Introduction to Fungi, Third Edition

MAGNAPORTHACEAE
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intracellular stores such as the endoplasmic
reticulum; this forms a complex with the
calcium-binding protein calmodulin, and the
calcium calmodulin complex activates a calmodulin-dependent
protein kinase. The DAG component
directly activates protein kinase C (Fig.
12.48). Both the IP 3 and DAG branches of this
signalling pathway thereby lead to the activation
of protein kinases which phosphorylate transcription
factors.
3. Mitogen-activated protein kinase (MAPK)
pathways have been proposed to be involved at
various points in appressorium induction and
maturation in M. grisea (Xu & Hamer, 1996;
Thines et al., 2000). A mitogen is an extracellular
substance which stimulates nuclear division
or cell differentiation; here it is a stimulus
for appressorium formation. The MAP kinase
encoded by PMK1 responds to a surface signal and
interacts with the cAMP pathway in a manner
not yet entirely understood, to initiate appressorium
formation (see Fig. 12.48). A second MAP
kinase, MPS1, is involved in penetration of the
epidermis from mature appressoria. A third MAP
kinase, OSM1, is involved in turgor regulation
during osmotic stress but plays no role in
appressorium functioning (Dixon et al., 1999).
MAP kinases are very highly conserved between
different fungi, to the extent that they are
functional when their genes are exchanged,
e.g. between M. grisea, Candida albicans and
Saccharomyces cerevisiae (Xu, 2000).
It should be noted that any one of the above
principal signalling cascades may act repeatedly
in the course of appressorium development, as
indicated in Fig. 12.44, and in other events such
as production of conidiophores and conidia.
Signal cascades acting repeatedly in the life
cycle of M. grisea use many shared components
and only a few specific ones at any one time
point. There is also considerable cross-talk
between the three types of signalling cascade
mentioned here (see Kronstad et al., 1998), and
equivalent signalling pathways are involved
in infection processes of many other fungal
pathogens and in other fundamental processes
such as yeast hyphal dimorphism, mating and
osmoregulation in most fungi examined to
date (Xu, 2000). They are also fundamentally
conserved across other eukaryotic life forms,
and some are even found in prokaryotes. For
this reason, signalling cascades are unlikely to
provide suitably specific targets for fungicides,
and interest in this aspect of signalling seems to
have waned somewhat in recent years.
12.9.5 Gaeumannomyces graminis
Gaeumannomyces graminis var. tritici causes take-all
disease of wheat, barley, rye and numerous
wild grasses. The pathogen is soil-borne and
Fig12.49 Melanized hyphopodia of
Gaeumannomycesgraminis arising from an
equally strongly melanized runner hypha
formed on the hydrophobic surface of a
plastic coverslip.