Introduction to Fungi, Third Edition

170 ZYGOMYCOTA
Fig 7.5 Sporangiophore of
Phycomyces as a cylindrical lens.
Upper portion: light ray L impinges
from the left and is refracted at
the first surface.The ratio of
‘path length’of the light ray in
the proximal part of the
sporangiophore to the path length
in the distal part is PM/DM.The
maximum value of the angle b is
about 20°.Lowerportion:
sporangiophore in section to show
the peripheral layer of cytoplasm
surrounding the central vacuole.
The values are estimates of the
refractive index of cytoplasm and
vacuolar sap. Diagram modified
from Bergman et al. (1969).
nature of the photoreceptors can be obtained by
studying the action spectrum of the response
over a range of light wavelengths. The phototropic
curvature of Phycomyces sporangiophores
has a similar action spectrum to the growth
response of the vegetative mycelium, which is
also stimulated by light. There are several clearly
defined peaks at 485, 455, 385 and 280 nm, i.e.
mostly in the blue part of the spectrum.
Although b-carotene is present in large amounts
in the growing zone of the sporangiophore, the
photoreceptor system is more likely to comprise
a flavin-type molecule and a pterin-type protein
(Flores et al., 1999; Galland & Tölle, 2003).
Mutants with less than 0.1% of the wild-type b-
carotene content remain fully photosensitive.
However, b-carotene is involved in the other
light-induced responses. The signalling chains
involved in transduction of the light signal are
only partially unravelled at present (Cerdá-
Olmedo, 2001).
Moss and Baker (2002) have described a
technique for demonstrating the phototropic
response of P. blakesleeanus in the laboratory.
7.2.3 Sporangiophore development in
Phycomyces
As the sporangiophore of Phycomyces develops it
rotates. Castle (1942) followed the growth and
rotation of the sporangiophore by attaching
Lycopodium spores as markers and tracking the
displacement of the markers. His findings are
illustrated in Fig. 7.6. After a period of apical
growth of the tubular sporangiophore (stage I),
the sporangium appears as a terminal swelling
and growth ceases. During this period (stage II),
growth is limited to sporangial enlargement.
In the next period (stage III) no further enlargement
of the sporangium occurs, and elongation
is also at a standstill. During stages IVA and IVB,
elongation of the sporangiophore is resumed and
growth is mainly localized in a zone somewhat
below the sporangium. During stage I the tip of
the sporangiophore rotates clockwise (as seen
from above looking down) through a maximum
angle of about 90°. There is no rotary movement
during stages II and III. When sporangiophore
elongation recommences in stage IVA, the direction
of rotation is now anti-clockwise (as seen from