Growth, Differentiation and Sexuality

phology and are affected only in cell separation
(Wedlich-Söldner et al. 2002b). This indicates that
functional endosomes are required for morphogenesis
whereas their motility is not. Indeed, recent
studies clearly demonstrate that microtubules
have only minor roles in determining shape and
bud formation in yeast-like cells (Fuchs et al. 2005),
whereas they are essential in nuclear migration and
mitosis (Steinberg et al. 2001; Straube et al. 2001).
With respect to the filamentous growth forms, however,
the situation is different. The overall distribution
of microtubules in yeast-like cells, conjugation
hyphae and bE/bW-induced filaments appears very
similar (Fig. 18.7). However, in the absence of microtubules,
growth of conjugation hyphae stops at
alengthofabout50–60μm, indicating that microtubules
are specifically required for long-distance
transport. Kin2, a kinesin-1 member in U. maydis
(Lehmler et al. 1997), cooperates with Myo5 in polar
growth, and evidence for a role of both motors in
polarized secretion exists (Schuchardt et al. 2005).
These data suggest that microtubules and F-actin,
in combination with associated components such
as molecular motors, need to cooperate in a functional
network to promote filamentous growth.
VI. Concluding Remarks
The elucidation of signalling networks which operate
during mating and morphogenesis in U. maydis
has solved a number of old mysteries but has
raised even more new questions. The most prominent
ones concern the biotrophic phase: what are
the signalling inputs provided by the plant, and
how is the fungus able to juggle with three different
types of MAP kinases likely to be connected
to the same upstream components in determining
temporal and spatial control of signalling? How
are the signalling pathways interconnected during
pathogenic development? How is the plethora
of different shapes adopted by the fungus during
the biotrophic phase determined and controlled
by cytoskeletal elements? What is the trigger for
the change in growth direction during the initial
phase of infection? Which are the genes allowing
U. maydis to adapt to its new environment in the
host, and why is it that U. maydis needs the host
plant at all to complete its sexual cycle? Although
these questions all focus on the fungal partner, it
is quite clear that the plant host is still a black
boxandneedstobeincludedmuchmoreextensively
in future investigations. How is host speci-
Regulatory and Structural Networks in Ustilago maydis 387
ficity determined? Which compounds are provided
by the plant for the proliferation of dikaryotic hyphae?
What are the molecular events underlying
tumour development? It is expected that the available
genome sequence of both partners will greatly
speed up the discovery of genes involved in these
processes. Since mutants can be efficiently generated
in U. maydis through reverse genetics (Brachmann
et al. 2004; Kämper 2004), and since large
collections of transposon-tagged maize lines exist,
thechancesarehighfornewinsightsintothisfascinating
interaction of a fungal pathogen with its
host.
Acknowledgements. Ourworkreferredtointhisarticleis
supported by grants from the DFG and the BMBF, Germany.
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