Activities of phenol oxidizing enzymes of ectomycorrhizal fungi in axenic culture and in symbiosis with Scots pine (Pinus sylvestris L.)

Abstract

The ectomycorrhizal fungi Suillus granulatus and Paxillus involutus were grown in both liquid culture (LC) and in symbiosis with Pinus sylvestris seedlings under sterile conditions. In LC, the activities of tyrosinase, laccase and peroxidase, and also the oxidation rates of litter- and lignin-derived phenolics were determined. LC mycelia of S. granulatus showed intracellular tyrosinase, laccase, and peroxidase activities. In the culture fluid, extracellular laccase and tyrosinase activities were found. P. involutus predominantly produced intracellular laccase. LC mycelia of S. granulatus suspended in buffer oxidized a range of phenolic acids and mono- and diphenolic compounds by intracellular mechanisms more effectively than did mycelia of P. involutus. In addition, LC mycelia of S. granulatus oxidized p-cresol also by extracellular mechanisms. In symbiosis with Scots pine, S. granulatus and P. involutus increased the level of peroxidase in the fungus/root homogenate and in the nutrient solution of the mycorrhizal plants. Polyphenol oxidase activities were only found with S. granulatus as the mycorrhizal symbiont, and were possibly responsible for the high rate of p-cresol oxidation by this mycorrhizal association. The results suggest that fungal phenol oxidizing enzymes of ectomycorrhizas can substantially contribute to humification and detoxification processes in soil.