Fungi Structure and Metabolism

Chapter 3 Structure of Fungi

• Course Learning Outcomes:
  • Explain modern taxonomy and phylogeny of fungi.
  • Compare and contrast major taxonomic groups.
  • Identify and describe morphological features of yeasts and filamentous fungi.
  • Describe metabolic processes, growth, and reproduction patterns.
  • Describe symbiotic relationships between fungi, plants, and humans.
  • Explain disease cycles, epidemiology, and control of fungal pathogens.
  • Provide examples of the economic importance of fungi.

Major Sections

• Structure of a fungal hypha
• Fungal ultrastructure
• Hypha as part of a colony
• Structure of yeasts
• Fungal walls and wall components
• Septa
• Fungal nucleus
• Cytoplasmic organelles
• Fungal cytoskeleton and molecular motors

Morphology and Structure

• Yeasts: unicellular, nucleated, and rounded.
• Molds: multicellular, filamentous.
• Hyphae: fine, branching, usually colorless threads.
• Mycelium: tangled web formed by intertwining hyphae.
• Hypha: tube with a rigid wall containing protoplasm.
• Fungal cell walls: contain chitin or glucan, unlike plant cell walls (cellulose).

Structure of Fungal Hyphae

• Hyphae grow at tips (extension zone), up to 30 \,\mu m long in fast-growing species (e.g., Neurospora crassa, which can extend at up to 40 \,\mu m/min).
• Older cells: behind the actively growing tips, loaded with nutrients.
• Most fungi: multinucleate and multicellular with septa, or aseptate (coenocytic).
• Hyphae of most fungi: have cross walls (septa) at fairly regular intervals.
• Septa: absent from hyphae of most Oomycota and Zygomycota, except where they isolate old or reproductive regions.

Fungal Ultrastructure - Summary

• Fungal hyphae: surrounded by a complex wall of chitin, glucan, or both.
• Plasma membrane: firmly attached underneath the cell wall.
• Fungal cell: has membrane-bound vesicles (from Golgi bodies) but no other major organelles.
• Apical vesicle cluster (AVC): collection of vesicles at the hyphal tip, consisting of actin microfilaments and microtubules.
• Spitzenkörper (“apical body”): dark particles/spots at the apex of septate fungi.

Cell Organelles

• Mitochondria: powerhouse of the cell.
• Endoplasmic reticulum and ribosomes: assist in protein synthesis.
• Nuclei: one or more; can be multinucleate (coenocytic), sometimes diplonucleated.
• Septa: presence/absence affects nuclei definition.
• Dolipore septa: special thick-walled septa.
• Golgi bodies: transport channel.
• Cells also have vesicles and vacuoles.

Dolipore Septa

• Narrow central channel (100-150 nm diameter) bounded by glucan flanges.
• Parenthosomes: bracket-shaped membraneous structures on either side, with pores for cytoplasmic continuity but prevent passage of major organelles.
• Function: isolate compartments or allow organelle passage.
• Can be degraded: for mass nutrient translocation.

Hypha as Part of a Colony

• Colonies develop from a single germinating spore, producing a germ tube (young hypha) that grows and branches.
• Circular outline: characteristic of colony development.
• Some fungi: grow as budding, uninucleate yeasts.

Unicellular Yeast & Pseudohyphae

• Microscopic morphology of yeasts: includes cell wall, cell membrane, endoplasmic reticulum, nucleus, nucleolus, storage vacuole, Golgi apparatus, ribosomes, and mitochondrion.
• Budding and bud scars: are visible.

Fungal Cell Structure

• Vacuole: Cytoplasm less dense in older parts.
• Mitochondrion.
• Rough endoplasmic reticulum.
• Growing tip.
• Nuclei: hypha is coenocytic (aseptate).
• Cell wall.
• Cell membrane.
• Golgi apparatus.
• Nucleus.

Cell Walls of Fungi

• Rigid: provide structural support and shape.
• Different: in chemical composition from prokaryotic cell walls.
• Thick layer: of polysaccharide fibers (chitin or cellulose).
• Thin outer layer: of mixed glycans.
• Most fungi: cell walls contain chitin, unlike plants (cellulose).

The Plasma Membrane

• Typical bilayer: of phospholipids with embedded proteins.
• Enriched: with lipids and proteins.
• Relative rigidity: gives stability.
• Selectively permeable: barrier.
• Fluid mosaic model: based on eukaryotic membranes.

Plasma Membrane

• Cell processes: cell movement and transduction.
• Unique: contains ergosterol (main sterol), unlike animals (cholesterol) and plants (phytosterols like β-sitosterol).

Endoplasmic Reticulum

• Folded transport network.
• Rough ER (RER): studded with ribosomes for protein synthesis.
• Smooth ER: no ribosomes, synthesizes cell membranes, fats, and hormones.
• RER: transports materials from nucleus to cytoplasm and cell exterior.

Ribosomes

• Sites: of protein synthesis.
• 80S: large 60S subunit and small 40S subunit.
• 70S.
• Membrane-bound: attached to ER.
• Free: in cytoplasm.
• Also: found within mitochondria.

Golgi Complex

• Transport: organelle; modifies proteins from ER, transports them to plasma membrane via secretory vesicles.

Mitochondria

• Double membrane: inner folds (cristae) and fluid (matrix).
• Cellular respiration: ATP/Energy production.
• Generate energy: ATP generated by electron transport chain.
• Round or elongated: scattered throughout cytoplasm.
• Cristae: tubular inner folds that hold enzymes and electron carriers for aerobic respiration.

The Nucleus

• Small: (1-2 μm to 20-25 μm diameter).
• Double membrane: with pores, like all eukaryotes.
• Haploid: chromosome numbers ranging from 6 - 20.

The Nucleus

• Prominent: organelle of eukaryotic cells.
• Nuclear envelope: double membrane structure separated by a narrow space, penetrated by nuclear pores.
• Pores: allow material transport in/out of the nucleus.

The Nucleus (cont'd)

• Nucleolus:
  • Found in nucleoplasm
  • Site of RNA synthesis
  • Collection area for ribosomal subunits
• Chromatin:
  • Membrane-bound structure that houses genetic material.
  • Made of DNA and proteins: dense material.

Cytoplasm and Cytoskeleton

• Cytoplasm: substance inside the plasma membrane but outside the nucleus.
• Cytosol: fluid portion of cytoplasm.
• Cytoplasmic streaming: movement of the cytoplasm throughout a cell.
• Organelles: eukaryotic cells/fungi lie in cytoplasmic matrix.
• Filaments: form the cytoskeleton (microfilaments, microtubules, and intermediate filaments).

Cytoskeleton

• Role: in cell shape and cell movement.
• Internal organization: provides dynamic structural framework for transporting organelles, cytoplasmic streaming, and chromosome separation during cell division.
• Three main elements:
  • Microtubules: polymers of tubulin proteins
  • Microfilaments: contractile protein actin
  • Intermediate filaments: provide tensile strength
• Fungal cell wall: complex and flexible, composed of chitin, α- and β- linked glucans, glycoproteins, and pigments.
• Glycan vs. Glucan: Glycan - any polysaccharide or oligosaccharide while glucan - any polysaccharide that is a polymer of glucose.

Chapter 7 Fungal Metabolism and Products

7.1 Energy from Glucose and Non-Sugar Substrates

• Energy source: Fungi obtain energy by oxidizing compounds, with glucose as primary carbon source.
• Aerobic respiration: Like all eukaryotes, fungi break down glucose into carbon dioxide, metabolic water, and ATP.
• Net result: C6H{12}O6 + 6O2 \rightarrow 6CO2 + 6H2O + 38 ATP

Figure 7.2: Embden-Meyerhof Pathway and Tricarboxylic Acid Cycle

• Major pathways: Embden-Meyerhof pathway and tricarboxylic acid cycle generate energy from sugars.
• Pentose phosphate pathway: Provides some energy but mainly for biosynthesis (5-carbon sugars for nucleic acids).
• Important cofactors:
  • Flavin adenine dinucleotide (FADH2)
  • Nicotinamide adenine dinucleotide phosphate (NADPH) - used to donate electrons and hydrogens in enzyme-catalyzed reactions

Cellular Respiration

• Four-stage process: Glucose is oxidized to carbon dioxide, and oxygen is reduced to water.
• Energy storage: Released energy stored as ATP (36 to 38 ATP per glucose).
• Stages:
  1. Glycolysis: Partial oxidation of glucose to 2 pyruvate molecules in cytosol.
  2. Formation of Acetyl CoA: Pyruvate enters mitochondrial matrix, oxidative decarboxylation to 2 Acetyl CoA molecules (catalyzed by pyruvate dehydrogenase).
  3. Krebs Cycle (TCA or Citric Acid Cycle): Acetyl CoA fully oxidized in the tricarboxylic acid cycle.
     • Acetyl CoA combines with oxaloacetate (4-carbon) to form citrate (6-carbon).
     • Two CO_2 molecules released, oxaloacetate recycled.
     • Energy stored in ATP, NADH, and FADH2.
  4. Electron Transport System and Oxidative Phosphorylation: ATP generated when electrons are transferred from NADH and FADH2 to molecular O_2.
     • O2 is reduced to H2O.
     • Occurs in inner membrane of mitochondria.

Glycolysis and the Citric Acid Cycle

• Glycolysis: Generates two acetyl CoA molecules from one glucose molecule (ten enzyme-catalyzed reactions).
• Products: Glycolytic pathway and citric acid cycle produce six CO_2 molecules, 10 NADH molecules, two FADH2 molecules, and 38 ATP per glucose molecule.
• NADPH: A cofactor that donates electrons and hydrogens in enzyme-catalyzed reactions.
• FADH2: A redox cofactor created during the Krebs cycle and used in the electron transport chain.
• ATP: Carries energy in its phosphate bonds; breaking a phosphate bond releases energy.

Gluconeogenesis: Generating Sugars from Non-Sugar Substrates

• Sugar needs: Sugars are needed for the synthesis of fungal walls, nucleic acids, and storage compounds.
• Gluconeogenesis: Reversal of the Krebs cycle, used when fungi grow on non-sugar substrates.
• Process: Starting from glyoxylate, converting to oxaloacetate, and reverse metabolism (Figure 7.5).
• Glyoxylate cycle: Short-circuited form of the TCA cycle.

Figure 7.5: Role of the Glyoxylate Cycle

• Glyoxylate cycle: Generates sugars for biosynthesis when fungi grow on non-sugar substrates (acetate or organic acids).

Secretion of Organic Acids as Commercial Products

• Commercial importance: Fungi are important commercial sources of organic acids.
• Citric acid: Aspergillus niger converts most sugar to citric acid (beverage, food, pharmaceutical, and cosmetic industries) - Approximately 1.6 million tons produced each year, with China accounting for 35-40%.
• Other acids: Other Aspergillus species produce gluconic, malic, itaconic, and gallic acids.
• Fumaric acid: Rhizopus nigricans produces large amounts.
• Fumaric and kojic acids: Rhizopus oryzae produces
• Lactic acid: Other Rhizopus species produce
• Acetic acid: Approximately 7,000,000 tons produced annually, with only 190,000 tons produced by microbes.
• Examples: Aspergillus for citric acid and Lactobacillus for lactic acid production.
• Gluconic acid: Naturally in fruits and honey; produced by Aspergillus niger. Used as food additive, imparting a refreshing sour taste, also used in pickling.

Organic Acids Production

• Lactic acid production: Approximately 150,000 tons annually; global consumption expected to rise to 500,000 tons a year due to polymer/plastic use.
• Polylactic acid (PLA): Lactic acid can be polymerized to polylactic acid (PLA), forming a sustainable bioplastic.
• Uses of lactic acid: food (flavoring and preservative), cosmetic (moisturizers/rejuvenation), pharmaceutical (I.V. solutions/drug delivery).
• Itaconic acid production: 15,000 tons annually, mainly by Aspergillus terreus.

Primary & Secondary Metabolites

• Primary metabolites: Intermediates/end products of common metabolic pathways (sugars, amino acids, organic acids, glycerol, etc.) for normal cellular functions.
• Secondary metabolites: Diverse compounds formed by specific pathways; not essential for growth but can confer an advantage (e.g., antibiotics, fungal toxins).

Secondary Metabolites: Penicillin

• Discovery: Alexander Fleming in 1929 from Penicillium chrysogenum, preventing Staphylococcus spp. growth.
• Activity: A broad-based antibiotic active against Gram-positive bacteria.
• Still a front-line antibiotic: after more than 60 years of use.
• Breakdown: Penicillins are susceptible to breakdown by plasmid-encoded β-lactamases from enteric bacteria, neutralizing penicillins and causing allergic reactions in some patients.
• Cephalosporins: Used in cases of penicillin breakdown, from Cephalosporium acremonium.
• Commercial source: Cephalosporin is now commercially obtained from strains of Streptomyces spp.

Mycotoxins

• Definition: Poisonous secondary metabolites produced by many filamentous fungi in the phylum Ascomycota.
• Toxicity: Toxic to humans and animals depending on their toxicity levels and concentration.
• Problem source: Improper storage of food/feed products, grains, and nuts.
• Production time: Can be produced during preharvest and postharvest of crops.
• Examples: aflatoxin, ochratoxin, ergot, phallotoxins, and amatoxins (by Amanita spp.).

Major Mycotoxins and US/EU Limits

• Table summarizing mycotoxins, fungal species, food commodities, and US/EU limits in \,\mu g/kg:

  Mycotoxin	Fungal Species	Food Commodity	US FDA (\,\frac{\,\mu g}{kg})	EU (EC 2006) (\,\frac{\,\mu g}{kg})	Comment
  Aflatoxins	Aspergillus flavus, A. parasiticus	Maize, wheat, rice, peanut, sorghum, pistachio, almond, ground nuts, tree nuts, figs, cottonseed, spices	20 for total	2-12 for B1, 4-15 for total	200 for figs
  Aflatoxin M1	Metabolite of aflatoxin B1	Milk, milk products	0.5	0.025 in infant formulae
  Ochratoxin A	Aspergillus ochraceus, Penicillium verrucosum	Cereals, dried vine fruit, wine, grapes, coffee, cocoa, cheese	Not set	2-10
  Fumonisins	Fusarium verticillioides, F. proliferatum	Maize, maize products, sorghum, asparagus	2000-4000	200-1000
  Zearalenone	Fusarium graminearum, F. culmorum	Cereals, cereal products, maize, wheat, barley	Not set	20-100
  Deoxynivalenol	Fusarium graminearum, F. culmorum	Cereals, cereal products	1000
  Patulin	Penicillium expansum	Apples, apple juice, and concentrate	50	10-50

Ergot Toxin

• Fungus: Claviceps purpurea produces sclerotia (ergots) in place of grain in infected cereals/grasses.
• Ergots: Contain alkaloids called ergot.
• Ergotism: Caused by ergot toxin:
  • Convulsive ergotism: Affects the nervous system, causing violent convulsions.
  • Gangrenous ergotism: Blood capillaries contract, leading to oxygen starvation and tissue damage.
• Medical uses: Ergot alkaloids relieve certain migraines and control hemorrhaging after childbirth.
• Ergotamine: Is lysergic acid, which can be chemically altered to produce the hallucinogenic drug LSD (lysergic acid diethylamide).

Ergotism

• Historically known as: "holy fire" or "St. Anthony's fire."
• Outbreaks: France (1093), Russia (1926), Ireland (1929), France (1953), India (1958), and Ethiopia (1973).

Aflatoxin

• Producers: Mainly Aspergillus flavus and A. parasiticus, normally present in soil and various organic materials.
• Strains:
  • A. flavus: produces aflatoxins B1 (AFB1) and B2 (AFB2).
  • A. parasiticus: produces AFB1, AFB2, G1 (AFG1), and G2 (AFG2).
• Favorable conditions: Stored grains and oil-rich crops, such as peanuts and cottonseed.
• Affected Crops: cereals (maize, rice, barley, oats, and sorghum), peanuts, ground nuts, pistachio nuts, almonds, walnuts, and cotton seeds.
• Effects: Carcinogenic, teratogenic, hepatotoxic, mutagenic, and immunosuppressive, primarily affecting the liver.

Aflatoxins Contamination and Effects

• Aflatoxin absorption: Aflatoxins absorbed from the gut pass to the liver, causing liver cancer.
• Milk contamination: Milk can be contaminated with aflatoxin M1 (AFM1), detectable 12–24 hours after a cow consumes feed contaminated with AFB1; the concentration of AFM1 correlates to AFB1 levels in feedstuffs.
• Contaminated dairy products: AFM1 can also be detected in dairy products like cheese.
• Human symptoms: Acute aflatoxicosis is characterized by vomiting, abdominal pain, pulmonary and cerebral edema, coma, convulsions, and death.
• Animal effects: Gastrointestinal dysfunction, reduced reproduction, lowered milk and egg production, and anemia.

Sporidesmin

• Source: Found in spores of Pithomyces chartarum, a saprotroph growing on dead leaf sheaths at the bases of pasture grasses.
• Location: Common in New Zealand, Australia, and South Africa, causing facial eczema in sheep and cattle.
• Symptoms in infected cattle: Blistering sores on exposed body parts (face, udders) and damage to internal organs.

Patulin

• Producers: Many species of Penicillium and Aspergillus, including Penicillium expansum (common apple-rot fungus).
• Mode of Infection: P. expansum causes a soft, watery rot when spores enter the apple skin through wounds.
• Effects: Can cause edema and hemorrhaging when ingested and is carcinogenic in experimental animals.
• Recommendation: It is unwise to eat any part of a rotted apple.

Ochratoxins

• Discovery: Discovered in 1965 in South Africa; produced by Aspergillus ochraceus, Penicillium verrucosum, and other Penicillium species.
• Most important toxin: Ochratoxin A (OTA).
• Agricultural Commodities: Corn, wheat, barley, flour, coffee, rice, oats, rye, beans, peas, and mixed feeds. Also wine, grape juice, and dried vine fruits.
• Animal-derived products: Can contaminate meat and milk; found in human milk.
• Major contributors to intake: Coffee and wines are major contributors to OTA intake.
• Effects: Acutely nephrotoxic and hepatotoxic; linked to Balkan Endemic Nephropathy (BEN).

Roquefort Cheese and Sick Building Syndrome

• Roquefort cheese: Produced from goats’ milk and inoculated with Penicillium roqueforti. Contains low levels of roquefortine, but it’s not considered hazardous.
• Sick building syndrome: Associated with dampness and condensation, encouraging the growth of several mould fungi, including Stachybotrys chartarum.
• Toxin production: Stachybotrys chartarum produces the toxin trichothecene.
• Historical impact: This fungus was implicated in the death of thousands of horses in the Soviet Union in the 1930s when the animals were fed on contaminated hay.

Chapter 11 Fungal Ecology

Ecology

• Definition: The study of organisms in relation to each other and their environment.
• Fungi's role: Primary agents of decomposition in terrestrial and aquatic environments.
• Functions: Breaking down and recycling cellulose and hemicelluloses, decompose wood, degrade natural and man-made materials, and can cause diseases in plants, animals, and humans.

Coprophilous Fungi

• Definition: Dung-loving fungi, are saprobic fungi that grow on animal dung.
• Substrate for study: Succession in fungal communities is best studied on cow dung.
• Colonization Process:
  • Initially, lower subdivision fungi like Pilobolus (a zygomycete) colonize the dung.
  • Later stages are dominated by climax species in Basidiomycetes, such as mushrooms.
• Ecological Significance: Dung is a valuable source of organic matter, providing a habitat for saprotrophs and a site for studying ecological succession.

Saprotrophic Fungi - Pioneers

• Distribution: Several species of Mucor, Rhizopus, and other Zygomycota are commonly found in soils or fecal-enriched materials and in the rhizosphere (root zone) of plants.
• Nutrient Utilization: These fungi utilize sugars and other simple soluble nutrients.
• Pioneer Concept: Pioneer fungi colonize a substrate first, followed by other fungi.
• Rapid Response: Spores of pioneer fungi germinate and grow rapidly in response to soluble nutrients.
• Asexual/Sexual Reproduction: Within a few days, they produce asexual dispersal spores or sexual resting spores.
• Characteristics: Pioneer fungi typically have a short exploitative phase but high competitive ability and cannot degrade complex structural polymers like cellulose.

Saprotrophic Fungi and Polymer Degradation

• Sapstain fungi: Colonize newly felled trees and are typical pioneer saprotrophs. Sapstain is a discoloration considered a fault in timber.
• Extended Growth Phase: Several fungi have an extended growth phase on major structural polymers like cellulose, hemicelluloses, or chitin.
• Cellulose Degradation Example: When cellulose-rich materials (e.g., cereal straw) are buried in soil, they are colonized by fungi such as Fusarium and Trichoderma spp.
• Chitin Degradation Example: Chitinous materials are colonized by another group of fungi, such as Mortierella spp. (Zygomycota) in soil or Chytridium confervae (Chytridiomycota) in freshwater habitats.
• Cellophane Case: Cellophane material buried in soil is colonized by cellulolytic chytrids (e.g., Rhizophlyctis rosea).

Fungi that Degrade Recalcitrant Polymers

• Lignocellulose Degradation: Fungi that degrade more resistant polymers, such as lignocellulose (cellulose complexed with lignin), often predominate in the later stages of decomposition.
• Basidiomycota Examples: Several of these are Basidiomycota, including Mycena galopus, a common small toadstool in woodland leaf litter, and fairy ring fungi.

Secondary (Opportunistic) Invaders

• Opportunity for growth: At many stages in the decomposition of natural materials, there are opportunities for secondary invaders to grow and feed on the breakdown products released by enzyme action (e.g., Thermomyces lanuginosus).

The Fungal Community of Composts

• Initial composition: A mixture of weak parasites and pioneer saprotrophic fungi are found in the first few days. Many were present on the original material, including leaf-surface fungi (e.g., Cladosporium), with maximum growth at 57–60°C.
• Thermophilic fungi: A few thermophilic fungi also grow in the first few days.
• Cellulolytic colonizers: Cellulolytic species of Ascomycota and mitosporic fungi colonize after peak-heating and grow over the next 10–20 days (e.g., Aspergillus fumigatus with max growth at 55°C, or Thermomyces lanuginosus with max growth at 62°C).
• Temperature Decrease: As the temperature falls below 35–40°C, the thermophilic fungi start to decline, though A. fumigatus remains active.
• Mesophilic Colonization: The compost is then colonized progressively by mesophilic fungi (e.g., Fusarium and Basidiomycota such as Coprinus cinereus, with max. growth at 40°C).
• Recalcitrant Polymer Degraders: Coprinus spp. represent the degraders of recalcitrant polymers. They utilize lignocellulose and are highly antagonistic to many other fungi, damaging their hyphae on contact by a process termed hyphal interference.
• Commercial Mushroom Introduction: At this stage, spores of the commercial mushroom, Agaricus bisporus (edible mushroom), can be introduced into the compost once this has cooled to below 30°C.
• Nutrient Balance: An adequate supply of mineral nutrients is essential for composting and for the decomposition of organic matter in general: carbon-to-nitrogen (C:N ratio of approximately 10 : 1).

Lignocellulose and Biotechnology of Wood-Decay Fungi

• Abundance and Potential: Lignocellulose (cellulose complexed with lignin) is abundant as a byproduct of the wood-processing industries and also in crop residues, presenting potential as a cheap commercial substrate.
• Cellulose Degradation: Cellulose can be degraded to sugars, which could be used to produce fuel alcohol by microbial fermentation as an alternative to fossil fuels.
• Lignin Degradation: Phanerochaete chrysosporium (white rot fungus) produces lignin peroxidase to degrade lignin, offering a potential solution to degrade lignin from cellulose.
• Further Utilization: Cellulose can be further digested to sugars and used in industries.
• Bioremediation: White-rot fungi and their enzyme systems also have potential for bioremediation of land contaminated by aromatic pollutants, and many other processes.

Chapter 12 Fungal Biology: Fungal Interactions

12.1 Terminology of Species Interactions

• Competition: One species excludes another by exploiting resources (space, substrate) faster or more efficiently.
  • Sometimes called exploitation competition.
• Antagonism: One species excludes or replaces another by directly affecting it through:
  • Antibiotic production
  • Parasitism
  • Toxins
  • Waste products
  • Sometimes called interference competition or combat.
• Commensalism: Two species coexist to the benefit of one or both (mutualism).
  • Examples:
    • Lichens (Algae & Fungi)
    • Mycorrhizae

12.2 Antibiotics and Their Roles in Species Interactions

• Well-known fungal antibiotics:
  • Penicillins from Penicillium chrysogenum: Broad-spectrum antibiotics.
  • Cephalosporins from Cephalosporium: Effective against Staphylococcus and Streptococcus.
  • Griseofulvin from Penicillium griseofulvum: Antifungal.
• Recently discovered antibiotics:
  • Fusidic acid from Fusidium coccineum: Effective against Gram-positive bacteria.
  • Fumagillin from Aspergillus fumigatus: Used against parasitic protozoa in veterinary medicine.
  • Sordarin from Sordaria (Ascomycota): Used to control fungal infections in humans.
• Fungi produce: 1000+ known antibiotics.

Antibiotics in Natural Environments: Control by Fluorescent Pseudomonads

• Fluorescent pseudomonads: Bacteria found on plant roots at high population levels.
  • Detected using “King’s B agar” (iron-deficient medium).
  • Produce iron-chelating compounds (siderophores) to capture iron.
    • Siderophores chelate ferric ion (Fe^{3+}).
    • Reabsorbed through a specific membrane protein, and Fe^{3+} is reduced to Fe^{2+} within the cell.
• Control Mechanism: Pseudomonas fluorescens and P. aureofaciens effectively control fungi by depriving them of siderophores.
  * Iron is essential for various proteins and pigments in both plants and pathogenic fungi.

Antibiotics and Disease Control by Trichoderma Species

• Action Mechanism: Trichoderma species coil around hyphae of other fungi (e.g., Rhizoctonia solani), penetrating and disrupting them.
• Antibiotic Production: Produce volatile and nonvolatile antibiotics active against fungi, bacteria, or both.
• Trichodermin: Well-known antibiotic produced by Trichoderma spp.

Biocontrol Formulations of Trichoderma

• Commercial Use: Several commercial formulations of Trichoderma spp. are used to control other fungi.
• TrichodexTM: (T. harzianum) for control of Botrytis cinerea (gray mold) on grapes.
• TrichodowelsTM: Trichoderma spp. are also impregnated into dowels to control wood-rot fungi in trees (marketed in New Zealand).
• Rhizosphere Competence: Trichoderma harzianum strain T-22 is a “rhizosphere competence” strain:
  • Colonizes the entire root system.
  • Persists throughout the crop's life.
  • Protects the crop against infection.

Trichodex

• Pathogen Controlled: Botrytis cinerea is a ubiquitous and damaging fungal pathogen.
• Major Pest in Nurseries: Major pest in container and bareroot nurseries.
• Usage: Trichodex is used to control B. cinerea in grapes and other crops worldwide.

Hyphal Interference

• Behavior: Several Basidiomycota antagonize other fungi at points of contact.
• Speed: Occurs rapidly (within minutes) after hyphal contact.
• Localization: Localized to a single hyphal compartment.
• Containment: Damage is contained by zones of dense, coagulated cytoplasm on either side of the contact point.

Mycoparasites: Fungi That Parasitize Other Fungi

• Mycoparasites: (In Zygomycotina) parasitize other fungi.
• Necrotrophic: Invade and destroy other fungal cells, then feed on the dead cell contents.
• Biotrophic: Establish a specialized feeding relationship by:
  • Producing haustoria to penetrate and absorb nutrients from living fungal hyphae.
  • Using nutrients to produce sporulating structures on the host colony.
• Process:
  • Spores germinate, and the germ tube penetrates the host.
  • The germ-tube tip produces an appressorium on the host surface.
  • A penetration peg enters the host to form a haustorium.

Haustoria – Fungi Inside Fungal Cells

• Appressorium (ap) and branched haustorium: of the mycoparasite Piptocephalis unispora (Zygomycota) in a fungal host.
• Hw: host wall
• ol and il: outer layer and inner layer of Piptocephalis wall
• Extrahaustorial membrane (e): haustorium is surrounded by a continuous membrane.

Potential Biocontrol Agents

• Alternative to Chemical treatments: Biocontrol agents serve as alternative to chemical or synthetic fungicides.
• Effective species: The mitosporic fungus Verticillium biguttatum is a biotrophic mycoparasite with potential for biocontrol of specific Rhizoctonia solani strains that cause black scurf disease of potatoes (Solanaceae).

Biotrophic Mycoparasites

• Ampelomyces quisqualis: a naturally occurring biotrophic mycoparasite.

Chapter 15 Fungal Parasites of Insects & Nematodes

Insect-Pathogenic Fungi

• Prevalence: Fungi commonly attack insects, nematodes, and other invertebrates in natural environments.
• Population Regulation: They act as natural population regulators, helping to keep insect and nematode pests in check.
• Biocontrol Agents: Some insect-pathogenic and nematode-destroying fungi can be exploited as biocontrol agents; some are available commercially as alternatives to chemical pesticides.
• Entomopathogenic: Fungi are specifically adapted to parasitize insects and depend on them for survival.
• Common Species: Beauveria and Metarhizium species are commonly found in natural environments and have strong potential for practical control of insect pests, especially in glasshouses and other protected cropping systems.
• Beauvericin: Beauveria bassiana produces beauvericin toxin, which extensively invades insect tissues before death.
• Oosporein: Beauveria spp.