The molecular biology of appressorium turgor generation by the rice blast fungus Magnaporthe grisea
Glycogen metabolism in M. grisea
Glycogen
Lipid metabolism during appressorium turgor generation
Abundant within the spores of M. grisea
lipid bodies surrounded by single unit membrane in M.grisea
Mobilized very quickly on germination
During appressorium development
Appressorium mature
Glycogen rosettes= to accumulate within appressoria during their development
biochemical analysis-prove that presence of triacyglycerol lipase activity is induced during appressorium maturation
At the onset of turgor generation, glycogen quickly disappears from the appressoria during melanization and turgor generation
regulatory subunit PKA mutant-rapide lipid degradation in the appressorium which completed melanization of infected cell
Glycogen mobilization= regulated by the cAMP
∆cpkA mutants show retarded degradation of glycogen during
4 genes to encode intracellular triacylglycerol lipases
glyoxylate cycle required for pathogenicity
conidial germination
initiation of appressorium development
cAMP regulates triacylglycerol lipase activity
Degraded by (enzymes)
glycogen phosphorylase
amyloglucosidase
required for full
virulence of the fungus
pls1 mutant
non-pathogenic and produces completely non-functional appressoria,
accumulates glycogen deposits within infected cells
encodes a tretraspanin
Required for controlling the translation of
turgor into physical force for penetration hypha production.
lipolysis appear in vacuole during turgor generation
lipid bodies coalesce and taken up by vacuoles by process that same as autophagocytosis
lipid bodies accumulate at the germ tube and in the incipient appressorium
penetration hyphae emerge
requires the MST12-encoded transcription factor,
occurs independent of turgor generation
The Process of Appresorium Turgor Generation
occurs due to influx of water into infected cell
Free water essential pre-requisite - generation of cells
solutes accumulates within the cells - glycerol accumulate at high conc.
Appressoria forms on leaf surface.
Appressorium development in M. grisea
Introduction
Trehalose Metabolism in M.grisea
3-cell tear-shaped conidia land on surface of rice leaf -> germinate immediately on contact with rice leaves, spore adhere tightly to the hydrophobic surface by secreting mucilage at apex of spore -> extension of narrow germ tube from conidia -> mitosis in germ tube -> germ tube swell at apex and flattens against surface of rice leaf -> form appressorium (swollen dome shaped).
common sotrage product
Act as metabolites and cellular protectant from desiccation
Trehalose (T6P) syn using UDP-Glucose and G6P of substrate and directly converted into trehalose.
TPS1 mutant responsible for appressoria turgor generation
tps1 mutant pleitropic effects
in M.grisae, tps 1 mutant unable to grow on acetate or lipids.
Conclusions
TRE1 & NTH1 - gene responsible for trehalose metabolisms.
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mutants lacking NTH1 reduced in virulence due to decrease ability invasive growth within tissue
TRE1- main trehalose activity during spore germination.
NTH1 - expressed during conidiogenesis and spore germination
Rice blast disease caused by Magnaporthe grisea is prevalent in temperate-flooded and tropical upland (rain-fed) rice cropping system.
Besides rice, affects also barley and wheat (wheat blast).
Defined morphogenetic developmental steps -> develop appressorium on plant leaves surface -> generate turgor pressure -> narrow penetration hypha at base of cell -> physical breakage of plant leaf cuticle -> infection
Cytorrhysis (permanent and irreparable damage to cell wall) experiments performed by applying increasing concentrations of polyethylene glycol to appressorium. Aim to determine rate of cell collapse to estimate the turgor within appressorium.
Identified pressure appressorium reached 8 MPa during plant infection.
Appressorium develop due to absence of external nutrients and presence of cuticular waxes.
Signalling pathway for regulating appressorium development is cAMP pathway triggered after fungus attached to leaves surface.
Mutants (no adenylate cyclase cannot accumulate cAMP) cannot form appressorium but if second-site mutation in regulatory subunit of pKA -> restore appressorium independent of cAMP.
Therefore, lacking pKA form small, misshaped and non-functional appressorium.
Pmk1 MAP kinase by PMK1 gene (one of the MAPK cascade) need for appressorium development because mutant (No Pmk1) arrest growth before appressorium formed.
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