Fungi

 

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Fungi

Fungi come from Latin word 'fungus' which means mushroom. Fungi are achlorophyllous, (saprophytic, parasitic, ,symbiotic), thallophytic, eukaryotic and spore bearing organisms. The cell is surrounded by a well defined cell wall which is made up of chitin with fungal cellulose. The reserve food material is in the form of glycogen or fat. Due to the absence of chlorophyll pigment, they cannot perform photosynthesis. The branch of science which deals with the study of fungi is called mycology.

 The founder of mycology - Pier Antonio Micheli (1729). 

 Father of modern mycology - Anton de Barry.

Characters of fungi: 

1. Fungi are multicellular (except yeast), eukaryotic, non chlorophyllous, non vascular, non-flowering, heterotrophic, spore bearing, organisms. 

2. The plant body is thallus.

3. The branched filamentous body of fungi is called mycelium. The unit of mycelium is called hypha but not cell. 

4. Chlorophyll pigments are absent. 

5. They have heterotrophic modes of nutrition. They may be saprophytic, parasitic or symbiotic. 

 6. Cell wall is made up of fungal cellulose or chitin. 

 7. The reserve food material is in the form of glycogen and oil. starch is absent. 

8. They reproduce by vegetative, asexual and sexual methods. 

 9. The vegetative reproduction takes place by fragmentation, budding, fission, etc.

10. Asexual reproduction takes place by the formation of different types of spores likes chlamydospores, oidia, zoospores, conidia, sporangiospore etc. 

11. Sexual reproduction takes place by gametangial copulation, somatogamy, spermatogamy etc. 

12. Sexual reproduction involves three events that are plasmogamy, karyogamy and meiosis. 

 13. The sex organs are usually unicellular. 

14. Sexual spores are oospores, ascospores, basidiospores, zygospores etc. 

15. Embryo is not formed after fertilization. Examples: Mucor, Rhizopus, Yeast, Albugo, Aspergillus, Agaricus, Alternaria etc.

Classification of fungi: 

Fungi are classified into four classes. 

1. Phycomycetes: It is further classified into two sub-classes.

 a. Oomycetes (water mould):

   ● Mostly parasitic

   ● Multinucleated (coenocytic), aseptate, hyphae.

   ● Asexual spore zoospores,

   ● Sexual diploid spore oospores,

   ● Eg , Phytophthora, Albugo. 

b. Zygomycetes (conjugation fungi): 

  ● Mostly saprophytic 

  ● Mycelium aseptate 

  ● Multinucleated 

  ● Asexual spores are chlamydospores, oidia, 

  ● Sexual spores are diploid zygospores.

2. Ascomycetes ( sac fungi): 

  ● Mostly terrestrial, saprophytic or parasitic, 

  ● Presence of branched, septate mycelium (except yeast) 

  ● Asexual reproduction by conidia, oidia, chlamydospores, 

  ● Presence of ascus (special types of sporangia) which contains ascospores (usually 8) 

  ● Sexual reproduction takes place by gametangia 

  ● Eg., Penicillin, Aspergillus, yeast etc. 

3. Basidiomycetes (club fungi): 

  ● Mostly saprophytic 

  ● Presence of septate, branched, mycelium. Mycelium is of two types i.e, primary mycelium and secondary mycelium. 

  ● Primary mycelium (monokaryotic) is formed by germination of basidiospores. 

  ● Secondary mycelium (dikaryotic) is formed by the fusion of two primary mycelium 

  ● Asexual reproduction by budding, oidia, conidia, chlamydospores 

  ● Sexual reproduction by conjugation of two nuclei. 

  ● Four basidiospores are developed exogenously from basidium 

  ● Examples mushroom, rust (Puccinia), smut (Ustilago). 

4. Deuteromycetes( Fungi imperfecti): 

   ● Saprophytes or parasites 

   ● Due to the absence of sexual reproduction, (perfect stage),they are known as fungi imperfecti.

   ● Mycelium is well developed, branched, septate, and multinucleated. 

   ● Asexual reproduction takes place by chlamydospores, conidiospores,

   ● Examples: Alternaria, Fusarium etc.

Mucor 

 The members of mucor are found in moist leather, bread, jam, rotten vegetables, fruits etc. They form small masses of white cottony threads. The mycelium of Mucor bears loosely arranged, aseptate coenocytic hypha. Mycelium bears basically three types of hypha that is prostate (grow horizontally on outer surface of substratum), subterranean (penetrates the substratum and absorb their requirements) and erect which grow vertically upward and they take part in asexual and sexual reproduction. 

 Hyphae of mucor is bounded by double layered covering in which outer covering is cell wall which is made up of fungal cellulose or chitin. The cell membrane is found inside the cell wall. cytoplasm is a coenocytic that has many nuclei. Additionally it bears small vacuoles, reserve food material in the form of glycogen as well as other some cell organelles like mitochondria, golgi body, ribosomes, endoplasmic reticulum etc.

Reproduction 

Mucor reproduce by 

1. Vegetative reproduction 

2. Asexual reproduction 

 3. sexual reproduction 

Vegetative reproduction: It takes place by fermentation. During unfavorable conditions the mycelium of Mucor breaks into small segments called fragments. On coming to a favourable condition each fragment gives new mycelium. 

Asexual reproduction: In mucor asexual reproduction takes place by following different methods. 

1. Sporangiospores 

2. Chlamydospores 

3. Oidia 

Sporangiospore: It is very common method of asexual reproduction. Sporangiospore are non-motile spores developed in sporangium. During this reproduction some erect hyphae of mucor are specialised called sporangiophore. Large numbers of nuclei, reserve food material with other cytoplasmic content, are migrated to the tip of such hypha. So a globular vessel-like structure is formed at the tip of hyphae called sporangium. The sporangia get differentiated into the peripheral fertile portion called sporoplasm and inner sterile, vacuolated, dome shaped portion called columella. Nuclei of sporoplasm divide to form large numbers of nuclei. The cleavage occurs in Sporoplasma and a large number of multinucleated spores are formed called sporangiospore. These spores become free after breaking down the wall of sporangium. After getting a favourable condition each spore germinates into a new hyphae of mucor.

Chlamydospores: 

 Chlamydospores are thick walled resistance spores formed during unfavourable conditions. During such conditions, mycelium of mucor becomes septate and protoplasts accumulate to form round and thick walled structures i.e., chlamydospores. After getting a favourable condition each chlamydospores germinate into new hyphae of mucor.

Oidia:

Oidia are thin walled spores formed by certain aquatic species of mucor in high concentration of sugar or with higher acidic medium. The chain-like structure of oidia is called torula stage.

Sexual reproduction: 

 Sexual reproduction in mucor occurs by the fusion of opposite stained hypha. Two types of species are found in mucor called heterothallic and homothallic. In heterothallic species, conjugation occurs between the opposite stained hypha of two mycelium but in homothallic species conjugation occurs between the branches of same mycelium. During conjugation two opposite stains come very close to each other and develop progametangia opposite to each other. Many nuclei and cytoplasm migrate to progametangia. Then each pro-gametangium is divided into two parts in which apical multinucleated part is gametangium and basal stalk cell is called suspensor. Then fusion of the contact wall of gametangia occurs and plasmogamy is followed by karyogamy. Thus, many diploid zygospores are formed.

After some time, meiosis occurs in these diploid nuclei and many haploid nuclei are formed. Among these haploid nuclei only one is survival, but others are degenerated. This survival haploid nucleus multiplies by repeated mitotic division and a mass of haploid nuclei are formed. The zygospore wall breaks and a swollen portion comes out in the form of a germ tube called promycelium. At the last tip of promycelium swells up called sporangium. Large numbers of spores are formed inside sporangium. These spores become free after breaking the wall of sporangium. In suitable condition, each spore germinates into new hyphae.

 

Yeast 

 Yeast is unicellular, microscopic, non- mycelial, oval or spherical, saprophytic fungus. It takes part in fermentation of sugar to alcohol and CO2. It mostly grows in rotting fruits, sweets, nectarines of flowers, cane juice, organic decaying matter etc. 

 Cell is bounded by double layered covering called the cell wall and cell membrane. The cell wall is made up of chitin. Inner to the cell membrane there is protoplasm. The protoplasm consists of nucleus, vacuole, cell organelles like mitochondria, golgi bodies, endoplasmic reticulum, ribosomes, reserved food material (glycogen, oil globules) in cytoplasm. Nucleus lies at one end of the vacuole.

Reproduction: 

 Two types of reproduction are found in yeast. 

1. Asexual reproduction 

2. Sexual reproduction 

Asexual reproduction: it occurs by following ways. 

  a. Budding 

  b. Fission 

  c. Endospores 

a. Budding: It is a common method of vegetative reproduction. Under favourable conditions, the yeast cell gives rise to one or more outgrowths which gradually increase in size. The nucleus divides and migrates to the outgrowth that forms a bud. The bud separates from the parent cell and becomes independent. Sometimes due to repeated budding, yeast appears in one or more chains.

b. Fission: During fission, the yeast cell elongates and it's nucleus divides into two. These nuclei move apart. The cell wall is formed in the middle of the cell and divided into two cells.

c. Endospores: Endospores are thick walled resistant spores formed during unfavourable conditions. During such conditions the protoplast of yeast is divided into 4 parts and each part secretes a wall and behaves as endospores. After getting favourable conditions, they come out by breaking the wall of the mother cell, and germinate into new yeast cells.

Sexual reproduction: 

In yeast sexual reproduction occurs by the method of conjugation. It starts at the end of the growing season. On the basis of the technique of sexual reproduction, the life cycle is divided into 3-types.

 1.)Haplobiontic life cycle (Saccharomyces octosporus) During this type of life cycle the diploid phase is very short while the haploid phase is dominant and elaborated. 

 At the time of sexual reproduction two haploid somatic cells come together. A conjugation tube is formed between these two cells.The nuclei of both cells fuse in the conjugation tube forming a diploid zygote nucleus. The zygote cell acts as an ascus. This diploid nucleus divides by meiosis and finally forms 8- haploid ascospores.These ascospores are released by the rupture of the ascus wall and each ascospore germinate into haploid somatic cells which also divide by fission.

2.) Diplobiontic life cycle (S. ludwigii ) 

 In this life cycle the haploid phase is very short and separated only by ascospores which also remain within the ascus and soon undergoes fusion to form diploid zygote.

Somatic cells are diploid which multiply by budding. Finally its somatic cells directly function as ascus. It’s nucleus divides by meiosis forming four haploid nuclei. Each nucleus is converted by cytoplasm to form ascospores. Therefore 4-ascospores are formed in each ascus. Fusion of two 2-ascospores takes place inside the ascus forming diploid zygote cell. The diploid zygote germinates by a germ tube which rupture the ascus wall. The germ tube becomes diploid mycelium from which diploid buds are detached. During favourable conditions each diploid cell acts as ascus. It divides by meiosis to form a haploid ascospores. Thus the diplobiontic life cycle is completed.

 3.)Haplo-diplobiontic life cycle (S. cerevisiae) 

 In this type of life cycle haploid and diploid phases are equally important. Generally haploid cells reproduce by budding under suitable conditions. Two somatic cells fuse to form zygote. This diploid zygote multiplies by budding producing diploid cells for several generations. When there is shortage of food, water and suitable temperature, these diploid cells behave as asci. The diploid nucleus divides by meiosis forming four haploid daughter nuclei which are covered by cytoplasm to form ascospores. These ascospores rupture the ascus wall, release and germinate to produce new haploid cells of yeast.

Economic importance of Fungi 

 Economically, fungi are very much important. Some activities are highly useful to humans and other organisms but some other activities are highly harmful. Some of them are discussed below.

Useful activities: 

1.)As food, some members of fungi are used as food. E.g. Agaricus campestris, A. bisporus, Amanita vaginata, Morchella etc.

 

2.)As antibiotic: Some member of fungi are used for the preparation of some antibiotics:

  Antibiotics                Fungi 

 Penicillin                 Penicillium notatum

 Proliferin                 Aspergillus proliferans

 Ergotine                   Clavicepes purpurea

3.)In industry, some fungi are used in industry for the preparation of different products. 

 i) Saccharomyces cerevisiae are used in breweries for the preparation of alcoholic beverages like beer, wine, whiskey, etc. 

 ii.) Penicillium camembertile is used to give specific flavor to cheese. 

iii. Enzymes production: some enzymes are produced from fungi like;

 Fungi.                           Enzymes . 

Saccharomyces            Invertase

Rhizopus.                     Pectic enzyme

Aspergillus.                  Amylase

4. In agriculture: 

The saprophytic members of fungi breakdown dead and decaying organic matter into simple inorganic forms which are absorbed by plants as their basic requirements. Mycorrhiza fungus, associated symbiotically with roots of higher plants, supplies minerals to plants.

5. Growth hormone: plant growth hormone Gibberellin produced from fungus Gibberella fujikuroi.

6. Organic acid production: fungi are used for the production of various organic acids like;

      Fungus.                  Organic acid

Aspergillus niger.        Citric acid

Aspergillus niger.        Oxalic acid

Rhizopus oryzae.         Lactic acid

 

Harmful activities: 

Some activities of fungi are highly harmful to both plants and animals. Some are

1. Food spoilage: Some saprophytic fungi (Mucor, Rhizopus, Aspergillus, Fusarium) cause the spoilage of food material. 

 2. Destruction of lather, paper, clothes: Some saprophytic fungi (Mucor, Rhizopus, Aspergillus, Alternaria, Fusarium etc) cause destruction of paper, lather, clothes etc. 

3. Contamination of optical instruments: The fungus Helminthosporium causes the destruction of optical instruments. 

4. Diseases to human beings: Some fungi cause different diseases to human beings.

     Diseases.                       Fungi

  Ringworm of skin.          Trichophyton rubrum

  Cryptococcosis.             Cryptococcus neoformans

5. Diseases to plants: 

   Disease.                                   Pathogen

  Early blight of potato.          Alternaria solani

  Late blight of potato            Phytophthora infestans

  White rust of crucifers.         Albugo candida 

  Rust of wheat.                     Puccinia graminis

6. Poisonous fungi: Some fungi are deadly poisonous to humans and other animals. Some examples are Helvella spp, Amanita phalloides.

Mushroom 

 Mushroom is a saprophytic fungus with basidiocarp as a fruiting body and forms basidiospores on basidium. It grows on rotten wood, decaying organic matter, moist straw, soil rich in organic substances etc. Only edible species are commonly called mushrooms, and the poisonous species are known as toadstools.

Structure: 

 Vegetative part lie below the soil surface which includes primary mycelium and secondary mycelium. The primary mycelium is branched, septate and monokaryotic (uni-nucleated) which is formed by the germination of basidiospores. It is short lived. 

 Secondary mycelium is branched, filamentous formed by fusion of two primary mycelium. It is dikaryotic (binucleated). Badidiocarp (fruiting body) is produced by secondary mycelium. Reproductive part is the basidiocarp (fruiting body). It consists of rhizomorph, stipe and pileus.

 Rhizomorph: the secondary mycelium below the soil surface is called rhizomorph. Stipe is short, stalk. Velum is present in the form of a ring called annulus. Pileus is the fertile, uppermost, umbrella shaped cap. Large numbers of gills (lamellae) are present on the lower side of pileus. Each gill consists of trama, sub- hymenium and hymenium. The hymenium is outermost layer consists of basidium. The two nuclei of each basidium fuse to form diploid nucleus which divide by meiosis to form four haploid nuclei called basidiospores. Later basidiospores lie exogenously on sterigma.

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