Terry L. Highley

Mechanism of brown-rot decay: Paradigm or paradox☆

Interest in understanding how brown-rot fungi degrade wood has received increasing attention in recent years because of a need to identify novel targets that can be inhibited for the next generation of antifungal wood preservatives. Brown-rot fungi are unique in that they can degrade holocellulose (cellulose and hemicellulose) in wood without first removing the lignin. Furthermore, they degrade holocellulose in an unusual manner, causing a rapid decrease in degree of polymerization at low weight loss. Despite increased research effort, the mechanism of brown-rot decay remains unclear. Furthermore, this research has not pointed to biochemical targets for inhibition and development of new wood preservatives. In reviewing the brown-rot literature, it became apparent that many beliefs about brown-rot decomposition of wood are based more on tradition or conjecture than on facts. In some cases, these misconceptions have become near dogma. They cloud our understanding of brown-rot decay and as a result may contribute to a misdirection of research efforts. The purpose of this paper is to attempt to identify and clarify some of these misconceptions.

Chitinase and laminarinase production in liquid culture by Trichoderma spp. and their role in biocontrol of wood decay fungi

Abstract This paper describes macro- and microassay methods to assess the production of the lytic enzymes laminarinase and chitinase, by a range of Trichoderma isolates, and investigates the effect of nutrient composition, glucose amendment and the addition of basidiomycete cell wall material on the production of these lytic enzymes. Large interstrain and interspecies differences exist in the levels of production of both enzymes. Total activities of the enzymes are greater when isolates are cultured in malt medium, but specific chitinase and laminarinase activities are higher in low nutrient conditions. Glucose repressed the production of both laminarinase and chitinase, although this effect was not common to all Trichoderma isolates for the latter enzyme. Cell walls of both N. lepideus and T. versicolor induced increased chitinase activity but this effect was Trichoderma species specific. Mycoparasitism involving lytic enzymes has been described as the mechanism of action of Trichoderma species in the biological control of commercially important plant pathogens. However, little information is available on the significance of this mechanism for the biological control of wood decay fungi. The importance of mycoparasitism as a mechanism in the biological control of wood decay fungi is briefly discussed.

Micromorphology of degradation in western hemlock and sweetgum by the brown-rot fungus Poria placenta

The micromorphologtcal changes in sweetgum and western hemlock caused by the brown-rot fungus Poria placenta are studied by transmission electron microscopy (TEM). Attack is predominantly initiated by hyphae growing in the cell lumen rather than by penetration. Hyphae are often attached to the wood cell wall by hyphal sheaths. The 82 layer is intensely degraded while the S3 layer remains relatively unattacked when attack is initiated from hyphae in the lumen. Occasionally intense degradation in localized areas of the middle lamella and cell corners occurs without noticeable degradation of the surrounding secondary wall. Degradation by penetrating hyphae differs from that of nonpenetrating hyphae in that degradation is localized to the immediale vicinity of the hyphae. Evidence that P. placenta produces lignin-degrading agents is demonstrated by the destniction of the lignin lamellar structure visualized by KMnO4 fixation.

Induction of polygalacturonase and the formation of oxalic acid by pectin in brown-rot fungi

Extracellular polygalacturonase (PG) production was estimated in vitro, using liquid cultures of three species of brown-rot decay fungi (Postia placenta, Gloeophyllum trabeum and Serpula incrassata), by cup-plate assay, assay of reducing sugars, and decrease in viscosity. Although all three experimental assays demonstrated that PG was induced by pectin in all three fungi, decrease in viscosity gave the best correlation with decay capacity in soil block tests. PG activity, determined as an increase in reducing sugar activity, was greatest in G. trabeum and weakest in S. incrassata. The optimum pH for PG activity was between pH 2.5 and 4.5. Oxalic acid production was also enhanced by pectin and functioned synergistically with PG activity. We conclude that these fungi produce PG that is best induced by pectin and that PG activity exceeds production of xylanase and endoglucanase activity in vitro. Polygalacturonase is likely to act synergistically with oxalic acid to solubilize and hydrolyse the pectin in pit membranes and middle lamellae. Thus, production of PG and oxalic acid should facilitate early spread of hyphae and enhance the lateral flow of wood-decay enzymes and agents into adjacent tracheids and the wood cell wall, thus initiating the diffuse decay caused by brown-rot fungi.

Comparative durability of untreated wood in use above ground

Abstract Cross-brace units constructed of 10 different softwoods and nine different hardwoods were exposed on a test fence in Wisconsin for up to 22 years. Sapwood was included for all species and heartwood for some. The objective of this study was to determine the above-ground longevity of these woods against decay. The wood was classified into above-ground decay resistance groups. The longevity spans apply only to structural components similar in cross-sectional size to the test units used in this study. Millwork and fencing components may fit into this category. Woods estimated to last more than 20 years above ground, and thus classified as most resistant, included the heartwood of Douglas-fir, western white pine, redwood, Eucalyptus sp., red and white oak, lodgepole pine, ponderosa pine, western red cedar, and the sapwood of redwood, white oak, and red oak. No woods fell into the nonresistant class (⩽ 7 years expected average life). The remaining woods were classified moderately resistant or resistant based upon service lives of 8 to 13 and 14 to 19 years, respectively.

Electron microscopy of cellulose decomposition by brown-rot fungi

The degradation of isolated cellulose by 11 brown-rot fungi was observed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). SEM showed random growth of all the fungi over the fiber surface and that none of the fungi penetrated into the fibers by means of bore holes. Erosion troughs or depressions in the fiber surface at points of hyphal contact were not visible. When viewed by TEM, hyphae of some fungi were present in fiber walls and lumen. However, cellulose within fibers was extensively degraded whether or not they had been invaded by hyphae, indicating that the initial dissolution of cellulose is accomplished by a small diffusable depolymerizing agent. An extracellular matrix or sheath encased hyphae of all the fungi. Sheaths often spread away from hyphae and sometimes encircled the fibers. Cytoplasmic material from autolyzed cells was commonly observed in sheaths, and often the material totally permeated the fiber. This suggests that some cytoplasmic organelles, such as microbodies, may be involved in the production of the agent employed by brown-rot fungi to degrade cellulose.

Cellulolytic Activity of Brown-Rot and White-Rot Fungi on Solid Media

Solubilization of dyed crystalline and noncrystalline (acid-swollen) cellulose and of isolated cotton cellulose by brownand white-rot fungi was studied. Cellulose azure (dyed acid-swollen cellulose) was solubilized by all fungi. Dyed microcrystalline cellulose was solubilized by all of the white-rot fungi and only the Coniophoroid brown-rot fungi. There was no relationship between decay capacity and the production of cellulase äs measured by the degradition of cellulose azure. Isolated cotton cellulose exposed to brown-rot fungi over soil or agar was extensively depolymerized, and solubility in alkali increased before any weight loss occurred. The degree of polymerization (DP) of isolated cellulose exposed to white rot remained relatively high even at high weight losses, and alkali solubility increased only slightly with degradation.

Protection of southern pine from fungal decay and termite damage with N,N-naphthaloylhydroxylamine☆

The design of environmentally benign methods for preserving wood in service requires an understanding of the precise sequence of the biochemical events that occur as wood is colonized. We hypothesize that in-situ precipitation of existing calcium ions in association with pectin in wood may prevent the cascade of biochemical events involved in fungal colonization. Preliminary experiments showed that pretreatment of wood blocks with the selective water-soluble calcium-precipitating agent N,N-naphthaloylhydroxylamine (NHA) inhibited decay caused by brown-rot and white-rot fungi as well as damage caused by eastern subterranean termites.

Decay Resistance of Wood Removed from Poles Biologically Treated with Trichoderma

Soil block ter mapping of microbial inhabitants. The results indicate that material from pole interiors colonized by Trichoderma is able to resist decay by Lentinus lepideus and Antrodia carbonica. Any decay prevention Southern yellow pine Trichoderma was lost however when the wood was steam sterilized prior to exposure to the basidiomycetes. The implications of the results for the usc of biological control of internal decay in creosoted poles are discussed.

Micromorphology of Degradation in Western Hemlock and Sweetgum by the White-Rot Fungus Coriolus versicolor

The micromorphological changes in sweetgum and western hemlock sawdust caused by the white-rot fungus Coriolus versicolor were studied by transmission electron microscopy (TEM). Degradation by C. versicolor differed in sweetgum and hemlock. Sweetgum was attacked by hyphae from both the lumen and the cell corners. Hemlock was attacked only from the lumen. The compound middle lamella of hemlock was not attacked until removal of the secondary wall was completed; the cell corners were particularly resistant to degradation. In sweetgum, the middle lamella and cell corners were severely degraded without appreciable degradation to adjacent cell wall material; delignification of the cell wall occurred without substantial structural alteration of the remaining cell wall. In contrast, in hemlock, lignin was degraded with the simultaneous destruction of the cell wall. These results support earlier findings that, because of the slower degradation of softwood lignin, C. versicolor degrades softwoods slower than it degrades hardwoods.