Variability in nucleus number in basidiospore isolates of Rhizopogon roseolus and their ability to form ectomycorrhizas with host pine roots

Abstract

Rhizopogon roseolus (Corda) Th. M. Fr., known as “shoro” in Japanese, is a hypogeous basidiomycete that is an ectomycorrhizal symbiont of Pinaceae. In this study, we examined the mycelial organization and karyological phase of the primary and secondary mycelia of this mushroom, and evaluated their ability to form ectomycorrhizal association with host pine roots. Variability in nuclear number was recognized in not only basidiospore isolates but also in hybrid strains obtained from crosses between the isolates. Binucleate cells were the most frequently found in both the basidiospore isolates and hybrid strains. PCR-restriction fragment length polymorphism (RFLP) analysis was performed on two basidiospore isolates, a hybrid strain, and a tissue culture isolate from the fruiting body. The RFLP patterns of the two basidiospore isolates differed, although the total calculated size of the fragments for each isolate was close to the expected size. The compatibility RFLP patterns in the hybrid strain and tissue culture isolate were the same and the total calculated size was up to twice that expected. When unisporic isolates were aseptically inoculated onto the roots of P. thunbergii, typical ectomycorrhizas were formed on the root systems. A semi-thin section from the ectomycorrhiza formed by a unisporic isolate showed the presence of mantle layers surrounding the root and Hartig net hyphae, which developed between the epidermal and cortical cells. This is the first study to show that ectomycorrhizas can be formed by basidiospore isolates, in which the mycelial organization was revealed, and which were karyologically characterized as homokaryon by RFLP.