14: Microbial Diversity - Mutualistic Interactions

Microbial Diversity BR19920 - Lecture 14: Mutualistic Interactions

General Information

  • Instructor: Gareth Griffith (gwg@aber.ac.uk, @AberMycol)
  • Lecture Date: 2025

Practical MCQ Exam Details

  • Date: Wednesday, 26th Feb 10:10 AM (TODAY!! after this lecture)
  • Location: Edward Llwyd Labs (see timetable for specific lab: 101, 114, 201, 202, 301)
  • Format: Exam conditions, closed-book
  • Questions: 35 multiple-choice questions (5 possible answers, only one correct)
  • Allowed: Calculator
  • Weighting: 25% of module mark
  • Extra credit: Extra 1% if all correct.
  • IER Requirements: If any IER requirements, please details to me via email

Biotrophic Pathogens

  • Examples: Rusts, Smuts, Powdery mildews
  • Infection Mechanism: Form appressoria to infect and feeding haustoria
  • Characteristics: Highly evolved; do not kill the host

Fungal Structures

  • Haustorium: A specialized structure of parasitic fungi for nutrient absorption from host cells.
  • Haustorial mother cell
  • Neckband
  • Extrahaustorial membrane: A membrane surrounding the haustorium within the plant cell.
  • Extrahaustorial matrix

Stem Rust (Puccinia graminis)

  • Host: Cereal crops
  • Life Cycle: Heteroecious (two hosts: Barberry and grasses)
  • Spore Types: Three spore types
  • Race: Ug99 (Uganda 99) is spreading across Africa/Asia
  • Historical Context: In 17-19th C, removal of alternate host (barberry) inhibited sexual cycle

Coffee Rust (Hemileia vastatrix)

  • Discovery: First discovered in Kenya (1861) but spread to Asia.
  • Impact: From 1869-1890, Sri Lanka coffee exports dropped by >95% as farmers switched to tea
  • Historical Significance: Reason why tea is consumed in the UK

Powdery Mildews

  • Obligate biotrophs
  • Structures: Conidia, Ascospores, Ascus, Cleistothecium, Conidiophore, Mycelium, Haustoria
  • Infection of rose leaves, buds, twigs
  • Overwintering structures: Cleistothecia and mycelium
  • Sexual reproduction: Ascogonium and Antheridium

Powdery Mildew Resistance

  • Resistance screening of barley varieties to powdery mildew (Blumeria graminis var hordei)

Cereal Powdery Mildew (Blumeria spp.)

  • Host Range: Very specific
    • Wheat: B. graminis var. tritici
    • Barley: B. graminis var. hordei

Host-Pathogen Interaction

  • Blumeria spores sense host surface within minutes of landing
  • Host must detect fungus quickly.
  • Fungal attack is stopped by programmed cell death
  • Structures: Multiple primary germtubes, Primary germtube, Secondary germtube and appressorium, Spider thread

Rice Blast Disease (Magnaporthe oryzae)

  • Reference: Dangol et al. (2019). Plant Cell, 31: 189–209
  • Mechanism: H2O2 Fenton reaction
  • Susceptible: Haustoria formed
  • Resistant: Programmed cell death

Symbiosis

  • Definition (De Bary):
    • Mutualism: +/+
    • Neutralism: 0/0
    • Parasitism: +/-
  • Considerations:
    • Outcome vs Mechanism
    • Cost / benefit analysis - Variable/Hard to measure
    • Mutualism vs Competition

Key Figures

  • Anton de Bary (1831-1888)
  • Rev. Miles Joseph Berkeley (1803-1869)

Cost/Benefit Analysis in Mutualism

  • Reference: JANZEN (1985)
  • Example: Oak, Squirrel, and Acorns
    • Bury + Recover: +/-
    • Bury + Forget: 0/+
    • Acorns germinate

Smelly Cuckoos

  • Question: Parasites or mutualists?
  • Reference: Canestrari et al. (2014), Science, 21st Mch, v343, pp1350-1353
  • Finding: Cuckoo chick secretions deter predators of crow chicks

Lichen Symbiosis

  • Mutual benefit is obvious
  • Ascomycete apothecia

Plant Roots and Fungi/Bacteria

  • Situation is less clear for some fungi/bacteria inhabiting plant roots

Mycorrhizas

  • Final Step in Nutrient Cycle: ‘Close the loop’
  • System Components: Fungus & Root
    • DECOMPOSER SUBSYSTEM
    • PLANT SUBSYSTEM
  • Prevalence: >75% of plant species are mycorrhizal
  • Mycorrhizas

Why Mycorrhizas?

  1. Surface area (SA) : Volume ratio
    • Roots (>30mm diam); Hyphae (2-10mm diam)
    • >10x greater SA per unit biomass (halving the cell diameter doubles the SA/Vol ratio)
  2. Exoenzymes: Access to diverse nutrient sources
  3. Plant growth not limited by carbon: Exchange C for nutrients

Nutrient Exchange in Mycorrhizas

  • FUNGUS PLANT
    • N, P, water C
    • trehalose sucrose
  • Efficient “haustorial” transfer interface
  • Use of radiolabels
  • MASS FLOW
    • Higher C demand
    • Higher N,P demand

Types of Mycorrhizas

  • Up to 30% of ‘root’ biomass is fungal
  • Structure
    • Intracellular?
  • Main nutrients
  • Hosts
  • Fungal symbiont
    • ECTO (sheathing)
    • AMF
    • Ericoid
  • MYCORRHIZAL TYPES
    • Glomeromycota
    • Basidiomycota
    • Ascomycota
    • Arbuscules
    • Coils

Ectomycorrhizas

  • Only colonize outer root tissues (sheathing mycorrhizas)
  • Cause change in root morphology
  • Infect many tree /shrub species
  • Fungi involved are mainly basidiomycetes
  • Structures: Sheath, Hartig net

Species-Specific Ectomycorrhizal Morphology

  • e.g. Amanita muscaria

Arbuscular Mycorrhizal Fungi (AMF)

  • Phylum: Glomeromycota (e.g. Glomus)
  • Ancient (400 MY)
  • Obligate biotrophs / Wide host range
  • Form intracellular arbuscules
  • arbuscules inside host cell
  • spores found in soil
  • Appressorium (entry)
  • Arbuscules (feeding)
  • Vesicles (storage)
  • AMF mycelium extends into soil (spores formed here)
  • Some species for lipid vesicles inside roots formerly called VAM (vesicular-arbuscular mycorrhizas)

Ericaceous Mycorrhizas

  • Intracellular
  • Coils are short-lived (3-4 wks)
  • e.g. Hymenoscyphus
  • Hymenoscyphus fructigenus
  • Structures:
    • hyphal coil
    • cortical cell
    • endodermis of root
    • lysed hyphal coil

Mycorrhizal Interfaces

  • Ecto
    • intercellular fungus
  • Eric
    • HAUSTORIUMI
    • intracellular fungus
  • AMF
    • intracellular fungus
  • host
  • perifungal membrane
  • interface

Herbivore Digestion

  • Herbivores need access to lignocellulose energy BUT no cellulases
  • Cellulolytic GUT MICROBES -Bacteria / Protozoans / Rumen fungi
  • Most animals have mutualistic associations with microbes
  • Fungus Farming = Inside -out guts

Termites

  • Major agents of wood decay in tropics
  • Can cycle up to 25% NPP in savannah ecosystems
  • Old World termites (Macrotermitinae) have species- specific Termitomyces ‘Fungus Gardens’ (Agaric)
  • New World termites have protist gut symbionts, Hence their larger body size
  • Examples:
    • Macrotermes
    • Acanthotermes
    • Anoplotermes

Termitomyces and Old World Termites

  • Tall earth nests (= fermenters)
  • Termitomyces titanicus (2.5kg mushrooms)
  • Termites collect and chew wood pulp to feed the fungal ‘comb’
  • Termitomyces forms swollen hyphae which are eaten by the termites

Termite-Fungus Symbiosis

  • FORAGING
  • COLLECTION
  • COMMINUTION
  • REDUCED
  • MICROBIAL
  • FLORA
  • CHITINASES + ANTIFUNGALS
  • IN SALIVA
  • Fungal
  • enzymes
  • continue to
  • N RECYCLING
  • FAECES
  • FOOD STORE
  • act in the
  • termite gut
  • COMB
  • MIXED WITH TERMITOMYCES)
  • TERMITES
  • FEED FROM BELOW)
  • FUNGUS
  • WHITE ROT
  • OCCASIONAL
  • FRUIT BODIES

Termite Nest Environment

  • WARM AIR
  • COOL AIR
  • $30°C ±2°C$
  • COMBS
  • WOOD PULP STORE
  • Convective
  • Cooling

Evolution of Fungus Farming

  • Ambrosia Beetle
  • Termites
  • Ants
  • Evolution of 'tasty' swollen hyphae

Fungus-Farming Ants

  • Attine ants (New World) and Attamyces
  • Dump
  • chambers

Ant-Fungus-Bacteria Interactions

  • The ants carry Streptomyces bacteria
  • Escovopsis: Ascomycete parasite of the fungus garden
  • Specific antibiotic to kill pathogen
  • Promotes Attamyces growth
  • ATTINE ants + Attamyces fungus

Significance of Ant-Fungus Mutualism

  • Fungus farmers show way to new drugs
  • In a mutually beneficial symbiosis, leaf-cutting ants cultivate fungus gardens, providing both a safe home for the fungi and a food source for the ants.
  • This 50-million-year-old relationship also includes microbes that new research shows could help speed the quest to develop better antibiotics and biofuels.
  • The bacteria churn out an antibiotic that protects the ants' fungal crops from associated parasitic fungi (such as Escovopsis).
  • Currie suggests that the newfound bacterial and fungal enzymes might be efficient at digesting cellulose because they have evolved for centuries along with the ant-fungal symbiosis.