Unit 3: Fungi and its Associations

1. General characteristics of Fungi

Definition: Fungi are a kingdom of eukaryotic, achylophyllous (lacking chlorophyll), heterotrophic (saprophytic, parasitic, or symbiotic) organisms. Their bodies are typically filamentous (hyphae), and their cell walls are made of chitin. The study of fungi is called Mycology.

• Nutrition: Heterotrophic, meaning they cannot make their own food. They obtain nutrients by absorption after secreting digestive enzymes onto their food source.
• Thallus: The body of a fungus is called a thallus. In most fungi (except yeasts), it is made of a network of thread-like filaments called hyphae. The entire network of hyphae is called a mycelium.
• Cell Wall: The rigid cell wall is made primarily of chitin (a polysaccharide, also found in insect exoskeletons).
• Reserve Food: Food is stored as glycogen and oil. (No starch).
• Habitat: Fungi are ubiquitous (found everywhere), but they prefer dark, moist environments with organic matter.

2. Affinities with plants and animals

Fungi were once classified as plants but are now in their own kingdom. Genetically, they are more closely related to animals than to plants.

Affinities with Plants (Resemblances)

• Presence of a rigid cell wall (plants have cellulose, fungi have chitin).
• They are non-motile (fixed in one place).
• They show asexual and vegetative reproduction methods (like spore formation, fragmentation) similar to lower plants.
• They absorb nutrients in solution (like plants absorbing minerals).

Affinities with Animals (Resemblances)

• Heterotrophic: They lack chlorophyll and cannot perform photosynthesis.
• Reserve Food: They store food as glycogen (the same as animals).
• Cell Wall Chemistry: They synthesize chitin, a substance common in the animal kingdom (e.g., arthropods) but absent in plants.
• Metabolic Pathways: Certain synthetic pathways (e.g., for the amino acid lysine) are similar to those in animals.

Differences from Plants

• Lack chlorophyll.
• Lack differentiation into true roots, stems, leaves.
• Cell wall is chitin, not cellulose.
• Store glycogen, not starch.

3. Thallus organization (Growth forms and range)

The fungal thallus ranges from a single cell to a complex, organized mycelium.

1. Unicellular (Yeasts):
   • The thallus is a single, spherical, or oval cell (e.g., *Saccharomyces*).
   • They reproduce asexually by budding or fission.
2. Filamentous (Molds):
   • The thallus is a mycelium, a tangled mat of filaments called hyphae.
   • Hyphae Structure:
     • Aseptate (Coenocytic): Hyphae are "lower fungi" (like Oomycota, Zygomycota). They lack cross-walls (septa), so the filament is a multinucleate, continuous tube (e.g., *Rhizopus*, *Albugo*).
     • Septate: Hyphae are "higher fungi" (Ascomycota, Basidiomycota). They are divided by cross-walls (septa) into individual cellular compartments. The septa are porous, allowing cytoplasm to flow.
3. Dimorphic Fungi:
   • These fungi can switch between two forms:
     1. A yeast-like (unicellular) form.
     2. A mycelial (filamentous) form.
   • The switch is often triggered by environmental factors like temperature. Many human pathogens are dimorphic (e.g., *Candida albicans*, *Histoplasma capsulatum*).
4. Mycelial Modifications:
   • Plectenchyma: Organized fungal tissue. Can be:
     • Prosenchyma: Loosely woven hyphae, parallel to each other.
     • Pseudoparenchyma: Tightly interwoven, isodiametric cells that resemble plant parenchyma.
   • Sclerotium: A hard, dense, resting mass of mycelium that survives harsh conditions.
   • Rhizomorphs: Thick, root-like cords of aggregated hyphae, used for translocation and colonization (e.g., *Armillaria* - honey fungus).

4. Cell structure

Fungal cells are eukaryotic.

• Cell Wall: Outermost layer. Composed of chitin and glucans. Provides shape and protection.
• Plasma Membrane: Inside the cell wall. Contains ergosterol (the fungal equivalent of cholesterol, and a target for antifungal drugs).
• Cytoplasm: Contains all the standard eukaryotic organelles:
  • Nucleus: True, membrane-bound nucleus. Fungal cells can be uninucleate, binucleate (dikaryotic), or multinucleate.
  • Mitochondria: For cellular respiration.
  • Ribosomes: 80S type.
  • Endoplasmic Reticulum & Golgi Apparatus: Involved in protein synthesis and secretion (especially of digestive enzymes).
  • Vacuoles: Large, central vacuoles are common, used for storage and turgor.
• Septa (in higher fungi):
  • Ascomycota: Have simple septa with a central pore.
  • Basidiomycota: Have complex dolipore septa, a barrel-shaped pore with "parenthesome" caps on either side.

5. Nutrition

All fungi are heterotrophic and use absorptive nutrition. They secrete extracellular enzymes (exoenzymes) to digest food, then absorb the small, soluble molecules.

1. Saprophytes (or Saprotrophs):
   • They feed on dead organic matter (e.g., fallen leaves, dead logs, bread).
   • They are nature's primary decomposers, essential for nutrient cycling.
   • Example: *Rhizopus* (bread mold), *Agaricus* (mushroom), *Penicillium*.
2. Parasites:
   • They obtain nutrients from a living host, often causing disease.
   • Obligate Parasites: Can *only* survive on a living host (e.g., *Puccinia* - rust fungus).
   • Facultative Parasites: Can live as saprophytes but can *also* infect a host (e.g., *Phytophthora*).
   • They often develop specialized absorptive structures called haustoria that penetrate the host cell.
3. Symbionts (Mutualists):
   • They live in a mutually beneficial relationship with another organism.
   • Example 1: Lichens (Fungus + Alga). The fungus gets food from the alga; the alga gets protection, water, and minerals from the fungus.
   • Example 2: Mycorrhiza (Fungus + Plant Root). The fungus gets sugars from the plant; the plant gets enhanced water and mineral (especially phosphorus) uptake from the fungus.

6. Reproduction

Fungi reproduce by all three methods. The entire fungal life cycle is the holomorph, which has two parts: the asexual stage (anamorph) and the sexual stage (teleomorph).

1. Vegetative Reproduction

• Fragmentation: A piece of the mycelium breaks off and grows into a new individual.
• Fission: A unicellular yeast cell splits into two (e.g., *Schizosaccharomyces*).
• Budding: A small "bud" forms on a yeast cell, grows, and detaches (e.g., *Saccharomyces*).

2. Asexual Reproduction (Anamorph)

Involves the formation of spores (mitospores) by mitosis. These are for rapid dispersal.

• Sporangiospores: Spores produced *inside* a sac-like structure called a sporangium. Common in Zygomycota (e.g., *Rhizopus*).
• Conidia: Spores produced *externally* (not in a sac), usually in chains, at the tip of a specialized hypha called a conidiophore. Common in Ascomycota and Basidiomycota (e.g., *Aspergillus*, *Penicillium*).
• Oidia / Arthrospores: A hypha breaks up into its individual cells, which act as spores.
• Chlamydospores: Thick-walled, resting spores formed from vegetative cells.

3. Sexual Reproduction (Teleomorph)

Involves the formation of sexual spores (meiospores) after meiosis. This introduces genetic variation. The process has three distinct phases:

1. Plasmogamy: Fusion of the protoplasm of two gametes or hyphae. The nuclei do *not* fuse.
2. Karyogamy: Fusion of the two haploid nuclei (n+n) to form a diploid (2n) zygote nucleus.
3. Meiosis: The diploid nucleus immediately undergoes meiosis to produce haploid (n) spores.

Sexual Spore Types (named after the group):

• Oospores: Formed in Oomycota.
• Zygospores: A thick-walled resting spore formed by the fusion of two gametangia. Found in Zygomycota (e.g., *Rhizopus*).
• Ascospores: Typically 8 spores produced *inside* a sac-like structure called an ascus. Found in Ascomycota (e.g., *Saccharomyces*, *Aspergillus*).
• Basidiospores: Typically 4 spores produced *externally* on a club-shaped structure called a basidium. Found in Basidiomycota (e.g., *Agaricus*).

7. Classification (outline, Ainsworth)

G.C. Ainsworth (1973) proposed a widely used classification system for fungi, primarily based on the morphology of the thallus and reproductive structures (especially sexual spores).

Ainsworth's Outline:

Kingdom: MYCOTA (Fungi)

• Division 1: MYXOMYCOTA (Slime Molds)
  • Lack cell walls in their vegetative (plasmodium) stage.
• Division 2: EUMYCOTA (True Fungi)
  • Have cell walls.
  • Subdivision 1: MASTIGOMYCOTINA
    • "Lower fungi" that produce motile spores (zoospores).
    • e.g., Oomycota (*Phytophthora*). (Note: Oomycota are now known to be unrelated to true fungi).
  • Subdivision 2: ZYGOMYCOTINA
    • Aseptate (coenocytic) mycelium.
    • Sexual spore: Zygospore.
    • e.g., *Rhizopus*, *Mucor*.
  • Subdivision 3: ASCOMYCOTINA (Sac Fungi)
    • Septate mycelium.
    • Sexual spore: Ascospore (in an ascus).
    • e.g., *Saccharomyces* (yeast), *Aspergillus*, *Penicillium*.
  • Subdivision 4: BASIDIOMYCOTINA (Club Fungi)
    • Septate mycelium (with dolipore septa).
    • Sexual spore: Basidiospore (on a basidium).
    • e.g., *Agaricus* (mushroom), *Puccinia* (rust).
  • Subdivision 5: DEUTEROMYCOTINA (Fungi Imperfecti)
    • An "artificial" group.
    • Includes fungi with septate mycelium for which no sexual stage (teleomorph) has been discovered.
    • They reproduce only by conidia (anamorph).
    • (Note: With DNA analysis, most of these are now being reclassified as Ascomycota or Basidiomycota).

8. Nature of associations

This refers to the symbiotic relationships fungi form with other organisms, as described under Nutrition.

• Parasitism (+/-): Fungus benefits, host is harmed (e.g., *Puccinia* on wheat).
• Mutualism (+/+): Both partners benefit (e.g., Lichens, Mycorrhiza).
• Commensalism (+/0): Fungus benefits, host is unaffected (e.g., some fungi living on skin).

9. Lichens: Classification, Structure, Reproduction

Definition: A lichen is a stable, mutualistic association between a fungus (the mycobiont) and a photosynthetic partner (the phycobiont, which is either a green alga or a cyanobacterium).

Classification of Lichens

Lichens are classified based on two main criteria:

1. Based on the Fungal Partner:
   • Ascolichens: The fungal partner is an Ascomycota. This is the vast majority (98%) of all lichens.
   • Basidiolichens: The fungal partner is a Basidiomycota. These are rare.
2. Based on Thallus Morphology (Growth Form):
   • Crustose: A flat, crust-like thallus, tightly attached (embedded) to the substrate (rocks, bark). Very difficult to remove.
   • Foliose: A leaf-like, lobed thallus, attached to the substrate by small root-like structures called rhizines. (e.g., *Parmelia*).
   • Fruticose: A shrub-like, branched, and erect or hanging thallus. (e.g., *Usnea*).

Structure of Lichens

A cross-section of a typical (foliose) lichen thallus shows distinct layers:

1. Upper Cortex: A protective outer layer made of tightly packed fungal hyphae (pseudoparenchyma).
2. Algal Layer: Below the cortex. A layer of algal cells (phycobiont) intermingled with loosely woven fungal hyphae (mycobiont). This is the photosynthetic zone.
3. Medulla: The main body of the thallus, composed of loosely woven fungal hyphae. This layer is for water/gas exchange and storage.
4. Lower Cortex: A bottom protective layer, similar to the upper cortex.
5. Rhizines: Fungal hyphae that extend from the lower cortex to anchor the lichen to the substrate.

Reproduction in Lichens

• Vegetative Reproduction (Asexual): This is the main method and ensures *both* partners are dispersed together.
  • Fragmentation: A piece of the thallus breaks off and grows into a new lichen.
  • Soredia: Powdery, microscopic granules that are "packages" of a few algal cells wrapped in fungal hyphae. They are formed in specialized patches (soralia) and are easily dispersed by wind.
  • Isidia: Small, coral-like outgrowths from the upper cortex that contain both partners. They break off for propagation.
• Sexual Reproduction:
  • This is reproduction of the fungal partner only.
  • Most (Ascolichens) form fungal fruiting bodies (e.g., apothecia) that produce ascospores.
  • When these ascospores germinate, the new fungal hyphae must find a compatible algal partner *by chance* to form a new lichen. This is a very inefficient process.

10. Mycorrhiza: Ectotrophic, Endotrophic, their types and significance

Definition: Mycorrhiza ("fungus-root") is a mutualistic association between a fungus and the roots of a higher plant.

The Mutualism:

• Fungus gets: Sugars (carbon) from the plant's photosynthesis.
• Plant gets: Greatly enhanced water and mineral uptake (especially phosphorus and nitrogen), and protection from root pathogens. The fungal hyphae act as a massive extension of the plant's root system.

1. Ectotrophic Mycorrhiza (Ectomycorrhiza or ECM)

• Description: The fungus forms a dense sheath or mantle of hyphae on the *outside* of the plant root.
• The hyphae do not penetrate the plant's root cells.
• Instead, they grow *between* the root cortex cells, forming a network called the Hartig net. This is the site of nutrient exchange.
• Fungi involved: Mostly Basidiomycota (e.g., *Amanita*, *Boletus*).
• Plants involved: Common on temperate trees like pines, oaks, and birches.

2. Endotrophic Mycorrhiza (Endomycorrhiza)

• Description: The fungus does not form a dense outer mantle.
• The hyphae grow *into* the root cortex and penetrate individual plant cells.
• Inside the cells, they form highly branched, tree-like structures called arbuscules, which are the primary site of nutrient exchange.
• They do *not* break the plant cell's plasma membrane (they just invaginate it).

Types of Endomycorrhiza:

• Arbuscular Mycorrhiza (AM): (Also called VAM - Vesicular-Arbuscular Mycorrhiza).
  • This is the most common type of mycorrhiza, found in >80% of all land plants (including most crops like corn, wheat, and vegetables).
  • They form arbuscules (for nutrient exchange) and often vesicles (for lipid storage) inside the root cells.
  • Fungi involved: A specific group called Gascomycota.
• Orchid Mycorrhiza: A specialized form required by orchids. The fungus provides the tiny, nutrient-poor orchid seed with carbon to germinate.
• Ericoid Mycorrhiza: Found in plants of the Ericaceae family (e.g., blueberries, rhododendrons) growing in acidic, nutrient-poor soils.

Significance of Mycorrhiza

• Agriculture & Horticulture: Inoculating soils with mycorrhizal fungi can significantly improve crop yield, reduce the need for phosphate fertilizers, and increase drought tolerance.
• Forestry: Essential for the growth and health of forest trees. Many trees cannot survive in new environments without their specific mycorrhizal partner.
• Ecology: They are a fundamental link in terrestrial ecosystems, connecting plants and facilitating nutrient cycling.
• Land Reclamation: Mycorrhizal plants are more successful at colonizing and stabilizing barren, polluted, or eroded soils.