Growth, Differentiation and Sexuality

mon signal transduction pathway. In Magnaporthe
grisea, Colletotrichum lagenarium and Cochliobolus
heterostrophus, strains containing mutations in
FUS3 orthologs are defective in appressoria formation
(Xu and Hamer 1996; Lev et al. 1999; Takano
et al. 2000), and fail to colonize host plants when
inoculated through wound sites. In addition, these
MAP kinase mutants were reported to be impaired
in conidiogenesis, aerial hyphae formation, and female
fertility. Although unreported in the MAP kinase
mutants constructed in plant pathogens, our
prediction is that these mutants may also be hyphal
fusion defective and fail to make an interconnected
mycelial network, an aspect that may be important
for pathogenesis.
One of the downstream components of the
pheromone response pathway in S. cerevisiae
is STE12, which encodes a transcription factor
(Errede and Ammerer 1989). Activation of Ste12
bytheMAPkinasepheromoneresponsepathway
culminates in the activation of a number of genes
associated with mating and cell fusion (Errede
and Ammerer 1989; Zeitlinger et al. 2003). In A.
nidulans, mutations in the ste12-like transcription
factor steA resulted in mutants affected in sexual
development (Vallim et al. 2000). In M. grisea,
strains containing a deletion of the SteA ortholog
(MST12) formed defective appressoria, and were
unable to form invasive hyphae within the plant
when inoculated into wound sites (Park et al.
2002). In N. crassa, a strain containing a mutation
in the SteA ortholog, pp-1, is very similar in
phenotype to a mak-2 mutant, and is defective in
hyphal and germling fusion (Jacobson, Fleißner
and Glass, unpublished data). Live cell imaging
and microscope observations of the N. crassa nrc-
1/mak-2/pp-1 mutants indicate that they are blind
to self (mutants neither attract nor are attracted
to hyphae/conidia of identical genotype). Furthermore,
the nrc-1 and mak-2 mutants do not form
CATs (Roca et al. 2005). These data suggest that the
pheromone response MAP kinase pathway is either
involved early in the germling or hyphal fusion
processes, or is required for rendering conidia and
hyphae competent to undergo fusion. Genome
comparisons show conservation of genes required
for mating in S. cerevisiae within the genomes of
filamentous fungi, such as N. crassa (Glass et al.
2004). However, the role of most of these genes in
either mating and/or anastomosis is unknown.
In Fusarium graminearum, mutations in another
MAP kinase, MGV1, the homolog of SLT2 of
S. cerevisiae, resulted in a mutant that was female-
Anastomosis in Filamentous Fungi 131
sterile and also failed to form heterokaryons (Hou
et al. 2002). However, the step at which hyphal fusion
was blocked in the MGV1 mutant was not evaluated.
In S. cerevisiae, theSLT2 MAP kinase pathway
is downstream of the FUS3 MAP kinase pathway,
and is required for remodeling the cell wall
(Buehrer and Errede 1997; Xu 2000). The polarisome
component Spa2, which interacts with Ste11
and Ste7 of the pheromone response MAP kinase
pathway, also interacts with Mkk1 and Mkk2 of
theSlt2cellwallintegrityMAPkinasepathway
(Sheu et al. 1998; Fig. 7.4). The activation of the
pheromone response MAP kinase pathway induces
cells to form projections oriented toward the gradient
of pheromone secreted by a mating partner,
whichrequiresnewcellwallsynthesisviaactivation
ofthecellwallintegrityMAPkinasepathway.These
data suggest that the cell wall remodeling MAP kinase
pathway may be required also for anastomosis
in filamentous fungi. In A. nidulans and M. grisea,
mutations in the MAP kinase gene orthologous to
the slt2 gene in S. cerevisiae affect conidial germination,
sporulation, and sensitivity to cell walldigesting
enzymes (Xu et al. 1998; Bussink and Osmani
1999). However, possible anastomosis defects
in these mutants were not tested.
5. Initiation of Branch Formation/Pegs
and Polarization
Initiation of branch formation is associated with
the formation of an incipient Spitzenkörper at
the branch point (Riquelme and Bartnicki-Garcia
2004); peg formation during hyphal fusion is also
preceded by formation of a new Spitzenkörper and
subsequent tip growth. In plant pathogenic rust
fungi, Uromyces and Puccinia, the Spitzenkörper
was associated with germ tube elongation and
direction of growth, but disappeared during appressorium
differentiation. However, a structure
similar in appearance to the Spitzenkörper was
observed during the formation of the penetration
peg (Dijksterhuis 2003).
In S. cerevisiae, a structure termed the “polarisome”isbelievedtobeanalogousinfunction,
and perhaps in some protein components to
the Spitzenkörper described in filamentous ascomycete
fungi (Harris et al. 2005). The polarisome
is composed of scaffolding components (such as
Spa2), signaling molecules (such as Rho proteins)
and actin regulators (such as the formin Bni1;
Bidlingmaier and Snyder 2004). The formation of
the polarisome and subsequent polarized growth