THE KINGDOM FUNGI

 THE KINGDOM FUNGI

“Fungi are non-chlorophyllous, multicellular (except yeast) eukaryotes having fungus cellulose or chitinous cell wall, body mycelium consists of hyphae, absorptive heterotrophs.”

TAXONOMIC STATUS: (PLANT LIKE CHARACTERS) Previously fungi were regarded as plants because of the following characters:

• Having cell wall
• Lacking centriole
• Non-motile

Animal Like Characters: By showing the following characters, fungi resemble more animals than plants.

• They are heterotrophs (chloroplast is absent).
• Lack cellulose in their cell wall.
• Fungi are different from all other organisms.

They have characteristics of mitosis called nuclear mitosis, during which the nuclear membrane does not break, and the spindle is formed within the nucleus.

Conclusion: Fungi are neither completely plants nor animals and are assigned to a separate kingdom: Fungi.

THE BODY OF FUNGUS: Vegetative Structure: The fungi have a very simple body. In the majority of fungi, the body consists of a network of branched thread-like structures called the hyphae. When the hyphae form a mass, it is known as mycelium. In some lower fungi, the body does not form mycelium. They are single-celled structures, for example, Synchytrium and Saccharomyces. The cell wall of mycelium does not consist of true cellulose. It is composed of chitin or fungal cellulose.

The mycelium is of two types:

• Coenocytic Mycelium: In zygomycetes, the mycelium is unseptate and multinucleate; it is called coenocytic mycelium.
• Septate Mycelium: In Ascomycetes and Basidiomycetes, the mycelium is septate, and hyphae are divided into cells. The cells may be uninucleate or multinucleate. The cells of mycelium are filled up with colorless cytoplasm, which contains nuclei and vacuoles. The food is stored in the form of glycogen or oil; starch is absent.

NUTRITION IN FUNGI: Fungi are nonchlorophyllous and absorptive heterotrophs. They are classified into the following groups based on their mode of nutrition.

• Saprotrophs (Saprobes / Decomposers)
• Parasites
• Predators
• Symbiotic fungi

Saprotrophs: (Saprobes/Decomposers) The saprophytic fungi obtain their food from dead organic compounds. Saprobic fungi anchor to the substrate by modified hyphae called rhizoids. When rhizoids contact the substrate, they secrete digestive enzymes which digest dead organic matter and convert it into simple compounds.

Parasites: These are fungi that depend on a living host for nutritional requirements, known as parasitic fungi. They obtain food from the host through special hyphae called haustoria. Parasitic fungi may be obligate or facultative.

• Obligate Parasite: They can grow only on their living host and cannot be grown without it, e.g., mildews and rust.
• Facultative Parasites: They can grow parasitically on their host as well as by themselves on artificial growth media, e.g., Aspergillus, Trichophyton.

Predators: These fungi trap other living organisms like nematodes, protozoans, etc., and use them as food. They show many parasitic adaptations, such as Arthrobotrys traps soil nematodes by forming a constricting ring, while other predators secrete sticky substances.

Symbiotic Fungi: Symbiosis is an association between two organisms in which neither partner is harmed. Fungi form two key mutualistic symbiotic associations (both partners get benefits).

• Lichen: It is a mutualistic symbiotic association between fungi and certain phototrophs, either green algae or cyanobacterium, or sometimes both. Lichen is an important group that includes about 400 genera and 1600 species. They vary in color, shape, and overall growth form. Most of the visible part of lichen consists of fungi, while algae present within the hyphae provide protection to the algal partner from strong light and desiccation, and itself gets food from the algae. They are ecologically very important as bioindicators of air pollution.

• Mycorrhizae:

• Myco - Fungi; (Rhizae – Root)
• Some fungi form an association with the roots of vascular plants. This association is mutualistic, in which both organisms live together on mutual bases, and both get benefits.

hyphae of fungi increase the soil contact and help in the absorption of important substances, like phosphorus, zinc, copper, and other nutrients. The plant gets all these substances, and as a result of which the plant supplies organic compounds to the Fungus. In this way fungi and higher Plants live together.

There are two types of mycorrhizal fungi.

• Edomycorrhizae
• Ectomycorrhizae

Edomycorrhizae: When the hyphae of fungi penetrate into the inner side of root cells, these are called endomycorrhizal fungi, such as orchids.

Ectomycorrhizae: When the fungal hyphae surround the roots and remain between the cells, but do not penetrate into the cells, these are called ectomycorrhizal fungi, e.g., Pinus, fir, oak, etc.

Reproduction: Most fungi can reproduce asexually as well as sexually (except imperfect fungi in which sexual reproduction has not been observed).

• Asexual Reproduction: Asexual reproduction takes place by spores, conidia, fragmentation, and budding.

Spores: Spores are produced inside the reproductive structures called sporangia, which are cut off from the hyphae by complete septa. Spores may be produced by sexual or asexual processes, are haploid, non-motile, and do not need water for dispersal. Spores are a common means of reproduction in fungi.

Conidia:
Conidia (singular conidium) are non-motile, asexual spores which are cut off at the end of modified hyphae called conidiophores, and not inside the sporangia, usually in chains or clusters.

Fragmentation:
Fragmentation is simple breaking of mycelium of some hyphal fungi, each broken fragment giving rise to a new mycelium.

Budding:
Unicellular yeasts reproduce by budding (an asymmetric division in which tiny outgrowth or bud is produced which may separate and grow by simple relatively equal cell division).

Sexual Reproduction:
Details of sexual reproduction vary in different groups of fungi, but fusion of haploid nuclei and meiosis are common to all. When fungi reproduce sexually, hyphae of two genetically different but compatible mating types come together. Their cytoplasm fuses, followed by nuclear fusion of nuclei (karyogamy). In Basidiomycetes and Ascomycetes, nuclear fusion does not take place immediately after the fusion of cytoplasm (plasmogamy); instead, the two genetic types of haploid nuclei from two individuals may coexist and divide in the same hyphae for most of the life of the fungus. Such a fungal hypha/cell having 2 nuclei of different genetic types is called dikaryotic also heterokaryotic hypha/cell.

Classification of Fungi:
There are four major divisions / phyla of fungi.

• Zygomycota
• Ascomycota
• Basidiomycota
• Deuteromycota

Zygomycota:

• Zygomycota is a group of Sapnophyte fungi.
• Zygomycota is the smallest group of fungi includes about 600 species group bread molds.
• These fungi have coenocytic mycelium i.e., it is multinucleate and unseptate. The septa are formed only at the formation of sporangia or gametangia.
• In these fungi, there is complete absence of motile cells.

Asexual Reproduction:

• Asexual reproduction takes place by spores, produced in sporangia.
• Sexual reproduction takes place by the fusion of isogametes, due to which zygospores are formed, so the group is called zygomycota. e.g. Mucor, Rhizopus.

Life Cycle Of Zygomycota: In the life cycle of zygomycota, asexual and sexual reproduction occur.

Asexual Reproduction: It takes place during favorable conditions. Many erect branches arise from the mycelium, called sporangiophores. At the tip of each sporangiophore, a rounded body is formed, into which the cytoplasm, nuclei, and oil globules are transferred from the mycelium. The oil globules arrange themselves to form a layer, and the rounded body divides into two parts.

The upper larger part is known as sporangium, while the smaller lower part is known as columella. In sporangium, many spores are formed. After spores' maturation, the columella exerts pressure on the sporangium, so its wall ruptures, and spores are liberated out. The spores are very small in size, light in weight, and dry. Whenever they find food material, they germinate into new mycelium of fungus.

Sexual Reproduction: Sexual reproduction occurs by the fusion of gametangia. At the time of sexual reproduction, two hyphae of opposite strains (+ and -) come parallel to each other. Each hypha produces an outgrowth, the papilla, which divides into two cells. The lower cell is called the basal cell or suspensor cell. The upper cell is called gametangium. In each gametangium, a gamete is formed. These gametes are externally similar, so they are known as isogametes. The isogametes are fused together; this process is called conjugation. As a result of conjugation, a zygospore is formed.

Germination Of Zygospore: The zygospore consists of two layers: the outer layer is thick and spiny, called exospore, and the inner layer is thin and smooth, known as endospore. At the time of germination, the exospore ruptures, and the endospore germinates into a tube-like body, called promycelium.

At the tip of promycelium a sporangium is produced. The nucleus and reserved cytoplasm of zygospore are migrated into the sporangium. The nucleus divides by meiosis into many nuclei, then cytoplasm also divides into many pieces, each piece of cytoplasm surrounds the nucleus, in this way many spores are formed. After the maturation of spores, the wall of sporangium ruptures and all spores become free. They remain in air and by the availability of organic food substance, they germinate into new mycelium of fungus.

Ascomycota: The Ascomycetes (Gr. askos, bladder; myles, fungi) or the sacfungi constitute a class of fungi. They include about 2,500 genera and 35,000 species which have to almost half of the entire number of Fungi. Among the known examples of Ascomycetes are: the yeasts, the common green and blue molds, the powdery mildews, the morels, the cup-fungi and the truffles.

Occurrence: Ascomycetes occur in variety of habitats almost throughout the year. Some of them are parasitic. Many members cause serious plants-diseases, such as peach leafcurl, brown rot of stone fruits apple scab, bitter-rot of apple, powdery mildews ergot of rye. Others are saprobic (saprophytic). A few are hypogeal (Gr. hypo, below; ge, earth) and stain permanently underground. A large majority of the Ascomycetes are coprophilous (Gr. kepros, dung; philein, to love), growing on dung of certain animals. Fruiting season is short, lasting for three or four weeks.

Somatic Structure: The mycelium is usually well developed and profusely branched except in yeasts (Saccharomycetes), which are unicellular and where the mycelium is absent. The hyphae are septate each cell may be uninucleate or multinucleate. The septa, however, perforated and permit the streaming of the protoplasm from cell to cell. The cell wall contains a large amount of chitin. The mycelium in the fruiting bodies is often organized into fungi tissues, which may prosenchymatous or pseudoparenchymatous. In some, the hyphal cells separate with age forming oidia or become thick-walled, forming chlamydospores.

Asexual Reproduction: Asexual reproduction takes place by means of conidia, spores cut off by septa at the end of modified hyphae called conidiophores. Many conidia are multinucleate.

Sexual Reproduction:
In Ascomycota sexual reproduction takes place by the formation of male and female reproductive organs. The male organs are called antheridia and female are called ascogonia. They are usually developed at the apex of hyphae. The ascogonium forms a special structure, called trichogyne. It acts as a fertilization tube through which male nuclei from the antheridium is transferred into ascogonium. By the fusion of male and female nuclei zygote is formed.

Formation Of Ascus:
In Ascomycetes when male and female gametes are fused together, zygote is formed. This zygote is changed into a special structure, called ascus. Sometimes ascogonia of certain species are changed into asci without fertilization. In the lower fungi of Ascomycetes the zygote nucleus divides by meiosis and forms 4 to 8 nuclei, which form ascospores. At maturity the ascus bursts at certain point and ascospores are liberated.

Ascocarp:
Ascocarps or fruity bodies are made up tightly interwoven monokaryotic and dikaryotic hyphae. Within an ascocarp, on special fertile layer of dikaryotic hyphae, the asci are formed. The ascocarps of the cup fungi and the morels are open, with the asci lining the open cups called apothecium, other ascocarps are closed or have a small opening at the apex called cleistothecium and perithecium respectively; the ascocarps of neurospora, an important organism of genetic research, are of this latter kind.

Yeast: (Saccharomyces)
Systematic Position:
Class - Ascomycetes
Sub-class - Protoascomycetes (Hemiascomycetes)
Order - Endomycetales
Family - Saccharomycetaceae
Genus - Saccharomyces

Occurrence And Habit:
The yeasts first described by Leuwenhoek in 1680, are the simplest and the most widely distributed of the Ascomycetes. They are unicellular and particularly abundant in sugary substances, containing small amounts of nitrogen and sulphur. Such as the nectar of flowers, the surface of fruits like grapes, date-palm (khajoor), sugar-cane, as also fruit juices, exuded sap of plants and the soil of wine-yards, etc. Some of them are pathogenic on plants causing leaf-curl diseases of various species, while others are even pathogenic on man and animals.

Fermentation:
Yeasts obtain energy by a process known as alcoholic fermentation of sugar. During the process, the yeast cells absorb sugar and decompose it by enzyme (zymase) with the formation of carbon dioxide and alcohol (ethyl) with traces of glycerine, succinic acid and acetic acid, as shown by the following equation.

$\text{C}_6\text{H}_{12}\text{O}_6 + \text{zymase} \rightarrow 2\text{C}_2\text{H}_5\text{OH} + 2\text{CO}_2$
Ethyl alcohol | Carbon dioxide

Fermentation takes place when there is insufficient supply of oxygen.

Economic Importance: Yeast is particularly useful because of their ability to ferment carbohydrates; hence the name Saccharomycetes. Saccharomyces cerevisiae (The Beer Yeast) is the best-known species extensively cultivated in bakeries, breweries, and distilleries. Alcohol and carbon dioxide, brewers and bakers use yeasts extensively in their industries. Different yeasts give different flavors to the alcohol, cider, wine, beer, ale, brandy, rum, and other liquors.

Because of their high vitamin contents (e.g., vitamin B, B2, and G) and digestible compounds, especially proteins, fats, carbohydrates, enzymes, etc., yeasts are particularly valuable as food (e.g., Cryptococcus), as also in the treatment of certain skin and intestinal diseases. Some species of yeast cause food spoilage, giving yeasty flavor, while others are pathogenic, causing plant diseases, of tomato fruits (Nematospora lycopersici), garden beans (Nematospora phasoli), cotton (Nematospora gossypii). Several species of the genus Torula are pathogenic on man, causing several diseases, best known of which is blastomycosis or torulosis, which attacks the skin and central nervous system; yeast vaginitis causes vaginal thrash, while “cryptococcosis and moniliasis” are caused by Cryptococcus neoformans and Candida albicans. Cryptococcosis involves the central nervous system and may cause meningitis, abscess, or brain tumor, while Moniliasis involves nails and cuticle but sometimes also involves pericardium of heart and lungs.

Somatic Structure: Like other Ascomycetes, the mycelium in yeast is not a branched filamentous structure but consists of isolated cells. The cells of Saccharomyces are very minute, measuring 1/100th of a millimeter (10µ) and are either spherical, elongated, rectangular, or slightly oval. They possess a definite cell wall, which is made up of polysaccharides, chitin, phosphoric acid, and glycogen. Inside is the protoplasm which in the older cells is differentiated into an outer ectoplasm and an inner endoplasm. The endoplasm surrounds a large central (nuclear) vacuole, which represents the nucleus, traversed by chromatin fibrils. The nucleus contains a normal nucleolus on one side, but the chromatin matter is relatively small.

According to the most recent view, the enlarged nuclear vacuole encloses a small nucleolus, which is connected to the centrosomes by means of chromosomes. The centrosome lies at one end and is surrounded by centrioles and heterochromatin. Other inclusions such as mitochondria, starch grains, oil globules, glycogen grains, and grains of volition (which is ribonucleic acid - RNA or one of its salts) are embedded in the ectoplasm.

Though ordinarily unicellular, the yeast cells sometimes adhere in chains, forming a pseudomycelium. When grown on gelatin, individually, the yeast cells are colorless, but when grown on solid media, they form colonies, which may appear white, cream-colored, or brownish. These colony characteristics are used in classifying yeasts.

Nitrogen: Yeasts, like other fungi, lack chlorophyll and are, therefore, heterotrophic in their mode of nutrition. They are mainly saprophytic and live in sugary substances containing small amounts of nitrogen and sulfur. The yeast cells manufacture two types of enzymes, which completely destroy the sugar in the surrounding medium. One of these enzymes is invertase, an extracellular enzyme, which passes out of the other enzyme, zymase, which remains within the yeast cell and converts invert sugar into alcohol and carbon dioxide.

Vegetative Reproduction: Vegetative reproduction takes place by:

• Budding:
  In S. cerevisiae, the common bread yeast, the vegetative reproduction takes place by budding. When the food supply is in abundance, the mother cell gives rise to a small outgrowth, which gradually increases in size till it is as large as the mother cell itself. Sometimes, a bud may repeat the process of budding a number of times.

• Fission:
  In some species, such as Schizosaccharomyces octo-sporus, which is very common on grapes, the vegetative multiplication takes place by fission or cell.

Division:
During fission, the nucleus of the mother cell divides into two daughter nuclei, and this is followed by a transverse cytokinesis that results in the formation of two daughter cells of approximately equal sizes.

Sexual Reproduction:
This occurs under conditions unfavorable for active vegetative growth, such as scarcity of food or water. Sexual union in yeasts takes place either between two somatic cells or between two ascospores, which assume the function of gametes. Two gametes unite and form a diploid zygote cell. Eventually, the zygote forms an ascus, which contains four to eight or more ascospores of variable shapes, though often globose or oval. The resulting ascospores function directly as new yeast cells.

Basidiomycota:
The basidiomycetes have about 16,000 named species. More is known about some members of this group than about any other fungi. Among the basidiomycetes are not only the mushrooms, toadstools, puffballs, jelly fungi, and shelf fungi, but also many important plant pathogens among the groups called rusts and smuts. Many mushrooms are used as food, but others are deadly poisonous. Still other species are poisonous to some people and harmless to others.

Vegetative Structure:
The mycelium of basidiomycota fungi is advanced. It is branched and septate. The cell wall is composed of chitin. The mycelium is intracellular or intercellular. Each cell contains cytoplasm and nucleus. The mycelium with uninucleate cells, called primary mycelium (monokaryotic mycelium), and with two nuclei is called secondary mycelium (dikaryotic mycelium).

Life Cycle of Basidiomycota:
In this division of fungi, the reproductive bodies are called basidiospores. These spores are produced by special organs, called basidia. In basidiomycetes, new mycelium is formed by the germination of basidiospores. In the beginning, the mycelium is uninucleate or monokaryotic, called primary mycelium. After some time, two different hyphae come in contact. The cytoplasms of the cells are fused together while nuclei are not fused. This is called plasmogamy. By this process, dikaryotic mycelium is formed. It contains two nuclei in each cell and is also called secondary mycelium. The dikaryotic mycelium takes part in reproduction.

Formation of Basidia and Basidiospore:
At the time of reproduction, special spores are produced in the cells of dikaryotic mycelium, called chlamydospores or teliospores. These spores remain inactive for a particular time, after that they start germination. The two haploid nuclei of chlamydospore fuse together to form a diploid nucleus. This process is called karyogamy. The chlamydospores form a tube-like structure, called basidium. The diploid nucleus divides into four nuclei. The basidium divides into four cells; each nucleus is migrated into each cell. From each cell of basidium, a small stack arises, called sterigma, the tip of sterigma becomes swollen, the nucleus of cell is migrated into this swollen end, it is called basidiospore. In this way, four basidiospores are formed. After maturation, the basidiospores are separated from sterigma. They can germinate during favorable conditions and develop into mycelium.

Deuteromycota (Fungi Imperfecti):
Most of the fungi imperfecti are Ascomycota that have lost the ability to reproduce sexually. The fungi that are classified in this group, however, are simply those in which the sexual reproductive stages have not been observed. There are some 17,000 described species of fungi imperfecti. Even though sexual reproduction is absent among fungi imperfecti, there is a certain amount of genetic recombination. This becomes possible when hyphae of different genetic type fuse, as sometimes happens spontaneously. Within the heterokaryotic hyphae that arise from such fusion, genetic recombination of special kind called parasexuality may occur. In parasexuality, the exchange of portions of chromosomes between their genetically distinct nuclei within a common hypha takes place. Recombination of this sort also occurs in other groups of fungi and seems to be responsible for some of the production of new pathogenic strains of wheat rust.

Among the economically important genera of fungi imperfecti are penicillium and aspergillus. Some species of penicillium are sources of the well-known antibiotic penicillin, and other species of the genus give the characteristic flavors and aromas to cheese. Species of Aspergillus are used for fermenting soya sauce and soya paste processes in which certain bacteria and yeast also play important roles. Citric acid is produced commercially with members of this genus under highly acidic conditions. In addition, the enrichment of livestock feed by the products of fermentation of other species is being investigated.

Land Adaptations of Fungi:
Following characteristics enable fungi to live on land.

• Hyphae that absorb water and soluble nutrients also anchor the plant.
• Thick-walled drought-resistant spores are produced in large numbers.
• Instead of forming gametes having flagella, special gametes are produced which need no water for fertilization.

Economic Importance:
In many ways, bacteria and fungi are similar in their importance to man. Like bacteria, fungi show both harmful and useful activities to human beings.

Useful Fungi:

• Food:
  Many kinds of edible fungi in the form of mushrooms are a source of nourishing and delicious food dishes. Some of them are poisonous and popularly called toad-stools or death-stools. Yeasts, another kind of fungi, are utilized in the baking industry. Others are used in brewing and in cheese and organic acid-producing industries.

• Medicines:
  Fungi have explored a new field in medicine by producing antibiotics like penicillin, chloromycetin, terramycin, neomycin, etc.

• Soil Fertility:
  They maintain soil fertility by decomposing the dead organic matter, e.g., Mycorrhizal fungi.

Harmful Fungi:

• Food Spoilage:
  Fungi cause tremendous amounts of spoilage of foodstuff by many of the saprophytic fungi.

• Human Diseases:
  Fungi cause a number of diseases in human beings like aspergillosis (ear, lungs, disease), moniliasis (skin, mouth, gums disease). Most of the fungi that cause skin diseases in humans, including athlete’s foot and ringworm, are also fungi imperfecti.

• Plant Disease:
  Fungi destroy many agricultural crops, fruits, ornaments, and other kinds of plants. Some of the diseases are loose smut of wheat, downy, and powdery mildews, etc.

• Spoilage:
  Many fungi spoil leather goods, wool, books, timber, cotton, etc.

Economic Losses Due to Fungi:
Fungi are responsible for many serious plant diseases because they produce several enzymes that can break down cellulose, lignin, and even cutin. All plants are susceptible to them. Extensive damages due to rusts and smut diseases of wheat, corn, and rice prompted mass displacement and starvation to death of many people. Powdery mildews (on grapes, rose, wheat, etc.), ergot of rye, red rot of sugarcane, potato wilt, cotton root, apple scab, and brown rot of peaches, plums, apricots, and cherries are some other common plant diseases caused by fungi.

Fungi also cause certain animal diseases. Ringworm and athlete’s foot are superficial fungal infections caused by certain imperfect fungi. Candida albicans, a yeast, causes oral and vaginal thrush. Histoplasmolysis is a serious infection of the lungs caused by inhaling spores of a fungus.

which is common in soil contaminated with bird’s feces. If infection spreads into the bloodstream and then to other organs (which is very occasional), it can be serious and even fatal. Aspergillus fumigatus causes aspergillosis, but only in a person with a defective immune system, such as AIDS, and may cause death. Some strains of Aspergillus flavus produce one of the most carcinogenic (cancer-causing) mycotoxins (toxins produced by fungi), called aflatoxins. Aspergillus contaminates improperly stored grains, such as peanuts and corn, etc. Milk, eggs, and meat may also have small traces of aflatoxins. Any moldy human food or animal forage product should be discarded. Ergotism is caused by eating bread made from purple ergot-contaminated rye flour. The poisonous material in the ergot causes nervous spasms, convulsions, psychotic delusion, and even gangrene.

Saprobic fungi are not only useful recyclers but also cause incalculable damage to food, wood, fiber, and leather by decomposing them. 15-50% of the world’s food is lost each year due to fungal attack. Wood-rotting fungi destroy not only living trees but also structural timber. Bracket/shelf fungi cause lots of damage to stored cut timber as well as stands of timber of living trees. A pink yeast (Rhodotorula) grows on shower curtains and other moist surfaces.