Please enable JavaScript.
Coggle requires JavaScript to display documents.
The molecular biology of appressorium turgor generation by the rice blast…
The molecular biology of appressorium turgor generation by the rice blast fungus
Magnaporthe grisea
INTRODUCTION
Rice blast disease
disease of cultivated rice
widespread in temperate-flooded and tropical-upland (rain fed) rice cropping systems
caused by ascomycete called
Magnaporthe grisea
Besides rice
, M. grisea can also infect other forms of host (large number of grass hosts, including important cereals, such as barley and wheat)
Rice blast symptoms become clearly visible 4–5 days after initial infection
How does the infection occur?
M. grisea undergoes a series of
defined morphogenetic developmental steps
Leads to
appressorium development
Appressoria
produced on the surface of rice leaves
causes plant infection primarily by physical breakage of the leaf cuticle
Appressoria generate substantial
turgor
Incipient cytorrhysis experiments
were performed by applying increasing concentrations of polyethylene glycol to appressoria of M. grisea, and then determining the rate of cell collapse
Incipient cytorrhysis assay to measure appressorium turgor generation
Results : generate up to 8MPa of pressure during plant infection
As a result of the turgor, the
appressorium produces
a
narrow penetration hypha
at the base of the cell, which is
forced
through the
underlying cuticle
and later
develops into invasive hyphae
that
fill the epidermal cells of the leaf
Journal focuses on?
biochemical pathways
that lead to generation of appressorium turgor
genetic determinants responsible
for pressure generation by infection structures of the plant pathogen
process of turgor generation
by appressoria of the rice blast fungus
Appressorium development by M. grisea
Rice infection by M.grisea is initiated when conidia land on the surface of a rice leaf
Spores germinate immediately on contact with the rice leaf and adhere tightly to the hydrophobic surface by means of a spore tip mucilage that is released from the apex of the spore
Germination proceeds by extension of a narrow germ tube that emerges from the conidium
Then, germ tube apex starts to swell at its apex and flattens against the surface of the rice leaf
The germ tube apex then develops into a swollen dome-shaped cell, called the appressorium
M.grisea appressoria form in response to hard, hydrophobic surfaces, absence of external nutrients, and presence of constituents of cuticular wax
cAMP response pathway is triggered soon after the attachment of the fungus on the leaf surface
Mutants lacking adenylate cyclase, which are unable to accumulate cAMP, do not form appressoria
Appressorium development
Initiation of appressorium - need cAMP signaling
Appressorium morphogenesis - requires the presence of Pmk 1 MAP kinase pathway
Once formed, appressoria become separated from the germ tube and conidium by a thick septum and become melanin-pigmented
The process of appressorium turgor generation
Water flows into appressorium against a concentration gradient generated by the accumulation of compatible solute within the appressorium.
Biochemical analysis of M. grisea appressoria, solutes accumulate within these cells = (glycerol accumulates to concentrations in excess of 3M)
accumulation due to vapour-pressure psychrometry, for generation of substantial turgor pressure.
Appresorium develop in the absence of external nutrients - glycerol and other solutes within appressorium must be accumulated
de novo
froms torage product present in conidia.
in conidia, predominant storage = accumulation of lipids, glycogen and the disaccharide trehalose
found either degrade rapidly / transported to germtube apex
mannitol accumulate within spores
due to rapid influx of water into the infected cell.
occurs in dewdrops on the surface of a rice leaf
essential pre-requisite for the generation of cells = free water.
Lipid metabolism during appressorium turgor generation
spore germination = lipid bodies are mobilized quickly from the conidium & accumulating in the germ tube apex
Spore germination = lipid bodies accumulate at germ tube apex & in the incipient appressorium
Appresorium maturation = lipid bodies coalesce & taken up by vacuoles (by process resembles autophagocytosis)
Presence of triacylglycerol lipase activity
Triacylglycerol lipase activity = cAMP-regulated
In ∆cpkA mutant, lack of catalytic subunit of PKA, lipase activity is substantially decreased and, lipid bodies fail to coalesce or undergo degradation during appressorium morphogenesis
Regulatory subunit PKA mutant perform rapid lipid degradation in the appressorium, which completed before melanization of the
With Pmk1 MAP kinase, lipid bodies to appressorium under control, due to mutants lacking PMK1, lipid bodies fail to move to the germ tube apex during the initiation of appressorium development
M. grisea
encodes at least 4 genes predicted to encode intracellular triglycerol lipases
rapidly producing glycerols in lipid droplets which transported to appressorium development
Mutants lacking of the ICL1 encode isocitrate lyase shown to reduced in virulence
temporal regulation effect on appressorium development
spore germination retarded: germ tube extension & appressorium morphogenesis
cell wall biosythesis & compatible solute generation required the glyoxylate cycle active in fungus during appressorium development
high during appressorium morphogenesis, penetration peg form & imvasive growth of M. grisea
capable in causing rice blast sypmtoms in delayed manner compared to wild-type M. grisea
glyoxylate required for pathogenicity as their need to develop initially within glucose-deficient environment
Colletorichum lagenarium
-produced melanin-pigmented appressoria
-suggest beta-oxidation may play important role in physiology of appressoria
CONCLUSION
Rapid generation of turgor
appressoria
accumulate
high concentrations of
compatible solutes
such as glycerol
synthesized in large quantities in the appressorium
The availability of the full M.grisea genome sequence, coupled with the ability to perform gene functional analysis at a relatively high throughput, means that there is a
clear opportunity to perform a systems biology approach to the dissection of fungal pathogenesis in M. grisea
and, in particular, to
resolve the mechanism
by which
appressorium turgor is generated
Importance of
turgor generation
to appressorium
capable of penetrating into inert plastic membranes
extra-cellular depolymerizing enzymes
act to accelerate the process on living plant tissue
M. grisea has evolved a
remarkable mechanism
involving production of appressoria
for attachment to the rice leaf surface
for generation of mechanical force to penetrate the rice leaf cuticle
Trehalose metabolism in M. grisea
Non-reducing trehalose
common storage product within microbial cell - act as stress metabolism & cellular protectant from dessication
in many eukaryotes, its meant sugar metabolism can be regulated
In
M. grisea
T6P is synthesized by using UDP-glucose & glucoss-6-phosphate as substrates then directly converted into trehalose
M. grisea tps1 mutants non-pathogenic but, was found in appressorium turgor generation
tps1 mutants displayed a number of pleiotrophic effects:
unable to grow on glucose or other rapid fermentable carbon source
unable to utilize acetate or lipid as sole carbon sources
glucose utilization can be restored as presence of alternate carbon source and complex nitrogen such as yeast extract and peptone
gluconeogenesisbmay affected by lose of T6P synthase activity as M. grisea tps1 mutant unable to grow on acetate or lipids
Two trehalose-encoding genes:
TRE1
encode trehalose main activity during gspore germination
dispensable for pathogenesis, producing rice blast symptomsthat isogenic to wild-type strain of fungus
encode unusual trehalose that not simiar to other either neutral or acidic trehalases
similar to genes found in
N. crassa
NTH1
encode natural trehalose as highly expressed during conidiogenesis & spore germination
mutants lack NTH1 reduced in virulence
In
S. cerevisae
multienzyme complex responsible for T6P synthesis
T6P synthase activity lead to uncontrolled influx of glucose in glycolysis: glucose become toxic to the cells
Glycogen metabolism in M.grisea
Glycogen is abundant within the spores of M.grisea and is metabolized very quickly on germination
Glycogen rosettes are then found to accumulate within appressoria during their development
But glycogen quickly dissappears from the appressoria during melanization and turgor generation
Glycogen mobilization appears to be regulated by the cAMP response pathway
Glycogen is degraded by 2 major enzyme activities
Amyloglucosidase
Glycogen phosphorylase