The Morphogenesis and Possible Evolutionary Origins of Fungal Sclerotia

Abstract

Summary 1. Fungal sclerotia are able to survive adverse conditions for long periods and they are formed by many important plant pathogens. An understanding of the factors involved in their initiation and development may lead to a method of repressing their formation in nature, thereby reducing the chances of survival of fungi that depend on them as persistent resting stages in their life‐cycles. Also, data on sclerotial morphogenesis may be applicable to other multihyphal fungal structures. 2. There are three types of sclerotial development. The most primitive and least common is the loose type, which is illustrated by Rhizoctonia solani. The sclerotium forms by irregular branching of the mycelium followed by intercalary septation and hyphal swelling. When mature, it consists of loosely interwoven hyphae that are rich in food reserves and darkly pigmented. The main types of development are terminal and lateral. The former develops from the coalescence of initials that are produced by a well‐defined pattern of branching at the tip of a hypha or tips of closely associated hyphae, e.g. Botrytis cinerea. Lateral sclerotia are formed by the interweaving of side branches of one or several main hyphae. When only one main hypha is involved the sclerotium is of the lateral, simple type, e.g. Sclerotinia gladioli. If several main hyphae give rise to a sclerotium, the term strand type has been used. Sclerotium rolfsii is the classical example. 3. There is a considerable literature on the effects of environmental conditions on the initiation, development and maturation of sclerotia but few attempts have been made to interpret the data. Phenolics and/or polyphenol oxidases have been found to be connected with morphogenesis of the protoperithecium of Neurospora crassa, the perithecium of Podospora anserina and of Hypomyces sp. and the basidiocarp of Schixophyllum commune. A close correlation has been shown between melanin synthesis and microsclerotial development by Verticillium but there appears to be no literature on the role of phenolics and polyphenol oxidases in the morphogenesis of sclerotia. Possibly these substances may inhibit growth of the apices of main hyphae by changing the permeability of the membrane, by inducing a thickening of the cell wall at the tip or by reducing the plasticity of the wall. Such a check in growth could trigger‐off the formation of initials close to the margin of the colony or elsewhere in the culture. Sulphydryl groups and disulphide bonds are of great significance in morphogenesis of organisms and are probably involved in sclerotial initiation. The formation of a large number of hyphal branches is a prerequisite for sclerotial initiation and mycelial branching is possible only if there is plasticity of hyphal walls. The ability of the wall to be moulded is possibly related to changes in the sulphur linkages of the protein of the protein‐carbohydrate complexes of the cell wall and could be influenced by sulphur availability or the activity of specific enzymes. 4. After a sclerotial primordi...

Induction of sclerotia in Sclerotium rolfsii by short low-temperature treatment

“…The chill-induced formation of sclerotia associated with the mycelial bands agrees with the known connection of hyphal strands and localized branching to sclerotial formation in S. rolfsii (Townsend & Willetts, 1954;Willetts, 1972;Henis et al, 1973). The results presented here support the view that restriction of mycelial growth leads to sclerotial formation (Henis et al, 1965;Wheeler & Waller, 1965;Willetts, 1978).…”

“…The absence of a second ring of mature sclerotia on twice-chilled colonies suggests that the same treatment applied twice can both start and stop events that lead to sclerotia in agreement with the arguments of Willetts (1972Willetts ( , 1978. A cold period might start sclerotial formation by slowing hyphal growth and removing apical dominance at the colony edge and, when repeated, interfere with development of those initials as it started the process anew at the colony edge some distance away.…”

“…In Hypomyces solani f. sp. cucurbitae a brief cold treatment stimulated the formation of primordia of perithecia (Wilson, 1968), which developmentally are similar to some sclerotia (Willetts, 1972). If interference with apical dominance of hyphae at the growing edge of a colony triggers sclerotial formation (Willetts, 1978), then a short restriction of growth by low temperature might affect sclerotial formation.…”

Isolation and Partial Characterization of an Extracellular Branched d-Glucan from Monilinia fructigena

“…A possibly identical polysaccharide named sclerotan (see Marshall, 1974) was isolated from sclerotia of Sclerotinia libertiana (= S. sclerotiorum) by Japanese workers. More recently, Willetts (1971Willetts ( , 1972 mentioned the copious production of extracellular mucilage by both S. sclerotiorum and Monilinia ( = Sclerotinia) fructicola but no characterization was attempted. A polysaccharide secreted by the latter species was also the subject of a brief report by Feather & Malek (1972).…”

“…In view of the probable importance of extracellular polysaccharides in the overall biology of the Sclerotiniaceae (Willetts, 1971(Willetts, , 1972 and the possible commercial use of such Organism and growth conditions. A monoconidial isolate of Monilinia fhctigenn ( A T c c~~ I 06), maintained on potato/carrot agar, was used for all studies of polysaccharide secretion.…”