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Understanding how Magnaporthe oryzae invades rice plants (5. Carbon and…
Understanding how
Magnaporthe oryzae
invades rice plants
1. Introduction
Rice blast disease is one of the most devastating of all cereal diseases worldwide
M.oryzae
has advanced particular administrative instruments to react to supplement free condition of the rice surface
Blast disease indicates show up as at first little necrotic sores and become bigger and combine
2. Appressorium formation by
M. oryzae
Formation of the appressorium depends on environmental cues
Hydrophobicity
Plant cutin monomers
Nutrient starvation
Hardness of contact surface
M. oryzae
can form appressoria on artificial plastic surfaces away from the host plant
Mutants lacking critical multi-functional fatty acid β-oxidation protein Mfp1- show a substantial decrease in virulence
mutants lacking carnitine acetyl transferase-responsible for transport of acetyl CoA across the mitochondrial and/or peroxisomal membrane
Appressorium morphology
Unicellular
Dome-shaped
Generate cellular turgor that translated to mechanical force
Development of appressorium steps:
Conidium attachment to host surface
Spore germination
Germ tube development
Appressorium formation
Penetration peg emergence
Invasive growth in host plants
Nutrient-depleted condition (nitrogen source) can effect on
M.oryzae
gene expression, growth and development
Appressorium turgor is generated by accumulation of compatible solutes to a very high concentration within infection cell which lead to enormous pressure build-up
3. Cell cycle control of appressorium differentiation in
M. oryzae
conidial germination --> single round of mitosis in germ tube --> appressorium formation --> appressorium differentiation --> cytokinesis
Conidia (tear-drop shaped) inside contains 3 cells and each cells consist a nucleus. From the pointy end will develop germ tube where nucleus will moved into germ tube and inside undergoes mitosis. 1 out of 3 daughter nuclei will move to the swollen germ tube tip (will then form appressorium) but the rest 2 will return to the cells.
Trigger for appressorium to develop required the nucleus to enter synthesis (S) phase where DNA replication happens.
Proven by:
(1) use hydroxyurea which can inhibit DNA replication
--> germ tube undifferentiated BUT fungi will always reached S phase upon spore germination.
(2) MoNIM1 mutant
. NIM means can initiate DNA replication (S phase) hence MoNIM1 cannot replicate DNA (because cannot initiate S phase mutated NIM) but can still progress to mitosis (M) phase with unreplicated DNA.
Spores do not grow (no enter S phase because did not pass G1 phase) --> no hooking or swelling of germ tube tip --> no form appressorium --> supports theory need DNA replication.
(3) Blocking mitosis at later M phase
no affect the appressorium formation. Conclusion: formation occurred at early stage of mitosis. Initiation of appressorium need S phase.
Appressorium differentiation needs control point of G2-M phase
(1) NimA mutant
. NIMA express protein kinase (help trigger from G2 into M phase). Mutant (no G2-->M phase), germ tube can become hooked or swollen but no appressorium differentiation. So, differentiation needs the G2-->M phase.
(
2) Blocking mitotic exit (M-->G1 phase)
no affect the appressorium differentiation. Penetration peg needs mitotic exit to occur.
Cytokinesis
Site of septum formation aka septin collar
Actomyocin contractile ring initiate septum formation after M phase
5. Carbon and nitrogen starvation in
M. oryzae
infection
Understanding how
M.oryzae
responds to starvation stress
Appresorium development occurs in
absence
of nutrients
Many
genes expressed
during appressorium development:
1)
MPG1
hydrophobin
2)
Hex1
Woronin body associated gene
3)
MSP1
gene
Provide
genetic control
of plant infection
Nitrogen source
regulated by GATA-factor encoding gene
NUT1
(a functional homologue of AreA/nit-2 genes of A. nidulans and N. crassa)
Nitrogen Source Utilisation Pathway
AreA/nit-2
genes=transcriptional activators
1) Ammonium
absent
--> GATA proteins
activated
-->bind to the promoter regions of genes involved
2) Ammonium
present
--> GATA proteins
inhibited
by a transcriptional repressor NMR to binds to the promoter.
Hence, it blocks their ability to undergo transcriptional activation.
During nutrient starvation
MPG1
promoter shows the presence of GATA motifs and might represent putative binding sites for the
Nut1
transcription factor where nitrogen catabolite repression (NCR) regulator.
Δnut1
mutants are impaired to grow a variety of nitrogen sources including nitrate, nitrite, formamide, histidine and uric acid.
These pathways are subject to NCR under control of
Nut1
.
However,
Δnut1
mutants can use other nitrogen sources such as proline, glutamate and alanine.
Effect of
AreA
-like GATA factors on
pathogenicity
depend upon the duration of the biotrophic phase of infection.
NPR1 and NPR2
(for nitrogen pathogenicity regulation) genes involved in regulating nitrogen metabolism during nitrogen or carbon starvation and required for pathogenicity.
During rice infection,
genes were found to be expressed are:
1)
NUT1
2)
MPG1
3)The integral membrane protein
PTH11
4)Tetrahydroxynaphalene reductase
T4HR
5)Alternative oxidase
AOX
6) Neutral trehalase
NTH1
SPM1
gene
encodes a putative subtilisin serine protease involved in conidiation, normal appressorium development and invasive growth
Δnpr1
and
Δnpr2
mutants,
Δspm1
mutants
do not use various secondary nitrogen sources and regulate MPG1 expression. Transcriptional regulation of SPM1 is also subject to NCR
The
importance of nitrogen starvation
is to identify genes that may be elevated in expression during the early pre-penetration stage of fungal development.
4. Infection-associated autophagy
is a cell-cycled mediated regulation.
Autophagy:
cell survival mechanism because nutrient starvation --> breakdown own cytoplasm or organelles (recycle).
(1)
no nutrients during conidia germination (on surface which promote appressorium develop) ---> increase in autophagosomes in conidia.
(2)
autophagy no occur if no appressorium formation or nutrient sufficient.
Inhibits autophagy --> no conidia die --> no need to recycle nutrients --> no appressorium differentiation and not enough turgor pressure for penetration peg --> no tissue invasion --> non-pathogenic --> no cause rice blast anymore.
Mitophagy and pexophagy
not important
because deleted genes still can cause plant disease. As long as autophagy still functioning for bulk recycling of entire cell content and regulation of
glycogen metabolism
during conidiogenesis.
6. Trehalose-6-phosphate synthase, an NADPH-dependent genetic switch in
M. oryzae
trehalose synthesis is mediated by trehalose-6-phosphate
synthase (T6PS) encoded by the TPS1 gene (TPS1 controls expression of the negative regulators of NCR)
trehalose-6-phosphate synthase (Tps1) integrates control of
glucose-6-phosphate metabolism and nitrogen source utilisation
necessary for nitrate reductase. Tps1 directly binds to NADPHand regulates a set of related transcriptional co-repressors,
comprising the three proteins, Nmr1, Nmr2, and Nmr3, which can each bind NADP
7. Reactive oxygen species generation during
appressorium differentiation in
M. oryzae
M.oryzae
will overcome the host defence system during infection: reactive
oxygen species, ROS.
ROS fungal production NOX1 and NOX 2 NADPH oxidises essential for appressorium function and pathogenicity of M. oryzae
Other proteins have been implicated as cellular redox sensors. Eg:metallothionein
8. Concluding remarks
Rice blast fungus
undergoes a specific sequence of morphogenetic events that lead to the development and
action of appressoria
at the leaf surface.
Infection cells requires the perception of
external
physical responses,
absence
of nutrients,
recycling
the contents of cells
Cell cycle checkpoints govern the progression of
appressorium development
and that
infection-associated autophagy
is essential for the biological role of these cells.
To determine
how autophagy is initiated
and how it is
linked to appressorium
morphogenesis.
To investigating the relationship between
the signalling pathways that are known to regulate appressorium development, the regulation of cell cycle control, and the precise mechanisms by which environmental stress is perceived and elicits a response.
Appressorium differentiation --> death of conidium
