Growth, Differentiation and Sexuality

Fig. 13.3. Graphical representation of Eq. (13.1), showing
the ratio N|N0 as a function of the volume of gravisusceptors.
N Number of particles per unit volume after sedimentation
(flotation), N0 number of particles per unit volume
prior to sedimentation (flotation), h distance of sedimentation
(flotation). Equation (13.1) was calculated for sedimenting
particles of density Š=1.21 g/cm 3 ,orforfloating
particles of density Š=0.79 g/cm 3 , a distance h of 10 mm,
a temperature of 300 K, an assumed density of the cytoplasm
of 1 g/cm 3 , and for gravitational accelerations of 1×g,0.2×g
and 0.02 × g (gravitropic threshold of Phycomyces). After
Grolig et al. (2004)
separate. At 0. 02 × g, however, which represents
the gravitropic threshold of Phycomyces (Galland
et al. 2004), only particles of diameter >4 μm will
form a substantial gradient.
When gravisusceptors sediment or float, they
generate a force, F, which can be calculated as:
F = g × V × n × (Šc − Šgs) (13.2)
where g is the earth’s gravitational acceleration
(9.81 m/s 2 ), V the volume of a gravisusceptor, n the
number of gravisusceptors, Šc the density of the cytoplasm,
and Šgs the density of the gravisusceptor.
The potential energy, E, ofasedimentingor
floating gravisusceptor is given by:
E = F × d (13.3)
where F is the static force (Newton), and d the
distance (m) over which the gravisusceptors
are displaced. The potential energy of a gravisusceptor
needs to exceed the thermal noise
Photomorphogenesis and Gravitropism 247
(3|2kT = 6.21 ×10 −21 J at 300 K). For example, at
the gravitropic threshold of Phycomyces, which is
near 2 ×10 −2 × g (Galland et al. 2004), the potential
energy generated by floating lipid globules
would amount to 10 −18 J, which is still 360 times
above the thermal noise. The estimated potential
energies are also sufficiently high to explain the
adherence of Phycomyces to the so-called sine law
of gravitropism (Galland et al. 2002). For small inclination
angles of the sporangiophore of 1–2 ◦ ,the
gravitropic stimuli are according to the sine law 1.7
to 3.4 ×10 −2 × g, which is just above the absolute
gravitropic threshold and thus above the thermal
noise (see above). An energy of about 10 −16 J could
be sufficient to open 10 6 mechanosensitive Ca 2+
channels (Howard et al. 1988). Such considerations
appear relevant, in view of the reasonable
possibility that the graviperception of fungi may
involve ion transport and the requisite channels.
B. Basidiomycota
Gravitropism is ubiquitous among Basidiomycota,
and is manifested by the lamellae as well as by
the stipe. The lamellae (gills) of the pileus of agarics
display positive gravitropism, i.e., they grow
parallel to, and in the same direction as the vector
of the earth’s gravitational acceleration when
they are displaced from the plumb line. A displacement
of vertical lamellae by as little as 5 ◦
can reduce the spore dispersal by some 50%; an
inclination angle of 30 ◦ may even completely abolish
spore dispersal (Buller 1909). Stipes that are
inclined bend upward, thus displaying negative
gravitropism, a response that Flammulina velutipes
completes in about 12 h (Kern and Hock 1996). Basidiomycota,
such as Polyporus brumalis and Flammulina,
that were cultured during microgravity in
satellites or space shuttles displayed disoriented
and twisted growth, and sometimes even abnormal
fruiting body formation (Zharikova et al. 1977;
Kern and Hock 1996).
The gravisensitive zone seems to be restricted
to the apex of the stipe (Haindl and Monzer 1994),
which in Coprinus cinereus comprises the upper
20%–30% (Greening et al. 1997). Gravitropic curvature
is caused by differential elongation growth
of the upper and lower flanks of the stipe. In Flammulina,
the outer flank grows at an increased rate
while the inner one shows a decreased growth rate
(Monzer et al. 1994). In Coprinus, theouterflank
shows a rapid increase in growth rate while the in-