Search

Sphaerostilbe repens gives rise to aggregated organs composed of aerial coremia and rhizomorphs which grow submerged in the medium. These two types of organ are structurally continuous. The sequence of events leading to organ differentiation under controlled laboratory conditions has been microscopically divided into four stages: 1. The vegetative phase which is characterized by a regular distribution of hyphae in culture media. 2. The aggregating phase in which a local cellular growth leads to the formation of a prosenchymatous mass. 3. The differentiating phase is distinguished by a parallel orientation of the hyphae in the middle of the aggregate. This is followed by an upward and downward growth of numerous hyphae giving rise to primordia of coremia and rhizomorphs respectively. At the periphery of the aggregate, cells become compact, isodiametric and vacuolated to produce the cortex of the rhizomorph, whereas in the middle of the organ, medulla in continuity with the coremium is filled with loosely woven and randomly oriented hyphae. 4. The elongating phase appears as an autonomous growth of organs which elongate by their apices. [ABSTRACT FROM AUTHOR]

The differentiating stages of coremia and rhizomorphs in Sphaerostilbe repens were studied by transmission electron microscopy. Vegetative mycelium is characterized by highly cytoplasmic cells rich in ribosomes and mitochondria and with few vacuoles as well as endoplasmic reticulum. Cell walls are thin attaining a maximum thickness of 0.10 μm. During the aggregating phase a prosenchymatous mass of randomly oriented cells is produced by localized elongation and branching of the filaments. The hyphae in this region have the appearance of actively metabolising cells. In the course of the differentiating phase, numerous hyphae of the median zone of the aggregate grow upward and downward to give rise to coremium and rhizomorph primordia respectively. The individual hyphal tips lay parallel to each other and cells of the growing apices retain their meristematic characteristics. At the periphery of the aggregate and to a lesser extent in the subapical rhizomorphic zone, cells reduce their cytoplasmic density as a consequence of a decrease in the number of ribosomes. These cells also increase in size and become isodiametric and vacuolated. During cellular differentiation walls increase steadily in thickness and at the elongating phase they reach 0.30 μm in the rhizomorphic cortex. Mucilaginous material is progressively deposited around hyphae and in the most differentiated zones, coalesce to fill interhyphal spaces. This extracellular matrix seems to play a role in maintaining cohesiveness of the aggregated organs. The tissue in the process of differentiation is scattered with cells highly enriched in mitochondria and with cells virtually undifferentiated. Accumulation of microfilaments takes place in the differentiating zone localized behind the immersed meristematic apex. These structures might be involved in wall synthesis. Glycogen rosettes accumulate in the vegetative mycelium surrounding the aggregating centers, suggesting the possibility of supplying energy during the differentiating processes. The vacuolar system, represented by autophagic vacuoles which are present until the differentiation phase, presumably may also participate in the biochemical changes that occur during aggregation. Coremial cells are characterized by an increase in wall thickness, a highly sinuous plasma-membrane as well as large amounts of mucilaginous compounds accumulated between hyphae, but in all other respects they resemble the cells of actively growing vegetative hyphae. [ABSTRACT FROM AUTHOR]

Changes in the biochemical composition of isolated cell walls were analysed during the differentiation of coremia and rhizomorphs in Sphaerostilbe repens. Differentiation was accompanied by exclusively quantitative variations of the wall components: the content in carbohydrates, chitin and free amino sugars increased; on the contrary, amino acids, uronic acids, lipids and mineral substances decreased. Carbohydrates were composed of glucose, galactose and mannose; glucosamine was the main component of amino sugars. The predominant amino acid in the walls was cysteine the amount of which increased during hyphal aggregation, while quantities of the sixteen other determined amino acids decreased. Mineral matter was present in large quantities in the walls of the fungus, especially in vegetative mycelium. Iron, phosphorus and calcium were the most abundant elements. Possible relations between the variations in chemical composition of the wall and the capability of hyphae to aggregate are discussed. [ABSTRACT FROM AUTHOR]