Fungi

positives of fungi

• direct benefits to human health

• human nutrition

• food industry

• ecosystems

• mutualism / symbioses

• biological control of crop damage

• chemical control of crop disease

• fungal genes/genomes

direct benefits to human health

• Antibiotics 

• Steroids 

• Heterologous gene expression 

human nutrition

• Bread/beer 

• Quorn 

• Mushrooms 

food industry

• Fungal enzymes 

• Citric acid 

• Palm oil replacement 

ecosystems

formation of organic forest soils from leaf litter and wood

mutualism / symbioses

• Endophytes in plants can repel insect pests 

• Lichens comprise fungi and algae or cyanobacteria 

• Mycorrhizal associations with plant roots (many trees are dependent on these) 

biological control of crop damage

green alternatives to pesticides

fungal genes/genomes

• Largest number of completely sequenced eukaryotic genomes 

• Diverse enzymes for degradation of polymers 

negatives of fungi

• human diseases

• plant diseases

• degradation

human diseases

• 2 million deaths per year 

• Eg skin diseases, allergies, mycotoxins 

plant diseases

• Main cause of crop losses worldwide 

• Contribute greatly to chronic problems with global food security 

degradation

• Building materials (wood, paint, plaster) 

• Petrochemical fuel 

• Food 

what are fungi

• Represent one of the three major evolutionary branches of multicellular organisms 

• There is no fungal synapomorphy (Bruns' law) 

• Opisthokonts (sister to animals) 

• Estimated millions of species, but only 90,000 have been described 

nutrition of fungi

• Heterotrophs 

• Absorb nutrients from outside their cells (saprophytic) 

• Most produce many extracellular degradative enzymes to produce this 

roles of fungi ecologically

• Decomposers 

• Parasites (e.g. pathogens to plants or animals) 

• Symbionts (mutualists) 

morphology

• Typical form = hyphae (network of filaments) 

  • Comprise tubular cell walls surrounding plasma membrane and cytoplasm 

  • Hyphae form mycelium (interwoven mass) 

  • Facilitates flexible, exploratory growth and nutrient absorption 

  • Maximises SA:VOL 

  • Grows by adding to hyphal length from a growing tip 

• Yeasts are usually unicellular and reproduce by budding 

hyphae

• Most hyphae are divided into cells by walls called septa 

  • Are able to specialise and take on different roles 

  • Increases versatility 

  • Septa become plugged on damage and a new hypha grows behind the damaged cell 

• Pores allow organelles to pass through 

• Some hyphae have no septa (coenocytic) and is multinucleate\ 

  • More vulnerable to damage 

spores, hyphae and colonies

• Colonies develop from a single germinating spore 

• They produce a germ tube (young hypha) that grows and branches behind the tip 

• In turn these branch behind their tips 

• Eventually a circular (agar) or spherical (in 3D) colony forms 

• Chemotactic (growth towards or away from a chemical stimulus) 

• Anastomosing (fusing) of hyphae can happen at the centre of a colony where nutrients run out 

  • Can only happen between compatible mating types or within an individual 

hyphal anastomosis

• Neutral 

  • Hyphae do not recognise each other 

  • In different anastomosis groups 

• Compatible 

  • Hyphae of a single compatibility group attach and fuse by tip to tip contact 

  • A network with cytoplasmic continuity is created 

• Incompatible 

  • Hyphae of 2 different compatibility groups fuse 

  • Cytoplasmic death of fused hyphae 

mycelium

• Shifts nutrients and water through hyphae 

• Apical growth zone (outside) 

• Storage zone (middle) reserves glycogen and lipids 

• Senescence zone (centre) often dark pigmentation and lysis 

spitzenkorper

The Spitzenkörper is a structure found in fungal hyphae that is the organizing center for hyphal growth and morphogenesis. It consists of many small vesicles and is present in growing hyphal tips, during spore germination, and where branch formation occurs. Its position in the hyphal tip correlates with the direction of hyphal growth. The Spitzenkörper is a part of the endomembrane system in fungi.

specialisation of hyphal structures

• Adapted for infection 

  • Produce an appressorium which is capable of adhesion, generating pressure 

  • Produces enzymes which attack the host cuticle 

  • Can also form mycelial cords which enable long distance spread and colonisation (transmission of nutrients or water) 

• Adapted for mutualism and parasitism 

  • Produce haustoria inside the living host cells which are used for nutrient absorption 

  • Host membrane invaginates to accommodate these 

  • Rich in hexose and amino acid transporters 

cell wall

glucans and chitin

spores

• Numbers produced can be vast 

• Specialised to be carried by wind/water/vectors 

• Can be sexual/asexual 

fossils and history

Don't fossilise well 

• Best known evidence is from the Devonian period 

• Likely they co-evolved with early land plants 

More closely related to animals than plants 

• Resemble one of the earliest branches around animals  

• Common ancestor appears to be a protozoan of the choanoflagellates 

true fungi

• mycota

• eumycota

• fungi

main lineages

• phylum chytridiomycota

• phylum zygomycota

• phylum glomeromycota

• phylum ascomycota

• phylum basidiomycota

phylum chytridiomycota

• The only true fungi to produce motile, flagellate zoospores 

• Can undergo prolonged amoeboid crawling using flagella 

• Primary colonisers and degraders of organic matter 

• Often anchored by rhizoids (function like hyphae) 

• Some are obligate intracellular parasites of animal, algae or plants 

• Batrachochytrium dendrobatidis 

  • One of the few organisms known to cause extinction via outbreak of disease 

  • Spores burrow into skin and develop into a thallus 

  • Spores released from a sporangium 

phylum zygomycota

• Hyphae lack cross walls (coenocytic) 

• Produce a thick walled zygospore, formed by sexual process involving two gametangia 

• Produce asexual spores (sporangia) by cytoplasmic cleavage from a sporangium 

• Parasites of nematodes, small invertebrates, other fungi 

• Some (Zygomycoses) are human pathogens 

• Saprotrophs in soil, dung, compost, ripe fruits 

• Rapid growth 

lifecycle

• A sporangium releases spores 

  • Can go through asexual reproduction to produce more fungi 

• If a + and – mating type meet they make a single cell to form a zygosporangium 

  • This is called heterokaryotic as it has 2 different mating types 

• A zygosporangium forms (resting sporangium) 

• Karyogamy occurs to form a diploid nuclei 

  • This can germinate to form a new sporangium (go through meiosis)

  • Releases haploid spores 

phylum glomeromycota

• 160 known species 

• Ecologically highly significant (80% vascular plants have them in roots as part of a mutualistic partnership) 

• Most form arbuscular mycorrhiza (grow between root cells and form large swollen vesicles 

• Intracellular tree-like branching structures (arbuscules for nutrient exchange) 

• Obligate biotroph (cannot normally be grown away from host plant) 

• Produce large spores up to 400 micrometres in diameter 

phylum ascomycota

• 75% of fungi described to date 

• Produce sexual spores (ascospores) in sacklike ASCUS 

• Sexual stage is borne in a fruiting body or ASCOCARP 

• Diverse morphologically 

• Include some of the most devastating plant pathogens 

• Over 40% form symbiotic relationships with green algae or cyanobacteria called lichens (96% lichens contain them) 

• Some form mycorrhizae 

• Some exist in plants between cells as harmless endophytes 

lifecycle

• Spores germinate and become conidia 

  • These can reproduce asexually 

• If mating type – land on already growing mating type + they can fuse (plasmogamy) 

• Forms a dikaryotic hyphae (dikaryotic Ascus) 

• Undergoes karyogamy  

  • A diploid nucleus forms 

• Meiosis occurs and 4 haploid nuclei become 8 ascospores 

• The spores are released from the asci 

  • They germinate to form mycelium and undergo sexual reproduction and grow more spores) 

phylum basidiomycota

• 37% of described species 

• Diverse fruiting body (basidiocarp) 

• Basidium is a cell in which karyogamy happens (becomes diploid -> meiosis to form basidiospores) 

• Complex dolipore spetum 

• Can degrade complex lignin (so are wood composers) 

• Two main groups of plant pathogens (rusts and smuts) 

lifecyle

• Spores germinate and form mycelia 

• Compatible mating type mycelia fuse together to form a dikaryotic mycelium 

• Grow almost entirely as a dikaryotic mycelia (into a mushroom) 

• Sexual reproduction occurs in the basidiocarp (found in gills) 

  • Becomes diploid 

  • Basidia branch out 

  • Meiosis occurs 

• Release spores called basidiospores 

more on phylum basidiomycota

• Spores grow on the edge of sterigmata 

• Have clamp connections to support the organism as it grows as dikaryotic  

  • Have to have 2 nuclei in each cell 

  • Looks like a lateral bulge 

Morphological groups 

• hymenomycetes  

  • Spores dispersed from sterigmata 

• Gasteromycetes 

  • Basidia open into cavities within the fruit body 

• Hemibasidiomycetes 

  • Rusts form teliospores 

  • Smuts form thick-walled chlamydospores 

  • Plant parasites 

microsporidia

• Intracellular parasites (mostly of animals) 

• Unicellular 

• Do not have mitochondria 

  • Had them but disappeared (still have mtDNA) 

• Linked to fungi via molecular phylogenetics 

infection of cells via polar tube eversion

• Shoot into a cell 

• Inject their cell contents into host cell 

• Once inside a host cell they can proliferate (meront) 

  • The meront may hijack cell processes e.g. nuclear replication and membrane production 

• Eventually leave cell  

cryptomycota

• Uniflagellate 

• Lack chitin 

• Seasonal cycles 

• Likely important in seas 

  • Nested in many key aquatic food networks and ecological interactions 

symbiosis

long term close association of different species, often leading to adaptation and coevolution

parasitism

symbiosis in which one species benefits at the expense of the other

rust parasitism

• Fungus infection:  

  • Rust fungi are often extremely host specific and have adaptations that are unique to individual hosts. 

  • They often rely on host cues to trigger infection and use very specific signals to manipulate plants during infection. 

• Coevolution of host and parasite:  

  • Plants continually evolve resistance to rusts, often with specific mechanisms that evade the fungal manipulation 

  • Rusts then evolve new mechanisms to infect hosts. This is a ‘classic’ system for coevolution. 

 

mutualism

symbiosis in which both species benefit

lichen mutualism

• Benefits to the alga:  

  • Fungi provide protection from desiccation and environmental stress

  • Fungi can provide nutrients from otherwise un obtainable substrates 

• Benefits to the fungus:  

  • Photosynthate – sugar and other energy providing nutrients 

  • In cases with cyanobacteria, then nitrogen is also captured 

biogeographic patterns in fungi

• Globally distributed fungi 

  • Aeroflora - the global fungi of the air 

  • Global plant pathogens (can be problematic as spread across the globe e.g. banana disease wiping out the Gros Michel) 

  • Global human pathogens 

  • Global saprophytes 

  • Globally distributed fungi can still show patterns of differentiation (but not always) 

• Locally unique fungi 

• Host specific or generalist fungi 

• Human driven dispersal of fungi 

why don’t all fungi spread

• Disposal restricted 

  • e.g. underground (only spread when dug up by an animal) 

• Climate restricted 

  • Can't grow everywhere 

• Host restricted 

  • Inside host 

adaptations for dispersal

• ballistospores

• cooperative dispersal

• spore morphology

• sporocarp morphology

ballistospores

• At the end of the sterigma 

• A droplet (Buller's drop) forms at the end of the sterigma 

• When the droplet gets to the right size, the spore is broken off and flies away (due to shift in mass, as the droplet moves to cover spore, changing centre of mass) 

cooperative dispersal

• Fungal spores generate their own air currents by dispersing together 

• They increase their overall travel distance  

• Improve navigation around obstacles 

• Cooperation can be between and within fruiting bodies 

fungi sensing the environment

• Must survive where they are or work to escape 

• Must respond to changes  

  • Can't run away like animals and are completely dependent on the external environment for nutrition 

• Light sensitivity 

  • Used for getting out of the ground, switching morphology, effective dispersal and finding correct environments 

  • Spores released towards light 

  • Can change pigments to reduce light stress 

• Oxygen sensing 

  • Fermentation in the absence of oxygen is the key driver in many ecological and host-pathogen interactions 

• Carbon dioxide sensing 

  • Find carbonic acids 

• Temperature sensing and response in fungi 

  • Yeast-hyphal switching 

  • Colder = hyphae 

  • Hotter = yeast (unicellular) 

• thigmotropism  

  • Response to physical stimuli 

• Spitzenkörper 

  • Central to the growth response to the environment in most fungi 

  • Disruption of it breaks many thigmotropic responses (will not hug the object) 

fungi and the host

• chemotaxis (sensing host)

• penetrating outer host defences (invading the host)

• invading host

chemotaxis (sensing host)

• Zoosporangium releases zoospores 

• Zoospores locate at the root tip and encysted 

• Random swimming motion (of fungi) becomes directional with a chemical stimulus 

Penetrating the outer host defences (invading the host) 

• Make appressoria 

• Sticks to host (adhesive) 

• Forms in response to many triggers, e.g.: 

  • Surface features 

  • Hardness 

  • Chemicals 

invading the host

• Necrotrophs 

  • Cause wall breakdown 

  • Produce cell wall degrading enzymes 

  • Secondary walls are rarely penetrated 

  • Produce secondary metabolites (often toxins) 

  • Use enzymes to degrade host defences and encourage hosts to destroy themselves via toxins 

• Biotrophs 

  • Effect minimal wall disruption 

  • Rely on signalling from both penetration structures and haustoria to avoid host defences 

fungi gaining nutrition from the host

• Fungi rely on both communication and efficient enzymes to gain nutrition from hosts 

• Sugar transporters are critical for fungi living with plants 

• Iron grabbing siderophores are key in animal environments where iron is low  

• Necrotrophs and saprotrophs rely on extensive degrading enzymes to obtain nutrition. Necrotrophs also exploit toxins to force hosts to 'leak' nutrients (toxins cripple cells to reduce defences as well) 

  • Various modes of action 

  • Membrane lipid peroxidation 

  • Inner mitochondrial membrane 

  • Chloroplasts 

  • Histone deacetylase 

• Saprophytes are able to get access to carbohydrate nutrients  

fungi manipulating hosts

• Insect host manipulation is well known (climbing, positioning, mating) 

• Plant host manipulation is often subtle and largely invisible inside plant cells (can be dramatic e.g. anther replacement) 

• Can secrete compounds into brains to change behaviour 

• Can use their own network (hyphae) to communicate and use the organism as a puppet 

• Infect sexual signs (become hyperactive and eventually more aggressive)