🍄 Introduction to Kingdom Fungi

Kingdom Fungi includes a large group of eukaryotic, heterotrophic, non-vascular organisms that absorb nutrients from decaying matter or living hosts. Fungi lack chlorophyll and hence cannot perform photosynthesis.

Fungi exist as unicellular (yeast) or multicellular (molds, mushrooms) organisms. Their body is made up of thread-like structures called hyphae, which collectively form a mycelium.

Fungi constitute a unique kingdom of heterotrophic organisms showing great diversity in morphology and habitat. They are cosmopolitan and can be found in air, water, soil, and on plants and animals. They thrive in warm and humid places. Common examples include mushrooms, toadstools, yeasts, and moulds like the ones on bread.

🧬 General Characteristics

  • Structure: With the exception of unicellular yeasts, fungi are filamentous. Their bodies consist of long, slender, thread-like structures called hyphae. A network of hyphae is known as a mycelium.
  • Coenocytic Hyphae: Some hyphae are continuous tubes filled with multinucleated cytoplasm (aseptate).
  • Septate Hyphae: Others have cross-walls or septae.
  • Cell Wall: The cell walls of fungi are composed of chitin and polysaccharides.
  • Nutrition: Fungi are heterotrophic and can be:
  • Saprophytes: Absorb soluble organic matter from dead substrates.
  • Parasites: Depend on living plants and animals (e.g., Puccinia, the wheat rust fungus).
  • Symbionts: Live in association with other organisms, like with algae as lichens, or with roots of higher plants as mycorrhiza.

⚗️ Reproduction in Fungi

  • Vegetative: Can occur by fragmentation, fission, and budding.
  • Asexual: Occurs via spores like conidia, sporangiospores, or zoospores.
  • Sexual: Involves oospores, ascospores, and basidiospores. The sexual cycle has three steps:
  1. Plasmogamy: Fusion of protoplasms between two gametes.
  2. Karyogamy: Fusion of two nuclei.
  3. Meiosis: Occurs in the zygote to produce haploid spores.
  • Dikaryon: In Ascomycetes and Basidiomycetes, plasmogamy is not immediately followed by karyogamy. This results in an intervening stage where a cell has two nuclei (n + n), known as a dikaryon, and the phase is called a dikaryophase.

🧩 Classes of Fungi

1️⃣. Phycomycetes (Algal Fungi)

  • Habitat: Found in aquatic habitats, on decaying wood, and as obligate parasites on plants.
  • Mycelium: Aseptate and coenocytic.
  • Asexual Reproduction: By motile zoospores or non-motile aplanospores.
  • Sexual Reproduction: Can be isogamous, anisogamous, or oogamous. A zygospore is formed.
  • Examples: Mucor, Rhizopus (bread mould), Albugo (parasitic fungi on mustard).

2️⃣. Ascomycetes (Sac Fungi)

  • Structure: Mostly multicellular (e.g., Penicillium) but rarely unicellular (e.g., Yeast). Mycelium is branched and septate.
  • Asexual Reproduction: Spores are called conidia, produced exogenously on special mycelium called conidiophores.
  • Sexual Reproduction: Spores are called ascospores, produced endogenously in a sac-like ascus. Asci are arranged in fruiting bodies called ascocarps.
  • Examples: Aspergillus, Claviceps, Neurospora (used in genetic work), Morels and Truffles (edible).

3️⃣. Basidiomycetes (Club Fungi)

  • Common Forms: Mushrooms, bracket fungi, or puffballs.
  • Mycelium: Branched and septate.
  • Asexual Reproduction: Asexual spores are generally not found; vegetative reproduction by fragmentation is common.
  • Sexual Reproduction: Sex organs are absent. Plasmogamy occurs by the fusion of two somatic cells to form a dikaryotic structure, which gives rise to a basidium. Karyogamy and meiosis occur in the basidium, producing four basidiospores exogenously.
  • Examples: Agaricus (mushroom), Ustilago (smut), Puccinia (rust fungus).

4️⃣. Deuteromycetes (Imperfect Fungi)

  • Defining Feature: Known as 'imperfect fungi' because only their asexual or vegetative (vegetative) phases are known. When sexual forms are discovered, they are moved to other classes.
  • Reproduction: Reproduce only by asexual spores known as conidia.
  • Mycelium: Septate and branched.
  • Role: Many are decomposers of litter and help in mineral cycling.
  • Examples: Alternaria, Colletotrichum, Trichoderma.

🌾 Economic Importance of Fungi

Beneficial Fungi Harmful Fungi
Aspergillus – Citric acid production Puccinia – Wheat rust
Penicillium – Source of antibiotic penicillin Ustilago – Smut disease in crops
Yeast – Alcohol fermentation and baking Candida – Causes thrush in humans
Neurospora – Used in genetic studies Rhizopus – Spoils food

🧠 Summary

  • Fungi are eukaryotic heterotrophs with chitin walls.
  • Classified into four main groups: Phycomycetes, Ascomycetes, Basidiomycetes, and Deuteromycetes.
  • Play key roles in decomposition, industry, and disease causation.

💡 Solved Examples

Q1. Why are fungi placed in a separate kingdom?
A1. Fungi differ from plants as they are heterotrophic and possess chitinous cell walls.

Q2. What are coenocytic hyphae?
A2. Multinucleate, aseptate hyphae found in Phycomycetes.

Q3. Which fungus is used in genetic research?
A3. Neurospora crassa is the 'Drosophila of the Plant Kingdom'.

Q4. What is the industrial importance of Penicillium?
A4. It produces the antibiotic penicillin, discovered by Alexander Fleming.

Q5. Why are Deuteromycetes called 'imperfect fungi'?
A5. Because their sexual stage has not been observed.