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Regulation of appressorim develpoment in pathogenic fungi (Appressorium…
Regulation of appressorim develpoment in pathogenic fungi
Introduction
Appressoria: simple terminal swellings at the tips of germ tubes that emerge from spores on the leaf surface.
In
Magnaporthe oryzae
(rice blast disease) and
Colletotrichum
species (anthracnose disease), appressoria are:
melanin-pigmented
septate structures that initially form at the tips of
germ tubes, but then differentiate into dome-shaped fully differentiated infection structures
-a single-celled structures
composed of numerous cells
M. oryzae
is an ascomycete and
U. maydis
a basidiomycete
developmental process requiring perception of physical and chemical cues from the plant leaf surface, coupled with control of both nuclear and cell division.
Fundamental morphogenetic processes
Developing the next generation of anti-penetrant fungicides
targeting plant-based methods to control
some of the most important cereal diseases
Appressorium maturation and cuticle rupture
Why appresoria change their axis of polarity and restablish polarised growth at thr interface between fungus and plant?
focus turgor in
the appressorium
isotropic expansion of the cell, into physical force at the base of the infection cell
generation and protrusion of the penetration peg into the cuticle
Appressorium pore
infection point where the hyphae penetrates
M. oryzae and Colletotrichum species
Appressorium is not clearly visible
Thinner cell wall
Absence of melanin
Appressorium is visible by ultra-structural
analysis
appressorium pore is the site of
remodelling of the actin cytoskeleton
rapid F-actin polymerisation during penetration peg formation
leading to rapid polarised growth of
the penetration hypha
Re-modelling of actin requires
morphogenetic septin GTPases
Appressorium
pore of M. oryzae and is composed of four core septins
Sep3
Sep4
Sep5
Sep6
Importance of the septin ring
scaffolding actin, leading to formation of a toroidal Factin network at the base of the appressorium
lateral diffusion barrier, tethering in place proteins implicated in F-actin polymerisation,such as the Las17 component of the arp2/3 complex
actin membrane interactions at the cortex of
cells, were found to be located within the septin ring
ezrin, radixin, moesin (ERM) domain proteins
BAR domain proteins
control of membrane curvature
generation
Eukaryotic cells
undergo membrane
curvature generation in order to generate invaginations
associated with
endocytosis
cellular
protrusions, such as lamellipodia found in epithelial cells
require membrane curvature to be stimulated
rapid membrane biogenesis
F-actin polymerisation.
important-control of appressorium polarisation in M.
oryzae
septin-mediated appressorium repolarisation
reactive oxygen species burst catalysed by the Nox2 NADPH
. Nox2 and its regulatory subunit NoxR
required for septin ring formation
a second NAPDH oxidase, encoded by 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 of the NOX2 and
NOXR genes is sufficient to prevent plant infection and appressoria development
mutation of
NOX1
o arrest of the penetration process
Reactive oxygen
species (ROS) generated within the appressorium
stimulate cytoskeletal remodelling.
React on gelsolin, which are involved in actin severing and formation of free barbed ends that stimulate rapid F-actin
polymerisation
Appressorium tugor generation
Rapid synthesis of glycerol and other polyols
During maturation of appresorium in
M. oryzae
Required in turgor generation
Can still be retained even in the absence of melanin
Formation of thick differentiated melanin layer on inner side of appresorium cell wall (may not occur in non-melanised fungi)
Melanin: maintain tugor pressure due to lowering the porosity of the appressorium cell wall
Give structural rigidity and tugor generation
In fungi such as
C. graminicola
and
M. oryzae
In
Colletotrichum graminicola
melanin biosynthesis is inhibited
tugor pressure accumulates
Penetration of intact leaves and artificial substrates occur
Melanin may not provide barrier for osmolytes, in the way it does in
M. oryzae
Melanin is not required for solute accumulation and tugor generation (non-melanised fungi)
Cell collapse assays (cythorrhysis) using Mach-Zehnder interferometry
Experiment with soybean rust fungus,
Phakopsora pachyrhizi
High tugor pressure, 5.13 MPa in non-melanised appressoria
Hyaline (non-pigmented) appressoria of rust fungi can generate tugor in absence of melanin in the cell wall
Melanin plays a structural role
Albino mutants, lacking CgPKS1 polyketide synthase gene (1,3,6,8-tetrahydroxy-naphthalene biosynthesis)
Prone to rupture
Impaired in ability ti cause disease
The penetration peg as site of effector delivery
Involves deployment of
effector proteins
suppress plant immunity responses
facilitate proliferation of the pathogen within plant
tissues
Ultra-structural analysis of
C.
higginsianum
has detected effectors within the appressorium pore
how the penetration peg allows rapid deployment of
effectors early during the infection process.
effectors are likely to be present in both
Colletotrichum orbicular
and
M. oryzae
An essential pre-requisite for focal secretion of effectors at the penetration peg and extending primary infection hypha
communication between the extending hyphal tip and the fungal nucleus, which is still within the appressorium on the leaf surface.
In
U. maydis
, a recent study has shown that a retrograde, early endosome-mediated, long-distance signalling pathway
effector secretion from the hyphal tip during plant tissue colonisation
necessary for transcriptional regulation of effector genes
Early appressorium development.
Spore land on surface of host
M. oryzae adhere to hydrophobic, waxy leaf cuticle.
Upon hydration, spore germinates and germ tube emerge from end of conidium.
Flattening its tips, hooking and begin to differentiate into unicellular appressorium
Entry of conidial nucleus into DNA replication is necessary for initiation of appresorium development and inhibiting DNA replication, either with hydroxyurea or by generation of temperature-sensitive nim1 mutant and thus rendered its non-pathogenic
Appressorium maturation and melanisation is controlled by entry of the nucleus into G2 mitosis.
Cytokenesis occur and contractile actomysin ring forms at neck of apprssorium, differentiates pre-penetration structure
Autophagy is stimulated and turgor generation up tp 6-8 MPa and penetrate into rice cuticle.
1 more item...
in U. maydis, cell cycle arrest is necessary for an infective filament of the fungus to be able to penetrate plant tissue
U. maydis undergoes a self/nonself recognition process on the corn leaf surface and two monokaryotic sporidia fuse to form an infectious dikoryotic filament.