Decay mechanisms of brown-rot fungi

Brown-rot fungi, e.g. the dryrot fungus (Serpula lacrymans), are the most harmful microorganisms in wood in service in Finland and in temperate regions. Brownrot fungi cause wood decay primarly by attacking the carbohydrates of the cell walls, leaving lignin essentially undigested. At the initial stage of the decay, the brown-rot fungi seem to operate by a mechanism which cause extensive changes in the wood cell wall structure, leading to a rapid decline in the strength properties. It has been suggested that brown-rot produce a low molecular degradation agent which is capable of penetrating into the cell wall structures. Research on the brown-rot decay mechanism has focused on identifying the low molecular weight compounds enchancing cellulose depolymerization in the initial stages of brown-rot decay. The production of extracellular hydrogen peroxide by brown-rot fungi was qualitatively and quantitatively detected by using chromogen ABTS (2,2-azinobis(3ethylbenzthiazoline-6-sulphonic acid)) and horseradish peroxidase. Two brownrot fungi, Poria placenta and Serpula lacrymans were found to produce hydrogen peroxide on solid spruce sawdust medium. The production of hydrogen peroxide by P. placenta was observed in liquid culture media containing either amorphous or crystalline cellulose as a carbon source. The production of hydrogen peroxide and oxalic acid occured to be simultaneous on crystalline and amorphous cellulose, and the highest amount of hydrogen peroxide was detected on amorphous cellulose. The production of hydrogen peroxide by P. placenta dependent on the formation of acid pH of the culture medium. The accumulation of hydrogen peroxide was preceeded by a drop of pH of the culture medium, which was due to the production of oxalic acid. As a small diffusible molecule, hydrogen peroxide can act as degradation agent providing reactive hydroxyl or other oxygen radicals through the Fenton type of reaction which leads to the degradation of wood cellulose. The enzymatic hydrolysis of wood polysaccharides by Gloeophyllum trabeum was detected by following the production of cellulases, hemicellulases and extracellular protein on spruce sawdust or microcrystalline cellulose media. The production of endo-β-1,4-glucanase and endo-β-1,4-xylanase was most pronounced on both media. Brown-rot fungi differ from other cellulolytic fungi by lacking enzyme activites needed for the enzymatic degradation of crystalline cellulose. The endoglucanase activities produced by P. placenta were most