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Image Source: Department of Biology, University of Miami
Structure of fungal hypha
Hypha is characterized as a tube-like structure with a rigid wall that contains a moving slug of protoplasm. The length of the hypha varies in different fungal species; however, the diameter ranges from 2 to 30 micrometers and depends on the species and growth stage of the organism. The growth occurs at the tip of the hypha and the tip of the hypha contains a tapered region called extension zone. Apart from the growing tip region, the rest of the part of hypha ages accordingly and breaks down due to autolytic enzymes. The presence of cross walls is an important feature in most of the fungi. These cross walls are called septa (singular septum). However, members of the oomycete and Zygomycota lack the septa in their hypha. The hypha is also surrounded by a complex wall-like structure which thin at the apex region but the thickness increases later on. The plasma membrane is closely attached to the cell wall which made the hypha safe from plasmolyze.
Hypha as a part of Colony
The development of fungal colonies occurs from a single germinating spore. The spore produces a germ tube (small hypha) which grows further and forms branches. As the original hypha and the first-order branches grow, they produce further branches behind their tips. These branches diverge from one another until, eventually, the colony develops a characteristic circular outline. Then, in the older parts of a colony where nutrients have been depleted, many fungi produce narrow hyphal branches that grow towards one another instead of diverging, and fuse by tip-to-tip contact, involving localized breakdown of their walls. This process of hyphal anastomosis creates a network for the pooling and remobilization of protoplasm to produce chlamydospores or other, larger differentiated structures.
Structure of yeast
Yeasts are single-cell fungi that grow by the process called budding. Apart from that, these organisms lack hyphae. The common examples of yeasts are Saccharomyces cerevisiae, Cryptococcus spp., Sporobolomyces roseus, etc. Apart from these, there are several fungi that can be present in both the hypha and unicellular form. These are called dimorphic fungi; an example of this kind of fungi is Candida albicans. The general structure of a yeast cell consists of a single nuclear along with a typical range of cytoplasmic organelles. These organelles include the endoplasmic reticulum, Golgi apparatus, mitochondria, vacuoles, lipid bodies, etc. Most yeasts divide by budding from one or more locations on the cell surface. During this process a small outgrowth appears at the bud site, then the bud progressively elongates before expanding into a rounded form, by the synthesis of new wall components over the whole of the cell surface. When the bud has nearly reached its final size, the nucleus of the mother cell migrates towards the bud site and divides, so that one nucleus remains in the mother cell and a second nucleus enters the daughter cell. The final separation of the two cells is achieved by the development of a septum. By using fluorescent dyes that bind to chitin, we can count the number of bud scars on the cell surface. This reveals that Saccharomyces cerevisiae is multipolar budding yeast – it always buds from a different point on the cell surface, never from a previous bud site, whereas some other yeasts exhibit bipolar budding – the buds always arise at the same positions, often at the poles of the cell.
Fungal Cell wall
The fungal cell wall plays an important role in providing a structural barrier to the cell. Moreover, it also protects the cell from osmotic lysis. It is a kind of molecular sieve, which permits the entrance of specific molecules inside the cells and arrests the entrance of toxic molecules. The cell wall of fungi often contains pigments like melanin which provides protection against ultraviolet radiation and lytic enzymes. The backbone of most of the fungal cell wall is made up of polysaccharides. The cell wall components can be divided into two groups on the basis of the chemical nature of the components such as the structural (fibrillar) polymers that consist predominantly of straight-chain molecules, providing structural rigidity, and 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. 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-acetylglucosamine residues, whereas the fungal glucans are branched polymers, consisting mainly of β-1,3-linked backbones with short β-1,6-linked side chains. The Zygomycota typically have a mixture of chitin and chitosan, polymers of uronic acids such as glucuronic acid, and mannoproteins. The Oomycota (which are not true fungi) have little chitin, and instead, they have a mixture of a cellulose-like β-1,4-linked glucan and other glucans.
The extrahyphal matrix is a discrete polysaccharide capsule that is present outside of the cell wall. Both yeasts and hypha can be surrounded by this layer. The layer is made up of polysaccharides or glycoprotein which can easily be removed by providing simple chemical treatments such as washing. These extracellular matrix materials can have important roles in the interactions of fungi with other organisms. For example, the yeast Cryptococcus neoformans is a significant pathogen of humans; its polysaccharide capsule masks the antigenic components of the cell wall so that the fungus is not engulfed by phagocytes and can proliferate in the tissues.
Septa (cross-walls) are found at fairly regular intervals along the length of most hyphae in the Ascomycota, Basidiomycota, and mitosporic fungi, although the septa are perforated to allow continuity of the protoplasm. If part of the hypha is damaged, then a Woronin body or a plug of coagulated protoplasm rapidly seal the septal pore to localize the damage. Then the hypha can regrow from a newly formed tip behind the damaged compartment, or in some cases, a new tip can grow into the damaged compartment. Septa might help to provide structural support to hyphae, especially in conditions of water stress. But one of their main roles seems to be to enable differentiation. By blocking the septal pores, a fungal hypha is transformed from a continuous series of compartments to a number of independent cells or regions that can undergo separate development. Septa can be seen by normal light microscopy, but electron microscopy reveals several different types of the septum. The Ascomycota and mitosporic fungi have a simple septum with a relatively large central pore, ranging from 0.05 to 0.5-micrometer diameter, which allows the passage of cytoplasmic organelles and even nuclei. The Basidiomycota also has simple septa when they are growing as monokaryons (with one nucleus in each cell). But they often have a more complex dolipore septum when strains of different mating compatibility groups fuse to form a dikaryon, with two nuclei in each compartment. The dolipore septum has a narrow central channel (about 100–150 nm diameter) bounded by two flanges of amorphous glucan. On either side of this septum are bracket-shaped membraneous structures termed, which have pores to allow cytoplasmic continuity but which prevent the passage of major organelles.
The fungal nucleus is generally small; however, the diameter ranges from 2 micrometers to 25 micrometers on the basis of the species. Similar to other eukaryotes the nucleus is covered by a porous nuclear membrane. There are several features in the nucleus which are specifically found in fungi. For example:
- The nuclear membrane and the nucleolus remain intact during most stages of mitosis, whereas in most other organisms the nuclear membrane breaks down at an early stage during nuclear division.
- In fungi there is no clear metaphase plate; instead, the chromosomes seem to be randomly dispersed, and at anaphase, the daughter chromatids pull apart along two tracks, on spindle fibers of different lengths.
- The third point of difference is that fungi have various types of spindle-pole bodies. They are responsible for microtubule assembly during nuclear division.
Most of the fungal nuclei are haploid with chromosome numbers ranging from about 6 to 20.
The Plasma Membrane
As in all eukaryotes, the fungal plasma membrane consists of a phospholipid bilayer with associated transmembrane proteins, many of which are involved directly or indirectly in nutrient uptake. The membrane also can anchor some enzymes. In fact, the two main wall-synthetic enzymes, chitin synthase, and glucan synthase are integral membrane proteins; they become anchored in the membrane in such a way that they produce polysaccharide chains from the outer membrane face. The fungal plasma membrane is unique in one important respect – it typically contains ergosterol as the main membrane sterol, in contrast to animals, which have cholesterol, and plants which have phytosterols such as sitosterol. The Oomycota also have plant-like sterols. But some plant-pathogenic fungi such as Pythium and Phytophthora spp. are unable to synthesize sterols from nonsterol precursors and instead need to be supplied with sterols from the host. Ergosterol is the primary target of several fungicides that are used to control plant-pathogenic fungi. Ergosterol is also a primary target of several antifungal drugs that are used to treat human mycoses.
Gogli, Endoplasmic reticulum, and Vesicles
Fungi have a secretory system, consisting of the endoplasmic reticulum (ER), the Golgi apparatus (or Golgi equivalent), and membrane-bound vesicles. Proteins destined for export from the cell are synthesized on ribosomes attached to the ER, then enter the ER lumen and are transported to the Golgi. During their progressive transport through the Golgi cisternae, proteins undergo various modifications, including partial cleavage and reassembly, folding into a tertiary structure, and the addition of sugar chains (glycosylation). Then the proteins, or glycoproteins, are packaged into vesicles that bud from the maturing face of the Golgi and are transported to the plasma membrane for secretion. This intricate postal system sorts and delivers proteins to specific destinations, including the enzymes (pectinases, cellulases, proteases, etc.) that are destined for export to degrade polymers in the surrounding environment. The secretory system is involved in at least some aspects of fungal tip growth because glycoproteins are only produced in the Golgi and are transported in vesicles to the sites of wall growth. Additionally, the Golgi is important for commercial production, and release from the cell, of foreign (heterologous) gene products. The ability of a protein to enter the ER is determined by a signal sequence at the N-terminus, which is subsequently removed. Without this sequence, the protein will remain in the cell.
The vacuolar system of fungi has several functions, including the storage and recycling of cellular metabolites. For example, the vacuoles of several fungi, including mycorrhizal species, accumulate phosphates in the form of polyphosphate. Vacuoles also seem to be major sites for the storage of calcium which can be released into the cytoplasm as part of the intracellular signaling system. Vacuoles contain proteases for breaking down cellular proteins and recycling of the amino acids, and vacuoles also have a role in the regulation of cellular pH. All these important physiological roles are in addition to the potential role of vacuoles in cell expansion and in driving the protoplasm forwards as hyphae elongate at the tips. Vacuoles often are seen as conspicuous, rounded structures in the older regions of hyphae, but recent work has shown that there is also a tubular vacuolar system extending into the tip cells. It is an extremely dynamic system, consisting of narrow tubules which can dilate and contract, as inflated elements travel along with them in a peristaltic manner.
Fungal Biology by Jim Deacon, 4th Edition, Blackwell Publishing.
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