Effect of copper on tyrosinase activity and polyamine content of some ectomycorrhizal fungi

Abstract

Several species of ectomycorrhizal fungi were grown on solid and liquid media in order to compare their tyrosinases and polyamine contents in the presence of copper (Cu) stress. Fungi with naturally brown pigmented hyphae ( Pisolithus tinctorius and Boletinellus merulioides ) yielded measurable extracellular tyrosinase at both 0 and 60 μg ml −1 Cu. Of fungi which develop brown pigmentation in response to Cu stress ( Suillus pictus and Suillus granulatus ), only S. pictus produced the enzyme and then only at 60 μg ml −1 Cu. Fungi with other than brown hyphae ( Piloderma bicolor and Cenococcum graniforme ) did not produce extracellular tyrosinase. Intracellular tyrosinase of P. tinctorius and S. granulatus increased in activity due to Cu stress but substrate affinity ( K m ) remained similar. This indicates an increase in the amount of enzyme synthesized and suggests that tyrosine may act in detoxification by irreversibly binding Cu ions. Tyrosinase from control cultures of P. tinctorius had a greater substrate affinity than that from cultures of S. granulatus . Addition of the polyamines putrescine and spermidine to the reaction mixture from P. tinctorius activated and repressed tyrosinase, respectively. Polyamine content of P. tinctorius and B. merulioides was not altered by Cu stress, while that of the two Suillus species was decreased by the addition of Cu to the growth medium. The decrease in fungal polyamine content indicates a change in permeability of the cells both to Cu and to the extracellular signals for polyamine biosynthesis. Our results show that there are probably fundamental differences between stress induced and natural hyphal pigmentation.