Growth, Differentiation and Sexuality

mediate light regulation in development. The
combination of genetics and genome sequence
analysis will prove invaluable for the molecular
understanding of C. cinereus photomorphogenesis.
Considering its role as model of other edible
basidiomycetes, this line of research could hold an
additional commercial value.
2. Cryptococcus neoformans
The basidiomycete Cryptococcus neoformans is
a heterothallic yeast and a human pathogen
(Casadevall and Perfect 1998; Hull and Heitman
2002). Blue light inhibits mating and haploid
fruiting in Cryptococcus neoformans (henceforth,
Cryptococcus). A close inspection of the Cryptococcus
genome has allowed scientists to identify
several photoreceptor genes, including one gene
for an opsin, one gene for a phytochrome, and
one gene for the Neurospora WC-1 photoreceptor.
Each of these genes was disrupted but only the
inactivation of the gene similar to wc-1 showed
a blind phenotype, with equal mating reactions
in dark and light (Idnurm and Heitman 2005a;
Lu et al. 2005). A gene with similarities to the
Neurospora wc-2 gene was identified by two independent
approaches: by sequence similarities (Lu
et al. 2005), and by the isolation of an insertional
mutant with a blind-mating phenotype (Idnurm
and Heitman 2005a). Mutations in any of the
Cryptococcus wc genes resulted in a blind-mating
phenotype (Idnurm and Heitman 2005a; Lu
et al. 2005), and their overexpression resulted in
a stronger light-dependent inhibition of mating
(Lu et al. 2005). In addition, mutants in any of the
wc genes were more sensitive to UV light (Idnurm
and Heitman 2005a). The Cryptococcus WC-1
protein contains a putative chromophore-binding
domain but does not contain a Zn finger domain,
unlike the Neurospora WC-1. However, a putative
Zn finger is present in the Cryptococcus WC-2
protein (Idnurm and Heitman 2005a; Lu et al.
2005). The Cryptococcus wc-1 and wc-2 genes
are expressed at very low levels in the dark but
the wc-2 gene is induced by light (Idnurm and
Heitman 2005a; Lu et al. 2005). In addition, the
two WC proteins were shown to physically interact
in a yeast two-hybrid assay (Idnurm and Heitman
2005a). These results suggest that the Cryptococcus
WC proteins will form a complex that will bind
the promoters of light-regulated genes through
the WC-2 Zn finger, in a mode of action similar
to the Neurospora white collar complex (Idnurm
Photomorphogenesis and Gravitropism 245
and Heitman 2005a; Lu et al. 2005). Surprisingly,
the Cryptococcus wc mutants showed reduced
virulence in a murine model, indicating that the
WC proteins form a complex involved in the
regulation of development and virulence in this
pathogenic fungus (Idnurm and Heitman 2005a).
III. Gravitropism
The effect of gravity on plants and fungi is usually
associated with gravitropism, i.e., the directed
growth of elongating organs parallel or antiparallel
to the gravity vector. While gravitropism no
doubt represents the most apparent gravireaction,
gravity exerts a substantial influence on morphogenesis,
too – that is, on the shape and form of
plants and fungi. One example is the peg formation
in seedlings of several plants, including Mimosa
and Eucalyptus and various Cucurbitaceae.
The peg is a protuberance, a special hook-like organ,
at the base of the hypocotyl that serves to
removetheseedcoatatthetimeofgermination.
That the morphogenesis of this organ is under
control of gravity becomes apparent from the observation
that clinostatted seedlings of Cucurbita
develop two, rather than only one peg (Takahashi
1997). Prolonged clinostatting can also induce dramatic
changes in flower morphology, as zygomorphic
flowers can take on a radial symmetry (Rawitscher
1932). The space-filling pattern and morphologyoftheplantrootsystemcriticallydepend
on the hierarchy of primary, secondary and tertiary
roots, which each possess characteristic gravitropic
(liminal) setpoint angles (Hart 1990). The important
role of gravity for regular morphogenesis has
been documented even for fungi. Under conditions
of weightlessness, the agaric fungus, Polyporusbrumalis,
for example, develops flattened fruiting bodies
with twisted and irregular pedicles (Zharikova
et al. 1977), and the cytological fine structure of hyphae
from the stem of agaric fungi changes during
gravitropic bending (see below).
Graviperception is ubiquitous in the fungal
kingdom, and is manifested as gravimorphogenesis
and gravitropism. Fruiting bodies usually
grow vertically and reorient in a few hours after
displacement from the plumb line. Even though the
gravitropism of different classes of fungi has been
described since more than a century (Table 13.2),
the cellular and molecular mechanisms underlying
gravioriented growth are still poorly known, and