Fungal Interactions: Mechanisms and Exploitation

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.
• 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

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

Biocontrol Formulations of Trichoderma

• Several commercial formulations of Trichoderma spp. are used to control other fungi.
• Example: TrichodexTM (T. harzianum) for control of Botrytis cinerea (gray mold) on grapes.
• Trichoderma spp. are also impregnated into dowels (TrichodowelsTM) to control wood-rot fungi in trees (marketed in New Zealand).
• 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

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

Hyphal Interference

• Hyphal interference is a behavior of several Basidiomycota that antagonize other fungi at points of contact.
• Occurs rapidly (within minutes) after hyphal contact.
• Localized to a single hyphal compartment.
• 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 mycoparasites: Invade and destroy other fungal cells, then feed on the dead cell contents.
• Biotrophic mycoparasites: 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 the Piptocephalis wall.
• The haustorium is surrounded by a continuous membrane (the extrahaustorial membrane, labelled e).

Potential Biocontrol Agents

• Biocontrol agents are an alternative to chemical or synthetic fungicides.
• 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 is a naturally occurring biotrophic mycoparasite.
• Being developed for commercial control of powdery mildew fungi in glasshouse crops.