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Understanding how Magnaporthe oryzae invades rice plants (Reactive oxygen…
Understanding how
Magnaporthe oryzae
invades rice plants
Appressorium formation by
M. oryzae
Unicellular
Dome-shaped structure
Generate turgor
Cause rupture of rice cutical to entry the plant tissue
Depends on environment cues
Hydrophobocity
Hardness of contact surface
Plant cutin monomers
Nutrient starvation
Mechanism of invasion
Nutrient starvation at leaf surface
Formation of appressorium and subsequent pathogenic development
Differentiated (6-10 hours) = Darkly pigmented
Due to formation of melanin layer in the cell wall
For turgor generation
Appressorium turgor generate
By accumulation of compatible solutes (glycerol)
High glycerol (High pressure)
Cause physical force
Allow penetration appressorium hyphae to rupture host plant cuticle
Cell cycle control of appressorium differentiation in
M. oryzae
Appressorium differentation
single round of mitosis in the germ tube following conidial germination
conidium is a tear-drop shape cells with three cells
each cells contained single nucleus
innitiation of appressorium development
require S-phase control point
requires control at G2-M transition
mechanism
germination starts from the tappering end of the spore
nucleus pass through the germ tube where it undergoes mitosis
one of the daughter nuclei moves to the swollen germ tube tip while the other moves back into spore cell from which germination originated
cytokinesis occurs and the side of cell division is spatially uncoupled from the site of mitosis
in order to initiate appressorium development, DNA replication are needed
Hydroxyurea (chemical inhibitor for DNA replication)
Arrest growth of
M. oryzae
with undifferentiated germ tube
Infection-associated autophagy
Autophagy
cell survival mechanism
triggered by nutrient starvation
result in:
breakdown of cytoplasm or organelles
recycling of their constituent
When absent of nutrient, lead to large increase of autophagosome
However, in PmK1MAP kinase mutant, does not occur autophagy
Because it unables to form appressoria
By inhibiting autophagy in
M. oryzae
, it prevents conidial cell death
Also completely impaired the ability of fungus to cause rice blast
Introduction
The cause losses of 10-30% global rice yield/year
Need to overcome by understanding biology of rice blast
Develop new/ durable disease control
Rice blast is common cereal disease
Reactive oxygen species generation during appressorium differentiation in
M. oryzae
Plant will produce reactive oxygen species (ROS) as defense against infection
ROS can trigger programmed cell death and toughen plant cell walls.
M. oryzae evolved effective method of ROS detoxification
SSD1 gene important in initial initiation of infection by avoiding the plant defense response
SSD1 gene is regulator of cell wall biogenesis
Catalase B encode by CATB gene is for maintenance of fungal cell wall integrity during plant invasion
also involve in cell wall biogenesis, differentiation during appressorium formation, hypha production
MMT1 metallothionein involve in process of oxidative cross-linking within the fungal cell wall development and re-construction during plant invasion
MMT1 metallothionein act as powerful antioxidant
Carbon and nitrogen starvation
Appressorium development occurs in the absence of exogenous nutrients
responds to starvation stress and modulates gene expression
gene expressed during appressorium development
MPG1 hydrophobin
Hex1 Woronin
body associated gene
MSP1 gene
regulated by nutritional
conditions
Nitrogen source utilisation
regulated by the wide domain GATA-factor encoding gene NUT1
functional homologue of AreA/nit-2 genes
of A. nidulans and N. crassa
Nut1 mutants are impaired to grow a variety of nitrogen sources
AreA/nit-2 genes are transcriptional activators of the
nitrogen source utilisation pathway
nitrogen absent
GATA proteins are activated
GATA bind to the promoter regions
of genes involved in
nitrogen source utilisation
e.g r nitrate
assimilation
nitrogen present
GATA binding site on promoter regions of nitrogen
source scavenging genes is inhibited by a transcriptional
repressor NMR
repressor NMR blocks the ability to undergo transcription activation
nitrogen catabolite repression (NCR)
regulated in part by NUT1
NUT1 is positive regulator of nitrate reductase
NIAD is well known to be induced by nitrate
and repressed by glutamine and ammonium( in other filamentous fungi)
NUT1 dispensable for pathogenicity
e.g sequence analysis of the
MPG1 promoter shows the presence of GATA motifs
important in its regulatio during nutrient starvation
might represent putative binding sites for the Nut1 transcription factor
potential wide domain regulators
NPR1 and NPR2 (for nitrogen pathogenicity
regulation
previously known:
to be involved in regulating nitrogen metabolism
during nitrogen/ carbon starvation
and required for pathogenicity
unlinked to NUT1
Gene expressed during rice infection
global nitrogen regulator NUT1
pathogenicity
genes
MPG1
PTH11 ( integral membrane
protein)
T4HR(tetrahydroxynaphalene reductase)
AOX (alternative
oxidase)
NTH1 ( neutral trehalase)
gene induced during nitrogen starvation + involved in pathigenicity
SPM1
encodes a putative subtilisin serine protease
involved in;
invasive growth
conidiation
ormal appressorium development
Transcriptional regulation of SPM1(NCR)
SPM1, NPR1 and NPR2 mutants
do not use
various secondary nitrogen sources
regulate MPG1 expression
Trehalose-6-phosphate synthase (TPS1), an
NADPH-dependent genetic switch
to study how M. oryzae is able to integrate its response to changes in carbon,
nitrogen source and redox balance
Rice blast fungus - trehalose synthesis is mediated by trehalose-6-phosphate
synthase (T6PS) encoded by the TPS1 gene
the operation of functional appressoria and for invasive growth is required
TPS1
controls expression of the negative regulators of NCR, which have been designated NMR1,
NMR2, and NMR3
TPS1 mutant shows high levels of NMR1
transcript in nitrate-containing media, while expression of
nitrate and nitrite reductase genes reduced
integrates control of
glucose-6-phosphate metabolism nitrogen source utilisation
by regulate nitrogen source utilisation (generate NADPH)
directly binds to NADPH
regulates a set of related transcriptional co-repressor that bind to NADP (protein) : Nmr1, Nmr2, Nmr3
target deletion of Nmr-encodng gene surpresses non-pathogenic tps1 mutant
Tps1-dependent Nmr corepressors controls expressions of virulence-associated genes
regulated by two GATA factors + NUT1 (de-repressed during
appressorium-mediated plant infection)
