Unit2 - Subjective Questions

Definition: Fungi are eukaryotic, spore-bearing, achlorophyllous organisms that generally reproduce sexually and asexually, and whose filamentous, branched somatic structures are typically surrounded by cell walls containing chitin or cellulose.

General Characteristics:

• Eukaryotic: They possess a true nucleus enclosed by a nuclear membrane and membrane-bound organelles.
• Achlorophyllous: They lack chlorophyll and cannot perform photosynthesis. They are heterotrophic (absorptive nutrition).
• Cell Wall: Typically composed of chitin and glucans (except Oomycetes which have cellulose).
• Thallus: The body is usually filamentous, consisting of hyphae (collectively called mycelium).
• Reproduction: Reproduce by both asexual and sexual spores.
• Storage: Food is stored in the form of glycogen and lipids, not starch.

Based on the conversion of the thallus into reproductive structures, fungi are classified into two types:

1. Holocarpic Thallus:

• In these fungi, the entire vegetative thallus is converted into a reproductive unit.
• Vegetative and reproductive phases do not occur simultaneously.
• Example: Synchytrium endobioticum (Potato wart pathogen).

2. Eucarpic Thallus:

• Only a portion of the thallus forms the reproductive structure, while the remainder continues to function as the vegetative somatic phase.
• Vegetative and reproductive phases can occur simultaneously.
• This is the most common type in fungi.
• Example: Pythium, Phytophthora.

Plectenchyma: Fungal hyphae often become loosely or compactly woven together to form a tissue-like structure known as plectenchyma. It is of two types:

1. Prosenchyma:

• It is a loosely woven tissue.
• The component hyphae lie distinct from one another.
• They retain their individuality and are easily distinguishable.
• Usually found in the stipe of mushrooms.

2. Pseudoparenchyma:

• It is a compactly woven tissue.
• The hyphae have lost their individuality and cannot be distinguished.
• The cells appear isodiametric (oval or round) in cross-section, resembling the parenchyma of higher plants.
• Found in sclerotia and fruiting bodies.

1. Rhizomorphs:

• These are thick, root-like strands composed of aggregated hyphae.
• They have a hard, dark outer rind and a soft, white inner core.
• Function: They facilitate the survival of the fungus under adverse conditions and aid in the rapid spread of the pathogen through soil.
• Example: Armillaria mellea.

2. Sclerotia (Singular: Sclerotium):

• A hard, compact, resting body formed by the aggregation of pseudoparenchymatous tissue.
• They are resistant to unfavorable environmental conditions like high temperature and desiccation.
• They can remain dormant in soil for long periods and germinate to produce mycelium or fruiting bodies when conditions are favorable.
• Example: Claviceps purpurea (Ergot of rye), Rhizoctonia solani.

Haustoria (Sing. Haustorium):

• Specialized intracellular absorbing structures produced by obligate parasites (like rusts and mildews).
• They penetrate the host cell wall (but not the plasma membrane) to absorb nutrients.
• Shapes vary: Knob-like (Albugo), Branched/Finger-like (Erysiphe).

Appressoria (Sing. Appressorium):

• Flattened, cushion-like, or swollen hyphal tips formed by germ tubes or hyphae.
• They attach firmly to the host surface before penetration.
• A narrow infection peg grows from the appressorium to pierce the host cuticle and epidermis.
• Example: Colletotrichum, Puccinia.

Asexual reproduction (anamorph stage) does not involve the union of nuclei or sex organs. Methods include:

1. Fragmentation: The hyphae break into small fragments, each developing into a new individual (e.g., Rhizoctonia).
2. Fission: Simple splitting of a cell into two daughter cells by constriction (e.g., Fission yeasts).
3. Budding: A small outgrowth (bud) forms on the parent cell, nucleus divides, and one part migrates to the bud which eventually detaches (e.g., Saccharomyces).
4. Spore Formation: production of spores is the most common method.
   • Sporangiospores: Endogenous spores formed inside a sporangium (e.g., Rhizopus).
   • Conidia: Exogenous spores formed at the tip or side of conidiophores (e.g., Alternaria).
   • Chlamydospores: Thick-walled resting spores formed by modification of hyphal cells (e.g., Fusarium).

Sexual reproduction (teleomorph stage) involves the union of two compatible nuclei. It consists of three phases:

1. Plasmogamy:

• The union of two protoplasts brings two haploid nuclei () together in one cell.
• It results in a cell with two nuclei (dikaryon, ), specifically in higher fungi.

2. Karyogamy:

• The fusion of the two haploid nuclei to form a diploid zygote nucleus ().
• In lower fungi, this follows immediately after plasmogamy. In higher fungi, it is delayed.

3. Meiosis:

• Reduction division of the diploid nucleus () to form haploid nuclei ().
• This restores the haploid number of chromosomes and segregates genetic traits.

1. Planogametic Copulation: Fusion of two naked gametes. Can be isogamous (similar motile gametes) or anisogamous/oogamous (morphologically different gametes). Example: Synchytrium.

2. Gametangial Contact: Two gametangia (antheridium and oogonium) come in contact. The male nucleus migrates to the female gametangium through a fertilization tube. The gametangia do not fuse. Example: Pythium, Phytophthora.

3. Gametangial Copulation: The entire contents of two contacting gametangia fuse. The separating walls dissolve, resulting in a single cell. Example: Rhizopus (Zygomycetes).

4. Spermatization: Minute, male reproductive units called spermatia (non-motile) are carried to the receptive hyphae or trichogyne of the female organ. Example: Puccinia graminis.

5. Somatogamy: Fusion of vegetative somatic hyphae directly, without the formation of specific sex organs. Common in Basidiomycetes. Example: Agaricus.

Fungi are named according to the International Code of Nomenclature for algae, fungi, and plants (ICN).

Binomial System:
Introduced by Carolus Linnaeus. Each organism has two names:

1. Generic name (Genus): Noun, always capitalized.
2. Specific epithet (Species): Adjective, usually lowercase.

Rules:

• The scientific name must be in Latin.
• When printed, it should be in italics; when handwritten, it must be underlined separately.
• Citation: The name of the author who first described the fungus is usually appended (e.g., Puccinia graminis Pers.).
• Priority: The oldest validly published name is generally accepted.
• Typification: A 'Type' specimen must be designated and preserved in a herbarium.
• Example: Phytophthora infestans (Mont.) de Bary.

G.C. Ainsworth (1973) classified fungi into the Kingdom Fungi (Mycota), divided into two Divisions:

Division I: Myxomycota (Plasmodial forms, slime molds)

• Class: Plasmodiophoromycetes

Division II: Eumycota (True Fungi, filamentous)

• Subdivision 1: Mastigomycotina (Motile spores/zoospores present)
  • Classes: Chytridiomycetes, Oomycetes.
• Subdivision 2: Zygomycotina (Zygospores present, non-motile)
  • Class: Zygomycetes.
• Subdivision 3: Ascomycotina (Ascospores in Ascus)
  • Classes: Hemiascomycetes, Plectomycetes, Pyrenomycetes, Discomycetes.
• Subdivision 4: Basidiomycotina (Basidiospores on Basidium)
  • Classes: Teliomycetes, Hymenomycetes.
• Subdivision 5: Deuteromycotina (Sexual stage absent/unknown)
  • Class: Coelomycetes, Hyphomycetes.

Reasons Oomycota are distinct from True Fungi (Eumycota):

1. Cell Wall: Composed primarily of cellulose and beta-glucans, whereas true fungi have chitin.
2. Nuclear State: The vegetative mycelium is diploid (), whereas true fungi are typically haploid ().
3. Flagella: Zoospores are biflagellate with one tinsel (anterior) and one whiplash (posterior) flagellum (Heterokont). True fungi (like Chytrids) usually have only posterior whiplash flagella.
4. Biochemical: They synthesize lysine via the DAP (diaminopimelic acid) pathway, while true fungi use the AAA (alpha-aminoadipic acid) pathway.

Major Disease:

• Late blight of potato caused by Phytophthora infestans.

Characteristics of Zygomycota:

• Habitat: Mostly terrestrial, saprophytic, or weak parasites.
• Mycelium: Coenocytic (aseptate) and profusely branched.
• Asexual Reproduction: By non-motile sporangiospores produced endogenously in sporangia located on sporangiophores.
• Sexual Reproduction: By the formation of a resting spore called a Zygospore (formed by gametangial copulation).
• They lack motile cells (zoospores) in their life cycle.

Disease:

• Soft rot of sweet potato or fruits caused by Rhizopus stolonifer.

Distinguishing Features of Ascomycota:

• Septate Mycelium: Hyphae are septate with simple septal pores.
• Sexual Spores: Produce Ascospores endogenously within a sac-like structure called an Ascus (Plural: Asci).
• Asexual Spores: Typically Conidia formed exogenously on conidiophores.
• Karyogamy: Occurs within the young ascus.
• Dikaryophase: A short dikaryotic phase occurs before ascospore formation.
• Fruiting Bodies: Asci are often organized into fruiting bodies called Ascocarps (e.g., Cleistothecium, Perithecium, Apothecium).
• Examples: Erysiphe (Powdery mildew), Saccharomyces (Yeast).

General Characters:

• Most advanced group of fungi (Club fungi).
• Mycelium: Septate, often with a dolipore septum (barrel-shaped swelling around the pore).
• Clamp Connections: Hook-like structures formed to ensure proper segregation of nuclei during cell division in the dikaryotic mycelium.

Reproductive Structures:

• Basidium: The sexual reproductive organ (club-shaped).
• Basidiospores: Haploid sexual spores produced exogenously on the basidium (usually 4 spores per basidium) via sterigmata.
• Fruiting Body: Many form large fruiting bodies called Basidiocarps (e.g., Mushrooms).
• Examples: Puccinia (Rusts), Ustilago (Smuts), Agaricus (Mushrooms).

Definition: Deuteromycota is a form-phylum for fungi where the sexual stage (teleomorph) is either absent, unknown, or has not yet been discovered.

Why "Fungi Imperfecti"?

• In mycology, the "perfect" stage refers to the sexual stage.
• Since these fungi only reproduce asexually (usually by conidia) or by vegetative means, their life cycle is considered "imperfect."
• When the sexual stage of a Deuteromycete is discovered, it is usually reclassified into Ascomycota or Basidiomycota.
• Examples: Alternaria, Fusarium, Colletotrichum.

Lower Fungi (e.g., Mastigomycotina, Zygomycotina):

• Mycelium: Generally coenocytic (aseptate), except for reproductive structures.
• Reproduction: Often involves asexual reproduction within sporangia.
• Complexity: Structurally simpler.
• Classification: Includes Chytrids, Oomycetes, and Zygomycetes.

Higher Fungi (e.g., Ascomycota, Basidiomycota, Deuteromycota):

• Mycelium: Septate (divided by cross-walls).
• Reproduction: Complex fruiting bodies (Ascocarps, Basidiocarps) are often formed.
• Complexity: More advanced structural and evolutionary features.
• Dikaryophase: An extended dikaryotic phase () is common.

Taxonomy: Both belong to Basidiomycotina.

• Rusts: Order Uredinales. Obligate parasites.
• Smuts: Order Ustilaginales. Facultative saprophytes (can grow on culture media).

Symptoms:

• Rusts: Produce rusty, orange, yellow, or brown pustules on stems and leaves. The epidermis ruptures to release spores (e.g., Puccinia graminis - Stem rust of wheat).
• Smuts: Replace grain kernels or plant parts with black, charcoal-like powdery masses of spores (teliospores). The symptoms are often seen in floral parts (e.g., Ustilago tritici - Loose smut of wheat).

1. Heteroecious Fungi:

• These fungi require two taxonomically distinct hosts to complete their life cycle.
• Example: Puccinia graminis tritici (Stem rust of wheat) requires Wheat (primary host) and Barberry (alternate host).

2. Autoecious Fungi:

• These fungi complete their entire life cycle on a single host species.
• Example: Melampsora lini (Linseed rust) or Uromyces appendiculatus (Bean rust).

• Zoospores: Motile asexual spores equipped with flagella. They lack a rigid cell wall and swim in water films. Found in Mastigomycotina (e.g., Pythium).
• Aplanospores: Non-motile asexual spores produced endogenously within a sporangium. They have a distinct cell wall. Found in Zygomycotina (e.g., Rhizopus).
• Conidia: Non-motile asexual spores produced exogenously (externally) at the tip or sides of specialized hyphae called conidiophores. They are deciduous (fall off easily). Found in Ascomycota and Deuteromycota (e.g., Alternaria).

Characteristics:

• Thallus: A plasmodium (a multinucleate, motile mass of protoplasm lacking a cell wall).
• Habitat: Obligate endoparasites of vascular plants.
• Zoospores: Biflagellate with two anterior whiplash flagella of unequal length.
• Resting Spores: Formed in masses within the host cell.
• They are now often classified under Kingdom Protista but studied in plant pathology.

Disease:

• Club root of cabbage caused by Plasmodiophora brassicae.