MYCO | 1.3 Fungal Structure & Ultrastructure

Intermediate stage between yeast cells and true hyphae

Pseudohypha

T/F: There are specific structures fungi produce depending on their habitus or type of environment they’re found

TRUE

This is bc the type of environment determines the evolution of their form, e.g., Thallus of Rhizophlyctis, Plasmodia in Plasmodiophora

Structure used by fungi to invade internal cellular structures of plants; for feeding, preparation for reproduction

(G) Plasmodia

Body (structure) composed of matted mycelia, then mycelial network emanating from it; for general growth and nutrient absorption

(F) Thallus

The evolutionary history of fungi reflects _

how they were able to conquer many habitats through a diversity of their forms

Fungi have colonized a wide range of habitats, fulfilling important roles in diverse ecosystems through the _, often rooted in the ability of the fungal hypha to grow and form a mycelium

diversity of their forms

Fungi have colonized a wide range of habitats, fulfilling important roles in diverse ecosystems through the diversity of their forms, often rooted in the _

ability of the fungal hypha to grow and form a mycelium

Describe the various growth forms of fungi as shown in the figure

(1/4 20 um) A = Aseptate hypha of Mucormucedo

B = Septate branched hypha of Trichoderma

(10 um) C = Dividing yeast cells of Schizosaccharomyces

D = Budding yeast cells of Dioszegia

(10 um) E = Pseudohypha of Candida (intermediate stage between yeast cells and true hyphae)

F = Thallus of Rhizophlyctis

E = Plasmodia of Plasmodiaphora

_ branches to form a mycelium

Hypha

2 basic forms of fungi

1. Hyphal (filamentous)

2. Yeast

T/F: There are fungi with no base yeast or hyphal form in terms of their cellular ultrastructure

FALSE

All fungi have a base yeast or hyphal form in terms of their ultrastructure

Identify

(A) Yeast cells of Saccharomyces cerevisiae
(B) Growing hypha of Aspergillus niger

Basic fungal form

• Microscopic fungi consisting of solitary cells that reproduce by budding

Yeasts

Form of asexual reproduction where a small protrusion forms on parental cell’s surface, referred to as the bud, then the bud enlarges as cytoplasm and organelles are shared between parent and the bud, eventually forming a septum that leads to pinching off of the bud from the parent cell

Budding

The number of times a yeast cell reproduced can be estimated based on the _

number of scars from budding the parental cell has on its surface

Basic fungal form

• AKA filamentous or mold form

• Have long filaments called _, which grow by apical (tip) extension

Hyphal form

Yeasts reproduce via _

budding

Hyphae in hyphal fungi grow by _

apical (tip) extension

Elongated filaments with a moving protoplasm inside, growing via apical (tip) extention

Hyphae

Yeasts are considered fungi because they possess features of the kingdom, including _

cesaf

• Chitin in cell walls

• Extracellular digestion

• Saprophytic nutrition

• Asexual reproduction

• Food stored as glycogen

Some fungi are _, meaning they can alternate cell growth between yeast and filamentous forms

dimorphic

Example of possible triggers for dimorphism in fungi

• Temperature

• Humidity

• Presence of internal environment of a host

3 examples of dimorphic fungi

bch

• Blastomyces dermatitidis

  • Environment (nutrient-poor) = Hyphal

  • Host (37C, nutrient-rich) = Yeast

• Coccidioides immitis / posadasii

  • Environment (nutrient-poor) = Hyphal

  • Host (37C, nutrient-rich) = Yeast

• Histoplasma capsulatum

  • Environment (nutrient-poor) = Hyphal

  • Host (37C, nutrient-rich) = Yeast

Why is it important to study dimorphism in fungi?

• Medically, it is important because there are lots of fungi invading the human body that start out as filamentous, enter as yeast, then develop into filaments once inside the human body

• and understanding the pathogenicity, immune evasion, and tissue invasion mechanisms of these fungi can play a critical role in developing effective diagnostic tools and antifungal therapies to combat fungal infections, particularly in immunocompromised individuals.

Explain mechanism of Histoplasma capsulatum that causes histoplasmosis mostly in construction workers

• In the environment, H. capsulatum exists in its hyphal form, but this hyphal form has reproductive structures containing spores, spores are then released from repro structures, turning into yeasts once inside human body (due to human body temperature at 37C)

• H. capsulatum is typically found in its hyphal form in bat droppings. If these droppings are agitated due to construction work and you’re not wearing PPE, these spores can get released and be inhaled by workers

• Because spores are large, these are filtered out by nasal passage ways, but some spores are so tiny that they can actually reach the lung

• Once in the lung, they grow as yeast (due to human body temp 37C), proliferating inside the body, eventually finding their way in lymphatic tissues and thus spreading all over to different organs and organ systems

Dimorphic fungus that grows in soil exposed to bird feces or bat feces

Histoplasma capsulatum

T/F: Candida albicans can start out as filamentous then become yeast inside the body, or start out as yeast then become filamentous inside

TRUE

What happens when Candida albicans reaches the lung?

It can cause emphysema-like symptoms (feel like drowning) because C. albicans is invasive to alveoli, potentially leading to pneumonia due to internal water accumulation

T/F: Ultrastructurally, yeasts appear like a typical eukaryotic cell but with chitinaceous cell wall

TRUE

Several fungi grow as _ that divide via budding

budding, uninucleate yeasts

Describe the figure shown

Diagrammatic representation of a budding yeast, Saccharomyces cerevisiae, ~5 um in diameter

Yeasts are not different from hyphal fungi but represent a _

different growth form adapted to nutrient-rich environments

T/F: Hyphal growth is used in nutrient-poor conditions to move around and get as much nutrition as they could

TRUE

In a plate with isolated fungi, why is the central portion darker and outer portion lighter?

• Central portion = most mature and contains reproductive structures

• Outer portion = youngest, hyphae seeking nutrition

Some other fungi such as _ are dimorphic (with two shapes) and can thus switch between yeast and hyphal forms in response to changes in environmental conditions

Candida albicans

Depending on environmental conditions, temperature, etc., Candida albicans can grow as _

• Yeasts

• Pseudohyphae

• Hyphae

_ look like hyphae because it’s elongated, but there are bunch of cells clustered together

Pseudohyphae

T/F: There are yeasts capable of developing as yeast, hyphae, or pseudohyphae separately or simultaneously depending on the situation

TRUE

How does a dimorphic yeast (e.g., C. albicans) become medically problematic?

• If the yeast has different forms, then this allows it to traverse a variety of crevices in the human body

• It can create growth forms that will allow it to colonize as much crevices in the body

Trigger for development of pseudohyphal form of C. albicans

Consequence of environmental gradient because it’s an intermediate form that allows yeast to colonize the environment, which might have a different temperature or pH, or not big enough to form hyphae but good enough to form pseudohyphae to colonize species

Some other fungi such as C. albicans are dimorphic (with two shapes) and can thus _

switch between a hyphal and a yeast form in response to changes in environmental conditions

_ is essentially a tube with a rigid wall, containing a moving slug of protoplasm

Hypha

Protoplasm vs. Cytoplasm

• Cytoplasm = gel-like; excluding plasma membrane

• Protoplasm = all living components; everything apart from the cell wall; including plasma membrane

The fungal hyphal tip contains an organizing center for _

hyphal growth and morphogenesis

_ is the organizing center for hyphal growth and morphogenesis located in the fungal hyphal tip

Apical vesicular cluster (AVC), organized as spitzenkorper in dikarya (Asco- & Basidiomycota)

Describe the figure

• Figure shows the apical vesicular cluster (AVC) in growing hyphal tips

• Secretory vesicles are visible in all hyphal tips of (A) Oomycota (Water molds), (B) Zygomycota, (C) Asco- & Basidiomycota

• However, specialized chitosomes are prominent only in Asco- & Basidiomycota, organized as a spitzenkorper

An apical vesicular cluster (AVC) is only considered a spitzenkorper:

dvc

1. Once it reaches a particular density

2. Has several vesicle types

3. Chitosomes are microscopically observable

T/F: The secretory vesicles in apical vesicular clusters (AVCs) are prominent only in Asco- & Basidiomycota

FALSE

The secretory vesicles in apical vesicular clusters (AVCs) are prominent in all hyphal tips of Oomycota, Zygomycota, and Asco- & Basidiomycota

Specialized chitosomes within apical vesicular clusters (AVCs) are prominent only in Asco- and Basidiomycota and thus are organized as _

spitzenkörper (SPK)

T/F: Spitzenkorper solely determines the growth pattern of hyphal fungi

FALSE

Not necessarily because fungus is also capable of recognizing different stimulus at the hyphal tip

_ occupies the extreme apex of the hypha and directs growth and branching

Spitzenkorper

Spitzenkorper occupies the extreme apex of the hypha and _ (function)

directs growth and hyphal branching

Spitzenkorper in German means _

“pointed body with secretory vesicles organized by microfilaments”

Spitzenkorper is present in _ but is termed AVC in all others; it helps determine the direction of the growth and facilitates hyphal branching

Dikarya (Asco- & Basidiomycota)

_ helps determine the direction of growth and facilitates hyphal branching

Spitzenkorper in Dikarya (AVC in others)

Major reservoir of chitin synthase (enzymes that produce chitin)

Chitosomes

Describe the structure of a Spitzenkorper

• 2 layers

  • Outer layer = Macrovesicles

  • Core = Chitosomes, Ribosomes

  • Connecting the 2 = Microtubules, Actin microfilaments

What triggers the spitzenkorper to stop hyphal growth?

• Tactile stimulus, e.g., when there’s no more room to grow, thus fungi stop sending growth signals and try to change the direction of growth instead

• Chemical stimulus, e.g., encountering harmful chemicals potentially problematic for fungi to deal with, thus chemoreceptors stop growth

• Nutrient deprivation

What happens if you cut the hyphal tip?

• Plug the cut area with Woronin bodies to prevent protoplasmic leak

• Nuclei then streams very close to the area, producing a new hyphal tip extension on another direction

Describe the figure shown

Parallel panels of Neurospora crassa hypha, with Spitzenkorper clearly visible at the tip (stained red at bottom panel)

Hyphal types can be classified based on _

• Septation

• Cell arrangement

T/F: Fungal hypha cannot elongate without chitin produced by chitin synthase within chitosomes

TRUE

Self-assembling aggregates of the enzyme chitin synthase

Chitosomes

Each vesicle (of chitosome) contains sufficient _ to produce, after proteolytic activation (where protease cleaves portion of chitin synthase), a chitin microfibril composed of several chitin chains

inactive chitin synthase

• Activation causes a _ to appear in the chitosome

• The shell of the chitosome is opened/shed and an extended microfibril arises from the fibroid particle (R)

coiled microfibril (R = fibroid)

_ causes a coiled microfibril (R = fibroid) to appear in the chitosome

Proteolytic activation

Describe the figure shown

• Synthesis of chitin microfibrils by chitosomes

• Preparation from Mucor rouxii, incubated with substrates (UDP-N-acetyl-D-glucosamine) and activators (N-acetyl-D-glucosamine, acid protease)

• (A) Inactivated chitosomes

• (B) Ribbon-like chitin microfibrils produced after activation

Hyphal type

• Septae is not present

• Coenocytic mass is retained

• Cells are multinucleate

Coenocytic hyphae

T/F: More often than not, septate fungi is only a single cell with multiple compartments

TRUE

It’s a case-to-case basis. There are cases where they can develop into separate cells.

Hyphal type

• No true hyphae

• Has elongated yeast cells clustered together

Pseudohyphae

Hyphal type

• Septae separates cell nuclei into discrete compartments

Septate hyphae

Each vesicle of chitosome contains sufficient inactive chitin synthase to produce, after proteolytic activation, a _ composed of several chitin chains

chitin microfibril

The evolution of fungal hypha is thought to have begun from a _ form, choreographed by key innovations in eccda elongation, compartmentalization, communication, differentiation, & adhesion

coenocytic

Main reason why fungi evolved to have a hyphal form

To be able to colonize a variety of different environments, as each environment would have a different supply and cycling of nutrients, thus having hyphae that can grow, move, and traverse to different areas allows fungi to survive in some of the most extreme environments

T/F: Vesicles responsible for enzyme secretion are part of SPK

FALSE

Though, there are vesicles in SPK that facilitate delivery of these enzymes to hyphal tip

The evolution of a fungal hypha is thought to have begun from a coenocytic form, choreographed by key innovations in _ (5)

eccda

1. Elongation

2. Compartmentalization

3. Communication

4. Differentiation

5. Adhesion

_ are perforated crosswalls that compartmentalize the hyphal cells in septate fungi

Septa

T/F: Each vesicle of chitosome contains sufficient chitin synthase to produce, before proteolytic activation, a chitin microfibril composed of several chitin chains

FALSE

Each vesicle of chitosome contains sufficient chitin synthase to produce, after proteolytic activation, a chitin microfibril composed of several chitin chains

• _ develop as ingrowing rings from the lateral wall of the hypha

• Associated with this is the modification of the lateral wall, including _

• Septum

• Proliferation of glycoprotein reticulum (GR), glucan layer (G), proteins (P), chitin (C)

• Found in many members of Basidiomycota

• Characterized by

  • Large deposits of glucan lining narrow central pore,

  • Specialized perforated membranes called parenthosomes preventing major organelles from passing through septal pore

Dolipore septum

_ are found at fairly regular intervals along the length of most hyphae in the Ascomycota, Basidiomycota, & mitosporic fungi, which allow the continuity of the protoplasm

Perforated septa

T/F: Nuclei can pass through dolipore septum

FALSE

Nuclei is too big to pass through dolipore septum, but it can pass through normal hyphal septa

2 characteristics of dolipore septum

• Large glucan deposits lining narrow central pore

• Parenthosomes preventing major organelles from passing through the septal pore

Advantages of having septa in fungi

1. Isolation of damage or damage control

   1. If part of hypha is damaged, the septa can compartmentalize the injury to prevent protoplasmic leakage and maintain the integrity of the remaining hyphal network

   2. Con: movement of material is slower in septate fungi

2. Structural support

   1. Provides additional structural reinforcement to hyphae, making it more resilient against mechanical stress

_ prevents cytoplasmic loss at septal pores of Ascomycota

Woronin bodies

T/F: The dolipore septum of Basidiomycota prevents protoplasmic leakage by utilizing Woronin bodies to seal its specialized pore in case of damage

FALSE

Woronin bodies are exclusive to Ascomycota. Protoplasmic leakage in Basidiomycota is prevented by parenthosomes

Specialized organelle derived from peroxisomes found near the septae of filamentous Ascomycota; unit membrane + protein core (HEX-1) attached via Leashin to Woronin Sorting Complex

Woronin body

Components of a Woronin body

Unit membrane + Protein core (HEX-1) attached via Leashin to pore Woronin Sorting Complex

Function of Woronin bodies

Act as a seal that plugs septal pores during hyphal wounding, thus restricting cytoplasmic loss

Perforated septa are found at fairly regular intervals along length of most hyphae in _, which allow continuity of protoplasm

Ascomycota, Basidiomycota, mitosporic fungi

T/F: Dolipore septum is found in many members of Ascomycota

FALSE

Dolipore septum is found in many members of Basidiomycota

Specialized perforated membranes that prevent major organelles from passing through the septal pore

Parenthosomes

What happens when there is damage in the hyphal tip of an ascomycete?

• The portion of hypha damaged then lyses to ensure that the damaging agent remains and does not spread to other areas

• Woronin bodies then plug the septal pore

Structure in plants similar to Woronin bodies in Ascomycetes

Tyloses

Condition where cells are exposed to significantly lower external solute concentration than inside the cell, leading to water influx due to osmosis

Hypotonic shock

_ typically develops from a single germinating spore that produces a young hypha called the germ tube, which eventually develops into a mycelium

Hypha

T/F: Hypha is not an individual but almost always part of a colony

TRUE

T/F: There are hyphae that act as holdfasts (roots), stems, that generate reproductive structures

TRUE

Typical asexual life cycle of fungi

sghm

1. Spores (conidia) are produced by conidiophore (spore-producing organ)

2. Upon encountering a proper environment, spore germinates, producing germ tube

3. Germ tube then develops into hyphae

4. Hyphae then differentiates, forming a network called mycelia

T/F: Yeasts are very rarely produced from spores

TRUE

Spores must grow into hyphae first before they can form yeast