Introduction to Fungi, Third Edition

378 HYMENOASCOMYCETES: PYRENOMYCETES
because two members, Magnaporthe grisea and
Gaeumannomyces graminis, are important plant
pathogens. Seminal work on developmental and
molecular aspects of plant pathogenesis has
been done with M. grisea which will be the
main focus of this section.
Magnaporthe grisea causes rice blast disease
in which individual infections give rise to
spindle-shaped necrotic lesions on rice leaves.
The fungus is particularly common in South
East Asia, its probable centre of origin where
it has been known for centuries (Rao, 1994). Rice
blast has spread to virtually all rice-growing
areas, although it is more severe in cooler
climates. Considerable research efforts are
being directed at controlling rice blast, although
the output in the shape of fungicides against
M. grisea or the seeds of resistant rice cultivars
may well be beyond the financial means of the
small-scale agricultural systems found in many
countries in Asia and Africa where rice blast is
a problem.
In the field, the fungus is encountered mainly
in the anamorph state which used to be called
Pyricularia oryzae if growing on rice. In addition
to rice, M. grisea can attack wheat, barley and
various wild grasses on which the asexual state is
called P. grisea. Accordingly, the rice pathogen
should perhaps be renamed M. oryzae, but we feel
bound by convention to retain M. grisea since
that name is universally used. There are several
strains with different host spectra, and e.g. the
wheat blast strain in Brazil is genetically distinct
from rice blast strains present in the same
regions (Urashima et al., 1993). This is consistent
with the suggestion by Couch et al. (2005)
that the rice-infecting lineage of M. grisea arose
from a single host-switching event from Setaria
millet early in the history of rice cultivation
which began around 5000 BC. Sexual reproduction
is rare in the field especially with the rice
strains, so that discrete clones of M. grisea
populations are typically formed in many ricegrowing
areas (Zeigler, 1998). In tropical climates
with up to three cropping seasons each year, the
fungus can continuously infect fresh green
foliage, whereas in Southern Europe it overwinters
on rice stubble. Magnaporthe grisea is
haploid, with each nucleus containing about
seven chromosomes (Valent, 1997).
12.9.1 Conidium germination and
appressorium formation in
Magnaporthe grisea
A summary of the disease cycle is given in
Fig. 12.44. Good reviews have been written by
Howard and Valent (1996), Valent (1997) and
Tucker and Talbot (2001). The conidia of M. grisea
are three-celled, with each cell containing a
single nucleus. A mature conidium swells upon
hydration, and this causes the breakage of the
wall at the tip of the spore, releasing a drop
of mucilage stored in the periplasmic space
(Fig. 12.45a). This may already occur while the
spore is still attached to the conidiophore.
The exact chemical nature of this mucilage is
unknown, although it probably contains glycoproteins.
It attaches the spore firmly to the wax
of the host cuticle or other hydrophobic surfaces
(Hamer et al., 1988; Howard, 1994). Mucilage
release is a purely physical phenomenon which
does not require any de novo metabolic
activities.
Under suitable conditions, a single conidial
cell usually the basal or apical cell, more rarely
the central one emits a germ tube. Numerous
conflicting reports have been published on the
requirements for germination, but in our experience
conidia germinate readily in aqueous
suspension or in contact with any inert surface,
provided that they have been washed by centrifugation
and resuspension in water. Washing
removes an auto-inhibitor which prevents germination
of spores in dense suspensions (Kono et al.,
1991). Germination on the plant cuticle may
appear to be stimulated simply because the inhibitor
is lipophilic and dissolves into the cuticular
waxes, thereby becoming diluted from the
spore (Hedge & Kolattukudy, 1997).
In contrast to spore germination, commitment
to appressorium formation requires
the presence of specific environmental signals.
These are perceived after the germ tube has
formed a hook-like appressorium initial (Bourett
& Howard, 1990; deZwaan et al., 1999).
Appressoria are formed from hooks upon contact