Lecture: Structure/Function, Part A BIOL 319 – Spring 2017

Biology of Fungi

Fungal Structure

and Function BIOL 319

Overview of the Hypha (cont.)

Apical growth in Sclerotium rolfsii. Source: www.fungalcell.org BIOL 319

Overview of the Hypha

  The hypha is a rigid

tube containing

cytoplasm   Growth occurs at

the tips of hyphae   Behind the tip, the

cell is aging Diagram of hyphal ultrastructure. Source: Deacon, 2006

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Overview of the Hypha (cont.)   Many hyphae possess septa

  Septa contain pores through which

cytoplasm flows   Hyphae are actually interconnected

compartments, not individual cells

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Overview of the Hypha (cont.)

Diagram of colony growth from a single

germinating spore. Source: Deacon, 2006 BIOL 319

Overview of the Hypha (cont.)

Branching hyphal growth in a hyper branching mutant of Neurospora crassa. Source: www.fungalcell.org

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Lecture: Structure/Function, Part A BIOL 319 - Spring 2017

Overview of the Hypha (cont.)

Hyphal anastomosis. Source: Deacon, 2006 BIOL 319

Overview of the Hypha (cont.)

Hyphal anastomosis I - Neurospora crassa.

Source: www.fungalcell.org BIOL 319

Overview of the Hypha (cont.)

Hyphal anastomosis II - Neurospora crassa.

Source: www.fungalcell.org BIOL 319

Overview of the Hypha (cont.)

Hyphal anastomosis III - Neurospora crassa.

Source: www.fungalcell.org BIOL 319

Overview of the Hypha (cont.)

  Cell wall of hyphae are complex in

structure and composition   Thinner at apical (growing) end

  Plasma membrane closely associated with

inner portion of the wall

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Fungal Ultrastructure

  Zonation of organelles

in hyphae   Hyphae show a

defined polarity in the arrangement of organelles

Transmission electron micrograph of the hyphal

tip from Sclerotium rolfsii. Source:

www.bsu.edu/classes/ruch/msa/porter.html BIOL 319

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Lecture: Structure/Function, Part A BIOL 319 - Spring 2017

Fungal Ultrastructure (cont.)

  Apical tip   Extreme end - no organelles, but numerous

membrane-bound vesicles of differing electron densities (Golgi derived?), cell wall is dynamic and rather ‘plastic’ (site of synthesis)

  Chitin synthase is present   Apical vesicle cluster (AVC) - Spitzenkörper  

Actin microfilaments   Short zone following apex - no organelles, but

rich in mitochondria   Nuclei - distribution varies

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Fungal Ultrastructure (cont.)

Fungal Ultrastructure (cont.)

Spitzenkorper in various filamentous fungi. Source: Deacon, 2006

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Fungal Ultrastructure (cont.)   Yeast ultrastructure

  Typical cellular structures of a yeast

include those found in other eukaryotes

Diagram of yeast ultrastructure.

Source: Deacon, 2006

  Reproduction by

budding does impact the structure of the cell wall producing

  Bud scars on

the mother cell   Birth scars on the

newly-formed daughter cell

Scanning electron micrograph of the budding yeast,

Saccharomyces cerevisiae. Source: www.denniskunkel.com

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Fungal Cell Wall   Functions

  1. Structural barrier   2. Determines pattern of cell growth and

is partly dependent upon:   A. Chemical composition   B. Assembly of the wall components

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Fungal Cell Wall (cont.)

  3. Environmental interface of the fungus   4. Protects against osmotic

lysis  

5. Acts as a molecular sieve   6. Contains pigments for protection

  7. Binding site for enzymes   8. Mediates interactions with other organisms

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Lecture: Structure/Function, Part A BIOL 319 - Spring 2017

Fungal Cell Wall (cont.)

  Cell wall components

  Two major types of components   Structural polymers - polysaccharide fibrils that provide rigidity/integrity of the wall

  Matrix components - cross-link the fibrils as

well as coat/embed them

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Fungal Cell Wall (cont.) Structural (fibrillar) Matrix

Taxonomic Group Component(s) Component(s)

Chytridiomycota Chitin Glucan?

Glucan

Zygomycota Chitin Polyglucuronic acid

Chitosan Mannoproteins

Ascomycota Chitin Mannoproteins

- -(1→3)-glucan

-(1→3)-, -(1→6)-glucan

Basidiomycota Chitin Mannoproteins

-(1→3)-, -(1→6)-glucan - -(1→3)-glucan

Oomycota -(1→3)-, -(1→6)-glucan Glucan

Cellulose

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Fungal Cell Wall (cont.)

Diagram of hyphal wall architecture. Source: Deacon, 2006

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Fungal Cell Wall (cont.)

  Main wall components differ between the

major taxonomic groups of fungi [see Table 3.1, Deacon]

  Chitin - straight chain polymers of β-1,4-linked

N-acetylglucosamine residues; chitosan is de-acetylated chitin   Glucan - polymers of β-1,3-linked glucose

residues with short β-1,6-linked side

chains   Cellulose - β-1,4-linked

glucans   Matrix polymers   Glucouronic acids   Mannoproteins - mannose attached to protein

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Fungal Cell Wall (cont.)   Wall architecture

  Hyphae tend to have separate layers of

wall components   Layers actually grade into one another

  Components of one layer tend to be

covalently bond to those of another   Sub apical regions are relatively thicker

than apical region   Yeasts have less complex wall architecture

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Fungal Cell Wall (cont.)

  Extra hyphal

matrix - two types:   Defined zone of polysaccharide - capsule

  Diffuse area outside

hyphal wall Capsule of Cryptococcus neoformans.

Source: www.bcgsc.ca/gc/cryptococcus BIOL 319

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Lecture: Structure/Function, Part A BIOL 319 - Spring 2017

Septa

  Septa occur at generally regular

intervals along a length of a hypha   Perforations allow cytoplasm to flow

from one cell to another

Septum formation in Neurospora crassa. Source: www.fungalcell.org

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Septa (cont.)   Functions of septa

  Structural support of the hypha   Enables differentiation by dividing hypha

into different cells that can undergo separate modes of development

  Types of septa   Simple   Dolipore

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Fungal Nucleus (cont.)

General diagram of mitosis. Source: www.bioteach.ubc.ca/CellBiology/TheCellCycle BIOL 319

Septa (cont.)

  When a cell is damaged, a Woronin

body or coagulated cytoplasm serves a

plug to prevent loss of cytoplasm   Coenocytic fungi are more susceptible

to cellular damage

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Fungal Nucleus

  Double membrane bound organelle

ranging µ in size from 1-2 m to µ20-25 m

in diameter   Unique features of fungal nucleus

  Membrane remains intact during mitosis  

No clear metaphase plate   Various types of spindle-pole bodies

(microtubule-organizing centers)

depending upon species BIOL 319

Fungal Nucleus (cont.)   Ploidy

  Most fungi are haploid with the number of

chromosomes ranging from 6 to 20   Some fungi are naturally diploid

  Others alternate between haploid and

diploid states   Possible reasons for haploidy

  Multiple haploid nuclei can mask mutations  

Advantageous mutations can be selected BIOL 319

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Lecture: Structure/Function, Part A BIOL 319 - Spring 2017

Cytoplasmic Organelles

  Plasma membrane - phospholipid

bilayer   Involved in uptake of nutrients

  Anchorage for enzymes/proteins, e.g.,

chitin synthase, glucan synthase, etc.   Signal transduction   Differs in that it contains ergosterol

  Site of action for certain antifungal drugs  

Oomycota contain plant-like sterols BIOL 319

Cytoplasmic Organelles (cont.)

  Chitosomes - microvesicles that are

capable of synthesizing chitin

Fungal chitosomes. Source: Deacon, 2006

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Cytoplasmic Organelles (cont.)   Vacuoles

  Functions   Storage   Recycling of materials

  Contain proteolytic enzymes  

Regulation of cellular pH   Possible role in cellular expansion/growth

  Shape   Round   Tubular - may be involved in material transport

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Cytoplasmic Organelles (cont.)

  Secretory system

  Consists of the following:   Endoplasmic reticulum (ER)

  Golgi apparatus (or equivalent) - different in

than those found in animals, plants, and the Oomycota in that they lack cisternae

  Membrane-bound vesicles   Involved in fungal tip growth

  Commercially important in the production

of extracellular products BIOL 319

Cytoplasmic Organelles (cont.)

  Chitosomes - microvesicles that are

capable of synthesizing chitin   First noted from homogenized hyphae

  Able to self assemble   Controversial as to whether or not they are

an integral part of the plasma membrane   Function primarily within the region of the

apical tip

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Cytoplasmic Organelles (cont.)

  Endocytosis and vesicle trafficking -

data is still unclear if fungi have an

endosomal system like that found in

other types of eukaryotes

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Lecture: Structure/Function, Part A BIOL 319 - Spring 2017

Cytoplasmic Organelles (cont.)

Endocytosis in growing apical tips and in non-apical portions of true hyphae.

Source: www.fungalcell.org

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Cytoplasmic Organelles (cont.)

Model of fungal endosome system. Source: Deacon, 2006

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Fungal Cytoskeleton   Cytoskeleton functions:

  Transport of organelles  

Cytoplasmic streaming  

Chromosome separation   Three types of cytoskeletal filaments:

  Microtubules - composed of tubulin

  Microfilaments - composed of actin   Intermediate filaments - provide tensil

strength BIOL 319

Fungal Cytoskeleton (cont.)   All play a major role in hyphal tip growth

Cytoskeleton of fission yeast - microtubules (red), actin (green). Nuclei are blue. In color.

Source: www.kent.ac.uk/bio/mulvihill/Research/Default.htm BIOL 319

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