Biochemical Responses of Ten Ectomycorrhizal Fungal Isolates to Manganese

Manganese (Mn) is one of the heavy metals that pollute the environment. However, there are still few studies concerning Mn pollution. Our study focused on the ectomycorrhizal fungi (EMF), an important organism in the forest. We selected ten isolates of EMF, including Pt.2 (Pisolithus tinctorius 2), Cg.1 (Cenococcum geophilum 1), Br (Boletus reticulatus), Sb (Suillus bovinus), Pt.chdx (P. tinctorius chdx), Pt.104 (P. tinctorius 104), Sg (Suillus granulatus), Av (Amanita virgineoides), Sl.1 (Suillus luteus 1), and Sl.chdx (S. luteus chdx), to investigate their responses to Mn stress (0 to 3000 mg L−1) on liquid MMN medium. EMF isolates showed different responses to Mn stress after 60 days growth. According to their growth, they were classified into three groups: sensitive, moderately sensitive, and tolerant. Three isolates showed tolerance to Mn stress. The mycelia of the four sensitive EMF isolates grew slowly along with the increasing Mn level, and the growth was significantly inhibited in 3000 mg L−1 Mn. The accumulation of Mn in EMF mycelium was increased and the secretion of oxalic acid was stimulated by the increasing Mn concentration for all isolates. However, the sensitive isolates secreted significantly more oxalic acid than the tolerant isolates. A scanning electron microscope showed that hyphae were distorted and severely swollen and a transmission electron microscope revealed that Mn enriched in the cytoplasm and organelles of hyphae cells when EMF grew under 3000 mg L−1 Mn stress. In addition, we found that antioxidative enzyme activities, glutathione S-transferase (GST), and superoxide dismutase (SOD) were higher under the stress of Mn. In conclusion, we identified three isolates (Pt.2, Pt.104, and Br) that showed high Mn tolerance. This might demonstrate the great potential of those EMF in the reforestation of Mn mine lands.