Fungal selectivity of two mycorrhiza helper bacteria on five mycorrhizal fungi associated with Pinus thunbergii

The fungal selectivity of helper effect was revealed using four ectomycorrhizal fungi (Rhizopogon sp., Pisolithus tinctorius, Cenococcum geophilum and Suillus granulatus), and one ectoendomycorrhizal fungus (Wilcoxina mikolae). Previously, we isolated a rhizobacteria, Ralstonia basilensis and Bacillus subtilis, which have the ability to enhance the mycorrhizal symbiosis between S. granulatus and Pinus thunbergii. However, the characteristics of each bacterium on mycorrhizal fungi are still unclear. Therefore, we tried to examine the fungal selectivity of helper effect. A confrontation assay revealed that R. basilensis significantly promoted the in vitro hyphal growth of W. mikolae and S. granulatus, but it had no effect on Rhizopogon sp., P. tinctorius and C. geophilum. These results were consistent with the effects shown by R. basilensis on the mycorrhizal formation of these fungi. On the other hand, B. subtilis promoted the hyphal growth of W. mikolae and C. geophilum but suppressed growth of Rhizopogon sp. B. subtilis significantly stimulated the mycorrhizal formation of S. granulatus. Thus the effects of B. subtilis on hyphal growth and on mycorrhizal formation were inconsistent. These results suggest that R. basilensis and B. subtilis have fungal selective and different mechanisms in their roles as mycorrhiza helper bacteria.