Most fungal aggregations consist of parallel hyphae. The individual hyphae are recognisable as such and the term used to describe these tissues is “prosenchyma”, meaning loosely organised tissue. Some fungi develop more tightly organised tissue where the tightly packed cells resemble plant parenchyma. These tissues are referred to as “pseudoparenchyma”, meaning false parenchyma. The hyphal origin of the cells is less obvious in pseudoparenchyma.
Two types of structure are formed from pseudoparenchyma: sclerotia and stromata. The differences between the two types of tissue is simply that one bears or contains fruiting bodies, and the other does not.
During development, stromata resemble sclerotia, though in addition, some host plant tissue may be incorporated in stromata. Indeed, sclerotia may eventually form fruiting bodies following recovery from dormancy. The distinction between the two complex tissues is unclear.
The term sclerotium is used in a functional sense. The rounded bodies are able to survive periods of adverse environment. They remain dormant or quiescent during the stressful period and then germinate when conditions are more favourable.
A number of different types of sclerotia are found. The simplest sclerotia consist of a few cells and are microscopic. The most complex are the enormous sclerotia of Polyporus mylittae found in arid and semiarid regions of Australia. These sclerotia can reach 30 cm diameter.
Sclerotia develop from an initial cluster or knot of hyphae. As the knot expands, cells shorten, the hyphae become tightly packed: the structure changes from undifferentiated tissue to a structure where the outer layer becomes rind-like. Walls of the outer hyphae are thickened, and become melanised, and compartments lose their cytoplasm. In addition, deposits between cells of the rind reduce the apoplastic transfer of solutes.
Internal hyphae, the medulla, of the sclerotium are relatively swollen, with extended vacuoles. The hyphae have considerable dichotomous branching, and remain relatively thin walled. The medullary cells are surrounded by an extracellular matix, which becomes chystalline with maturity. Nutrients are stored in the inner tissue. The substratum may be partially enclosed in the sclerotium. The cells of the outer layer of the sclerotium are commonly melanised.
The structures formed by a single fungus may vary in dimensions and anatomy. It appears that the environment plays an important part in the development and survival of sclerotia, just as in other parts of the fungal thallus. Light, pH, reactive oxygen species and UV light appear to be involved in triggering the formation of sclerotia, and then regulating the development.
Germination of sclerotia is usually associated with improved environmental conditions, usually adequate moisture and temperature. The fungus Polyporus mylittae (see image above) appears to germinate following a drop in soil temperature as adequate reserves of water are contained within the structure. Sclerotia survive for many years. The precise trigger for germination may also involve either heat from summer or a fire. Some sclerotia of fungal pathogens such as pathotypes of Sclerotinia and Sclerotium, require specific chemical, probably nutritional, triggers indicating the presence of a potential host.
Germination may be by emergence of hyphae from within the sclerotium. However, a significant number form fruiting bodies which disperse spores.
Sclerotia and stromata are complex rounded hyphal structures which enable survival of the fungus through stressful times. Their formation appears to be under genetic control, and is not necessarily related to stress at the time of formation. The germination of sclerotia is probably triggered by the structure sensing relief from the stress.
Carlisle MJ & Watkinson SC 1994 The Fungi. Academic Press.