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Describe the processes that result in the differentiation of fungal cells into
yeasts, pseudohyphae and true hyphae
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Numerous fungi
have the ability to switch between these forms producing yeast, pseudohyphal
and hyphal morphologies
...
C
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As an opportunistic
fungus, this ability to switch phenotype has important roles in virulence
...
cerevisiae is a harmless, dimorphic fungus used to make bread
and beer, which lacks the ability to form true hyphae
...

Yeasts
Unicellular Saccharomyces cerevisiae exhibit three specialised yeast cell
types; ‘a’ and ‘α’ which are haploid cells and ‘a/α’ which is a diploid cell1
...
In comparison,
Candida albicans yeasts are diploid; however like S
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This growth is a highly dynamic and regulated process, heavily
controlled by the cell cycle clock that is conserved from fungi to humans
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Cdc42 is a Rho family GTPase and is an essential part of the budding
process, acting as a master regulator to establish/maintain cell polarity4
...
There are two
methods of growth utilised in budding yeast, the first of which is referred to as
apical expansion, where polarisation drives cell lengthening at the bud tip5
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Apical expansion

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depends upon these vesicles as they aid endo/exocytosis at the bud tip,
facilitating new cell wall growth7
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The mother-bud neck is situated between the mother cell and the
budding daughter cell, marking the site of division that will take place when
the daughter cell has reached full size
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The mother cell is depicted in pink and the new budding daughter cell in yellow
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The initial site of growth and subsequent positioning of the mother-bud neck
are controlled by bud-site selection genes, which in S
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Diploid C
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Axial Budding Pattern (Haploid)
Bipolar Budding Pattern (Diploid)
Rsr1 (Ras-related protein 1 (Bud1))
Bud2 (Rsr1 GTPase activating protein)
Bud5 (Rsr1 Guanosine exchange factor)
Bud3 (Bud site selection protein)
Bud7 (Bud site selection protein)
Bud4
Bud8
Bud10
Bud9
AXL1
Rax1
AXL2
Rax2
Table 1 – Table showing the S
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The proteins required specifically for an axial budding pattern (blue) set up
the axial landmark and mutations in those protein’s genes leads to a bipolar
budding pattern
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There are differences in the genes regulating C
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cerevisiae proteins in the
table are present in or have a homolog in C
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Axial budding pattern
Cdc42 becomes localised at the future bud site by Bud2/Bud5 proteins
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The localisation of Cdc42 causes the formation of a
septin ring from five septin subunits6 (figure 2)
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Bud4 is required to recruit/interact with
the remaining axial budding pattern proteins, resulting in the formation of an
axial landmark8
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These inherited septin
rings, along with the rest of the landmark, act as a guide for the new axial
budding site which will then form at an adjacent position9
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The

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cycle starts again as Bud2/5 and Rsr1 recognise the markings of Bud3/4 and
Axl1/2 at the bud scar site, causing them to localise at the new bud site (figure
2)
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cerevisiae yeasts
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Actin is also essential during cell division (figure 1)
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g
...
Formins are large actin binding
polypeptides which assist in the assembly of the actin ring from long actin
filaments
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Bipolar budding pattern
Cytoskeletal polarisation is also necessary for the growth of S
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The axial budding protein, Axl1, is repressed in bipolar
budding allowing Rax1, which accumulates at the bud site, to set-up the
bipolar landmark11 (figure 3)
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The Rax1 signal remains persistent
throughout the cell cycle, maintaining bipolar budding2
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cerevisiae yeasts
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The mother and daughter cells act differently from each other
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This pattern is linked to the localisations of Bud8 and Bud9; therefore
Rax1 is crucial for establishing the bipolar budding pattern
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Conversely Bud9 accumulates at the proximal pole and
requires Rax1, actins and septins to do so
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Due
to the more complex nature of bipolar budding, cell wall proteins e
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fks1
(glucan synthase), affect site selection whereas they do not in axial budding
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At the mother-bud neck, Rax1 localisation splits into a double
ring, comparable to septins, with one ring being inherited by each cell11
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I briefly mentioned that specialised yeast cells play different roles within the S
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The nature of their budding pattern is thought to help
facilitate these different roles (figure 4)
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When exposed to pheromones, released by the other mating type
cell, they fuse to form the a/α diploid cell1
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This locus is a key genetic regulator, determining

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which gene sets are activated and therefore the yeast cell type13
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Likewise α cells are MATα homozygous, with the α-specific gene set
activated
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a/α cells are heterozygous (MATa/MATα) forming a
regulatory species, a1/α2, necessary for the a/α phenotype
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Figure 4 – Diagram showing the axial and bipolar budding patterns, and how these
patterns support mating or foraging
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The bud scars (blue rings) help to
show the position of the newly budding cell in relation to the previous bud site
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In bipolar budding the daughter cell
always buds a new cell at a distal (opposite) position while the mother cell can bud at
either a distal or proximal (same end) position (adapted from2)
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cerevisiae and C
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g
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Although pseudohyphae
share similarities with both budding yeast and true hyphae, they all have a
distinct phenotype with morphological, genetic and cell-cycle organisational
differences3, as shown in table 2
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Therefore
extended polarised cell growth, stemming from the localisation of Cdc42, is
extremely important in pseudohyphal bud formation
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The protein kinase Swe1,
phosphorylates Cdc28 preventing its proper interaction with the G2-cyclin,
Clb2, leading to cell elongation12,14
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albicans have more diverse
pseudohyphal budding patterns than S
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In
contrast, S
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During cytokinesis
constriction occurs at septa at the mother-bud neck; however unlike budding
yeast, the two cells remained attached
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Attached elongated ellipsoidal
cells with constrictions
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Extent of growth depends on
conditions
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Remains in growing bud tip
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Tec1

Apical and isotropic growth,
with a clear transition phase
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Needed for polarisation
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Form at the mother-bud neck
and separate into a double ring
during cytokinesis
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Mitosis across the mother-bud
neck
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Mitosis across the mother-bud
neck at site of max constriction
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Needed for polarisation
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Form at a distance from the
mother-bud neck and separate into
a double ring during cytokinesis
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Mitosis across the site of septation
away from the mother-bud neck
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Lipids

Uniform lipid distribution
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Lipid polarisation
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Mainly highly polarised apical
growth with no transition phase
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Table 2 – Table showing some of the major differences and similarities between the three morphologies;
budding yeast, pseudohyphae and true hyphae
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cerevisiae, diploid C
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In
vivo, C
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However; hyphae formation can occur through the interplay between
environmental signals and transduction pathways, leading to the expression of

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transcription factors which subsequently control the spatial/temporal
expression of hyphal specific genes (HSGs)6,15 (figure 5)
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Tec1 not only helps to directly induce the expression
of HSGs but also helps to maintain their expression via inducing Cdc42
recruitment to the hyphal tip (figure 5)
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As in pseudohyphae, it is this
maintained long term polarisation that is crucial for hyphal formation,
preventing the switch to isotropic growth exhibited in yeast cells
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albicans but not in S
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17 of these peak during the S/G2 transition
and are important in switching on the hyphal formation program
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albicans
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A unique feature of hyphal formation is the presence of polarised lipid rafts
enriched in sterols16 (table 2)
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In
hyphae, septation occurs away from the mother-bud neck and disassembly of
septins does not occur, as phosphorylated Efg1 binds to and inhibits Ace2,
preventing the transcription of the septin degrading enzyme6
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Growth kinetics
Growth kinetics between the three phenotypes differ for a number of reasons,
including the regulation of growth mechanisms within the cell cycle
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As shown in figure 6 there is a clear
distinction between rates of surface expansion at the apical portion compared
to the proximal portion near the mother-bud neck, in both budding yeast and
hyphae
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The proximal
portion of the yeast bud (green) grew at a low but constant rate, whereas the equivalent portion in the
hyphae grew at a negligible rate
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At the hyphal tip the rate of growth is comparable
to that of the budding yeast; however hyphae do not undergo the transition to a lower growth rate
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Due to the length of hyphae, long distance transport is vital, supported by
another important feature; the spitzenkorper
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Hyphae are among the fastest growing
cells, ~20µm/min, partially due to the spitzenkorper moving forward with
growth and providing a constant/regulated source of vesicles resulting in the
linear, high growth rate at the hyphal tip18 (figure 6)
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In
hyphae the tip remains plastic while the sub-apical walls harden facilitating
parallel and prolonged apical growth
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Septation occurs ~hourly, dividing apical
and subapical compartments, the latter of which becomes vacuolated with cell
cycle arrest occurring in G13
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A plentiful nutrient supply,
allows the subapical compartment to accumulate cytoplasm and re-enter the
cell cycle, generating branches6
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The linear growth of a single hyphae combined with the
exponential production of branches results in the exponential growth of the
colony, called the mycelium19
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The total mycelium
length (blue) in C
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In S
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Pseudohyphae growth is also predominantly via linear apical growth; however
unlike in hyphae, compartmentalisation results in a lack of cytoplasmic
continuity with subapical cells remaining in the cell cycle20
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In conclusion, S
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albicans have helped us to elucidate the
complex nature of growth regulation and budding processes leading to the
differentiation of yeasts, pseudohyphae and true hyphae
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(Word count – 2,199)
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13 Herskowitz I
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Microbiol Rev
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