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The Molecular Biology of Appresorium Turgor Generation by the Rice Blast…

- - - - 1 - Three-celled, tear-drop shaped conidia land on surface of rice leaf (cAMP response pathway triggered)
        2 - The spore germinate instantly on contact with the rice leaf, adhering tightly to the hydrophobic surface by spore tip mucilage (adhesive) that is released from apex of the spore
        3 - Within an hour, a narrow germ tube extension emerges from the conidium
        4 - Within 4 hours, germ tube starts to swell at its apex, and flattens against the surface of rice leaf
        5 - Then, germ tube apex develops into a swollen dome-shaped cell, called appresorium
    - - As mitosis is always observed within the germ tubes of fungus before appresorium development, the process is tightly coupled to cell division
      - After formation of appresoria, it is separated from germ tube and conidium by thick septum
        
        Becomes melanin-pigmented
        (deposited in a layer within cell wall of appresorium)
      - A MAP kinase (mitogen-activated protein kinase) cascade involving the Pmk1 MAP kinase is required for production of appresoria
    - - Hard, hydrophobic surfaces
      - Absence of external nutrients
      - Presence of constituents of cuticular wax
    - - Mutants lacking adenylate cyclase (unable to accumulate cAMP, thus do not form appresoria)
        
        The mutation is from frequent reversion by a second-site mutation in the regulatory subunit of PKA (cAMP-dependent protein kinase A)
        
        This leads to cAMP-independent triggering of PKA and restoration of appresorium development
      - Mutants lacking catalytic subunit of PKA
        
        Form small, mis-shaped & non-functional appresoria
      - Mutants lacking PMK1 gene
        
        Arrest the growth before development of infectious structures
      - Mutants lacking melanin (due to mutations in gene-encoding enzymes in the dihydroxy-napthalene melanin biosynthetic pathway)
        
        Readily identified in M.grisea because of alteration in the colour of fungus, which is normally grey
      - buf1 Mutants (mutation of a gene-encoding dihydroxynapthalene-reductase)
        
        Shows buff colour, lacking this enzymatic activity (light brownish yellow colour)
      - Mutants of RSY1 gene (mutation of gene-encoding scytalone dehydratase)
        
        Shows a pink colour
      - Mutants of ALB1 gene (mutations in a polyketide synthase)
        
        Lead to albino mutant phenotype
- - - - because in a ∆cpkA mutant, lacking the catalytic subunit of PKA, lipase activity is substantially decreased and, additionally, lipid bodies fail to coalesce or undergo degradation during appressorium morphogenesis
    - - because in mutants lacking PMK1, lipid bodiesfail to move to the germ tube apex during the initiation of appressorium development
  - - - Mutants lacking the ICL1 gene encoding isocitrate lyase
        were recently shown to be reduced in virulence
      - a temporal regulation effect on the development of
        appressorium
      - spore germination retarded in ∆icl1 mutants, as was the subsequent germ tube extension and appressorium morphogenesis.
      - cell-wall biosynthesis and compatible solute generation required the glyoxylate cycle to be active in the fungus during appressorium development
      - very high during appressorium morphogenesis, penetration peg formation and invasive growth of M. grisea
  - - - required for appressorium-mediated plant infection by the antracnose fungus
      - produced melanin-pigmented appressoria
- - - - also required for full virulence of the fungus
      - pls1 mutant (non pathogenic) accumulates glycogen deposits within infected cells
        
        encodes a tretraspanin required for controlling the translation of
        turgor into physical force for penetration hypha production
        
        produces completely non-functional appressoria
        
        penetration hyphae emergence, which requires the MST12-encoded transcription factor independent of turgor generation
- - - - due to absence of T6P synthase activity
        
        leading to an uncontrolled influx of glucose into glycolysis
        
        leads to rapid utilization of ATP in the first two steps of glycolysis and a subsequent drop in ATP and phosphate levels within the cell
    - - TRE1
        
        encoding the main trehalase activity
        
        occurs during spore germination
        
        dispensable for pathogenesis
        
        Mutants lacking TRE1 produce rice blast symptoms that are identical to those of an isogenic wild-type strain of the fungus
        
        encodes trehalase that does not show significant
        similarity to other
        fungal neutral or acidic trehalases.
      - NTH1
        
        encode a neutral trehalase
        
        highly expressed during conidiogenesis and spore germination
        
        Mutants lacking NTH1 are reduced in virulence
        
        due to a decreased ability to perform invasive growth within rice tissues
  - - - ∆tps1 mutant and those of∆nth1 and∆tre1 trehalose mutants
        
        trehalose synthesis is needed for appressorium function but not for appressorium metabolism
- - - - generation of appressorium turgor and the genetic determinants
      - responsible for pressure generation by infection structures of this important plant pathogen.
- - - - accumulate high concentrations of compatible solutes
        
        e.g Glycerol
        
        responsible for rapid generation of turgor