Growth, Differentiation and Sexuality

250 L.M. Corrochano and P. Galland
latencies of the corresponding stretch responses
are of about 1-min duration, and thus are much
shorter than the gravitropic latencies obtained
after reorientation of the sporangiophore.
The gravisusceptors of Phycomyces must be
internal, because negative gravitropism persists in
sporangiophores submerged in water or in fluids
with a density exceeding that of the cytoplasm
(Dennison 1961). Because cytoplasm sediments
and the central vacuoles float slightly in horizontal
sporangiophores, they might participate in
gravisusception (Dennison and Shropshire 1984).
The central vacuoles contain octahedral protein
crystals of high density (1.27 g/cm 3 ) that rapidly
sediment upon reorientation of the sporangiophore,
and that participate in gravisusception.
Mutants lacking these protein crystals are affected
in gravitropism (Schimek et al. 1999; Galland et al.
2004). The crystals contain three proteins, and are
associated with pterin- and flavin-like pigments
(Eibel et al. 2000; Fries et al. 2002).
Beside the protein crystals, also apical lipid
globulesareinvolvedingravisusception.Instage-1
sporangiophores of Phycomyces, the lipid globules
are clustered in a special organelle, a complex of
lipid globules (CLG) that resides 110 μm below
the apex. When the sporangium is subsequently
formed, the lipid globules migrate into the columella.
Their role in the columella is presently less
clear, but it appears likely that even in this mature
stage they function as gravisusceptors. Sporangiophores
lacking the CLG display a greatly diminished
gravitropic response (Grolig et al. 2004). The
sedimentation of the octahedral crystals, and the
buoyancy of the lipid globules generate a potential
energy that each supersedes the thermal noise
by 3–4 orders of magnitude (Schimek et al. 1999;
Grolig et al. 2004). The density of the lipid globules
of Phycomyces (0.79 g/cm 3 )ismuchlowerthanthat
commonly found in oleosomes of plant material.
The specific density of oleosomes (spherosomes)
of peanuts, for example, is 0.92 g/cm 3 (Jack et al.
1967), and that of oleosomes of wheat aleuron can
be as high as 1.16–1.18 g/cm 3 (Quail 1979). It is
very apparent from such a comparison that the
lipid globules of Phycomyces, which have diameters
comparable to those of plant oleosomes, display
traits that are indispensable for gravisusception,
whereas those of plants display traits suitable
only for storage.
One prominent feature of the lipid globules is
the fact that they are in continual non-Brownian
motion. The molecular motors are presently
unknown, but the fact that they occur in a cage of
dense actin mesh indicates that an acto-myosin
system powers the motion. The lipid globules
contain β-carotene and appear deep yellow. In addition,
they carry pterin- and flavin-like pigments
that emit blue and green fluorescent light upon
excitation (Ogorodnikova et al. 2002; Grolig et al.
2004). Lipid globules (droplets) are ubiquitous
among oleaginous fungi (e.g., Mortierella ramanniana;
Kamisaka et al. 1999), and it appears likely
that graviperception mediated by buoyancy might
represent a mechanism that is rather widespread
in the fungal kingdom.
2. Kinetics, Dose Dependence and Threshold
Sporangiophores of Phycomyces that are placed
horizontally have a rather irregular gravitropic
latency of about 10–30 min, or even longer
when the conditions are suboptimal (Dennison
1961; Dennison and Shropshire 1984; Schimek
et al. 1999). Gravitropic bending of horizontal
sporangiophores is complete in 10–12 h (Schimek
et al. 1999). The dose dependence was determined
in long-term experiments in a clinostat
centrifuge, and the threshold was found to be
near 2 ×10 −2 × g (Galland et al. 2004). A mutant
that lacks the vacuolar protein crystals showed
a slightly elevated threshold. Upon reorientation
of the sporangiophore, the slowly ensuing bending
response of Phycomyces is preceded by very fast
molecular events that can be monitored spectroscopically.
A gravitropic stimulus elicits so-called
gravity-induced absorbance changes (GIACs)
that occur almost instantaneously, and that are
specific for early events of the transduction chain,
because they are altered in a gravitropism mutant
of genotype madJ (Schmidt and Galland 2000,
2004).
3. Cytoskeleton and Calcium
As in higher plants, the cytoskeleton appears to
play a major role in the gravitropism of the Phycomyces
sporangiophore, in which actin, myosin,
spectrin and integrin have been immunodetected
(Doucette et al. 1994). The apical lipid globules of
stage-1 sporangiophores are encased by a dense
mesh of actin filaments that fills the dome-like
structure of the apex. Inhibitor studies show that it
is the actin filaments that cause the non-Brownian
motion of the lipid globules (Grolig et al., unpublished
data). Injection of stage-4 sporangiophores