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Understanding how Magnaporthe oryzae invades rice plants (Cell cycle…
Understanding how
Magnaporthe oryzae
invades rice plants
Cell cycle control of appressorium
differentiation in
M. oryzae
Begins with a single round of mitosis in the germ tube after germination
Progression into nuclear division cam be impaired using inhibitors or conditional mutations
Initiation is controlled by S-phase
Hydroxyurea (DNA replication inhibitor)
Arrest growth with undifferentiated appresorium (no hooking and swelling)
Growth progressed after 2-4H on hydrophobic surface
Temperature sensitive mutant
Monim-1
Cells advance into mitosis with un-replicated DNA at restrictive temp.
Germling arrest growth prior to hooking and swelling
Nim-A
Germ tubes become hooked & swollen, but do not mature.
Differentiation controlled by G2-M border
MoBIM-1
Does not affect appressorium formation at restrictive temp.
Suggest that the commitment point for appressorium formation occurs during mitotic entry (G2-M)
CYC-1 and CYC-2
did not block appresorium differentiation
Bim-1
Non-functional appressoria at semi-restrictive temperature
Suggests penetration peg requires mitotic exit
Cytokinesis occurs after M-phase
Site of cell division is spatially uncoupled from the site of mitosis
Altering the spatial control does not affect the appressorium morphogenesis, but prevents plant infection
Correct organization and site of septum formation is important for appressorium function
Septin collar
Laid down pre-mitosis
Acts as a diffusion barrier
Retains key gene products assoc. with appressorium differentiation
defined by
Actomysin contractile ring
Occus post-mitosis
Linked to transit of daughter nucleus to incipient appressorium
Appressorium differentiation is always accompanied by collapse and death of the conidium
3 nuclei in the conidium are degraded prior to plant infection resulting in a single daughter nucleus
All other subsequent nuclei in the invasive hyphae are formed
Conical cell death is not completely understood
Strong experimental data suggests that it requires autophagy
Carbon and nitrogen starvation in M. oryzae infection
Appressorium development
occurs during: no exogenous nutrients
genes expressed
MPG1 hydrophobin
Hex1 Woronin body associated gene
MSP1
regulated by nutritional
conditions & transcriptional profiling
SPM1
putative subtilisin serine protease
involved in conidiation
Transcriptional regulation subject to NCR
NUT1,MPG1, PTH11, T4HR, AOX, NTH1
genes expressed during
rice infection
regulation of utilisation of nitrogen source
GATA-factor encoding gene NUT1
functional homologue of AreA/nit-2 genes (transcriptional activators)
no ammonia
GATA proteins are acti- vated and bind to the promoter regions
ammonia present
binding of GATA factor to promoter regions of nitrogen source scavenging genes is inhibited by a transcriptional repressor NMR
transcriptional repressor NMR blocks their ability to undergo transcriptional activation
regulate nitrogen catabolite repression (NCR)
Dnut1 mutants
impaired to grow a variety of nitrogen sources
use other nitrogen sources such
as proline, glutamate and alanine
positive regulator of nitrate reductase gene
NIAD
induced by nitrate
and repressed by glutamine and ammonium
activates expression of
nitrogen-regulated genes such as MPG1
dispensable for
pathogenicity
NPR1 and NPR2
regulating nitrogen metabolism
during nitrogen or carbon starvation and required for pathogenicity
Disease cycle of
M. oryzae
Conidial attachment to host surface
spore germination
germ tube development
Infection-associated autophagy
Autophagy : cell survival mechanism that is often triggered by nutrient starvation and which results in breakdown of cytoplasm or organelles and recycling of their constituents
large increase in autophagy can be observe in conidia during conidial germination on a surface inductive to appressorium formation and in the absence of nutrients
autophagy does not occur in Pmk1 MAP kinase mutant
inhibiting autophagy prevents conidial cell death and impairs rice blast disease due to non-functional appressoria
infection-associated autophagy in
M. oryzae
requires bulk recycling of the entire contents of the three-celled conidium to the appressorium
to fuel its further differentiation,turgor generation and subsequent growth during theinitial stages of tissue invasion
autophagy plays a role in regulation of glycogen metabolism during conidiogenesis
Reactive oxygen species generation during
appressorium differentiation in M. oryzae
The production of ROS surrounding infection sites is one of the earliest plant defence response that
M. oryzae
need to fight for them to successfully colonise plant host tissues
Reactive oxygen species (ROS) : toxic molecules that can trigger programmed cell death in infected plant cells and toughen the plant cell walls
cause rapid ROS detoxification (fungal evolved effective)
Deletion of
SSD1
gene (a regulator of cell wall biogenesis) cause reduce of colonisation
M. oryzae
on rice leaves
so, an appropriate assembly of the cell wall by
SSD1
is essential for the initial establishmentof infection by avoiding the induction of basal plantdefence responses
MMT1
gene encodes a 22 amino acids metallothionein in
M. oryzae
identified because it showed reduced expression in a Delta
pmk1
mutant
Delta
mmt1
mutants are non-pathogenic and unable to cause penetration-mediated penetration on the host surface
MMT1
metallothionein-like protein acts as a powerful antioxidant and probably plays a role during the process of oxidative cross-linking within the fungal cell wall during development and polarity re-establishment of plant infection
Trehalose-6-phosphate synthase, an
NADPH-dependent genetic switch in M. oryzae
integrate response to changes in carbon,
nitrogen source and redox balance.
regulatory mechanism
NADPH sensor protein, Tps1, a set of
NADP-dependent transcriptional co-repressors
NADPH and NADP (signal transducer)
trehalose synthesis
mediated by trehalose-6-phosphate
synthase (T6PS) encoded by the TPS1 gene
controls expression of the negative
regulators of NCR,
NMR1,
NMR2, and NMR3
integrates control of
glucose-6-phosphate metabolism and nitrogen source utilisation
regulation of the oxidative pentose phosphate
pathway
NADPH is generated by glucose-6-phosphate
dehydrogenase
directly binds to NADPH
regulates a set of related transcriptional co-repressors,
comprising Nmr1, Nmr2, and Nmr3, each can bind NADP
operation of functional appressoria
and for invasive growth
Dtps1
use nitrite and ammonium
but not nitrate as sole nitrogen source depends on carbon source available
high levels of NMR1
transcript in nitrate-containing media
Tps1-dependent Nmr corepressors
control the expression of a set of
virulence-associated genes
regulated by two additional
GATA & NUT1
de-repressed during
appressorium-mediated plant infection
Asd4,
essential for rice blast
disease
Appressorium formation by
M. oryzae
Appressorium
Unicellular, dome-shaped structure which generates cellular turgor and cause mechanical force to rupture of the rice cuticle and gain entry into plant tissue
Formation depends on enviromental cues
Hydrophobicity
Hardness of contact surface
Plant cutin monomers
Nutrient starvation
Turgor formation in appressorium
Once the appressorium differentiated, it become darkly pigmented
Due to a dinstict layer of melanin formed in the cell wall
Melanin also contributes for the turgor generation of appressorium
Turgor also generated by accumulation of compatible solutes. Eg: glycerol
High concentration of glycerol within infected cell lead to enormous pressure build up
When focused at leaf surface, cause physical force and pentration hypha at base aprressorium structure rupture the host plant cuticle,
Glycerol synthesized by mobilization of lipid bodies from three celled conidium to develop appressorium and rapid breakdown of lipid and glycogen
Penetration peg emergence
Invasive growth in host plant
(encodes for protein kinase neccesary for entry into M)
(encodes large subunit of anaphase-promoting complex)
(blocks mitotic exit)
(Explains the role of mitosis)