Growth, Differentiation and Sexuality

248 L.M. Corrochano and P. Galland
ner flank shows a delayed increase of growth rate
(Greening et al. 1997). In Flammulina, thegravitropic
bending of horizontally placed stipes is complete
in about 12 h (Kern and Hock 1996).
Inner hyphae of stipes contain large vacuoles
and high turgor pressure, whereas outer hyphae
contain smaller ones and moderate turgor. It is the
high turgor of the inner hyphae that causes the
elongation growth while the outer hyphae control
the differential relaxation that causes differential
growth, and thereby gravitropic curvature. Elongation
growth of hyphae of the stipe differs from
that of mycelial hyphae, in that cell extension occurs
along the entire length of the cell; mycelial
hyphae, by contrast, display apical growth (Kern
and Hock 1996).
Bending stress generated by the weight of the
pileus does not elicit gravitropism (Greening et al.
1993). The gravistimulus is perceived even after
removal of the cap; the graviperceptive and the
graviresponsive zone is thus located in the apex
of the stipe (Kern and Hock 1996). However, to
maintain elongation growth of the stipe for prolonged
periods, the presence of the cap is necessary.
A diffusible growth promoter of unknown
chemical identity plays a crucial role in the gravitropism
of Agaricus and Flammulina (Hagimoto
1963; Gruen 1979, 1982). The growth promoter
must be water-soluble, because stipes or segments
of stipes display gravitropism in air and under silicon
oil, but not, however, under water. Imbibition
under water abolishes tropism, but not, however,
elongation growth (Kern and Hock 1996). In order
to elicit gravitropic bending, one has to postulate
that the growth promoter forms a gradient in
horizontal stipes that increases from the upper to
the lower flank. The fact that the upper part of
thestipecontainsahighamountofwaterbetween
the hyphae fits this hypothesis well (Kern and
Hock 1996). In Coprinus, graviperception remains
unaffected by the Ca 2+ channel blockers verapamil
or by the calmodulin inhibitor calmidazolium,
although the efficiency of gravitropic bending is
reduced (Novak-Frazer and Moore 1993).
1. Gravisusceptors
Sedimentation of cell organelles such as observed
for plant statoliths (amyloplasts, or barium-sulfate
bodies in Chara rhizoids) has never been observed
in Basidiomycota, and their statoliths have remained
elusive. Upon reorientation (inversion) of
Coprinus, no displacement of glycogen granules
could be observed, and it was thus concluded that
they do not represent gravisusceptors (Borriss
1934). In horizontal stipes of Coprinus cinereus,the
cytoplasm of hyphae was displaced to the lower,
and vacuoles to the upper part. It is thus feasible
that the cytoplasm functions as a gravisusceptor
(Gooday 1985). A similar separation was reported
for sporangiophores of Phycomyces (see below),
but not, however, for other Basidiomycota.
In Flammulina velutipes, actin filaments play
an important role in graviperception. Gravitropic
curvature was suppressed by the actin filamentdisrupting
agent cytochalasin D, but not by the
microtubule inhibitor oryzalin (Monzer 1995). The
relatively high density of the nuclei (1.2 g/cm 3 )and
their close association with actin filaments suggest
a role for the nuclei as gravisusceptors (Monzer
1996). The density of nuclei of Flammulina is lower
than that of plant nuclei (1.32) but higher than that
of Neurospora (1.08). A density of 1.2 is sufficiently
high for a potential gravisusceptor Eq. (13.1), and
actin-associated nuclei thus represent interesting
contenders for gravisusceptors.
Reorientation of stipes of Flammulina causes
the formation of numerous microvacuoles in the
hyphae of the lower flank whereas the upper flank
retains a single major vacuole. It is likely that the
process of vacuolization is relevant for graviperception,
as the process of vacuolization correlates
withthedifferentialgrowthratesofthehyphaeof
the upper and lower flanks (Kern and Hock 1996).
2. Kinetics, Dose Dependence and Threshold
When Coprinus cinereus is placed horizontally,
gravitropic bending manifests itself after about
25 min (=reaction time; Kher et al. 1992). The
presentation time, i.e., the minimal stimulus time
for eliciting a response, for excised and clinostatted
stipes of Coprinus cinereus is 9.6 min (Hatton and
Moore 1992). In plants, the presentation time can
be as short as 10–15 s or as long as 4 min, leading
to gravitational threshold doses (expressed as
the earth acceleration g×time) ranging from 10
to 240 g s (Volkmann and Sievers 1979; Hatton
and Moore 1992). The corresponding value for
Coprinus would be 576 g s. Mechanical stress does
not affect the bending reaction (Greening et al.
1993).