Fungal Growth and Morphogenesis I Flashcards
What are fungi?
A group of eukaryotic organisms - cells have a nucleus - equal in rank to plants and animals.
What is the impact and significance of fungi?
- Major decomposers of organic matter, especially of structural polymers such as lignin, cellulose.
- Geoactive agents: rock and mineral dissolution; element cycling.
- Plant pathogens - fungi cause ~70% of all the major crop diseases.
- By forming mycorrhizas with roots of most (~90%) land plants, fungi play a vital role in plant and crop productivity.
- Many commercial products such as ethanol, organic acids, enzymes, antibiotics, etc., but also some potent human/animal toxins.
- Animal including human pathogens - with the increase in immunosuppressive conditions, including AIDS, fungi are emerging as a significant group of life-threatening human pathogens.
80% of all plants have a fungal symbiont.
Commercial products from fungal fermentation.
What are general fungal characteristics?
- Eukaryotic, typically grow as filaments termed hyphae (~5-10 μm in diameter), surrounded by a wall, and extend at their tips - apical growth. All the hyphae form the mycelium.
- Fungi are chemoorganotrophic (heterotrophic) - obtain C and energy from simple, soluble nutrients (sugars, amino acids, etc.) and by releasing extracellular enzymes to degrade polymers.
- Yeasts are fungi that are predominantly unicellular, though many can produce hyphae.
- Fungi are dispersed by means of spores produced by either an asexual or a sexual process.
Chitin is present in the wall.
Obtain carbon energy from organic compounds.
Fungus morphology?
- Typically, fungi grow as filaments termed hyphae (singular: hypha), about 5-10 μm in diameter.
- Hyphae are surrounded by a wall and extend at their tips - apical growth.
- The hyphae branch repeatedly to form the ‘body‘ of the fungus - the mycelium.
Many interactions within mycelium.
Where may macroscopic fruiting bodies arise from?
Macroscopic fruiting bodies may arise from the mycelium.
Sexual cycle.
Fruiting body made of gathered differentiated hyphae.
Mass of mycelium underneath.
What are features of yeasts?
Saccharomyces cerevisiae - brewer’s or baker’s yeast.
Candida albicans – thrush, candidiasis.
- Yeasts are fungi that are predominantly unicellular.
- Most yeasts reproduce by budding.
- Some can exhibit filamentous growth.
Saccharomyces- budding. Candida- filaments. Filaments integral to pathogenesis. Some plant pathogens show this too. Dimorphism/polymorphism.
How do Schizosaccharomyces pombe reproduce?
Fission yeast.
Reproduce by binary fission.
Make colonies like bacteria.
Model org. for cell division and replication.
Only found in lab, not in the environment.
What is pseudohyphal growth?
- Many yeasts, especially pathogens, exhibit pseudohyphal growth – Candida albicans, Candida tropicalis, Candida glabrata (and S. cerevisiae).
- Pseudohyphal growth is a distinct vegetative growth form – different to budding and from hyphal growth.
- Constricted at septal junctions.
- Cells synchronously divide.
S.cere can have filamentous growth under certain circumstances.
What is the fungal structure?
Hyphae possess a cell wall and contain eukaryotic organelles:
•often multinucleate.
•contains many mitochondria, ribosomes, Golgi, rough and smooth ER, vacuoles.
•membrane-bound vesicles.
•microtubules and microfilaments.
Vesicles for cell membrane synthesis and apical growth.
Cellular compartments.
What is the fungal wall?
- Complex and fibrillar with an amorphous matrix.
- True fungi have chitin in their walls – a polymer of n-acetyl glucosamine.
- Many fibrils align to form microfibrils of chitin of between 10 and 25 nm diameter.
Fibrils make strength.
Monomer- similar to cellulose in plants.
The cell wall of fungi, showing glycoproteins and mannoproteins in the layer of the wall and an inner layer of chitin. The presence of different types of glucans and chitin varies between different fungal species.
Mycelium compartments?
- The living part of the mycelium consists of the apical compartment and a few compartments behind it.
- Mycelium beyond ~ 5 septa is usually moribund (unless it has a specialist function).
- Compartments are usually 3-10μm wide and 50μm long.
- Apical cell is usually 300-400μm long, can be up to 2mm.
Intercalary compartments – shorter than apical compartment and containing 1 to several nuclei.
Apical compartment – contains 1 to > 75 nuclei.
What are septa?
- Septa are perforate cross walls within hyphae.
- Their structure varies from group to group.
- Protect the mycelium from catastrophic events - plugged septal pores reduce loss of cytoplasm from intercalary compartments.
- Can allow or prevent large organelles from passing through.
- Control sexuality.
Allow passage of nutrients, organelles, in cases of sexual reproduction.
Fungal tip growth?
- Hyphae extend at the tips, where extracellular enzymes are released and where new wall materials are made.
- Rate of tip extension can be rapid - up to 40 μm min-1.
- Extension is supported by the continuous movement of materials into the tip from older regions of the hyphae - a hypha is really continuously moving mass of protoplasm in a continuously extending tube.
- This unique mode of growth - apical growth - accounts for much of their environmental success – exploration, colonization, pathogenesis and symbiosis.
Neurospora is fast growing.
Are able to avoid each other so as not to compete or interfere.
What are mechanisms of apical growth?
The extreme tip of a growing hypha contains a body termed the Spitzenkörper (apical body). This is a cluster of small, membrane-bound vesicles embedded in a meshwork of actin microfilaments.
Spitzenkörper is always present in growing tips, disappears when growth stops, reappears when growth restarts, and changes position within the apex hyphae change direction.
Cytoskeleton and molecular motors?
Cytoskeletal components believed to be the driving force behind apical growth.
- Actin - microfilaments.
- Microtubules.
- F-actin – Actin filaments at hyphal tip, movement of vesicles (actin-myosin motor).
- Tubulin – Microtubules (absent from tip), movement of organelles including nuclei (tubulin-kinesin motor).
Tip- fusion of vesicles with membrane to build up/break down wall.
Driven by cytoskeletal motors.
Quick process.
