Developmental biology of sclerotia

Abstract

Sclerotia are loosely described as morphologically variable, nutrient-rich, multihyphal structures which can remain dormant or quiescent when their environment is adverse and then, when conditions improve, germinate to reproduce the fungus. A narrower concept of sclerotia distinguishes between ‘true sclerotia’ and ‘sclerotioid structures’, with the former tuber-like and detachable from the substratum at maturity. The possibility of ‘true sclerotia’ originating from sporogenous tissue is discussed. There are three main types of sclerotial ontogeny — loose, terminal and lateral. Many endogenous and exogenous factors are involved in sclerotial initiation, and these are generally similar to those involved in initiation of other multihyphal structures. Metabolic changes in the vegetative mycelium may cause a flux in a single, organic, carbon compound, which could then become the major carbon translocate from the nutrient source and provide the trigger for sclerotial initiation. Translocation is probably by turgor-driven mass flow along physiologically specialized conducting hyphae. Exudation of liquid droplets is characteristic of developing sclerotia. It may be involved in the maintenance of internal physiological balance. However, exudation is probably primarily concerned with the expulsion of water and not directly in the removal of organic and inorganic materials from sclerotial hyphae. Soluble compounds not needed for structural growth of sclerotia are converted to insoluble forms and deposited both intra- and extracellularly in sclerotial tissues. These deposits are utilized during periods of rest and germination. The physiological processes of initiation, translocation and exudation are reviewed. Sclerotia usually consist of a peripheral rind which encases a broad medulla of loosely interwoven hyphae. Sometimes a narrow cortex of close-fitting hyphae is present between the rind and medulla. The rind is a continuous layer of tightly packed hyphal tips. Reduction in rind permeability corresponds with wall thickening and pigmentation rather than formation of the rind as a continuous surface layer. The cortex, if present, is a region where reserves accumulate. Some cortical cell walls may be pigmented. The medulla forms the main part of the sclerotium. Intracellular reserves are present in medullary hyphae, and an extracellular matrix, which may be continuous or contain lacunae, is usually a characteristic feature of the medulla. Considerable data have accumulated on the ultrastructure and histochemistry of sclerotia. Until recently most studies have involved the use of chemical fixation techniques, but current freeze-substitution methods are providing new insights into sclerotial ultrastructure and histochemistry. The main cytoplasmic reserves present in sclerotia are glycogen, protein, polyphosphate and lipid. Sclerotial cell walls contain chitin and glucans, and the extracellular matrix contains glucans. The structure, composition and functions of sclerotial reserves are reviewed and discussed. Although sclerotia are alike functionally they have probably developed due to convergent evolution. Studies on sclerotia and conidia of the family Sclerotiniaceae could indicate that there are only two types of true sclerotia produced by this family: the tuberoid sclerotium characteristic of Sclerotinia sclerotiorum ; and the plano-convexoid sclerotium characteristic of Botryotinia (form genus Botrytis ). The derivation of these two types of sclerotia is discussed and compared with that of resting stromatic structures produced by other sclerotiniaceous genera, including Monilinia , and of sclerotia of some genera of the Basidiomycotina and Deuteromycotina.