Pectin degradation during colonization of wood by brown-rot fungi

Brown-rot decay results in rapid reduction in degree of polymerization of holocellulose, with concomitant strength loss without removing lignin. Development of new methods of wood protection will require focusing on early events in the sequence of fungal attack during colonization. Pit membranes (sapwood) of wood cell walls represent a readily available source of nonlignified carbohydrate, i.e., pectin and cellulose. Commercial pectinases (Pectinol) and Trichoderma sp. have been shown to degrade pit membranes and increase penetration of preservatives. Plant pathogens have been shown to degrade pectin by the synergistic action of oxalic acid and polygalacturonase (PG). Brown-rot fungi have also been shown to produce oxalic acid and pectinase during the decay process. The oxalic acid solubilizes the pectin by chelating the Ca++ and the PG hydrolyses the β−1,4 linkages. We have demonstrated the ability of Postia placenta, Gloeophyllum trabeum, and Serpula incrassata to use pectin as a sole carbon source and to produce oxalic acid and PG on both liquid media and wood. Aspergillus niger and Trichoderma sp. also produce PG on wood but no oxalic acid or weight loss. One key to pectin hydrolysis by plant pathogens has been shown to be fungal production of oxalic acid, which lowers the pH of the substrate and chelates calcium ions. Production of oxalic acid may serve a similar role during incipient wood decay as calcium oxalate has been found by scanning electron microscopy during both brown-rot and white-rot decay. Therefore, we hypothesized that in situ precipitation of existing calcium ions in wood may prevent the cascade of biochemical events involved in colonization of wood by brown-rot fungi, especially hydrolysis of pit membranes. Preliminary experiments in our laboratory have shown that brown-rot fungi, white-rot fungi, and termites are inhibited from effecting weight loss of wood following pretreatment of wood blocks with the selective water-soluble calcium-precipitating agent N, N-naphthaloylhydroxylamine (NHA). We hypothesize that pectin utilization is an essential step during incipient brown-rot decay that helps to initiate fungal metabolism and promote the spread of fungal hyphae through wood. Our long-term goal is to stop the spread of decay fungi by treating wood with calcium-trapping agents such as NHA, with the ultimate goal of developing targeted strategies for environmentally benign decay prevention.