Publisher Summary The physiology of fungal lipids can be defined as the relation of lipids to the process of life in fungi. Besides the essential presence of lipids as membrane components of the endoplasmic reticulum, plasmalemma and organelles, a variety of lipid-containing inclusions occur in plant cells, usually becoming most prominent in reproductive organs and spores. Lipids are also found as cell-wall components and on the surfaces of spores. Lipid inclusions are often abundant within vegetative mycelium and their ultrastructural distribution within vegetative hyphae has received less attention than that in reproductive organs. Reproductive structures show some characteristic patterns of lipid distribution in different taxonomic groups of fungi. Aspects of morphological organization and life cycle used in fungal taxonomy correspond very satisfactorily with wall chemistry. The relative ease of identification of lipid structures and the convenient manipulation of physical and nutrient growth conditions permitted by their hyphal organization, make fungi an ideal system for the productive correlation of structural and biochemical aspects of lipid physiology in growth and differentiation. The chapter discusses lipids and fungal membranes. The primary functions of membranes are to provide a permeability barrier and sites for active transport of ions and facilitated diffusion of hydrophilic molecules. Membranes consist mostly of lipid, protein, and some water. The lipid is a mixture in which phosphoglycerides usually predominate.

Physiology of fungal lipids: selected topics.

A correlative approach, involving light and electron microscopic, cytochemical, and biochemical techniques, was used to study the structure and function of microbodies in zoospores ofEntophlyctis sp. The same population of microbodies already existing in the zoosporangium appeared to be segregated into zoospore initials during cytoplasmic cleavage. Microbodies laid at the anterior end of zoospores and were part of an organized assemblage of organelles, the microbody-lipid globule complex. In the microbody-lipid globule complex, endoplasmic reticulum occurred on the surface of the lipid globules toward the zoospore's exterior, and the microbody, subtended by mitochondria, was appressed to the opposite surface of the lipid globule. The organization of the microbody-lipid globule complex changed as the zoospore swam and encysted. As lipid globules coalesced, the microbody-lipid globule complex became disorganized. After lipid globule coalescence was completed, the microbody-lipid globule complex regained its order, and several microbodies were clustered adjacent to a single lipid globule. The microbodies persisted even in the encysted zoospore, but they were found on all sides of the lipid globule. Microbodies isolated from zoospores contained catalase as well as malate synthase and isocitrate lyase, two enzymes of the glyoxylate cycle. When zoospores encysted greater activities of these glyoxylate cycle enzymes could be detected. The presence of glyoxylate cycle enzymes and the close association between the microbody and lipid globule suggest that microbodies function as glyoxysomes in zoospores and encysted zoospores. The functional significance of the morphological organization of the microbody-lipid complex is discussed in terms of energy production and the conversion of storage lipid into structural components of the cell.

Ultrastructure and isolation of glyoxysomes (microbodies) in zoospores of the fungusentophlyctis sp.

Peroxisomes are versatile and dynamic organelles that are essential for the development of most eukaryotic organisms. In fungi, many developmental processes, such as sexual development, require the activity of peroxisomes. Sexual reproduction in fungi involves the formation of meiotic-derived sexual spores, often takes place inside multicellular fruiting bodies and requires precise coordination between the differentiation of multiple cell types and the progression of karyogamy and meiosis. Different peroxisomal functions contribute to the orchestration of this complex developmental process. Peroxisomes are required to sustain the formation of fruiting bodies and the maturation and germination of sexual spores. They facilitate the mobilization of reserve compounds via fatty acid β-oxidation and the glyoxylate cycle, allowing the generation of energy and biosynthetic precursors. Additionally, peroxisomes are implicated in the progression of meiotic development. During meiotic development in Podospora anserina, there is a precise modulation of peroxisome assembly and dynamics. This modulation includes changes in peroxisome size, number and localization, and involves a differential activity of the protein-machinery that drives the import of proteins into peroxisomes. Furthermore, karyogamy, entry into meiosis and sorting of meiotic-derived nuclei into sexual spores all require the activity of peroxisomes. These processes rely on different peroxisomal functions and likely depend on different pathways for peroxisome assembly. Indeed, emerging studies support the existence of distinct import channels for peroxisomal proteins that contribute to different developmental stages.

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Peroxisomes and sexual development in fungi

Control of basidiocarp initiation in the cultivated mushroom, Agaricus bisporus (Lange) Sing., by carbon dioxide and micro-organisms in casing material was studied under defined conditions using growth tubes. Fruit bodies were not produced in sterilized materials except when activated charcoal was included in the casing layer. Here responses to carbon dioxide were similar to those obtained in unsterilized casing where carpogenesis was associated with a check of hyphal growth over a restricted carbon dioxide concentration range (100–1000 ppm).

Aseptic fruiting of the cultivated mushroom, Agaricus bisporus

Fungi are primarily soil organisms and as agents of decay produce extracellular enzymes degrading many natural polymers, especially polysaccharides (Section 2). The majority of fungi also utilize a wide range of growth substrates including poly-and oligosaccharides, hexoses, pentoses, many organic acids and alcohols, but cannot metabolize C1 compounds such as methanol (Cooney and Levine, 1972). Although most filamentous fungi are obligate aerobes, they exhibit high affinities for oxygen and thus grow at very low oxygen concentrations (Bull and Bushell, 1976). It is therefore difficult to demonstrate true anaerobic growth, which is found only in a few fungi such as Blastocladiella ramosa (Held et al, 1969), Mucor species (Bartnicki-Garcia and Nickerson, 1962), and Fusarium oxysporum (Gunner and Alexander, 1964). The capacity for fermentation in a limiting concentration of oxygen is, however, more common. For example, a lactic acid fermentation is found in aquatic lower fungi, an alcoholic fermentation in F. lini, and fermentations yielding lactic acid, ethanol, and CO2 in Rhizopus species (see Cochrane, 1976). Anaerobic growth or fermentation may demand specific nutritional requirements such as thiamine and nicotinic acid in Figure 1. Relationships between primary and secondary metabolism in filamentous fungi. The origins of major classes of secondary metabolites produced by filamentous fungi are shown in relation to the central pathways of intermediary metabolism. Apart from the 3-lactam antibiotics, no attempt is made to show the many fungal products that arise by further reactions between secondary metabolites of diverse origins. The biosynthesis of certain organic acids is discussed in some detail (Section 3.4) and illustrated in Figs. 5–7.

Regulation of Carbon Metabolism in Filamentous Fungi

The activity of the enzyme, isocitrate lyase, was repressed in glucose cultures of Schizophyllum commune and derepressed in acetate-grown mycelium of this basidiomycetous fungus. The maximum acetat...

W. S. Wegener

Papers

CO2 control of fruiting in Schizophyllum commune: noninvolvement of sustained isocitrate lyase derepression