Fungi

Introduction

The organisms of kingdom Fungi (singular: fungus) are eukaryotic, unicellular or multicellular, feed by absorption of nutrients from dead or decaying materials (saprophytic heterotrophs) or are parasitic to plants and animals or may form symbiotic relationship, mycorrhiza with higher plants and lichens with algae. True fungi have cell walls composed primarily of N-acetylglucosamine polymer, called chitin; glucans and mannans are also present in the cell wall. The unicellular fungi (Yeast) are oval in shape and the multicellular fungi (molds) form visible mycelia of long filamentous structure called hyphae. The study of fungi is called mycology.
Examples of a few forms of fungus:

  • Yeast (Saccharomyces) is unicellular fungi and mushroom (Agaricus) is multicellular.
  • Claviceps purpurea causes ergotism (St. Anthony’s fire) is parasitic on human.
  • The mycelia of oyster mushrooms (Pleurotus) secrete adhesives onto their hyphae in order to catch nematodes.

Characteristics of Fungus

Vegetative Characteristics

  • Fungal colonies are described as vegetative structure because the cells of the colony are involved in catabolism and growth.
  • The thallus (body) of a multicellular fungus consists of long filaments of cells joined together; these filaments are called hyphae (singular: hypha) which can grow immensely.
  • The hyphae may have cross-walls called septa (singular: septum; Ex: Ascomycetes and Basidiomycetes). These hyphae are called septate hyphae which may be uninucleated or multi-nucleated. The other type of hyphae does not contain septa and are called aseptate hyphae; they contain many nuclei, hence also called coenocytic hyphae (Ex: Zygomycetes).
  • The hyphae grow to form a filamentous mass called mycelium.
  • The portion of a hypha that obtains nutrients is called the vegetative hypha and the portion involved in reproduction is called reproductive or aerial hypha.

Reproductive Characteristics

  • Fungi reproduce by both asexual and sexual means.
  • Asexual and sexual reproduction occur by spore (asexual and sexual spore) formation.
  • The filamentous form reproduces asexually by fragmentation.
  • Asexual spores are formed by hyphae of one organism. These spores germinate to produce genetically similar organisms. Asexual spores are of different kinds. These are:
    Conidiospore, or conidium unicellular or multicellular spore that is not enclosed in a sac. Conidia are produced in a chain at the end of a conidiophore. Ex: Aspergillus.
    Fragmentation of septate hypha produces anthroconidia (Ex: Coccidioides immitis), conidia formed from buds coming out from parent cell is called blastoconidia (Ex: Candida
    albicans) and thick-walled hyphal segment form conidia called chlamydoconidia (Ex: C. albicalls).
    The other kind of asexual spore is called sporangiospore, formed within a sporangium (sac), at the end of an aerial hypha called a sporangiophore (Ex: Rhizopus).
  • Sexual spores are formed by nuclear fusion of two opposite mating strains of same species. These sexual spores germinate and give rise to organism which share genetic characteristics with both parental strains. Sexual spore forms in three steps:
    Step 1: Plasmogamy– A haploid nucleus of a donor cell penetrates the cytoplasm of a recipient cell. Fusion of two cells may form a temporary dikaryophase (as in ascomycetes and basidiomycetes). The gamete-producing bodies called gametangia.
    Step 2: Karyogamy– The nuclei of donor and recipient fuse to form a diploid zygote nucleus.
    step 3: Meiosis– The diploid nucleus gives rise to haploid nuclei (sexual spores), some of which may be genetic recombinants.

Nutrition

  • Fungi are chemoheterotrophs, and they absorb nutrients from decaying materials.
  • Almost all molds are aerobic, and most yeasts are facultative anaerobes.
  • They can grow at relatively high salt or sugar concentration.
  • Fungi require less nitrogen as compared to bacteria.
  • Fungi are often capable of metabolizing complex carbohydrates, such as lignin (a component of plant cell wall).
  • Glycogen is the primary storage polysaccharide in fungi.

Classification of Fungi

On the basis of mycelium, spore formation and division fungi are divided into four – divisions.

Division Zygomycota (Zygomycetes or Phycomycetes)

  • Members of this group are also called algal fungi or zygote fungi or water molds.
  • Most zygomycetes live on decaying plant and animal matter in the soil; a few are parasitic to plants, insects, animals, and humans.
  • The hyphae are coenocytic and have many haploid nuclei.
  • Asexual spores develop in sporangia at the tips of aerial hyphae and usually dispersed by wind.
  • In sexual reproduction, tough and thick-walled zygotes called zygospores is produced by fusion of two gametes. If the gametes are similar in morphology, then called isogamous or if dissimilar then called anisogamous or oogamous.
    Ex: Rhizopus stolonifer, the bread mold, grows on the surface of moist, carbohydrate-rich foods, such as breads, fruits, and vegetables. Mucor spp. is used with soybeans in the Orient to make a cheese called sufu. Mucorales causes zygomycosis disease in human.

Division Ascomycota (Ascomycetes, Sac fungi)

  • It includes molds with septate hyphae and some yeasts.
  • The ascomycete growing on dung are called coprophilous.
  • The asexual spores are usually conidia produced in long chains from the conidiophore. The conidia germinate to produce hyphae.
  • Sexual reproduction; an ascospore is formed from the fusion of the nuclei (morphologically similar or dissimilar) of two cells. These spores are produced in a sac-like structure called an ascus. The ascus arranged on ascocarp. The ascospore divide by meiosis and then mitosis. The ascus then opens to release spores which germinate to give hyphae.
    Ex: Neurospora crassa, the pink bread mold, used as research tool in genetics and biochemistry.
  • Claviceps purpurea parasitizes rye and other grasses, causing the disease ergot. Morels and buffles are edable.
  • Penicillium synthesize the well-known antibiotics penicillin and griseofulvin. Many Penicillium spp redifined as a new genus Talaromyces.
  • Aspergillus spp are pathogenic to plants or animals.
  • Some are useful to us like Aspergillus niger used to produce citric acid.
  • Microsporum canis causes ring worm.

Division Basidiomycota (Basidiomycetes, Club fungi)

  • The Basidiomycetes have septate and branched hyphae.
  • Most Basidiomycetes are saprophytes.
  • Asexual reproduction occurs by fragmentation. Asexual spores are generally absent. Some of the Basidiomycota produce asexual conidiospores.
  • Sexual reproduction involves a special structure called basidium, produced at the tip of hyphae. The basidium is club shaped hence its name club fungi. Two or more basidiospores are produced by the basidium. The basidiospores released germinate to form mycelium and then form complete fungi.
    Ex: Agaricus campestris (mushroom) have economic importance.
    Amanita phalloides, destroying angel mushroom produces phalloidin and α-amanitin toxins.
    Cryptococcus neoformans is pathogenic to human causes the disease called cryptococcosis, a systemic infection primarily involving the lungs and central nervous system.

Division Deuteromycota (Deuteromycetes, Fungi imperfecti)

  • When a fungus lacks the sexual phase (perfect stage), or it has been not observed, it is placed within the division Deuteromycota. That is, only asexual phase is known.
  • Most Fungi Imperfecti are terrestrial, a few are freshwater and marine.
  • The majority are either saprophytes or parasites of plants.
  • The mycelium is septate and branched.
  • The asexual spores are called conidia.
    Ex: Trichoderma is used commercially to produce the enzyme cellulase.
    Colletotricum causes leaf blight, shot hole, and irregular leaf spot in plants.
    Trichoderma spp are beneficial soil microorganisms used in sustainable agriculture. 

Mycorrhizae

  • It is a symbiotic relation of fungus and plant root.
  • Discovered by Albert Bernhard Frank in 1885.
  • These include both non-septate and septate fungi.
  • There are two main types of mycorrhizae. First, ectomycorrhiza and second, endomycorrhiza.
  • Ectomycorrhiza occurs on the outer surface of plant root. The fungi Cennococcum, Pisolithus and Amanita form such association with roots of pine tree.
  • Fungal hyphae penetrate the outer cortical cells of the plant root, where they grow intracellularly. Endotrophic mycorrhizae are found in wheat, corn, beans, tomatoes, apples, oranges, and many other commercial crops. The zygomycetes main form this kind of association.

Lichen

  • Lichens are the association between specific ascomycetes fungi and certain genera of either green algae or cyanobacteria.
  • In a lichen, the fungal partner is termed the mycobiont, and the algal or cyanobacterial partner, the phycobiont.
  • The fungus can get its organic carbon directly from the alga or cyanobacterium. It also uses the O2 produced during phycobiont photophosphorylation in carrying out respiration.
  • The fungus protects the phycobiont from excess light intensities, provides water and minerals to it, and creates a firm substratum within which the phycobiont can grow protected from environmental stress.
  • Crustose lichens, foliose lichens, and fruticose lichens are three morphologic groups of lichens.

References:

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