Agriculture Reference
In-Depth Information
fluorescent protein (GFP) in
M. grisea
this gene (
PMK1
) was mainly expressed in
appressoria and developing conidia (Bruno
et al.
, 2004).
Furthermore, fungi without clearly differentiated appressoria show
PMK1
homologue genes which are essential for fungal pathogenicity, such as the causal agent
of grey mould,
Botrytis cinerea
(Zheng
et al.
, 2000), the vascular wilt pathogen
Fusarium oxysporum
f.sp.
lycopersici
(Di Pietro
et al.
, 2001),
Gaeumannomyces
graminis
(Dufresne and Osbourn, 2001) and
Claviceps purpurea
, the causal agent of
ergot of grasses (Mey
et al.
, 2002). Obviously, the signal transduction via the MAP
kinase
PMK1
homologues play an important role in the pathogenicity of several plant
pathogenic fungi.
A further question arising is what kind of genes are regulated by this MAP
kinase pathway. Two genes (
GAS1
and
GAS2
) that were regulated by
PMK1
and
specifically expressed in appressoria of
M. grisea
could be identified (Xue
et al.
,
2002). Mutants deleted in one or both genes were reduced in appressorial
penetration. Interestingly, homologues of these genes were found in several
filamentous fungi but not in yeasts (Xue
et al.
, 2002). However, these two genes
may function as real virulence factors in fungal pathogens.
(b) Penetration by mechanical force
Appressoria create a high turgor pressure that allows the penetration peg to penetrate
the plant cell wall (for review, see Bastmeyer
et al.
, 2002). Well studied examples of
penetration by mechanical force are that of the rice blast fungus
M. grisea
(Howard
and Valent, 1996) and of
Colletotrichum graminicola
, the causal agent of
anthracnose of numerous grasses (Bechinger
et al.
, 1999). In both cases cell walls of
mature appressoria are melanized and melanin plays a crucial role in pathogenicity.
Experiments with melanin synthesis inhibitors and with melanin-deficient mutants
(Chumley and Valent, 1990) show that non-melanized appressoria have lost the
ability to penetrate the plant surface as well as artificial membranes. Melanin lowers
the porosity of appressoria and thus helps to increase the osmotic pressure - up to 8
MPa in appressoria of
M. grisea
(Howard
et al.
, 1991). How such high pressure can
be generated within living cells remained unclear until de Jong
et al.
(1997) showed
that glycerol accumulates in the appressoria to very high concentrations (more than
3.0 M). The melanized appressorium wall is impermeable to glycerol and thus leads
to the generation of turgor pressure. Both glycogen and lipid mobilization during
conidial germination and subsequent degradation contribute to glycerol biosynthesis
(Thines
et al.
, 2000).
(c) Penetration by enzymatic digestion
The destruction of the plant cell wall by numerous phytopathogenic fungi is believed
to be an important aspect of the infection process. Although there is serious debate
whether microbial cuticle degrading and cell wall degrading enzymes
(CWDEs) like
cutinases, cellulases and pectinases in general have a major impact on pathogenesis,
