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The molecular biology of appressorium turgor generation by the rice blast…

- - - - presence of two trehalase-encoding genes, TRE1 and NTH1 (encode a neutral trehalase)
        
        mutants lacking NTH1 are reduced in virulence
        
        TRE1 appears to be completely dispensable
        
        mutant lacking TRE1 produce rice bast symptom; identical to those of an isogenic wild-type strain of fungus
  - - - due to the absence of T6P ssynthase activity
        
        lead to an uncontrolled influx of glucose into glycolysis
        
        lead to rapid utilization of ATP in glycolyis
        
        drop in ATP & phosphate level within the cells
- - - - Triggered after attachment of conidia on leaf surface
        
        Mutant lacking adenylate cyclase
        ~ Unable to accumulate cAMP
        ~ Hence, unable to form appressorium
      - Mutants pathway
        
        Mutation due to frequent reversion by second-site mutation in regulatory subunit PKA (cAMP-dependent protein kinase A)
        
        Leads to
        ~ cAMP-independent triggering of PKA
        ~ restoration of appressorium development
        
        Lacks of subunit of PKA
        
        Hence, appressorium formed
        ~ small
        ~ mis-shaped
        ~ non-functional
      - MAP kinase
        (Mitogen-activated protein kinase)
        
        MAP kinase cascade involving Pmk1 MAP kinase is required for appressoria production
        
        Mutant lacking PMK1 gene arrest the growth before development of infectious structures
  - - - Lack of melanin
        
        Mutation in gene-encoding enzymes in the dihydroxynapthalene melanin biosynthetic pathway
        
        Hence, alteration in the fungus colour turns it grey
      - Example of M. grisea mutants
        
        buf1 mutant
        
        mutation at gene encoding diydroxynapthalene reductase
        
        Produce buff colour
        
        RSY1 mutant
        
        mutation at gene encoding scytalone dehydratase
        
        Produce pink colour
        
        ALB1 mutant
        
        Mutation at gene encoding polyketide synthase
        
        Lead to albino mutant phenotype
        
        The mutants are non-pathogenic
        
        Their appressoria produced unable to accumulate turgor
- - - - PMK1 mutants;
        lipid bodies fail to move to germ tube apex
  - - - ∆cpkA mutant (lacking the catalytic subunit of PKA):
        
        lipase activity :small_red_triangle_down:
        
        x coalesce/ x degradation of lipid bodies
      - regulatory subunit PKA
        mutant :
        
        carry out rapid lipid degradation in appressorium
      - produces glycerol from lipid droplets;
        glycerol is transported to developing appressorium
        
        Consequence;
        generation of fatty acids
        
        give rise to the requirement for fatty acid B-oxidation and activation of glyoxylate shunt and gluconeogenesis
        
        glyoxylate cycle; required for pathogenecity
        
        ICL1 mutant gene encoding for isocitrate lyase; reduced virulence
        
        in ∆icl1 mutants; spore germination was retarded along with germ tube extension and appressorium development