Introduction to Fungi, Third Edition

384 HYMENOASCOMYCETES: PYRENOMYCETES
Fig12.48 The three principal signalling cascades involved in appressorium differentiation in Magnaporthe grisea and in many other
differentiation processes in other eukaryotes.Cross-talk occurs between the individual pathways, but this is not shown here.
Abbreviations are as follows: CaM (calmodulin), cAMP (cyclic adenosine monophosphate),Cat. (catalytic), DAG (diacylglycerol),
GTP (guanosine triphosphate), IP 3
(inositol trisphosphate), MAPK (mitogen-activated protein kinase), PIP 2
(phosphatidylinositol
bisphosphate), Reg. (regulatory), TF (transcription factor).
12.9.4 Signalling and pathogenesis
in Magnaporthe grisea
Both signals for appressorium initiation surface
hydrophobicity and cutin monomers are
probably perceived at the plasma membrane of
the Magnaporthe germ tube. The actual receptors
are unknown at present, but the Pth11p plasma
membrane protein is likely to be one of them
(deZwaan et al., 1999). The transmission of signals
from the plasma membrane to the nucleus
occurs along several different routes (Fig. 12.48;
Dean, 1997; Tucker & Talbot, 2001) which are
briefly outlined below:
1. One membrane receptor receiving the
chemical stimulus 1,16-hexadecanediol acts via
a trimeric GTP-binding protein to activate an
adenylate cyclase which converts ATP into the
second messenger, cyclic AMP (cAMP). This, in
turn, activates a protein kinase A by releasing
its monomeric catalytic subunits from the
inactive tetramer, and the catalytic subunits
then phosphorylate regulatory proteins (transcription
factors) which enter the nucleus
and activate the genes required for specific
developmental steps.
2. The involvement of a second common
eukaryotic signalling cascade in appressorium
formation was suggested by Thines et al. (1997)
who noted that diacylglycerols could trigger
appressorium formation on normally non-inductive
hydrophilic surfaces such as glass. The
initial signal (hydrophobicity) is thus likely to
be transduced via phospholipase C which hydrolyses
the membrane lipid phosphatidylinositol
into inositol-triphosphate (IP 3 ) and diacylglycerol
(DAG). IP 3 acts by releasing Ca 2þ from