ABSTRACT
Vast quantities of litter comprised mostly of dead plant cell walls are continuously decomposed. The functioning of the ecosystem depends on this crucial process in which communities of microorganisms, with fungi being the principal player, break litter down into smaller molecules. During this process the mineral ions required by living organisms are dissociated from the organic substances with which they were complexed and released into the soil. With their hyphal tips shaped as spears, the fungi penetrate through the pit apertures into the cell lumen, secreting enzymes that break pectin in the middle lamella and effecting separation of cells in dead plant material. Eventually, the cell wall is broken down into myriad small molecular substances for further decomposition into carbon dioxide and water. The fungi eke out a living by absorbing the released nutrients as a source of carbon and energy. Cellulose and hemicelluloses are the chief polysaccharide constituents of litter and wood. However, these polymeric substances are encrusted with lignin, a highly refractory insoluble compound resistant to microbial attack. Hence the key process in the recycling of carbon in nature is lignin biodegradation. The most striking lignindegrading fungi are those that form fruiting bodies that project out from the woody trunk of trees (Figure 4.1). The isolation and identiœcation of fungi from decomposing litter, their growth in pure cultures, and the characterization of the enzymes and of compounds have provided some insights into this process. The scavenging activity occurs through a number of extracellularly produced glycoside hydrolases, extracellular hydrogen-peroxide-producing enzymes, and low-molecularweight oxidants, particularly hydroxyl radicals.
