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Unit 2 - Notes

PTH103 7 min read

Unit 2: Introduction to Fungi and Fungal-like organisms

1. Definition and General Characteristics

Definition of Fungi

Fungi are nucleated, spore-bearing, achlorophyllous (lacking chlorophyll) organisms that generally reproduce sexually and asexually. Their somatic structures are usually surrounded by cell walls containing chitin and glucans (except for Oomycetes, which contain cellulose). They are heterotrophic, obtaining nutrition via absorption.

General Characteristics

  1. Eukaryotic: Possess a true nucleus with a nuclear membrane and membrane-bound organelles (mitochondria, ER, etc.).
  2. Heterotrophic: Unable to synthesize their own food. They exist as:
    • Saprophytes: Feed on dead organic matter.
    • Parasites: Feed on living hosts.
    • Symbionts: Live in mutual association (e.g., Mycorrhiza).
  3. Absorptive Nutrition: They secrete extracellular enzymes to digest food outside the body and absorb the solubilized nutrients.
  4. Cell Wall: Primarily composed of chitin (-acetylglucosamine polymer) and glucans. Note: Fungal-like organisms (Oomycota) have cell walls primarily of cellulose and glucans.
  5. Reserve Food: Stored as glycogen and oil drops (never starch).
  6. Habitat: Cosmopolitan (found in air, water, soil, on plants and animals).
  7. Reproduction: Reproduce by means of spores (sexual and asexual).

2. Somatic Structures and Types of Fungal Thallus

The entire body of the fungus is called the Thallus.

Types of Fungal Thallus

  1. Plasmodial: A naked, amoeboid mass of protoplasm containing many nuclei but lacking a rigid cell wall. (e.g., Plasmodiophora - Club root of cabbage).
  2. Unicellular: The thallus consists of a single cell. (e.g., Saccharomyces - Yeast).
  3. Filamentous (Mycelial): The most common fungal body, composed of microscopic threads.

The Mycelium

  • Hypha (pl. Hyphae): A single thread-like filament.
  • Mycelium: A mass or network of hyphae constituting the body of a fungus.

Types of Hyphae based on Septation:

  1. Aseptate (Coenocytic): Protoplasm is continuous; nuclei are scattered in a common cytoplasm without cross-walls. Characteristic of "Lower Fungi" (e.g., Pythium, Rhizopus).
  2. Septate: Cross-walls (septa) divide the hyphae into compartments. Characteristic of "Higher Fungi" (Ascomycota, Basidiomycota).

Types of Septa:

  • Simple Septum: A plate with a central pore (Ascomycetes).
  • Dolipore Septum: A barrel-shaped swelling around the central pore (Basidiomycetes).

3. Fungal Tissues (Plectenchyma)

During certain stages (often reproduction), hyphae organize into loosely or compactly woven tissues called Plectenchyma.

Types of Plectenchyma:

  1. Prosenchyma: A loosely woven tissue where the component hyphae lie parallel to one another and their individuality is easily distinguishable.
  2. Pseudoparenchyma: A compactly woven tissue consisting of closely packed, more or less isodiametric (oval/round) cells. The individual hyphae lose their identity, resembling the parenchyma of higher plants.

4. Modifications of Thallus

Hyphae often modify to perform specific functions like survival, attachment, or absorption.

  1. Rhizomorphs: Aggregation of hyphae into thick, root-like strands with a hard rind and a soft growing tip. Resistant to adverse conditions (e.g., Armillaria mellea).
  2. Sclerotia (sing. Sclerotium): Hard, resting bodies formed by the aggregation of pseudoparenchymatous tissue. They withstand extreme temperatures and dormancy (e.g., Rhizoctonia, Sclerotium rolfsii, Ergot of Rye).
  3. Stroma: A compact somatic structure (mattress-like) on or in which fructifications (fruiting bodies) are formed.
  4. Haustoria: Specialized intracellular branches found in obligate parasites (biotrophs) used to absorb nutrients from host cells without killing them immediately (e.g., Powdery mildews, Rusts).
  5. Appressorium: A flattened, swollen tip of a germ tube or hypha used to attach to the host surface and facilitate penetration (e.g., Colletotrichum).
  6. Chlamydospores: Thick-walled, asexual resting spores formed by the modification of hyphal cells for survival (e.g., Fusarium).

5. Reproduction in Fungi and Fungal-like Organisms

Fungi reproduce via Holocarpic (entire thallus converts to spores) or Eucarpic (reproductive organs form on a portion of the thallus) methods.

A. Asexual Reproduction (The Anamorph)

Does not involve union of nuclei or sex organs. Rapid multiplication.

  1. Fragmentation: Hyphae break into pieces, each growing into a new mycelium.
  2. Fission: Simple cell division (Yeasts).
  3. Budding: Production of a small outgrowth (bud) from the parent cell (Yeasts).
  4. Sporulation (Spore formation):
    • Zoospores: Motile spores with flagella (sporangiospores).
      • Whiplash flagellum: Smooth, pointed.
      • Tinsel flagellum: Feathery.
      • Found in Oomycetes and Chytrids.
    • Sporangiospores (Aplanospores): Non-motile spores formed endogenously inside a sac called a sporangium (e.g., Rhizopus).
    • Conidia: Non-motile spores cut off exogenously at the tip or side of specialized hyphae called conidiophores (e.g., Alternaria, Cercospora).

B. Sexual Reproduction (The Teleomorph)

Involves union of two compatible nuclei. Occurs in three phases:

  1. Plasmogamy: Fusion of protoplasm (cytoplasm) of two cells, bringing two nuclei close together.
  2. Karyogamy: Fusion of the two nuclei to form a diploid zygote nucleus (2n).
  3. Meiosis: Reduction division reducing the diploid nucleus to haploid (n) state.

Sexual Spores:

  1. Oospores: Thick-walled resting spore formed by fertilization of an oogonium by an antheridium (Oomycota).
  2. Zygospores: Formed by the fusion of two morphologically similar gametangia (Zygomycota).
  3. Ascospores: Endogenous spores (usually 8) produced inside a sac-like structure called an Ascus (Ascomycota).
  4. Basidiospores: Exogenous spores (usually 4) produced on a club-shaped structure called a Basidium (Basidiomycota).

6. Nomenclature and Binomial System

Binomial System

Proposed by Carl Linnaeus (1753). Each organism has a scientific name consisting of two parts (Latinized):

  1. Generic name (Genus): A noun, always capitalized (e.g., Puccinia).
  2. Specific epithet (Species): An adjective, always lowercase (e.g., graminis).
    • Full citation includes the author's name: Puccinia graminis Pers.

International Code of Nomenclature for Algae, Fungi, and Plants (ICN)

Formerly ICBN. Key rules include:

  1. Priority: The oldest validly published name is the correct one (starting from Linnaeus, 1753).
  2. Typification: Every name must be associated with a "Type" specimen.
  3. Latin description: Historically required; now English is accepted (since 2012).
  4. Standardized Suffixes:
    • Division/Phylum: -mycota
    • Sub-division: -mycotina
    • Class: -mycetes
    • Sub-class: -mycetidae
    • Order: -ales
    • Family: -aceae

7. Classification of Fungi

Plant pathology widely uses the classification systems derived from Ainsworth (1973) or Alexopoulos and Mims (1979), though molecular phylogenetics is updating these groups.

Broadly, plant pathogens fall into three Kingdoms:

  1. Kingdom Protozoa: (Slime molds)
  2. Kingdom Chromista (Stramenopila): (Oomycetes/Water molds)
  3. Kingdom Fungi: (True Fungi)

Key Distinctions

  • Fungal-like (Chromista): Cell wall = Cellulose; Flagella = 2 (Heterokont: 1 whiplash + 1 tinsel); Spore = Oospore.
  • True Fungi (Eumycota): Cell wall = Chitin; Flagella = None (except Chytrids); Spores = Zygospore, Ascospore, Basidiospore.

8. Key to Phyla, Classes, Orders, and Diseases

A. Kingdom: PROTOZOA (Plasmodial, Phagotropic)

  • Phylum: Plasmodiophoromycota (Endoparasitic slime molds)
    • Class: Plasmodiophoromycetes
    • Order: Plasmodiophorales
      • Disease: Club root of cabbage (Plasmodiophora brassicae)
      • Disease: Powdery scab of potato (Spongospora subterranea)

B. Kingdom: CHROMISTA (Cell wall cellulose, biflagellate zoospores)

  • Phylum: Oomycota
    • Class: Oomycetes
    • Order: Peronosporales
      • Family: Pythiaceae
        • Disease: Damping off of seedlings (Pythium spp.)
        • Disease: Late blight of potato (Phytophthora infestans)
      • Family: Peronosporaceae (Downy Mildews)
        • Disease: Downy mildew of grapes (Plasmopara viticola)
      • Family: Albuginaceae (White Rusts)
        • Disease: White rust of crucifers (Albugo candida)

C. Kingdom: FUNGI (True Fungi, Cell wall Chitin)

1. Phylum: Chytridiomycota (Motile zoospores present)

  • Class: Chytridiomycetes
    • Order: Chytridiales
      • Disease: Wart disease of potato (Synchytrium endobioticum)

2. Phylum: Zygomycota (Non-motile, Coenocytic mycelium, Zygospores)

  • Class: Zygomycetes
    • Order: Mucorales
      • Disease: Soft rot of fruits / Bread mold (Rhizopus stolonifer, Mucor spp.)

3. Phylum: Ascomycota (Septate, Ascospores in Asci)

  • Class: Hemiascomycetes (Naked asci, no ascocarp)
    • Order: Taphrinales
      • Disease: Leaf curl of peach (Taphrina deformans)
  • Class: Plectomycetes (Closed ascocarp - Cleistothecium)
    • Order: Erysiphales (Powdery Mildews)
      • Disease: Powdery mildew of wheat (Blumeria graminis)
  • Class: Pyrenomycetes (Flask-shaped ascocarp - Perithecium)
    • Order: Hypocreales / Clavicipitales
      • Disease: Ergot of bajra/rye (Claviceps purpurea)
  • Class: Loculoascomycetes (Ascostroma - Pseudothecium)
    • Order: Pleosporales
      • Disease: Apple Scab (Venturia inaequalis)

4. Phylum: Basidiomycota (Septate with clamps, Basidiospores on Basidium)

  • Class: Teliomycetes (No basidiocarp, produce Teliospores)
    • Order: Uredinales (The Rusts)
      • Disease: Black stem rust of wheat (Puccinia graminis tritici)
    • Order: Ustilaginales (The Smuts)
      • Disease: Loose smut of wheat (Ustilago tritici)
  • Class: Hymenomycetes (Basidia in hymenium, Mushrooms)
    • Order: Agaricales
      • Pathogen: Armillaria mellea (Root rot)

5. Form-Phylum: Deuteromycota (Fungi Imperfecti)

Sexual stage is unknown or lacking. Classification based on asexual structures.

  • Class: Hyphomycetes (Conidia on conidiophores, no fruiting body)
    • Genera: Alternaria (Early blight of potato), Cercospora (Tikkan disease of groundnut), Fusarium (Wilts).
  • Class: Coelomycetes (Conidia in fruiting bodies like Acervulus or Pycnidium)
    • Order: Melanconiales (Acervulus)
      • Disease: Red rot of sugarcane (Colletotrichum falcatum)
    • Order: Sphaeropsidales (Pycnidium)
      • Disease: Leaf blight of brinjal (Phomopsis vexans)
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