OXALATE PRODUCTION BY FUNGI - ITS ROLE IN PATHOGENICITY AND ECOLOGY IN THE SOIL ENVIRONMENT

Oxalate secretion by fungi provides many advantages for their growth a nd colonization of substrates. The role of oxalic acid in pathogenesis is through acidification of host tissues and sequestration of calcium from host cell walls. The formation of calcium oxalate crystals weake ns the cell walls, thereby allowing polygalacturonase to effect degrad ation more rapidly in a synergistic response. There is good correlatio n between pathogenesis, virulence, and oxalic acid secretion. Solubili ty of soil nutrients is achieved by soil-living species, when cations freed by oxalate diffusing in clay layers increases the effective solu bility of Al and Fe. Oxalate retained in hyphal mats of mycorrhizal sp ecies increases phosphate and sulphate availability. The formation of calcium oxalate crystals provides a reservoir of calcium in the ecosys tem. The ability of oxalate to bind divalent cations permits detoxific ation of copper, particularly evident in wood preserved with copper sa lts. Oxalate plays a unique role in lignocellulose degradation by wood -rotting basidiomycetes, acting as a low molecular mass agent initiati ng decay. In addition, in white-rot fungi oxalate acts as a potential electron donor for lignin-peroxidase catalysed reduction and chelates manganese, allowing the dissolution of Mn3+ from the manganese-enzyme complex and thus stimulating extracellular manganese peroxidase activi ty. The biosynthesis and degradation of oxalate are discussed.