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Regulation of appressorium development in pathogenic fungi (Early…
Regulation of appressorium development in pathogenic fungi
Appressorium maturation and cuticle rupture
Appressorium pore
Distinct at the rest of infection cell, thinner than cell wall & absence of melanin in
M. oryzae
and
Colletotrichum sp.
Point at the base of the infection cell from which the penetration hyphae merges
Site of remodelling of the actin cytoskeleton
Requires morphogenetic septin GTPases [
Septin ring
5.9 mm
In
M. oryzae
Composed of
four core septins
Sep3
Sep4
Sep5
Sep6
Scaffolding actin, leads to formation of a toroidal F-actin network at the base of the appressorium
Lateral diffusion barrier,
which link place proteins in F-actin polymerisation
Las17
component of the
arp2/3 complex
Ezrin, radixin, moesin (ERM) domain proteins
located in septin ring required for actin membrane interactions at the cortex of cells
BAR domain proteins
implicated in the control of membrane curvature generation
Generate invaginations
Cellular protrusions
Oxygen species burst catalysed by the
Nox2 NADPH
oxidase
- septin-mediated appressorium repolarisation
NOX1
gene
Maintenance of the polarised growth
Organisation of the toroidal F-actin network at the base of the appressorium during penetration peg formation
Mutation of genes NOX1
: plant fails to elongate & breach thecuticle
ROS such as
gelsolin
actin depolymerising agent
inhibit by presence of ROS lead to penetration of peg formation
acts on signalling components that operate downstream of a turgor sensor, re-polarisation need to be triggered
acts directly with proteins, end the stimulate rapid F-actin polymerisation
Upstream
of the formation of the
hetero-oligomeric septin ring
Chm1
protein kinase implicated in septin phosphorylation
The penetration peg as site of effector delivery
ultra-structural analysis of
C.hingginsianum
penetration peg allow rapid deployment of effector early during infection processes
In
Colletotrichum orbiculare
&
M. oryzae
fecal secretion at penetration peg & extending primary infection hypha - communication between extending hyphal tip & fungal nucleus within appressorium of leaf surface
In
U. mydis
Show : retrograde, early endosome-mediated, long signaling pathway necessary for
transcriptional regulation of effector genes & effector secretion from hyphal tip
(plant tissue colonization)
INTRODUCTION
Appressoria
Single-celled structure
Compound appressoria composed of numerous cells
Collectively forms infection cushions
What is appressoria?
simple terminal swellings at the tips of germ tubes that emerge from spores on the leaf surface
in
Magnaporthe oryzae
and
Colletotrichum
sp.
melanin-pigmented, septate structures that initially form at the tips of germ tubes, but then differentiate into dome-shaped fully differentiated infection structures
the rice blast fungus M.
oryzae
(ascomycete) and the corn smut fungus
Ustilago maydis
(basidiomycete)
physical and chemical cues from
the plant leaf surface
control of both
nuclear and cell division
developing the next generation of anti-penetrant
fungicides
targeting plant-based methods to control
some of the most important cereal diseases
Appressorium turgor generation
Maturation of the appressorium in
Magnaporthe oryzae
rapid synthesis of glycerol and other polyols
turgor generation
formation of a thick differentiated melanin layer on the inner side of the appressorium cell wall
provide structural rigidity and resilience to the
infection cell
retard efflux
of glycerol from the rapidly expanding appressorium
a role to maintain
turgor pressure
Anthracnose pathogen of corn,
Colletotrichum
graminicola
Melanin is not required
(Mach-Zehn-der interferometry) cytorrhysis) analysis
penetration of intact
leaves and artificial substrates still occur
solute accumulation and turgor generation still occur
Melanin
Soybean rust fungus,
Phakopsora
pachyrhizi
hyaline
(non-pigmented)
appressoria, non-melanised appressoria generates high turgor, of up to
5.13 MPa
using transmitted light double-beam interference Mach-Zehnder microscopy
Albino mutants (without melanin)
lacking the
CgPKS1 polyketide synthase
gene involved in 1,3,6,8- tetrahydroxy-naphthalene biosynthesis
prone to rupture
impaired in their ability to cause disease
Without melanin
cell walls of appressoria must have evolved in differ-
ent ways to maintain turgor
non-melanised fungi may still
undertake mechanical appressorium-mediated infection
Future prospects
Appresorium morphogenesis
importance of cell cycle control
Operation of a widely conserved MAP kinase pathway for appressorium differentiation
How isotropic expansion of appressoria is translated into generation if invasive forces necessary to breach leaf surface.
Appresorium pore as key signaling hub during plant infection (rapid fungal growth , initial secretion of fungal effector & associated regulatory components)
Future experiment
How focal effector secretion regulated and the likely conservation of early endosome-mediated signalling
Identify turgor sensing mechanisms of appressoria & understand how this trigger regulated synthesis of ROS & lead to septin-mediated cytoskeletal re-organization.
Identify new component of regulatory networks of targeted genome editing and silencing during mutation.
Early Development of Appressorium
Development of appressorium occurs once spore lands on the surface of host
The three-celled conidium germinates within an hour upon attaching to the leaf surface
Upon hydration and surface contact, the spore rapidly germinates and sends out a germ tube
The germ tube extends before flattening at the tip, hooking, and beginning to differentiate into the unicellular appressorium
Each compartment of the three-celled conidium contains a single nucleus.
The cell from which the germ tube emerges undergoes a single round of nuclear division, before appressorium development
Entry of this conidial nucleus into DNA replication (S-phase) is necessary for initiation of appressorium development and inhibiting DNA replication
Appressorium maturation and melanisation is controlled by entry of the nucleus into G2 and mitosis
Only if mitosis occurs does the appressorium become fully functional
Cytokinesis occurs and a contractile actomyosin ring forms at the neck of the appressorium, which differentiates the cell from the rest of the pre-penetration
Autophagy is then stimulated within the conidium, such that all of the intracellular contents of the three-celled spore are degraded before being trafficked to the incipient appressorium
Appressorium formation relies on the perception of physical and biochemical cues at the leaf surface
M.oryzae
appressorium morphogenesis involves the Pmk1 MAP kinase pathway and cAMP response pathway