Targeting fungal proteinases to prevent sapstain on wood
L D Abraham, D E Bradshaw, A Byrne, P I Morris, C Breuil
Discoloration of wood, caused by a variety of sapstaining fungi, leads to periodic losses in the Canadian lumber export industry. Proteolytic enzymes are thought to be necessary for retrieval of nitrogen during fungal growth on wood. The major extracellular proteinase of Ophiostoma piceae, a representative sapstaining fungus, was purified to homogeneity and its inhibition pattern characterised. Classic serine proteinase inhibitors inhibited the activity of the purified proteinase in a dose-dependent manner. Several chelating agents, detergents, heavy metals and PQ-8, a currently used commercial antisapstain formulation, also caused a reduction in proteolytic activity in vitro. Selected inhibitory compounds were then tested for their effect on growth of Ophiostoma piceae in artificial media and on wood. Heavy metals and several chelators inhibited growth on media containing protein or inorganic nitrogen, suggesting that they were toxic to the fungus rather than specific to the proteinase. However, chymostatin and PQ-8 did appear to be specific proteinase inhibitors. These products caused a decrease in growth on a protein-supplemented medium which induces proteinase production, but had little effect on growth on medium containing inorganic nitrogen. Visual assessment of lodgepole pine sapwood samples inoculated with Ophiostoma piceae also identified PQ-8 as an effective inhibitor of fungal growth. Other pure chelators and serine proteinase inhibitors did not perform well on wood. While some compounds showed promise in these tests, definitive testing and commercial application are constrained by the current lack of stable, cheap, non-toxic, specific serine proteinase inhibitors.