Growth, Differentiation and Sexuality

246 L.M. Corrochano and P. Galland
Table. 13.2. Examples for gravitropism in fungi
Organism Organ Gravitropism Gravisusceptor Reference
Basidiomycota
Amanita Stem Negative Unknown Hofmeister (1863)
Gill Positive Unknown Moore (1991)
Coprinus Stem Negative Unknown Knoll (1909)
Stem Negative Cytoplasm? Gooday (1985)
Gills Positive Unknown Moore (1991)
Flammulina Stem Negative Nuclei? Monzer (1996)
Gill Positive Unknown
Fomes Stem Negative Unknown Buller (1922)
Tube Positive Unknown
Psalliota Stem Negative Unknown Buller (1909)
Gills Positive Unknown
Polyporus Gills Positive Unknown Sachs (1879)
Zygomycota
Phycomyces Sporangiophore Negative Octahedral crystals
and lipid globules
Schimek et al. (1999),
Grolig et al. (2004)
Pilobolus Sporangiophore Negative Unknown Horie et al. (1998)
Glomeromycota
Gigaspora Hyphae Negative Lipid globules? Döring and Grolig
(personal communication)
Glomus Hyphae Negative Lipid globules?
as a consequence our general understanding has
remained largely at a phenomenological level in
this field. In plants, graviperception is mediated
by statoliths (usually amyloplasts), heavy cell
organelles that sediment upon reorientation
and that generate potential energy. The search
for fungal statoliths (gravisusceptors), although
a century old, has only very recently received novel
input (see below). As in plant research, the hunt
for the hypothetical gravireceptor has, however,
remained unsuccessful and will continue.
A. Criteria for Gravisusceptors
Cell organelles or cell inclusions that function as
gravisusceptors must be able to generate potential
energy that exceeds the thermal noise of the cellular
environment. After reorientation of the plant
or fungal organ, gravisusceptors generate a gravitropic
signal. To achieve this, they must have the
potential either to sediment (statoliths) or to float
(“buoys”),sothataparticle gradientcanbe formed.
To do this, they require a density different from
that of the surrounding cytoplasm, and additionally,
a critical size to overcome the effect of thermal
motion, which counteracts the effect of sedimentation
or buoyancy and thus the formation of a particle
gradient. Whether or not a cell organelle qual-
ifies as a gravisusceptor can be determined with
a function that was originally introduced by Einstein,
who modified the Boltzmann distribution by
taking into account the earth’s gravitational field.
The function describes the ratio of sedimenting (or
floating) particles separating along a distance, h:
N|N0 = exp − � V(Šc − Šgs)gh|kT �
(13.1)
where N0 and N are the number of particles
separated by a distance h before and after sedimentation
(flotation), respectively, V is particle
volume, Šc and Šgs are the specific densities
(kg/m 3 ) of the cytoplasm and the gravisusceptor,
respectively, g is the constant of gravitational acceleration
(9.81 m/s 2 ), k is the Boltzmann constant
(1.38 ×10 −23 J/K), and T is absolute temperature
(K). A plot of Eq. (13.1) is shown in Fig. 13.3
for T = 300 K and Šgs = 791 kg/m 3 (density of
lipid globules of Phycomyces, seebelow).Avalue
of N|N0 = 1 indicates that the particles do not
separate, so that no gradient is generated; values
smaller than 1 indicate effective separation. It is
apparent from such a plot that particle separation
by sedimentation or by flotation over a distance h
of 10 μm occurs at 1 × g only for particles with
diameters above 0.8 μm (Grolig et al. 2004).
At 0. 2 × g, particles of diameter >2 μm would