Molecular
Interactions
between Fungi
and their Hosts
,MOLECULAR INTERACTIONS BETWEEN FUNGI AND THEIR HOSTS....................................................................................1
...........................................................................................................................1
H1: INTRODUCTION .........................................................................................................................................................1
1.1 INTRODUCTION TO FUNGI .................................................................................................................................................... 1
1.2 TAXONOMY AND NOMENCLATURE ......................................................................................................................................... 2
1.3 ASCOMYCOTA ................................................................................................................................................................... 3
1.3.1 Saccharomycotina ................................................................................................................................................ 4
1.3.2 Pezizomycotina ..................................................................................................................................................... 4
1.4 BASIDIOMYCOTA ................................................................................................................................................................ 5
1.5 ZYGOMYCOTA ................................................................................................................................................................... 6
1.6 GLOMEROMYCOTA ............................................................................................................................................................. 6
1.7 CHYTRIDIOMYCOTA ............................................................................................................................................................ 7
1.8 DEUTEROMYCETES (FORMA SPECIALIS OR FUNGI IMPERFECTI) ..................................................................................................... 7
1.9 OOMYCETES (FUNGAL-LIKE ORGANISMS) ................................................................................................................................ 7
H2: CHARACTERISTICS AND VIRULENCE FACTORS OF PLANT PATHOGENIC FUNGI ............................................................8
2.1 INTRODUCTION ................................................................................................................................................................. 8
2.2 SPREAD OF FUNGI .............................................................................................................................................................. 8
2.3 CONTACT WITH THE HOST PLANT........................................................................................................................................... 8
2.4 MECHANISMS OF HOST PENETRATION .................................................................................................................................... 9
2.4.1 Penetration through natural openings or weak spots .......................................................................................... 9
2.4.2 Penetration through damaging the protective layer .......................................................................................... 10
2.5 COLONIZATION OF THE HOST .............................................................................................................................................. 11
2.6 VIRULENCE FACTORS OF PLANT-PATHOGENIC FUNGI ................................................................................................................ 13
2.6.1 Tropisms.............................................................................................................................................................. 13
2.6.2 Cell wall degrading enzymes .............................................................................................................................. 13
2.6.3 Toxins .................................................................................................................................................................. 15
2.7 MEANS TO EVADE HOST RESPONSES..................................................................................................................................... 19
2.7.1 Elicitors and plant defense mechanisms ............................................................................................................. 19
2.7.2 Strategies to evade host responses .................................................................................................................... 19
2.8 IDENTIFICATION OF GENES INVOLVED IN PHYTOPATHOGENICITY .................................................................................................. 20
2.8.1 Mutagenesis of the fungus ................................................................................................................................. 20
2.8.2 High-throughput screening procedures: functional genomics ............................................................................ 21
2.8.3 Signal transduction pathways activated prior to infection ................................................................................. 25
H3: PLANT DEFENSE MECHANISMS ................................................................................................................................ 29
3.1 EXISTING OR CONSTITUTIVE DEFENSE MECHANISMS ................................................................................................................ 29
3.1.1 Mechanical defense mechanisms ....................................................................................................................... 29
3.1.2 Chemical defense mechanisms ........................................................................................................................... 30
3.2 INDUCED DEFENSE MECHANISMS ........................................................................................................................................ 31
3.2.1 Phytoalexins ........................................................................................................................................................ 31
3.2.2 Cell wall modifications ........................................................................................................................................ 31
3.3 PATHOGENESIS RELATED PROTEINS (PRPS) ........................................................................................................................... 32
3.3.1 Enzymatic PRPs ................................................................................................................................................... 32
3.3.2 Peptide or protein inhibitors ............................................................................................................................... 33
3.4 INDUCTION OF THE DEFENSE: ELICITORS................................................................................................................................ 35
3.4.1 Biotic elicitors ..................................................................................................................................................... 35
3.4.2 Abiotic elicitors ................................................................................................................................................... 39
3.5 SIGNAL TRANSDUCTION .................................................................................................................................................... 40
3.5.1 Perception: effects on the membrane ................................................................................................................. 40
3.5.2 Internally generated signal molecules ................................................................................................................ 41
3.5.3 Systemic signals: SA, JA, and ethylene ................................................................................................................ 42
3.6 RESISTANCE AND SPECIFICITY .............................................................................................................................................. 47
3.6.1 Introduction: plant resistance and fungal specificity .......................................................................................... 47
, 3.6.2 Plant immune system: the zigzag model ............................................................................................................ 48
3.6.3 Gene-for-gene resistance .................................................................................................................................... 49
3.6.4 Cladosporium fulvum and tomato interactions .................................................................................................. 51
3.6.5 Biotechnological applications using R gene and HR mechanisms ...................................................................... 54
H4: SYMBIOSIS ............................................................................................................................................................... 55
4.1 LICHENS......................................................................................................................................................................... 55
4.2 MYCORRHIZA.................................................................................................................................................................. 56
...................................................................................................................... 61
H1: INTRODUCTION ....................................................................................................................................................... 61
H2: CHARACTERISTICS AND VIRULENCE FACTORS OF HUMAN-PATHOGENIC FUNGI ....................................................... 63
2.1 CANDIDA ALBICANS .......................................................................................................................................................... 63
2.1.1 Virulence factor: dimorphism ............................................................................................................................. 66
2.1.2 Virulence factor: phenotypic switching ............................................................................................................... 80
2.1.3 Virulence factor: adhesion .................................................................................................................................. 83
2.1.4 Virulence factor: biofilm formation .................................................................................................................... 84
2.1.5 Virulence factor: production of enzymes ............................................................................................................ 91
2.2 CANDIDA GLABRATA ......................................................................................................................................................... 94
2.2.1 Virulence factor: Yapsins ..................................................................................................................................... 95
2.2.2 Virulence factor: adhesins .................................................................................................................................. 95
2.2.3 Virulence factor: biofilm formation .................................................................................................................... 96
2.2.4 Virulence factor: drug tolerance ......................................................................................................................... 96
2.2.5 Antifungal compounds........................................................................................................................................ 97
2.3 CANDIDA AURIS ............................................................................................................................................................... 98
2.3.1 Underlying mechanisms of multidrug resistance ................................................................................................ 99
2.4 CRYPTOCOCCUS NEOFORMANS ......................................................................................................................................... 104
2.4.1 Characteristics of Cryptococcus neoformans .................................................................................................... 104
2.4.2 Virulence factors of Cryptococcus neoformans ................................................................................................. 105
2.4.3 Cryptococcus gattii ........................................................................................................................................... 107
2.5 ASPERGILLUS FUMIGATUS ................................................................................................................................................ 108
2.5.1 Characteristics Aspergillus fumigatus ............................................................................................................... 108
2.5.2 Virulence factors in Aspergillus fumigatus ....................................................................................................... 108
2.6 DIMORPHIC FUNGI ......................................................................................................................................................... 109
2.6.1 Histoplasma capsulatum .................................................................................................................................. 110
2.6.2 Coccidioides immitis ......................................................................................................................................... 110
2.6.3 Blastomyces dermatitidis .................................................................................................................................. 111
2.6.4 Paracoccidioides brasiliensis ............................................................................................................................. 111
2.6.5 Dermatophytes ................................................................................................................................................. 111
H3: ANTIFUNGAL DRUGS AND DRUG RESISTANCE MECHANISMS ................................................................................ 112
3.1 ANTIFUNGAL DRUG – AN INTRODUCTION............................................................................................................................ 112
3.1.1 General introduction and history ...................................................................................................................... 112
3.1.2 The different groups of antifungals .................................................................................................................. 112
3.1.3 Novel antifungal discovery ............................................................................................................................... 113
3.2 ANTIFUNGAL DRUG RESISTANCE MECHANISMS ..................................................................................................................... 114
3.2.1 Azole resistance mechanisms ........................................................................................................................... 114
3.2.2 Polyenes resistance mechanisms ...................................................................................................................... 118
3.2.3 Echinocandins resistance mechanisms ............................................................................................................. 118
H4: HOST DEFENSE MECHANISMS................................................................................................................................ 120
4.1 FIRST CONTACT BETWEEN HOST AND PATHOGEN ................................................................................................................... 120
4.1.1 Routes of infection and initial contact .............................................................................................................. 120
4.1.2 Role of PAMPs and PRRs ................................................................................................................................... 120
, 4.2 INNATE AND ACQUIRED IMMUNITY .................................................................................................................................... 121
4.2.1 Detection of Candida by oral epithelial cells ..................................................................................................... 121
4.2.2 Detection of Candida by myeloid immune cells ................................................................................................ 125
4.2.3 The inflammasome in reaction to Candida albicans ......................................................................................... 126
4.2.4 Adaptive immunity against C. albicans in skin infections ................................................................................. 129
4.3 IMMUNOLOGICAL PATHWAYS AND FACTORS INVOLVED IN CAPA AND CAM ............................................................................... 129
H5: FUNGAL IMMUNE EVASION STRATEGIES ............................................................................................................... 131
5.1 EVASION STRATEGY: AVOID DETECTION ............................................................................................................................... 131
5.2 EVASION STRATEGY: MOLECULAR MIMICRY.......................................................................................................................... 131
5.3 EVASION STRATEGY: DOWNREGULATE COMPLEMENT ............................................................................................................. 131
5.4 EVASION STRATEGY: DETOXIFICATION AND PROTECTION ......................................................................................................... 131
5.4.1 More in depth – survival pathogenic fungi ROS ............................................................................................... 132
5.4.2 C. albicans first contact with the mucosal barrier: the Th17 response ............................................................. 135
5.5 EVASION STRATEGY: INTRACELLULAR SURVIVAL..................................................................................................................... 136
5.5.1 Histoplasma capsulatum .................................................................................................................................. 136
5.5.2 Coccidioides immitis ......................................................................................................................................... 137
5.5.3 Cryptococcus neoformans................................................................................................................................. 137
5.6 EVASION STRATEGY: ESCAPE FROM PHAGOCYTOSIS ............................................................................................................... 138
,H1: Introduction
1.1 Introduction to fungi
What is a fungus?
• eukaryotes; digests food externally and absorbs nutrients directly through cell wall and PM,
typically through secretion of digestive enzymes
→ the fact that they are eukaryotes, like us, makes it hard to find treatment for fungus cells
• heterotrophs; obtain energy and carbon from other organisms
o biotrophs: obtain nutrients from living hosts without killing them
o saprotrophs: decompose and absorb nutrients from dead hosts/organic matter
o necrotrophs: infect living host and kills host cells to obtain nutrients
• functions of fungi:
o decomposers; recycling nutrients
o parasites or pathogens; causing disease in plants, animals and humans
o symbionts; partners in symbiosis
o ZB in industry, pharmacology, food consumption, biotechnology, fermentation,...
• reproduction is complex and may occur in two well-differentiated stages within the life cycle
o teleomorph (sexual reproduction)
o anamorph (asexual reproduction)
Characteristics
Fungi are considered a separate kingdom, distinct from plants and animals. They share some
characteristics with other organisms, while others are unique to fungi.
Shared features:
- with other eukaryotes:
o membrane-bound nuclei with chromosomes that contain DNA with non-coding
regions (introns) and coding regions (exons)
o membrane-bound cytoplasmic organelles (mitochondria, ribosomes,...)
o characteristic range of soluble carbohydrates and storage compounds
→ sugar alcohols (mannitol), disaccharides (trehalose), polysaccharides (glycogen)
- with animals:
o heterotopic organisms; require preformed organic compounds as energy source
o lack chloroplasts
- with plants:
o cell wall and vacuoles
o reproduce both sexually and asexually and can produce spores
Unique features:
- dimorphic fungi can switch between yeast and hyphal phase
- fungal cell wall is made of a chitin-glucan complex; N-acetylglucosamine polymer
- lots of fungi spend most of their lives as a dikaryotic organism (not all fungi!)
1
, Structural organization
Most grow as hyphae; cylindrical thread-like structures & form connected network (mycelium)
- hyphae grow at their tips (apices) by branching
- can be septate = divided into compartments separated by cross walls (internal cell walls
called septa, formed in right angles to the cell wall to give hyphae its
shape), with each compartment containing one or more nuclei
→ septa can have pores that allow cytoplasm, organelles and sometimes nuclei
to pass through (e.g., dolipore septum in Basidiomycota)
- can be coenocytic = not compartmentalized
- some have developed specialized hyphal structures for nutrient uptake from living hosts
→ haustoria in plant-parasitic species
→ arbuscules in mycorrhizal fungi to penetrate into host cells
septate hypha coenocytic hypha dolipore septum haustoria
Fungal mycelia can become visible to the naked eye in different forms
- molds on substrates and damp surfaces or food
- colonies in laboratory settings
- fruit bodies (typically referred to as “mushrooms”)
→ apothecium; specialized structure important in sexual reproduction of ascomycetes
→ basidiocarps; fruit bodies of basidiomycetes
1.2 Taxonomy and nomenclature
• they are eukaryotes and share closer evolutionary relationship with animals than plants
→ possess shared and unique characteristics
• monophyletic group; meaning they all share a common ancestor and said ancestor is
included within the group (↔ polyphyletic; members from multiple unrelated lineages)
→ molecular phylogenetics (e.g., ribosomal RNAseq) confirms that fungi descend from
a single common ancestor
• phylogeny is unresolved with lots of ongoing conflicts
Oomycetes: not true fungi
historically misclassified as fungi due to similar morphology (hyphal growth, spore producing),
however, they differ from true fungi in several ways:
- belong to Stramenopiles, closer to diatoms and brown algae (vs fungi closer to animals)
- cell wall composition; made out of cellulose (vs chitin)
- reproductive structures are zoospores with two flagella (feature absent in true fungi)
An important example of oomycetes is Phytophthora infestans, causal agent of late blight in
potatoes (The Great Famine, Ireland)
2
Interactions
between Fungi
and their Hosts
,MOLECULAR INTERACTIONS BETWEEN FUNGI AND THEIR HOSTS....................................................................................1
...........................................................................................................................1
H1: INTRODUCTION .........................................................................................................................................................1
1.1 INTRODUCTION TO FUNGI .................................................................................................................................................... 1
1.2 TAXONOMY AND NOMENCLATURE ......................................................................................................................................... 2
1.3 ASCOMYCOTA ................................................................................................................................................................... 3
1.3.1 Saccharomycotina ................................................................................................................................................ 4
1.3.2 Pezizomycotina ..................................................................................................................................................... 4
1.4 BASIDIOMYCOTA ................................................................................................................................................................ 5
1.5 ZYGOMYCOTA ................................................................................................................................................................... 6
1.6 GLOMEROMYCOTA ............................................................................................................................................................. 6
1.7 CHYTRIDIOMYCOTA ............................................................................................................................................................ 7
1.8 DEUTEROMYCETES (FORMA SPECIALIS OR FUNGI IMPERFECTI) ..................................................................................................... 7
1.9 OOMYCETES (FUNGAL-LIKE ORGANISMS) ................................................................................................................................ 7
H2: CHARACTERISTICS AND VIRULENCE FACTORS OF PLANT PATHOGENIC FUNGI ............................................................8
2.1 INTRODUCTION ................................................................................................................................................................. 8
2.2 SPREAD OF FUNGI .............................................................................................................................................................. 8
2.3 CONTACT WITH THE HOST PLANT........................................................................................................................................... 8
2.4 MECHANISMS OF HOST PENETRATION .................................................................................................................................... 9
2.4.1 Penetration through natural openings or weak spots .......................................................................................... 9
2.4.2 Penetration through damaging the protective layer .......................................................................................... 10
2.5 COLONIZATION OF THE HOST .............................................................................................................................................. 11
2.6 VIRULENCE FACTORS OF PLANT-PATHOGENIC FUNGI ................................................................................................................ 13
2.6.1 Tropisms.............................................................................................................................................................. 13
2.6.2 Cell wall degrading enzymes .............................................................................................................................. 13
2.6.3 Toxins .................................................................................................................................................................. 15
2.7 MEANS TO EVADE HOST RESPONSES..................................................................................................................................... 19
2.7.1 Elicitors and plant defense mechanisms ............................................................................................................. 19
2.7.2 Strategies to evade host responses .................................................................................................................... 19
2.8 IDENTIFICATION OF GENES INVOLVED IN PHYTOPATHOGENICITY .................................................................................................. 20
2.8.1 Mutagenesis of the fungus ................................................................................................................................. 20
2.8.2 High-throughput screening procedures: functional genomics ............................................................................ 21
2.8.3 Signal transduction pathways activated prior to infection ................................................................................. 25
H3: PLANT DEFENSE MECHANISMS ................................................................................................................................ 29
3.1 EXISTING OR CONSTITUTIVE DEFENSE MECHANISMS ................................................................................................................ 29
3.1.1 Mechanical defense mechanisms ....................................................................................................................... 29
3.1.2 Chemical defense mechanisms ........................................................................................................................... 30
3.2 INDUCED DEFENSE MECHANISMS ........................................................................................................................................ 31
3.2.1 Phytoalexins ........................................................................................................................................................ 31
3.2.2 Cell wall modifications ........................................................................................................................................ 31
3.3 PATHOGENESIS RELATED PROTEINS (PRPS) ........................................................................................................................... 32
3.3.1 Enzymatic PRPs ................................................................................................................................................... 32
3.3.2 Peptide or protein inhibitors ............................................................................................................................... 33
3.4 INDUCTION OF THE DEFENSE: ELICITORS................................................................................................................................ 35
3.4.1 Biotic elicitors ..................................................................................................................................................... 35
3.4.2 Abiotic elicitors ................................................................................................................................................... 39
3.5 SIGNAL TRANSDUCTION .................................................................................................................................................... 40
3.5.1 Perception: effects on the membrane ................................................................................................................. 40
3.5.2 Internally generated signal molecules ................................................................................................................ 41
3.5.3 Systemic signals: SA, JA, and ethylene ................................................................................................................ 42
3.6 RESISTANCE AND SPECIFICITY .............................................................................................................................................. 47
3.6.1 Introduction: plant resistance and fungal specificity .......................................................................................... 47
, 3.6.2 Plant immune system: the zigzag model ............................................................................................................ 48
3.6.3 Gene-for-gene resistance .................................................................................................................................... 49
3.6.4 Cladosporium fulvum and tomato interactions .................................................................................................. 51
3.6.5 Biotechnological applications using R gene and HR mechanisms ...................................................................... 54
H4: SYMBIOSIS ............................................................................................................................................................... 55
4.1 LICHENS......................................................................................................................................................................... 55
4.2 MYCORRHIZA.................................................................................................................................................................. 56
...................................................................................................................... 61
H1: INTRODUCTION ....................................................................................................................................................... 61
H2: CHARACTERISTICS AND VIRULENCE FACTORS OF HUMAN-PATHOGENIC FUNGI ....................................................... 63
2.1 CANDIDA ALBICANS .......................................................................................................................................................... 63
2.1.1 Virulence factor: dimorphism ............................................................................................................................. 66
2.1.2 Virulence factor: phenotypic switching ............................................................................................................... 80
2.1.3 Virulence factor: adhesion .................................................................................................................................. 83
2.1.4 Virulence factor: biofilm formation .................................................................................................................... 84
2.1.5 Virulence factor: production of enzymes ............................................................................................................ 91
2.2 CANDIDA GLABRATA ......................................................................................................................................................... 94
2.2.1 Virulence factor: Yapsins ..................................................................................................................................... 95
2.2.2 Virulence factor: adhesins .................................................................................................................................. 95
2.2.3 Virulence factor: biofilm formation .................................................................................................................... 96
2.2.4 Virulence factor: drug tolerance ......................................................................................................................... 96
2.2.5 Antifungal compounds........................................................................................................................................ 97
2.3 CANDIDA AURIS ............................................................................................................................................................... 98
2.3.1 Underlying mechanisms of multidrug resistance ................................................................................................ 99
2.4 CRYPTOCOCCUS NEOFORMANS ......................................................................................................................................... 104
2.4.1 Characteristics of Cryptococcus neoformans .................................................................................................... 104
2.4.2 Virulence factors of Cryptococcus neoformans ................................................................................................. 105
2.4.3 Cryptococcus gattii ........................................................................................................................................... 107
2.5 ASPERGILLUS FUMIGATUS ................................................................................................................................................ 108
2.5.1 Characteristics Aspergillus fumigatus ............................................................................................................... 108
2.5.2 Virulence factors in Aspergillus fumigatus ....................................................................................................... 108
2.6 DIMORPHIC FUNGI ......................................................................................................................................................... 109
2.6.1 Histoplasma capsulatum .................................................................................................................................. 110
2.6.2 Coccidioides immitis ......................................................................................................................................... 110
2.6.3 Blastomyces dermatitidis .................................................................................................................................. 111
2.6.4 Paracoccidioides brasiliensis ............................................................................................................................. 111
2.6.5 Dermatophytes ................................................................................................................................................. 111
H3: ANTIFUNGAL DRUGS AND DRUG RESISTANCE MECHANISMS ................................................................................ 112
3.1 ANTIFUNGAL DRUG – AN INTRODUCTION............................................................................................................................ 112
3.1.1 General introduction and history ...................................................................................................................... 112
3.1.2 The different groups of antifungals .................................................................................................................. 112
3.1.3 Novel antifungal discovery ............................................................................................................................... 113
3.2 ANTIFUNGAL DRUG RESISTANCE MECHANISMS ..................................................................................................................... 114
3.2.1 Azole resistance mechanisms ........................................................................................................................... 114
3.2.2 Polyenes resistance mechanisms ...................................................................................................................... 118
3.2.3 Echinocandins resistance mechanisms ............................................................................................................. 118
H4: HOST DEFENSE MECHANISMS................................................................................................................................ 120
4.1 FIRST CONTACT BETWEEN HOST AND PATHOGEN ................................................................................................................... 120
4.1.1 Routes of infection and initial contact .............................................................................................................. 120
4.1.2 Role of PAMPs and PRRs ................................................................................................................................... 120
, 4.2 INNATE AND ACQUIRED IMMUNITY .................................................................................................................................... 121
4.2.1 Detection of Candida by oral epithelial cells ..................................................................................................... 121
4.2.2 Detection of Candida by myeloid immune cells ................................................................................................ 125
4.2.3 The inflammasome in reaction to Candida albicans ......................................................................................... 126
4.2.4 Adaptive immunity against C. albicans in skin infections ................................................................................. 129
4.3 IMMUNOLOGICAL PATHWAYS AND FACTORS INVOLVED IN CAPA AND CAM ............................................................................... 129
H5: FUNGAL IMMUNE EVASION STRATEGIES ............................................................................................................... 131
5.1 EVASION STRATEGY: AVOID DETECTION ............................................................................................................................... 131
5.2 EVASION STRATEGY: MOLECULAR MIMICRY.......................................................................................................................... 131
5.3 EVASION STRATEGY: DOWNREGULATE COMPLEMENT ............................................................................................................. 131
5.4 EVASION STRATEGY: DETOXIFICATION AND PROTECTION ......................................................................................................... 131
5.4.1 More in depth – survival pathogenic fungi ROS ............................................................................................... 132
5.4.2 C. albicans first contact with the mucosal barrier: the Th17 response ............................................................. 135
5.5 EVASION STRATEGY: INTRACELLULAR SURVIVAL..................................................................................................................... 136
5.5.1 Histoplasma capsulatum .................................................................................................................................. 136
5.5.2 Coccidioides immitis ......................................................................................................................................... 137
5.5.3 Cryptococcus neoformans................................................................................................................................. 137
5.6 EVASION STRATEGY: ESCAPE FROM PHAGOCYTOSIS ............................................................................................................... 138
,H1: Introduction
1.1 Introduction to fungi
What is a fungus?
• eukaryotes; digests food externally and absorbs nutrients directly through cell wall and PM,
typically through secretion of digestive enzymes
→ the fact that they are eukaryotes, like us, makes it hard to find treatment for fungus cells
• heterotrophs; obtain energy and carbon from other organisms
o biotrophs: obtain nutrients from living hosts without killing them
o saprotrophs: decompose and absorb nutrients from dead hosts/organic matter
o necrotrophs: infect living host and kills host cells to obtain nutrients
• functions of fungi:
o decomposers; recycling nutrients
o parasites or pathogens; causing disease in plants, animals and humans
o symbionts; partners in symbiosis
o ZB in industry, pharmacology, food consumption, biotechnology, fermentation,...
• reproduction is complex and may occur in two well-differentiated stages within the life cycle
o teleomorph (sexual reproduction)
o anamorph (asexual reproduction)
Characteristics
Fungi are considered a separate kingdom, distinct from plants and animals. They share some
characteristics with other organisms, while others are unique to fungi.
Shared features:
- with other eukaryotes:
o membrane-bound nuclei with chromosomes that contain DNA with non-coding
regions (introns) and coding regions (exons)
o membrane-bound cytoplasmic organelles (mitochondria, ribosomes,...)
o characteristic range of soluble carbohydrates and storage compounds
→ sugar alcohols (mannitol), disaccharides (trehalose), polysaccharides (glycogen)
- with animals:
o heterotopic organisms; require preformed organic compounds as energy source
o lack chloroplasts
- with plants:
o cell wall and vacuoles
o reproduce both sexually and asexually and can produce spores
Unique features:
- dimorphic fungi can switch between yeast and hyphal phase
- fungal cell wall is made of a chitin-glucan complex; N-acetylglucosamine polymer
- lots of fungi spend most of their lives as a dikaryotic organism (not all fungi!)
1
, Structural organization
Most grow as hyphae; cylindrical thread-like structures & form connected network (mycelium)
- hyphae grow at their tips (apices) by branching
- can be septate = divided into compartments separated by cross walls (internal cell walls
called septa, formed in right angles to the cell wall to give hyphae its
shape), with each compartment containing one or more nuclei
→ septa can have pores that allow cytoplasm, organelles and sometimes nuclei
to pass through (e.g., dolipore septum in Basidiomycota)
- can be coenocytic = not compartmentalized
- some have developed specialized hyphal structures for nutrient uptake from living hosts
→ haustoria in plant-parasitic species
→ arbuscules in mycorrhizal fungi to penetrate into host cells
septate hypha coenocytic hypha dolipore septum haustoria
Fungal mycelia can become visible to the naked eye in different forms
- molds on substrates and damp surfaces or food
- colonies in laboratory settings
- fruit bodies (typically referred to as “mushrooms”)
→ apothecium; specialized structure important in sexual reproduction of ascomycetes
→ basidiocarps; fruit bodies of basidiomycetes
1.2 Taxonomy and nomenclature
• they are eukaryotes and share closer evolutionary relationship with animals than plants
→ possess shared and unique characteristics
• monophyletic group; meaning they all share a common ancestor and said ancestor is
included within the group (↔ polyphyletic; members from multiple unrelated lineages)
→ molecular phylogenetics (e.g., ribosomal RNAseq) confirms that fungi descend from
a single common ancestor
• phylogeny is unresolved with lots of ongoing conflicts
Oomycetes: not true fungi
historically misclassified as fungi due to similar morphology (hyphal growth, spore producing),
however, they differ from true fungi in several ways:
- belong to Stramenopiles, closer to diatoms and brown algae (vs fungi closer to animals)
- cell wall composition; made out of cellulose (vs chitin)
- reproductive structures are zoospores with two flagella (feature absent in true fungi)
An important example of oomycetes is Phytophthora infestans, causal agent of late blight in
potatoes (The Great Famine, Ireland)
2
