Introduction to Fungi, Third Edition

ZYGOMYCETES: MUCORALES
169
mycelium. Elongation of the sporangiophore is
confined to a yellow-pigmented growing zone
(about 1 cm long) beneath the apex. In the
absence of light, sporangiophores grow vertically
as a negative response to gravity. Schimek et al.
(1999) have suggested that gravity may be
detected by a combination of at least two
mechanisms. Proteinaceous crystals located
inside vacuoles have a higher density than the
vacuolar sap and therefore sediment in response
to gravity, whereas a cluster of buoyant lipid
droplets less dense than the cytoplasm floats to
the apex of the sporangiophore. Both mechanisms
would be different from that found in
the fruit bodies of basidiomycetes such as
Flammulina, in which nuclei denser than the
surrounding cytoplasm seem to be the organelles
involved in graviperception (see p. 546).
7.2.2 Phototropism in Phycomyces
If a sporangiophore is subjected to unilateral
illumination it bends towards the light,
especially blue light. Phototropism in Phycomyces
is extremely sensitive, the lower threshold being
1nWm 2 , which is equivalent to the light
emitted by a single star at night (Cerdá-
Olmedo, 2001). Bending is the consequence of
a deceleration of about 6% in the growth rate
of the side proximal to the direction of light,
and an increase by the same rate on the distal
side (Fig. 7.4). Because the refractive index of
the sporangiophore contents exceeds that of air,
the sporangiophore functions as a cylindrical
lens, focusing unilateral light on the distal
wall of the sporangiophore, resulting in more
intense illumination of that side. Evidence in
support of the lens effect is the demonstration
that sporangiophores immersed in mineral oil
with a higher refractive index than that of the
sporangiophore contents function as a diverging
lens and bend away from the light. The illumination
of the edge of a sporangiophore by a narrow
beam of light from a laser is followed by bending
of the sporangiophore in a direction perpendicular
to the light beam (Meistrich et al., 1970).
Photoreceptors are located in the plasma
membrane (Fukshansky, 1993), and the transmission
of the signal leads to localized wall
softening and the synthesis of new cell wall
Fig 7.4 Sporangiophore development of Phycomyces
blakesleeanus in standard test tubes (about1.5 cm diameter).
The tubes were wrapped except for the tip of tube (a) or a
square on the right-hand side near the top of tube (b) In tube
(a), the sporangiophores have grown straight towards the
light, whereas in tube (b) they have bent towards the lateral
light source.
material (Herrera-Estrella & Ruiz-Herrera, 1983;
Ortega, 1990).
A central problem in studies of photoresponses
is the nature of the photoreceptor(s).
Two photoreceptors one for low and the
other for high light intensities are involved
in determining the phototropism in Phycomyces,
and there are also two receptors each for
light-induced microphore formation, macrophore
formation, and carotenoid biosynthesis
(Cerdá-Olmedo, 2001). A clue to the possible