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Regulation of appressorium development in pathogenic fungi (Early…
Regulation of appressorium development in pathogenic fungi
Appressorium Turgor Generation
Appressorium maturation
Rapid glycerol synthesis
Leads to turgor generation
Formation of thick differentiated melanin layer on inner side of appressorium cell wall
Required to retard efflux
of glycerol
Provide structural rigidity and resilience to
infection cell
Melanin maintain turgor pressure
Lowering the porosity of the appressorium cell wall
Colletotrichum graminicola
Turgor accumulates even when melanin biosynthesis is inhibited
Penetration of intact
leaves and artificial substrates still occurs
Cell collapse assays (cytorrhysis) analysis
Mach-Zehnder interferometry
Melanin not required
for solute accumulation and turgor generation
Albino mutants
Lack CgPKS1 polyketide synthase gene involved in n 1,3,6,8-
tetrahydroxy-naphthalene biosynthesis
Prone to rupture and impaired in their ability to cause disease
Soybean rust fungus,
Phakopsora pachyrhizi
High turgor, 5.13 MPa
Non-melanised appressoria
Carried out using transmitted light double-beam interference Mach-Zehnder microscopy
Hyaline (non-pigmented) appressoria of rust fungi
P. pachyrhizi
Generate turgor in the absence of melanin
Requires accumulation
of osmotically active polyols
Can be retained even in the absence of melanin
C. graminicola
M. oryzae
Early appressorium development
M.oryzae
a three-celled conidium germinates within an hour after land on leaf surface
spore rapidly germinates and sends out
a germ tube upon hydration & surface contact
the tube will extend 10–15 mm before flattening & hooking
the initiation development is control by:
hydrophobicity
surface hardness
physical cues
each single nucleus in the three-celled conidium will undergoes single round nuclear division
S-phase is necessary for initiation of appressorium development
hydroxyurea & nim1 mutant will inhibit DNA replication
maturation & melanisation are control by by entry of the
nucleus into G2 and mitosis
appressorium will fully function only if mitosis occurs
cytokinesis occurs and a contractile actomyosin ring forms
Autophagy
will stimulated within the conidium
intracellular contents of the three-celled spore are degraded before being trafficked to the incipient appressorium
Blocking autophagy
targeted mutation of any of the 16 genetic components of the non-selective macroautophagy pathway
U. maydis
cell cycle arrest is necessary for an infective filament of thefungus
will help in penetration into plant tissue
undergoes a self-/non-self-recognition process on corn leaf
two monokaryotic sporidia fuse to form an infectious dikaryotic filament
cell arrested resulted by :
cooperation of at least two distinct underlying mechanisms
relies on transcriptional regulation of a gene called HSL1
this gene encodes a protein kinase that modulates the G2 to M transition
Appressorium formation also relies on perception of
physical and biochemical cues at the leaf surface
Pmk1 MAP kinase pathway and the cAMP response pathway
necessary for appressorium
development
Pmk1 a number of potential receptors are involved in perception of surface signals
Mac1 adenylate cyclase interacts with Cap1, a cyclase-associated protein that activates adenylate cyclase
involved in re-modelling the actin cytoskeleton
PTH11, for example, a CFEM domain Gprotein coupled receptor
necessary for perception of the hydrophobic leaf surface by M. oryzae
if absence the appressoria do not form
RAS signalling is likely to act upstream of the Pmk1 and cAMP response
because generation of a dominant-active allele of Ras2 (RAS2G17V) leads to abnormal appressorium formation
Expression of the dominant M. oryzae RAS2G17V
allele in
Colletotrichum graminicola
and
C. gloeosporioide
wil led to aerial appressoria suggesting conservation of
the surface perception signalling mechanism
Pmk1 kinase cascade is composed of three protein
kinases, Mst11, Mst7 and Pmk1,
be scaffolded by a protein called Mst50,
interacts with Mst11,
activation and phosphorylation of its components, a phosphor-relay is triggered leading to detachment pf Pmk1
traversal to the nucleus
during appressorium maturatio
Pmk1 pathway
regulate microconidia formation
by
M. oryzae
Mcm1 is essential for their development
represent an alternative means of propogation
facilitate rapid spread within plant tissue
required for infection related morphogenesis
Introduction
Plant pathogenic fungi cause many of the world’s most
devastating crop diseases.
Each year significant expenditure is required
To ensure food security
To combat plant diseases
Problem is even more pressing in the developing world
Fungicides
means that disease outbreaks have serious consequences
farmers frequently face economic ruin
and the societal and economic impact of plant diseases
Plant pathogenic fungi
produce appressoria
various form
either single-celled structures
or compound appressoria composed of numerous cells
appressoria are simple terminal swellings at the tips of germ tubes that emerge from spores on the leaf surface
Example
Magnaporthe oryzae
Rice blast fungi
Colletotrichum
species
Anthracnose disease
This review is
of biology of appressorium development in plant pathogenic fungi
Compare and evaluate the recent findings
Study focus on 2 rice blast fungi
M. oryzae
an ascomycete
Ustilago maydis
a basidiomycete
Appressorium maturation and cuticle rupture
appressorium pore
point at the base of
the infection cell
penetration hypha
emerges.
M. oryzae and Colletotrichum species
absence of melanin
a much thinner cell wall
site of
remodelling of the actin cytoskeleton
penetration peg formation
rapid F-actin polymerisation
occurs
requires
morphogenetic septin GTPases
M. oryzae
A septin ring of
approximately 5.9 mm was
observed at the appressorium pore
necessary
for 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
Las17 component of the arp2/3 complex
ezrin, radixin, moesin (ERM) domain proteins
BAR domain proteins- the control of membrane curvature
generation
to generate invaginations
associated
composed of four core septins,
Sep4
Sep5
Sep6
Sep3
septin-mediated appressorium repolarisation
reactive oxygen species burst catalysed by the Nox2 NADPH
oxidase
Nox2 and its regulatory subunit NoxR
required for septin ring formation at the base of the
appressorium
second NAPDH oxidase, encoded by
the NOX1 gene
for maintenance of the polarised growth
organisation of the toroidal F-actin network at the base of the appressorium during penetration peg formation
Mutation
genes encoding any of the septin components and either of the NOX2 and NOXR genes
prevent plant infection
appressorium pore fails to differentiate from the rest of the infection cell
NOX1
arrest of the penetration process just after
differentiation of a stunted penetration peg
fails to
elongate and breach the cuticle
Reactive oxygen
species (ROS)
act directly on proteins such as gelsolin, which are involved in actin severing and formation of free barbed ends
stimulate rapid F-actin
polymerisation
acts on signalling components
operate down stream of a turgor sensor (or sensors)
formation of the hetero-oligomeric septin ring
Chm1
a protein kinase implicated in septin phosphorylation
The penetration peg as site of effector delivery
Plant infection involves deployment of effector proteins
that suppress plant immunity response
facilitate proliferation of pathogen
Essential pre-requisite for focal secretion of effectors
means of communication between is a means extending hyphal tip and the fungal nucleus
which still within appressorium on leaf surface
Ultra-structural analysis of
C.
higginsianum
has detected effectors
Focusing on how penetration peg allows rapid deployment of infection
Recent study has shown that a retrograde (early endosome- mediated, long-distance signalling pathway)
necessary for transcriptional regulation of effector genes and effector secretion from the hyphal tip
Future Prospects
Recent studies shows appressorium turgor is
generated than was hitherto appreciated
Some common themes in appressorium morphogenesis have also emerged
Importance of cell cycle control
Operation of a widely conserved
MAP kinase pathway
There is emerging picture of appressorium pore
As key signalling hub during plant
infection
A the site of initial secretion of fungal effectors & associated regulatory components
Future experiments-
Identify the turgor sensing
mechanisms of appressoria
Understanding of how this triggers regulated synthesis
of ROS and lead to septin-mediated cytoskeletal re-organisation
Determine precise mechanism by which the appressorium monitors turgor
Rapid progress in generating mutants by
high throughput targeted genome editing and silencing
Define precisely how focal effector secretion is regulated
The likely conservation of early endosome-mediated signalling