A fungus is a member of a large group of eukaryotic organisms that includes microorganisms such as yeasts and molds (British English: moulds), as well as the more familiar mushrooms. These organisms are classified as a kingdom, Fungi, which is separate from plants, animals, and bacteria. One major difference is that fungal cells have cell walls that contain chitin, unlike the cell walls of plants, which contain cellulose. These and other differences show that the fungi form a single group of related organisms, named the Eumycota (true fungi or Eumycetes), that share a common ancestor (a monophyletic group). This fungal group is distinct from the structurally similar myxomycetes (slime molds) and oomycetes (water molds). The discipline of biology devoted to the study of fungi is known as mycology, which is often regarded as a branch of botany, even though genetic studies have shown that fungi are more closely related to animals than to plants.
Mycology is the branch of biology concerned with the systematic study of fungi, including their genetic and biochemical properties, their taxonomy, and their use to humans as a source of medicine, food, and psychotropic substances consumed for religious purposes, as well as their dangers, such as poisoning or infection. The field of phytopathology, the study of plant diseases, is closely related because many plant pathogens are fungi.
Fungi perform an essential role in the decomposition of organic matter and have fundamental roles in nutrient cycling and exchange. They have long been used as a direct source of food, such as mushrooms and truffles, as a leavening agent for bread, and in fermentation of various food products, such as wine, beer, and soy sauce. Since the 1940s, fungi have been used for the production of antibiotics, and, more recently, various enzymes produced by fungi are used industrially and in detergents. Fungi are also used as biological pesticides to control weeds, plant diseases and insect pests. Many species produce bioactive compounds called mycotoxins, such as alkaloids and polyketides, that are toxic to animals including humans. The fruiting structures of a few species contain psychotropic compounds and are consumed recreationally or in traditional spiritual ceremonies. Fungi can break down manufactured materials and buildings, and become significant pathogens of humans and animals and also plants.
The true biodiversity of Kingdom Fungi, which has been estimated at around 1.5 million species, with about 5% of these having been formally classified. Fungi have been classified according to their morphology (e.g., characteristics such as spore color or microscopic features) or physiology. Advances in molecular genetics have opened the way for DNA analysis to be incorporated into taxonomy, which has sometimes challenged the historical groupings based on morphology and other traits.
The opisthokonts are a broad group of eukaryotes, including both the animal and fungus kingdoms, together with the eukaryotic microorganisms that are sometimes grouped in the paraphyletic phylum Choanozoa (previously assigned to the protist "kingdom"). Both genetic and ultrastructural studies strongly support that opisthokonts form a monophyletic group. Phylogenetic studies published in the last decade have helped reshape the classification of Kingdom Fungi, which is divided into one subkingdom (Dikarya), seven phyla (Ascomycota, Basidiomycota, Blastocladiomycota, Chytridiomycota, Glomeromycota, Microsporidia, Neocallimastigomycota), and ten subphyla (Zygomycota).
Taxonomists considered fungi to be members of the Plant Kingdom because of similarities in lifestyle: both fungi and plants are mainly immobile, and have similarities in general morphology and growth habitat. Like plants, fungi often grow in soil, and in the case of mushrooms form conspicuous fruiting bodies, which sometimes bear resemblance to plants such as mosses. The fungi are now considered a separate kingdom, distinct from both plants and animals, from which they appear to have diverged around one billion years ago.
Shared features with other eukaryotes: As other eukaryotes, fungal cells contain membrane-bound nuclei with chromosomes that contain DNA with noncoding regions called introns and coding regions called exons. Fungi possess membrane-bound cytoplasmic organelles and ribosomes of the 80S type. Shared features with animals: Fungi lack chloroplasts and are heterotrophic organisms. Shared features with plants: Fungi possess a cell wall and big vacuoles. They reproduce by both sexual and asexual means, and produce spores. Similar to mosses and algae, fungi typically have haploid nuclei. Shared features with euglenoids and bacteria: Higher fungi, euglenoids, and some bacteria produce the amino acid L-lysine in specific biosynthesis steps, called the α-aminoadipate pathway.
Unique features of Fungi: some species grow as single-celled yeasts that reproduce by budding or binary fission. Dimorphic fungi can switch between a yeast phase and a hyphal phase in response to environmental conditions. The fungal cell wall is composed of glucans and chitin. Fungi are the only organisms that combine these two structural molecules in their cell wall.
The cells of most fungi grow as tubular, elongated, and thread-like (filamentous) structures and are called hyphae, which may contain multiple nuclei and extend at their tips. Each tip contains a set of aggregated vesicles—cellular structures consisting of proteins, lipids, and other organic molecules—called Spitzenkörper. Both fungi and oomycetes grow as filamentous hyphal cells. In contrast, similar-looking organisms, such as filamentous green algae, grow by repeated cell division within a chain of cells.
Mycorrhizal symbiosis between plants and fungi is one of the most well-known plant–fungus associations and is of significant importance for plant growth and persistence in many ecosystems; over 90% of all plant species engage in mycorrhizal relationships with fungi and are dependent upon this relationship for survival. Lichens are formed by a symbiotic relationship between algae or cyanobacteria (referred to in lichen terminology as "photobionts") and fungi (mostly various species of ascomycetes and a few basidiomycetes), in which individual photobiont cells are embedded in a tissue formed by the fungus. Many insects also engage in mutualistic relationships with fungi. yeasts of the genera Candida and Lachancea inhabit the gut of a wide range of insects, including neuropterans, beetles, and cockroaches; it is not known whether these fungi benefit their hosts.
Fungal as pathogens and parasites
Many fungi are parasites on plants, animals (including humans), and other fungi. Serious pathogens of many cultivated plants causing extensive damage and losses to agriculture and forestry include the rice blast fungus Magnaporthe oryzae, tree pathogens such as Ophiostoma ulmi and Ophiostoma novo-ulmi causing Dutch elm disease, and Cryphonectria parasitica responsible for chestnut blight. Some carnivorous fungi, like Paecilomyces lilacinus, are predators of nematodes, which they capture using an array of specialized structures such as constricting rings or adhesive nets.
Some fungi can cause serious diseases in humans, several of which may be fatal if untreated. These include aspergilloses, candidoses, coccidioidomycosis, cryptococcosis, histoplasmosis, mycetomas, and paracoccidioidomycosis. Persons with immuno-deficiencies are particularly susceptible to disease by genera such as Aspergillus, Candida, Cryptoccocus, Histoplasma, and Pneumocystis. Other fungi can attack eyes, nails, hair, and especially skin, the so-called dermatophytic and keratinophilic fungi, and cause local infections such as ringworm and athlete’s foot. Fungal spores are also a cause of allergies, and fungi from different taxonomic groups can evoke allergic reactions.
The human use of fungi for food preparation or preservation and other purposes is extensive and has a long history. Mushroom farming and mushroom gathering are large industries in many countries. Fungi produce an enormous range of natural products with antimicrobial or other biological activities such as vitamins, and anti-cancer and cholesterol-lowering drugs. More recently, methods have been developed for genetic engineering of fungi, enabling metabolic engineering of fungal species that are potentially more efficient than production by the original source organisms.
Naturally penicillins such as penicillin G (produced by Penicillium chrysogenum) have a relatively narrow spectrum of biological activity, can be produced by chemical modification of the natural penicillins. Modern penicillins are semisynthetic compounds, obtained initially from fungal fermentation cultures. Other antibiotics produced by fungi include: ciclosporin, commonly used as an immunosuppressant during transplant surgery; and fusidic acid, used to help control infection from methicillin-resistant Staphylococcus aureus bacteria.
Baker's yeast or Saccharomyces cerevisiae, a single-celled fungus, is used to make bread and other wheat-based products, such as pizza dough and dumplings. Yeast species of the genus Saccharomyces are also used to produce alcoholic beverages through fermentation. Shoyu koji mold (Aspergillus oryzae) is an essential ingredient in brewing Shoyu (soy sauce) and sake, and the preparation of miso, while Rhizopus species are used for making tempeh. Several of these fungi are domesticated species that were bred or selected according to their capacity to ferment food without producing harmful mycotoxins. Quorn, a meat substitute, is made from Fusarium venenatum.
Certain mushrooms enjoy usage as therapeutics in folk medicines, such as Traditional Chinese medicine. Notable medicinal mushrooms with a well-documented history of use include Agaricus subrufescens, Ganoderma lucidum, and Cordyceps sinensis. Research has identified compounds produced by these and other fungi that have inhibitory biological effects against viruses and cancer cells. The shiitake mushroom is a source of lentinan, a clinical drug approved for use in cancer treatments in several countries, including Japan. In Europe and Japan, polysaccharide-K (brand name Krestin), a chemical derived from Trametes versicolor, is an approved adjuvant for cancer therapy.
There are many mushroom species that are harvested from the wild for personal consumption or commercial sale. Milk mushrooms, morels, chanterelles, truffles, black trumpets, and porcini mushrooms (Boletus edulis; also known as king boletes) demand a high price on the market. They are often used in gourmet dishes.
Many mushroom species are poisonous to humans, with toxicities ranging from slight digestive problems or allergic reactions as well as hallucinations to severe organ failures and death. Genera with mushrooms containing deadly toxins include Conocybe, Galerina, Lepiota, and most infamously, Amanita.
· Each HYPHA is:
o essentially a tube - consisting of a rigid wall and containing protoplasm
o tapered at its tip - this is the region of active growth (i.e. the extension zone).
· SEPTA (cross-walls), if present, can usually be observed down a light microscope
o some fungi possess septa at regular intervals along the lengths of their hyphae
o in others, cross-walls form only to isolate old or damaged regions of a hypha or to isolate reproductive structures.
o some septa possess one of more PORES - such septa divide up the hyphae into a series of interconnected HYPHAL COMPARTMENTS, rather than separate, discrete cells.
· The PLASMA MEMBRANE is closely associated with the hyphal wall and in some regions may even be firmly attached to it - making it difficult to plasmolyse hyphae.
· Each hyphal cell or compartment normally contains one or more NUCLEI. In species whose septa possess a large central pore, the number of nuclei within a hyphal compartment won't remain static because the nuclei are able to pass between adjacent compartments, via the central septal pore.
· Other CYTOPLASMIC ORGANELLES are those commonly found in all eukaryotic cells.
· The GROWING TIP is structurally and functionally very different from the rest of the hypha
o its cytoplasm appears more dense
o there are no major organelles at the extreme tip
o at the extreme tip there is an accumulation of membrane-bound vesicles - the APICAL VESICULAR CLUSTER (COMPLEX) (AVC) - which plays an important role in apical growth.
· VACUOLES may be visible in sub-apical hyphal compartments - although small at first, they grow larger and merge with one another; they store and recycle cellular metabolites, e.g. enzymes and nutrients.
· In the oldest parts of the hypha the protoplasm may breakdown completely, due either to AUTOLYSIS (self-digestion) or in natural environments HETEROLYSIS (degradation due to the activities of other microorganisms).
The Fungal Wall
· PROTECTS the underlying protoplasm;
· determines and MAINTAINS THE SHAPE of the fungal cell or hypha; if you remove the wall the resulting protoplast will always assume a spherical shape;
· acts as an INTERFACE between the fungus and its environment;
· acts as a BINDING SITE for some enzymes;
· possesses ANTIGENIC properties - which allow interactions with other organisms.
Chemical composition of the wall:
· POLYMERIC FIBRILS
o some cellulose
· AMORPHOUS MATRIX COMPONENTS
o heteropolymers (mixed polymers) of mannose, galactose, fucose and xylose
· The types and amounts of these various components vary amongst different groups of fungi and may even vary during the life cycle of a single species.
Arrangement of the wall components:
· The diagram above represents a section through the mature lateral wall of hyphae of Neurospora crassa.
· In general, the inner part of the wall consists of POLYMERIC FIBRILS embedded in an AMORPHOUS MATRIX and this is covered by further layers of matrix material.
· At the HYPHAL TIP the wall is thinner and simpler in structure, consisting of only TWO LAYERS - an inner layer of fibrils embedded in protein and outer layer of mainly protein.
· EXTRA LAYERS of wall material are deposited in the lateral walls behind the extending apex - strengthening the wall as the hypha matures.
· In the oldest parts of the hyphae (and in many fungal spores) LIPIDS and PIGMENTS may be desposited in the wall:
o LIPIDS serve as a nutrient reserve and help prevent desiccation
o PIGMENTS, such as MELANIN, help protect the protoplast against the damaging effects of UV radiation.
· N.B. Although represented as distinct layers in the diagram above, these four zones merge into one another.
Septa (cross-walls) can be seen by light microscopy. But electron microscopy has revealed that several different types of septa exist among the major taxonomic groups of fungi.
· In general, the hyphae of fungi belonging to these groups are not regularly septate (although there are some exceptions).
· But septa in the form of COMPLETE CROSS-WALLS are formed to isolate old or damaged regions of the mycelium or to separate reproductive structures from somatic hyphae.
· Hyphae of fungi belonging to these groups (and the Basidiomycota) possess perforated septa at regular intervals along their length.
· The septum consists of a simple plate with a relatively LARGE CENTRAL PORE (50-500 nm diameter) - this allows cytoplasmic streaming (the movement of organelles, incl. nuclei) between adjacent hyphal compartments.
· Cytoplasmic streaming enables sub-apical and intercalary (central) compartments of young hyphae to contribute towards growth of the hyphal tip - transporting nutrients and essential enzymes to the apex - so maximizing the capacity for somatic growth.
· Associated with each septum are spherical, membrane-bound organelles called WORONIN BODIES that ........
o are composed of protein;
o remain close to the septal pore and tend not to be disturbed by the cytoplasmic streaming taking place;
o tend to be of the same or larger diameter than the septal pore and are, therefore, capable of blocking the pore;
o will block the septal pore if the adjacent hyphal compartment is damaged or ageing and becoming highly vacuolated.
· Not all fungi belonging to the Acomycota possess Woronin bodies - those that don't often possess LARGE HEXAGONAL CRYSTALS OF PROTEIN in the cytoplasm that are capable of serving the same function, i.e. they can seal the septal pores of damaged or ageing hyphae.
· A number of mitosporic fungi possess septa with a single central pore, similar to that observed in the Ascomycota.
· But other mitosporic fungi may possess MULTIPERFORATE SEPTA.
· E.g. the septa of Geotrichum candidum (illustrated above) possess characteristic MICROPORES (approx. 9 nm diameter).
· The number of pores in each septum can vary up to a maximum of approx. 50.
· These micropores allow cytoplasmic continuity between adjacent hyphal compartments, but are too small to allow cytoplasmic streaming to occur to the extent observed in fungi possessing larger septal pores.
· The most complex type of septum is found in fungi belonging to the Basidiomycota.
· Each septum is characterized by a swelling around the central pore (DOLIPORE) and a hemispherical perforated cap (PARENTHOSOME) on either side of the pore - illustrated above.
· The perforated parenthosome allows cytoplasmic continuity but prevents the movement of major organelles.
· The plasma membrane lines both sides of the septum and the dolipore swelling, but the membrane of the parenthosome is derived from endoplasmic reticulum.
· Act as STRUCTURAL SUPPORTS
o The addition of plate-like cross-walls to what is essentially a long tube-like structure (hypha) will help stabilize it.
· Act as the FIRST LINE OF DEFENCE when part of a hypha is damaged
o Large-pored septa that have Woronin bodies or large proteinaceous crystals associated with them have the advantage that cytoplasmic streaming can occur between adjacent compartments.
o But at the same time a mechanism exists for rapidly sealing the septal pore under conditions of stress (e.g. if the hypha is damaged) thereby helping protect the mycelium.
· Facilitate DIFFERENTIATION in fungi
o Septa can isolate adjacent compartments from one another so that different biochemical and physiological processes can occur within them - these may result in differentiation of the hyphae into specialized structures, such as those associated with sporulation.
o It's unlikely to be coincidental that the most complex and highly differentiated sporulating structures we see are those produced by fungi possessing the most complex types of septa, i.e. fungi belonging to the Basidiomycota.
· You might like to begin by viewing some movies that show apical growth, hyphal branching and septum formation.
· To understand the mechanisms involved in apical growth of a hypha we need to look again at the HYPHAL TIP.
· We already know that the growing hyphal tip is structurally and functionally different from the rest of the hypha - see section on Hyphal Ultrastructure.
· BUT - the hyphal tip (like the rest of the hypha) is surrounded by a wall - although the wall may be thinner and simpler in structure than the mature lateral wall of the hypha further back - see section on the Fungal Wall.
· We also know that growth of a hypha is closely linked to the presence of vesicles which form the APICAL VESICULAR CLUSTER (AVC):
o when a hypha stops growing, these vesicles disappear
o when growth of the hypha resumes, the vesicles reappear.
· In addition - the position of the vesicles is linked to the direction of growth of a hypha:
o when a hypha is growing straight ahead, the vesicles are positioned in the centre of the hyphal tip
o movement of the vesicles to the left or right side of the hyphal tip is accompanied by a change in direction of growth of the hypha
· So it's clear that the vesicles play a key role in apical growth.
Vesicles of the AVC contain:
· wall PRECURSORS - the sub-units or buildng blocks of the wall polymers - e.g. uridine diphosphate N-acetylglucosamine, the sub-unit of chitin
· wall LYTIC ENZYMES - which help breakdown and separate wall components - e.g. chitinase, glucanase
· wall SYNTHASE ENZYMES - which help assemble new wall components and so increase the size of the wall - e.g. chitin synthase, glucan synthase.
TWO MODELS have been proposed to explain the mechanisms of apical growth - they differ in whether or not wall lytic enzymes are necessary.
According to this model, if the hypha is going to be able to extend at its tip, there will have to be:
· some softening (lysis) of the existing wall, and
· the synthesis and incorporation of new wall material.
But these processes will have to be finely balanced - otherwise, the wall may become too weak or too rigid for further growth
The following series illustrate what may happen:
1. Vesicles containing lytic enzymes or wall precursors move through the cytoplasm towards the hyphal tip, where they fuse with the plasma membrane, releasing their contents into the wall.
2. The lytic enzymes released into the wall attack the polymeric fibrils.
3. The weakened fibrils stretch out and become separated from one another due to the turgor pressure of the protoplasm.
4. Synthase enzymes and wall precursors build new fibrils and synthesise additional amorphous components of the wall.
5. The surface area of the hyphal wall increases. Fusion of the vesicles with the plasma membrane ensures that the fomer contribute to the increase in surface area of the latter.
Model 2 - steady state:
According to this model:
· lytic enzymes are NOT involved in apical growth
· because the newly formed wall at the extreme tip of the hypha is VISCOELASTIC (essentially fluid)
· so that as new wall components are added at the tip, the wall flows outwards and backwards (see adjacent diagram)
· and the wall then RIGIDIFIES progressively behind the tip by the formation of extra chemical bonds.
Although each hypha exhibits apical growth (i.e. extends at its tip), it doesn't continue growing as just a single filament - it will eventually BRANCH and as the branches become progressively longer they too will branch, as illustrated in this movie clip from the Fungal Cell Biology Group based at the University of Edinburgh.
· Hyphal branching is necessary for efficient colonization and utilization of the substrate upon which the fungus is growing.
· A branch arises when a NEW GROWTH POINT is initiated in the existing lateral wall of the hypha - this is accompanied by the ACCUMULATION OF VESICLES.
· Branch formation almost certainly involves wall lytic enzymes (model 1), since the branch will emerge through a mature, rigid area of the hypha's lateral wall.
· Branches normally EXTEND AWAY FROM ONE ANOTHER, filling the gaps between existing hyphae, because they're:
o responding to nutrient gradients - growing out of areas where nutrients have become limited around existing hyphae, into areas where nutrients are more plentiful
o growing away from areas which have become staled by the metabolic by-products of existing hyphae.
· The extent of hyphal branching, i.e. the density of a fungal colony (number of hyphal branches formed per unit area), is directly related to the concentration of nutrients in the substrate or growth medium:
o a sparsely branched colony (low hyphal density) will develop on a nutritionally weak substrate or growth medium
o a densely branched colony will develop on a nutritionally rich substrate or growth medium.
· RADIAL GROWTH of the colony is NOT influenced by the concentration of nutrients (within limits).
· So a colony will reach approximately the same diameter in a given time interval whether growing on a nutritionally rich or poor growth medium (again, within limits).
THE REASON? - BECAUSE:
· Existing hyphal tips at the colony margin (which determine the diameter of a colony) have priority over all other hyphal tips (i.e. the branches) for the available nutrients.
· Only nutrients in excess of those required by the marginal hyphal tips are available to support branching.
· Therefore, the more nutrients that are surplus to the colony margin's requirements, the greater the hyphal density.
General Characteristics of Fungal Spores
1. Spores represent microscopic dispersal or survival propagules produced by most species of fungi:
2. Fungal spores vary in size, shape and colour
3. Fungal spores may be unicellular or multicellular
4. Some spores possess a textured or ornamented surface
5. The protoplasm of most (not all) spores is surrounded by a rigid wall, which ..........:
· is often thicker and more multilayered than that of somatic cells or hyphae;
· may be impregnated with pigments (e.g. melanins) and lipids.
6. Spores often contain substantial amounts of nutrient reserves, which may take the form of .........:
7. They possess a relatively low water content.
8. While dormant they exhibit a low rate of metabolic activity.
9. They vary in the primary functions they serve, which may include:
· dispersal to a fresh site or host;
· survival at the same site;
· increasing genetic variation.
10. They also vary in the methods by which they are formed, released and dispersed.
Fungal spores exhibit TWO TYPES of dormancy, described as CONSTITUTIVE (endogenous) and EXOGENOUS.
· Commonly exhibited by SEXUAL FUNGAL SPORES.
· Is imposed by some INHERENT (ENDOGENOUS) CHARACTERISTIC of the spore itself which prevents it from germinating.
· Spores may fail to germinate even when environmental conditions appear favourable for growth.
· Some may require a period of ageing or a specific activation trigger, such as heat-shock or cold-shock.
· E.g. uredospores of Puccinia graminis, cause of rust disease in cereal crops:
o Ensure they don't germinate while in close proximity to one another and consequently compete for a limited supply of nutrients in the environment.
o Because they contain METHYL-CIS-FERULATE, a water-soluble and volatile inhibitor of germination.
o Germinate once thoroughly dispersed from one another and methyl-cis-ferulate has leached out of the spore and become diluted in the environment.
· Commonly exhibited by ASEXUAL FUNGAL SPORES.
· Imposed by an UNFAVOURABLE ENVIRONMENT (i.e. exogenous factors).
· Factors influencing dormancy include availability of moisture and nutrients, as well as temperature and pH.
· Spores germinate only if and when environmental conditions are favourable for growth.
· E.g. conidia of Aspergillus species.
· A phenomenon LINKED TO EXOGENOUS DORMANCY.
· It is the inhibition of fungal growth without any effect on viability of the fungus.
· Spores may fail to germinate in natural environments (e.g. soil or leaf surfaces) because of the activities of other micro-organisms.
· This inhibition may be due to INHIBITORY METABOLITES produced by other micro-organisms and/or COMPETITION for a limited amounts of nutrients available.
· The EFFECT IS REVERSIBLE - once the inhibitory substances are removed (or become diluted) or additional nutrients become available the spores will germinate (or the mycelium will resume growth).
Any viable spore should eventually germinate. If the spore is that of a mycelial fungus germination usually involves the production of one or more germ-tubes. But before emergence of a germ-tube many spores will require an exogenous supply of nutrients to be available, will undergo hydration and swelling, and will experience an increase in metabolic activity.
Availability of nutrients:
· Some spores are able to germinate in the absence of any exogenous nutrients in the environment because they possess sufficient ENDOGENOUS RESERVES (within the spore) to sustain initial growth of the germ-tube.
· Others must be supplied with one or more EXOGENOUS NUTRIENTS (e.g. a carbohydrate source) before they are able to germinate.
Hydration (water uptake):
· The presence of liquid water or a high relative humidity is essential for the germination of spores of most species - few spores are capable of germinating at low relative humidities.
· Since most spores have a low water content, hydration is an essential first step in the germination process.
· Water uptake is an ACTIVE PROCESS and requires a change in permeability of the spore wall.
Swelling is due to:
· Deposition of new wall material within the spore - some of which is destined to form the wall surrounding the developing germ-tube (see diagram below).
· The small vesicles accumulating near the plasma membrane are involved in the synthesis of new wall materials.
· As the germ-tube develops these vesicles become arranged to form a cresent-shaped zone at the tip of the germ-tube - known as the APICAL VESICULAR CLUSTER or COMPLEX (AVC).
· Emergence of the germ-tube through the spore wall is due to a combination of enzymic degradation of a small localised region of the spore wall and the physical pressure being exerted by the protoplasm.
· The germ-tube may emerge from a pre-determined thinner region of spore wall (GERM PORE) or from some random site.
Commercial Uses of Fungal Spores
Fungal spores possess a wide range of CONSTITUTIVE ENZYMES (normally present) that may be absent from somatic cells and hyphae, or that may be produced by somatic cells only in the presence of their substrates (i.e. they may only be INDUCIBLE).
The possession of such constitutive enzymes, combined with the increase in metabolic activity that accompanies germination, has resulted in spores being used as biological catalysts in a number of commercial CHEMICAL TRANSFORMATION processes.
Examples of chemical transformations include:
· The transformation of penicillins by spores of Fusarium moniliforme.
· The conversion of fatty acids to methylketones by spores of Penicillium roquefortii.
· Fungal spores are generally more stable than somatic cells and mycelia and can be transported more readily.
· Spores provide homogeneous suspensions - providing reproducible conversion of the substrate.
· Yields are usually high.
· Undesirable by-products are rarely formed.
· Only a very simple growth medium is usually required.
· Emergence of the germ-tube can be inhibited without affecting efficiency of the chemical transformation - allowing a single batch of spores to be used several times.
Definition of Growth in Fungi
- Growth may be defined as an irreversible increase in the volume of an organism, usually accompanied by an increase in biomass.
- Mycelial fungi exhibit extension growth of hyphae, accompanied by an increase in biomass.
- Unicellular fungi (e.g. yeasts) may exhibit an increase in individual cell volume, accompanied by an increase in biomass.
- But collectively, the number of yeast cells within a culture (i.e. cell concentration) may also increase, resulting in an increase in biomass of the culture as a whole.
Fungi may be cultured on SOLID or in LIQUID MEDIA:
- YEASTS are often cultured in LIQUID media
- MYCELIAL SPECIES may be cultured in LIQUID or on SOLID growth media.
Introduction to Sporulation
Many (not all) fungi are capable of reproducing BOTH SEXUALLY and ASEXUALLY - usually producing spores as a result.
Purpose of sporulation:
· GENETIC VARIATION.
Factors influencing the type of sporulation:
· GENOTYPE of the SPECIES - .
· GENOTYPE of the MYCELIUM - .
· EXTENT OF SOMATIC GROWTH - .
· SPECIFIC ENVIRONMENTAL CONDITIONS - e.g. temperature, light, specific nutrients.
Types of Asexual Spore
Fungi produce two major types of asexual spore: SPORANGIOSPORES and CONIDIA.
· ENDOGENOUS - formed and contained WITHIN a SPORANGIUM.
· Formed as a result of the CLEAVAGE OF PROTOPLASM around nuclei.
· Characteristic of fungi belonging to the CHYTRIDIOMYCOTA, OOMYCOTA and HYPHOCHYTRIDIOMYCOTA.
· EXOGENOUS - often formed at the tip of supporting hyphae called a CONIDIOPHORES
· Characteristic of MITOSPORIC FUNGI and fungi belonging to the ASCOMYCOTA and BASIDIOMYCOTA.