Introduction to Fungi, Third Edition

EUAGARICS CLADE
549
Fig19.19 Gravitropism of basidiocarps in
Flammulina velutipes growing on a branch of
gorse (Ulex europaeus).The branch was turned
clockwise by 90° 4 h earlier, and the upper
stipe regions are undergoing curvature
(arrows) to bring the pilei back into the
horizontal position.
congregate and are more abundant in the lower
sides of cells in horizontally displaced organs. In
Flammulina, Monzer (1996) considered that only
the nuclei, with a density of 1.22 g cm 3 , were
valid candidates for gravity-related sedimentation
to enable them to function as statoliths.
In the transition zone there may be up to 10
nuclei per hyphal segment. These nuclei are
enmeshed in filaments of F-actin, and Monzer
(1995) has suggested that tension of the actin
filaments associated with sedimentation of
nuclei is transmitted to the plasma membrane
and provides the trigger to initiate cellular
changes involved in the gravitational response.
Evidence supporting the involvement of actin
filaments is that the gravitropic response
is affected by the actin-depolymerizing drug
cytochalasin D, but not by treatment with
microtubule-inhibiting drugs.
Within 30 min of displacement of a basidiocarp
from a vertical to a horizontal position,
differences may be noted between the upper and
lower flank cells. Microvesicles, most likely
derived from the endoplasmic reticulum and
Golgi cisternae, are more abundant in the lower
flank cells. They fuse with the vacuoles, thereby
contributing to the volume increase and turgordriven
enlargement of these cells. Vesicles
containing wall precursor materials and
enzymes also develop, providing for enlargement
and stretching of the walls of lower flank cells.
There are some 1.2 million hyphae present in the
cross-section of a Flammulina stipe. They remain
strictly parallel to each other but do not show
anastomosis or direct contact with each other.
It is believed that each separate hypha in the
transition zone responds to the gravitational
stimulus. Possibly a growth factor which inhibits
the growth of upper flank cells is involved, but
no such substance has yet been demonstrated or
identified.
Detailed studies of gravitropism have also
been made using stipes of the coprophilous inkcap
Coprinus cinereus (Kher et al., 1992). Although
there are similarities to Flammulina, there are
also differences. For example, the response
time of Coprinus is much shorter than that
of Flammulina, and curvature extends along the
entire stipe rather than being restricted to
the transition zone. These differences reflect
the varied growth conditions of the two fungi
(Moore et al., 1996). Flammulina velutipes is
lignicolous and its basidiocarps are relatively
long-lived whilst C. cinereus is coprophilous and
its basidiocarps are evanescent.
Armillaria (42 spp.)
Most species are root pathogens, especially of
woody plants, and there is an extensive literature
on their biology and pathogenicity (see Shaw &
Kile, 1991; Holliday, 1998; Fox, 2000). In the past,
most collections were identified as A. mellea
(the ‘honey fungus’), but this is now regarded
as an aggregate of about 5 10 species, A. mellea