Biological Sciences
Fungi Cell Wall
The fungal cell wall is a rigid structure that surrounds the cell membrane of fungi. It provides support and protection for the fungal cell and is primarily composed of chitin, glucans, and other complex carbohydrates. The cell wall plays a crucial role in maintaining the shape and integrity of the fungal cell, as well as in interactions with the environment.
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7 Key excerpts on "Fungi Cell Wall"
eBook - ePub- Joseph Heitman, Barbara J. Howlett, Pedro W. Crous, Eva H. Stukenbrock, Timothy Yong James, Neil A. R. Gow(Authors)
- 2017(Publication Date)
- ASM Press(Publisher)
8 ). The cell wall provides a valuable source of most diagnostic antigens that are used to detect human fungal infections, and it represents a rich source of unique targets for chemotherapeutic treatment of pathogens. Therefore, fungi are in no small measure defined by, and live through, the interface of their cell walls. Recent progress has been considerable, yet some of the most important and elusive questions in fungal cell biology relate to basic aspects of fungal cell wall biosynthesis and function. This review focuses on the biosynthesis and functions of fungal cell walls with an emphasis on model pathogenic species where the most detailed information is often available.COMPOSITION AND STRUCTURE
Structural Organization and Cell Wall Layers
Fundamentally, fungal walls are all engineered in a similar way. The wall structure directly affects wall function and interactions with the environment including immune recognition by plants and animals. Fibrous and gel-like carbohydrate polymers form a tensile and robust core scaffold to which a variety of proteins and other superficial components are added that together make strong, but flexible, and chemically diverse cell walls. Most cell walls are layered, with the innermost layer comprising a relatively conserved structural skeletal layer and the outer layers more heterogeneous and tailored to the physiology of particular fungi. In most fungal species the inner cell wall consists of a core of covalently attached branched β-(1,3) glucan with 3 to 4% interchain and chitin (9 , 10 ). β-(1,3) Glucan and chitin form intrachain hydrogen bonds and can assemble into fibrous microfibrils that form a basket-like scaffold around the cell. This exoskeleton represents the load-bearing, structural component of the wall that resists the substantial internal hydrostatic pressure exerted on the wall by the cytoplasm and membrane. This branched β-(1,3):β-(1,6) glucan is bound to proteins and/or other polysaccharides, whose composition may vary with the fungal species (Fig. 1 ). However, yeast cells have bud scars that tend to have fewer outer cell wall layers covering them and therefore have exposed inner wall chitin and β-(1,3) glucan (11
- Vijay Kumar Thakur, Manju Kumari Thakur(Authors)
- 2015(Publication Date)
- Wiley-Scrivener(Publisher)
Although the cell wall composition of each species can vary greatly, the basic common carbohydrate constituents are mannans, glucans and chitinous polymers (chitin, chitosan and their complexes) [3,6] (Table 3.1). Glucans and mannans are usually the main constituents, being present in all fungi. Chitin is found in most fungal species, except Schizosaccharomyces pombe [3], but its content can vary significantly depending on the species and on the cultivation conditions [6] (Table 3.1). It is mostly found in a ring in the neck between a mother cell and its emerging bud, in the primary division septum, and also in the lateral walls of newly separated daughter cells [13]. Chitosan, the deacetylated form of chitin, is present as a cell wall component in many fungi, especially Zygomycetes [14] (Table 3.2), covering chitin microfibrils and protecting them against chitinase attack [44]. Chitin and chitosan can be present in the cell wall as free macromolecules or complexed to β-glucans, forming chitin-glucan complexes (CGC) or chitosan-glucan complexes (ChGC), respectively. Other carbohydrates present in the cell wall appear to be specific of fungal groups or species: polyuronides, galactans, etc. [3,45] (Table 3.2). Table 3.1 Main carbohydrate constituents of the cell wall of different fungal species (% CDW). Table 3.2 Carbohydrate cell wall constituents found in some fungal groups and species. The cell wall is a two-layered structure with an inner layer that surrounds the plasma membrane and an electron-dense outer layer that is in contact with the medium [1,6]
eBook - ePub- Sarah C. Watkinson, Lynne Boddy, Nicholas Money(Authors)
- 2015(Publication Date)
- Academic Press(Publisher)
turgor . The increase in internal pressure allows the cell to approach a condition of homeostasis in which water influx matches the increase in cell volume that occurs during growth.The wall is a highly dynamic structure, resisting expansion over much of its surface, but extending in specific regions including hyphal tips and yeast buds. The adaptive significance of the wall is controversial. It is important to avoid the chicken-and-egg trap of suggesting that the cell wall functions to resist the explosive effects of turgor, because the cell would not be pressurised without the resistive behaviour of its wall. The cell wall allows the cell to generate turgor pressure, so perhaps it is more fruitful to think about why turgor might be useful. We will come back to this issue later in the chapter.The fungal cell wall is a porous macromolecular composite assembled at the surface of the plasma membrane (Figure 2.2 ). It contains stress-bearing microfibrils of chitin , linear polymers of glucose, or glucans , and a variety of cell wall proteins (CWP). The chitin polymer is built from β-1 → 4-linked monomers of the amino sugar, N -acetyl-d -glucosamine (Figure 2.3 ). Adjacent chitin chains assemble into hydrogen-bonded antiparallel arrays, producing microfibrils that can reach lengths of more than 1 μm. Chitin microfibrils have tremendous tensile strength; when chitin is disrupted, the cell loses its osmotic stability and may rupture. Chitosan , or β-1 → 4-glucosamine, is a polymer of the deacetylated sugar that is produced by many fungi in addition to chitin. β-1 → 3-glucan is often the most abundant wall polymer (Figure 2.4 ). The β-1 → 3-glycosidic linkage in glucans twists the polymer and three glucan chains form a triple helix that is held together by hydrogen bonds. β-1 → 3-glucans are connected with β-1 → 6-glucans in the mature wall structure to produce a highly branched elastic network of polymers. The structural proteins in the cell wall are glycoproteins with N - and O -linked carbohydrates. These include mannoproteins
eBook - ePub- J. W. Deacon(Author)
- 2013(Publication Date)
- Wiley-Blackwell(Publisher)
In recent years it has become clear that fungal walls serve many important roles, quite apart from the obvious role of providing a structural barrier. For example, the way in which a fungus grows – whether as cylindrical hyphae or as yeasts – is determined by the wall components and the ways in which these are assembled and bonded to one another. The wall is also the interface between a fungus and its environment: it protects against osmotic lysis, it acts as a molecular sieve regulating the passage of large molecules through the wall pore space, and if the wall contains pigments such as melanin it can protect the cells against ultraviolet radiation or the lytic enzymes of other organisms. In addition to these points, the wall can have several physiological roles. It can contain binding sites for enzymes, because many disaccharides (e.g. sucrose and cellobiose) and small peptides need to be degraded to monomers before they can pass through the cell membrane, and this is typically achieved by the actions of wall-bound enzymes (Chapter 6). The wall also can have surface components that mediate the interactions of fungi with other organisms, including plant and animal hosts. All these features require a detailed understanding of wall structure and architecture.The major wall components
The primary approach to investigating the wall composition of fungi is to disrupt fungal cells and purify the walls by using detergents and other mild chemical treatments, then use acids, alkalis and enzymes to degrade the walls sequentially. Although relatively few fungi have been analysed in detail, these treatments show that fungal walls are predominantly composed of polysaccharides, with lesser amounts of proteins and other components. The major wall components can be categorized into two major types: (i) the structural (fibrillar) polymers that consist predominantly of straight-chain molecules, providing structural rigidity, and (ii) the matrix components that cross-link the fibrils and that coat and embed the structural polymers.The main wall polysaccharides differ between the major fungal groups, as shown in Table 3.1 . The Chytridiomycota, Ascomycota, and Basidiomycota typically have chitin and glucans (polymers of glucose) as their major wall polysaccharides. Chitin consists of long, straight chains of β–1,4 linked N
eBook - ePub- K V Krishnamurthy(Author)
- 2020(Publication Date)
- CRC Press(Publisher)
J. Biophys. Biochem. Cytol., 3, 669, 1957.21. Aronson, J. M., The cell wall, in The Fungi, Vol. I, The Fungal Cell, Ainsworth, G. C. and Sussman, A. S., Eds., Academic Press, New York, 1973, 49.22. Selvendran, R. R., Stevens, B. J. H., and O’Neill, M. A., Developments in the isolation and analysis of cell walls from edible plants, in Biochemistry of Plant Cell Walls, Brett, C. T. and Hillman, J. R., Eds., Cambridge University Press, Cambridge, 1985, 39.23. Wilkie, K. C. B., New perspectives on non-cellulosic cell-wall polysaccharides (hemicelluloses and pectic substances) of land plants, in Biochemistry of Plant Cell Walls, Brett, C. T. and Hillmann, J. R., Eds., Cambridge University Press, Cambridge, 1985, 1.24. McCann, M. C. and Roberts, K., Plant cell walls: murals and mosaics, Agro-Food-Industry Hi-Tech, 43, 1994.25. Jarvis, M. C., Structure and properties of pectin gels in plant cell walls, Plant, Cell and Environ., 7, 153, 1984.26. Baron-Epel, O., Gharyal, P. K., and Schindler, M., Pectins as mediators of wall porosity in soybean cells, Planta, 175, 389, 1988.27. Eberhard, S., Doubrava, N., Marfa, V., Mohnen, D., Southwiek, A., Darvill, A., and Albersheim, P., Pectic cell wall fragments regulate tobacco thin-cell-layer explant morphogenesis, Plant Cell, 1, 747, 1989.28. Fengel, D., Ultrastructural behavior of cell wall polysaccharides, Tappi, 53, 497, 1970.29. Hoffmann, P. and Parameswaran, N., On the ultrastructural localization of hemicellulose within delignified tracheids of spruce, Holzforschung, 30, 62, 1976.30. Parameswaran, P. and Liese, W., Ultrastructural localization of cell wall components in wood cells, Holz Roh-Werkstoff, 40, 139, 1982.31. Stone, J. E. and Scallen, A. M., A structural model for the cell wall of water swollen wood pulp fibers based on their accessibility to macromolecules, Cell. Chem. Technol., 2, 343, 1968.32. McCandless, E. L., Polysaccharides of the seaweeds, in The Biology of the Seaweeds
eBook - ePub- Leo H Arnold(Author)
- 2018(Publication Date)
- CRC Press(Publisher)
In place of cellulose, which is universally present in the cell walls of higher plants, they contain other β-glucans, in which (1→3) and (1→6) linkages predominate, and occasionally α-glucans; both these types are also found in the cell walls of filamentous fungi. Equally important in yeast cell walls are polysaccharides in which mannose is the predominant sugar component. Small amounts of chitin are present in most species, and amino sugars also occur as links between carbohydrate and polypeptide. The peptide content of yeast cell walls, though small, is intimately associated with the polysaccharides, and so are esterified phosphate groups. The significance of the lipid associated with the cell envelope is discussed in Volume I, Chapter 7. The problems presented by the chemistry of yeast cell walls have been approached in a number of ways and it will be useful to review these briefly before setting out our present knowledge in greater detail. The ready availability of bakers’ yeast made it inevitable that it would be used in much of the early chemical work. Since this work extended over many decades during which fermentation technology was greatly developed, and was carried out in many countries, there may well have been some differences in the yeast samples employed, but that these were sufficient to affect the gross chemical structure of the wall components seems unlikely. It is only with the development of more refined methods of determining chemical structure that it has become worthwhile to look for differences between yeast strains and to examine connections between wall chemistry and taxonomy. Similarly, attempts to relate ultrastructure to chemical composition have tended, until now, to concentrate on the cell walls that are best understood chemically, i.e., a few species of Saccharomyces
eBook - ePub- Thomas Dyer(Author)
- 2017(Publication Date)
- CRC Press(Publisher)
Chapter 4 Fungal Biodeterioration 4.1 Introduction Fungi are eukaryotes: their cells contain nuclei and organelles both of which are enclosed by membranes. This configuration of the cell is the same as that for plants and animals.Species of fungi can live in terrestrial or aquatic environments, including seawater. Superficially fungi may seem very like plants—both include unicellular and multicellular organisms, lack motility, and many of the multicellular forms share some common morphological features. However, in taxonomic terms, fungi occupy a separate kingdom to these other organisms. There are a number of reasons for this. Firstly, unlike plants, fungi obtain energy and carbon from organic compounds in the surrounding environment. Secondly, the cell walls of fungi contain the protein chitin, as opposed to plants, whose cell walls are cellulose.Fungi can take the form of both single and multicellular organisms. It is useful to discuss these forms separately, as the differences have some significance to some of the topics covered later in this chapter.4.1.1 YeastsYeasts differ from other forms of fungi both structurally and in their means of reproducing. Yeasts are unicellular, like bacteria, meaning that they are composed of single discrete cells. Unlike bacteria, yeasts are non-motile, and so only move when fluids in which they are present are mobile. Some yeast are capable of attaching to surfaces, which occurs through the formation of adhesive compounds, including sugars, at the cell surface [1 ].Like bacteria, some yeasts reproduce asexually through fission. However, it is more common for asexual reproduction to occur via budding. Budding involves the formation of a small bud on the side of the yeast cell. Once formed, the nucleus of the cell also splits to form two identical nuclei (mitosis
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