Production of polygalacturonase and increase of longitudinal gas permeability in southern pine by brown-rot and white-rot fungi

Hydrolysis of bordered and pinoid pits may be a key event during colonization of wood by decay fungi. Although pits are numerous, studies of pectin-hydrolyzing enzymes in wood decay fungi are scarce, probably because of the relatively low content (less than 4%) of pectin in wood and because of the primary focus on understanding the degradation of lignified components. Endopolygalacturonase (endo-PG) activity was estimated by cup-plate assay and viscosity reduction of pectin from liquid cultures of fifteen brown-rot and eight white-rot basidiomycetous fungi using sodium polypectate as the carbon source. Oxalic acid was estimated in liquid culture and related to mycelial weight of each fungus. Changes in longitudinal gas permeability of southern pine cores exposed to selected decay fungi in liquid culture were measured to determine the extent of hydrolysis of bordered pits. Twelve of fifteen brown-rot and six of eight white-rot fungi tested were positive for at least one of the polygalacturonase test methods. Accumulation of oxalic acid was detected in thirteen of fifteen brown-rot isolates and none of the white-rot fungi tested. Gas permeability of pine cores increased approximately fourfold among brown-rot fungi tested and eighteenfold among white-rot fungi tested. Scanning electron microscopy revealed bordered pit membrane hydrolysis in cores colonized by white-rot fungi, but only torus damage, weakening and tearing of the pit membranes, was observed in cores exposed to brown-rot fungi. We conclude that both brown- and white-rot decay fungi have the enzymatic capacity to hydrolyze pectin, damage bordered pit membranes, and increase wood permeability during colonization and incipient decay.