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Functional genomic characterization of plant infection by the rice blast fungus Magnaporthe grisea N.J. Talbot, D.M. Soanes, J.M. Jenkinson, Z.Y. Wang, L.J. Holcombe,
M.J. Gilbert, and G. Bhambra, The rice blast fungus, Magnaporthe grisea causes one of the most serious
diseases of cultivated rice, and understanding the early events of the
infection is of paramount importance if durable control measures are to
be developed. Magnaporthe grisea develops a specialised infection structure
called an appressorium which is used to penetrate the tough outer cuticle
of rice leaves allowing the fungus entry to the underlying tissue. Appressoria
are melanin pigmented, dome shaped cells which accumulate massive intracellular
turgor. Turgor is generated by accumulation of a compatible solute within
the appressoria to very high concentrations. M. grisea appressoria accumulate
glycerol as a major compatible solute during appressorial turgor generation,
and understanding the mechanisms by which glycerol is synthesised within
appressoria and how this process is genetically regulated is one of the
primary aims of our research. Appressoria of Magnaporthe grisea form in
dew drops on the surface of rice leaves in the absence of exogenous nutrients.
Therefore, glycerol is synthesised from precursors that are present within
un-germinated spores of the fungus. We have been examining the role of
trehalose, glycogen and lipids as sources for glycerol biosynthesis. Trehalose
is synthesised by the trehalose-6-phosphate synthase complex, and TPS1
encoding the enzyme T6P synthase is required for pathogenicity of M. grisea
and turgor generation. However, trehalose breakdown, which would be required
for glycerol synthesis, is dispensable for appressorium turgor generation,
and, therefore, it seems more likely that trehalose accumulation contributes
to appressorium function, either in a protective capacity or by directly
contributing to turgor. TPS1 also exhibits a regulatory effect on glycolysis,
and tps1 mutants are unable to grow on glucose. We have been examining
the mechanism by which this occurs by using a genetic screen and metabolite
profiling, and then determining how this may impact upon appressorium
function. Glycogen breakdown is catalysed by glycogen phosphorylase and
amyloglucosidase. Functional analysis of the GPH1 and AGL1 genes encoding
these enzymes shows a small effect on virulence indicating that glycogen
degradation via this route is, probably, not a significant contributor
to appressorial turgor generation.
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