From polarity establishment to fast hyphal growth in the filamentous fungus "Ashbya gossypii"

Fungi are an evolutionary successful group of
organisms with great ecological importance. The
impact of filamentous fungi on human welfare is
enormous. They cause the majority of economically
significant diseases of crop plants and are becoming
increasingly important as human pathogens.
The formation of hyphae (elongated, tubular cells)
is a key characteristic of filamentous fungi. Hyphal
growth allows them to spread rapidly through
various substrates in search of nutrients or mating
partners, or to penetrate host organisms. Elucidation
of mechanisms that control hyphal growth and
fungal morphogenesis may lead to the identification
of new fungicide targets, and further may increase
the understanding of other hyperpolarized cell
types such as neurons in animals, pollen tubes in
plants or rhizoids in algae.
Part I:
Ashbya gossypii and the budding yeast
Saccharomyces cerevisiae carry a very similar set
of genes. Yet A. gossypii is a filamentous fungus
displaying constantly polarized growth at hyphal
tips resulting in surface expansion rates that are up
to 30 times higher than in yeast cells. Polar growth
in budding yeast is tightly controlled in a cell cycledependent
manner. Cell polarity establishment,
polar growth and isotropic expansion all take
place during formation of a bud, therefore these
processes are difficult to separate experimentally.
In contrast, growing hyphal tips are ideal systems
to study sustained polar growth though the
knowledge about the organization of this highly
dynamic site is presently limited.
I found that factors that are involved in plasma
membrane vesicle fusion, control of the polarized
actin cytoskeleton and regulation of cell polarity
in A. gossypii occupy distinct subcellular regions
in the hyphal tip. Core cell polarity factors such as
AgCdc42, AgCdc24 and AgBoi1/2 were restricted
to different parts of the tip cortex, whereas the
formin AgBni1, the polarisome components
AgSpa2 and AgPea1 and the exocyst were also
found in the apical body, which is a vesicle-based
structure commonly observed in the apex of
growing fungal hyphae. Importantly, localization
of these proteins to different zones changes with
increasing elongation speed. Slow hyphae with
surface expansion rates close to bud growth in
S. cerevisiae display most polarity factors at the tip
cortex independent of the factor’s identity, and at
slow growth speeds apical bodies are absent. These findings suggest that fast growing hyphal tips are
subdivided into functional zones where different
sets of polarity factors exert their function. A model
of fast hyphal growth can be postulated based on
this hypothesis.
Part II:
Establishment of a fungal mycelium depends on
constant hyphal elongation and formation of new
hyphae, which arise by branching. Emergence of
a lateral branch requires establishment of a novel
axis of cell polarity at the hyphal cortex, an event
which might a priori not involve an identical set of
polarity factors needed for sustained polar growth.
Therefore, I looked for A. gossypii candidate genes
the deletion of which could generate mycelia with
severely reduced or even absent lateral branching
while tip expansion was unchanged. I found that
loss of AgGic1/2 resulted in such a phenotype.
AgGic1/2 also plays a role in the first polarity
establishment during germination since spores
lacking AgGic1/2 display delayed germ tube
emergence. The data reported in this PhD-thesis
clearly indicate that AgGic1/2 is important for cell
polarity establishment but dispensable for polarity
maintenance in A. gossypii. In S. cerevisiae the
homologous proteins ScGic1 and ScGic2 act
as effectors of the key polarity factor ScCdc42.
This molecular function is probably conserved
in A. gossypii since AgGic1/2 interacts with
AgCdc42 in a two-hybrid assay. Degradation of
AgGic1/2 is likely to be mediated by its carboxyterminal
PEST domains, which are not predicted
in the homologous yeast proteins ScGic1 and
ScGic2. AgGic1/2 may have a regulatory function
in branch initiation based on the finding that an
AgGIC1/2 allele lacking the part that encodes the
PEST domains induces an increase in branching
if expressed from a strong promoter. It is possible
that controlled degradation of AgGic1/2 constitutes
a simple mechanism that is involved in regulating
emergence of new lateral branches.
Part III:
It is generally believed that the vast number of small
GTP-binding proteins present in cells of higher
organisms today has evolved by gene duplications
of a common ancestor. Furthermore it is taken for
granted that after duplication mutations alter the
protein’s effector interactions, thereby changing
its function. In contrast to these assumptions we
show here for the two duplicated RHO1 genes
from the filamentous fungus Ashbya gossypii that
the different functions of the encoded proteins are
not due to different effector interactions. Instead
we found that both proteins are regulated by
different GAP-proteins and that GAP specificity is
determined by either a tyrosine or a histidine at a
single position in the switch I region of the two
Rho1 proteins. An analogous histidine residue is
found in some atypical GTP-binding proteins of
higher eukaryotes, suggesting that the evolutionary
mechanism we describe here might be a common
way for diversification of GTP-binding protein
function.