Growth, Differentiation and Sexuality

3. Gravimorphogenesis
Long-term space experiments have shown that the
presence of a gravitational field is of paramount
importance for the morphogenesis of plants and
fungi. The growth pattern and the morphogenesis
of Basidiomycota are severely affected under
weightlessness. Polyporus brumalis and Pleurotus
ostreatus raised under weightlessness showed
abnormal or no formation of fruiting bodies
in darkness, and irradiation-induced fruiting
bodies lacked a hymenium. Stipes of Flammulina
raised under weightlessness were flat, rather than
round (Kern and Hock 1996). Similar results were
also obtained in long-term space experiments
with Polyporus brumalis. In weightlessness and
darkness, albeit not in light, either the formation
of fruiting bodies was arrested, or twisted stems
developed that lacked a cap (Zharikova et al. 1977;
Kasatkina et al. 1980).
C. Ascomycota
Even a superficial inspection of the various fructification
organs of Ascomycota provides evidence for
their ubiquitous capacity of graviperception. The
upright-growing perithecia of Neurospora,theupward
orientation of the apothecia of Pezizazeae,
and the vertical fruiting bodies of Helvellaceae,
Helvelloidae and Morchellaceae bear ample witness
for gravitropism. In spite of this fact, there exists
almost no literature on the graviperception of this
group of organisms.
To assess whether single-celled organisms
possess the potential to perceive gravity, yeasts
were often employed in space experiments under
microgravity conditions. Attempts to detect
differences in the induction and the repair of DNA
lesions under earth and microgravity conditions
were without success for Saccharomyces (Pross
et al. 2000; Takahashi et al. 2001). Cultures of
Saccharomyces that were subjected to simulated
weightlessness in a special apparatus providing
low-fluid shear, the “rotating wall vessel bioreactor”,
displayed substantially altered expression
for clusters of genes that were either up- or
down-regulated. The genes contained promoter
sequences with similarities to the Rap1p transcription
factor binding site and the stress responsive
element (STRE; Johanson et al. 2002). These
experiments provide clues on how physical forces
acting on the cell surface could translate into
differential gene expression, and thus represent
Photomorphogenesis and Gravitropism 249
a model for gravimorphogenesis. A phenomenon
of gravimorphogenesis was observed in bioreactor
cultures of Saccharomyces cerevisiae that were
maintained for 8 days under microgravity in
a space laboratory. The proportion of randomly
distributed bud scars was about three times higher
in cells subjected to weightlessness (17%) than
in those maintained on earth (5%; Walther et al.
1996).
D. Zygomycota
Sporangiophores of Phycomyces blakesleeanus and
Pilobolus crystallinus display negative gravitropism
(Horie et al. 1998; Schimek et al. 1999). Mycelial hyphae,
zygophores and zygospores attached to suspensors
are agravitropic. Sporangiophores of Phycomyces
that grew in a satellite under weightlessness
displayed completely random and disoriented
growth (Parfyonov et al. 1979). The effectiveness
of gravitropism depends to some extent on the
developmental stage. Stage-1 sporangiophores of
Phycomyces, which lack sporangia, bend gravitropically
more slowly than do stage-4 sporangiophores,
which possess sporangia (Schimek et al.
1999; Grolig et al. 2004). In Pilobolus,gravitropism
is almost absent in stage-1, whereas it is well expressed
in stage-4 sporangiophores (Horie et al.
1998). The latter observation might be explained
bythefactthatthegrowingzoneinstage-1sporangiophoresofthisfungusismerely0.3mm
thick,
whereas it is about 2–3 mm in Phycomyces (Horie
et al. 1998).
1. Gravisusceptors
Flexure (bending stress) could potentially play
a role in graviperception. Thus, sporangiophores
respondtocellwallstressgeneratedbyflexureor
compression (Dennison 1961, 1964). A unilateral
force as low as 0.5 mg elicited a bending response
(Dennison and Roth 1967). Stretch elicits also
a transient growth response (diminution of growth
rate) when sporangiophores are stretched (elongated)
by a load of 5 mg; when the load is lifted,
a transient increase of growth rate is observed.
Both responses are adaptive, so that after a few
minutes the growth rate returns to the pre-stimulus
level (Dennison and Roth 1967). These data are
in agreement with the assumption that stretch
(flexure) occurring at the upper side of a horizontal
sporangiophore influences gravitropism. The