Growth, Differentiation and Sexuality

18 Regulatory and Structural Networks Orchestrating Mating,
Dimorphism, Cell Shape, and Pathogenesis in Ustilago maydis
M. Feldbrügge 1 ,M.Bölker 2 ,G.Steinberg 1 ,J.Kämper 1 , R. Kahmann 1
CONTENTS
I. Introduction ......................... 375
II. Signalling Networks ................... 376
A. Signalling Network During Mating . . . . . 376
B. Signalling Network
DuringMorphogenesis .............. 379
C. Signalling Network During
PathogenicDevelopment ............. 380
III. Transcriptional Cascades
for Pathogenic Development ............. 381
IV. Small GTPase Networks for Cytokinesis
and Dimorphism ...................... 383
V. Cytoskeletal Networks
for Morphology ....................... 385
VI. Concluding Remarks ................... 387
References ........................... 387
I. Introduction
The heterobasidiomycete smut fungi are characterized
by their narrow host range on defined
groups of grasses including valuable crop species
such as maize, sugar cane, barley and wheat.
Most smuts enter a long systemic phase before
symptoms develop in the inflorescences of their
respective hosts. Disease symptoms are sori
in which the flower tissue is replaced by black
teliospores. It is in these organs that massive
proliferation of the fungus takes place and that
the diploid teliospores are eventually produced.
Teliospores represent a resting stage which can
survive harsh environments. Under appropriate
conditions teliospores germinate, undergo meiosis
andproducehaploidprogeny.Ustilago maydis,the
focus of this article, is a member of the smut fungi
and infects maize plants. In contrast to the other
smuts, prominent tumors are induced on all parts
1 Max-Planck-Institut für Terrestrische Mikrobiologie, Abteilung
Organismische Interaktionen, Karl-von-Frisch-Strasse, 35043 Marburg,
Germany
2 Philipps-Universität Marburg, Fachbereich Biologie, Karl-von-
Frisch-Strasse 8, 35032 Marburg, Germany
of its host, except the roots. Disease symptoms
develop early and independently of flowering,
which is one of the reasons why U. maydis has
become an important model organism for the
study of fungal pathogenicity (Banuett 1995;
Bölker 2001; Kahmann and Kämper 2004).
In its haploid form, U. maydis is yeast-like
and proliferates by budding (Fig. 18.1, step 1). It
can be propagated in the laboratory but is unable
to cause disease when injected into the host
as pure culture. The infectious form is generated
when compatible haploid strains fuse and generate
a dikaryotic filament (Fig. 18.1, steps 2–4). In
U. maydis, this process is controlled by a tetrapolar
mating system consisting of the biallelic a locus
and the multiallelic b locus (Feldbrügge et al.
2004; see Chap. 17, this volume). The a locus encodes
a pheromone/receptor system which allows
haploid cells to sense each other’s presence and
to fuse (Bölker et al. 1992). The b locus codes
for a pair of homeodomain transcription factors,
bE and bW, which dimerize when derived from
Fig. 18.1. Life cycle of U. maydis. For details, see Introduction
The Mycota I
Growth, Differentation and Sexuality
Kües/Fischer (Eds.)
© Springer-Verlag Berlin Heidelberg 2006