Reserve substances inPaxillus involutus sclerotia

Summary The ectomycorrhizal fungus,Paxillus involutus, produces sclerotia in culture. These can be induced to form on agar medium by exposing mycelium grown at 25°C to various temperatures between6°C and 15°C. Sclerotia formed at 10°C and above were large and covered with drops of exudate, while those formed at 6°C or 8°C were very small and did not produce an exudate. Mature sclerotia were bounded by a compact rind and contained abundant storage reserves. Histochemistry of the larger sclerotia showed large quantities of protein stored as protein bodies in the cytoplasm, lipid present as small droplets, glycogen granules stored in the cytoplasm and polyphosphate present as small granules in the cytoplasm and in the protein bodies. Energy dispersive X-ray microanalysis confirmed the presence of phosphate in the granules and was used to map its distribution throughout the sclerotium. The smaller sclerotia induced at 8°C and below on the same medium had the same basic structure and composition, but lacked the complex phenolic cell network found in large sclerotia, and had abundant extracellular polysaccharides. The rind was not well developed and these small sclerotia are interpreted to have been arrested at an early stage of development.