly colonized the roots of P. thunbergii and produced a large number of ectomycorrhizas were selected. An inoculation method using mycelial homogenates was found to be effective at producing ectomycorrhizas on seedlings. When the mycelial homogenate was inoculated on pine seedlings in a greenhouse, fruiting bodies were produced 3 － 4 months after the inoculation. Microscopic observation after HCl-Giemsa staining revealed distinct patterns of meiosis, post-meiotic mitosis, and nuclear migration. Electron microscopic studies showed that the spore-discharge mechanism was statismospory, and showed that the major ultrastructural change in basidiospore concerned in the spore wall. Variability in the rate of spore germination was observed between individual fruiting bodies. The peak germination rate was recorded for spores from fruiting bodies showing beige color. A higher spore germination rate was observed on a five-fold dilution of modified Melin-Norkrans medium. When the primary mycelial strain derived from the basdiospore was inoculated on pine seedlings, ectomycorrhizas were formed. A detection medium was developed for the rapid observation of clamp connection formation as an indicator of secondary mycelia. When tissue culture and basidiospore isolates were cultured on this detection medium, clamp connection formation was consistently observed in the tissue culture isolates, but never in the basidiospore isolates. The basic information regarding the life cycle of R. roseolus obtained from this study will contribute to the development of a cross-breeding system for ectomycorrhizal mushrooms.