untitled

where the host fungi produce an extracellular poly-
saccharide (Chapter 12). In fact, the production of
an extracellular matrix is often influenced by growth
conditions. We noted in Chapter 1 that pullulan is
produced commercially from Aureobasidium pullulans;
in this case its synthesis is favored by an abundant
sugar supply in nitrogen-limiting growth conditions
(Seviour et al. 1984). Some of the other gel-like mater-
ials around fungi also have potential commerical roles.

Septa

Septa (cross-walls) are found at fairly regular intervals
along the length of most hyphae in the Ascomycota,
Basidiomycota, and mitosporic fungi, although the
septa are perforated to allow continuity of the pro-
toplasm. If part of the hypha is damaged, then a
Woronin body or a plug of coagulated protoplasm
rapidly seals the septal pore to localize the damage.
Then the hypha can regrow from a newly formed tip
behind the damaged compartment, or in some cases
a new tip can grow into the damaged compartment
(see Fig. 12.12). Clearly, these “damage limitation”
responses help to conserve the integrity of the hypha,
and might often be necessary when hyphae are eaten
by insects, or are attacked by parasitic fungi (myco-
parasites), or when hyphae of different vegetative
compatibility groups attempt to anastomose (Chap-
ter 9). However, there are several fungi and fungus-like
organisms that do not produce septa in the normal
vegetative hyphae – for example the Zygomycota
(Mucor, etc.) and the fungus-like Oomycota (Pythium,
Phytophthora, etc.). These aseptate fungi are more
vulnerable to damage.
Septa might help to provide structural support to
hyphae, especially in conditions of water stress. But one
of their main roles seems to be to enable differentia-
tion. By blocking the septal pores, a fungal hypha is
transformed from a continuous series of compart-
ments to a number of independent cells or regions that
can undergo separate development (Chapter 5).
Septa can be seen by normal light microscopy, but
electron microscopy reveals several different types of

septum. The Ascomycota and mitosporic fungi have a

simple septum(Fig. 3.10) with a relatively large central

pore, ranging from 0.05 to 0.5μm diameter, which

allows the passage of cytoplasmic organelles and even

nuclei. The development of these septa occurs remark-

ably quickly, usually being completed within a few

minutes. They develop as an ingrowing ring from the

lateral walls of the hypha, and this is associated with

localized modifications of the lateral walls themselves,

including a localized proliferation of a glycoprotein reti-

culum in the walls of Neurospora crassa.

The Basidiomycota also have simple septa when

they are growing as monokaryons (with one nucleus

in each cell). But they often have a more complex

dolipore septum(Fig. 3.11) when strains of different

mating compatibility groups fuse to form a dikaryon,

with two nuclei in each compartment. The dolipore sep-

tum has a narrow central channel (about 100 –150 nm

diameter) bounded by two flanges of amorphous glu-

can. On either side of this septum are bracket-shaped

membraneous structures termed parenthosomes,

which have pores to allow cytoplasmic continuity but

which prevent the passage of major organelles. Thus,

the Basidiomycota often have a more regular arrange-

ment of nuclei compared with other fungi. But we

will see in Chapter 5 that the septa are selectively

degraded when the Basidiomycota begin to form a

FUNGAL STRUCTURE AND ULTRASTRUCTURE 57

G C

P

Septal pore

GR

GR/P

LW

Fig. 3.10Diagrammatic representation of a simple
septum of Neurospora crassa. The septum develops
as an ingrowing ring from the lateral wall of the
hypha. Associated with this is a modification of
the lateral wall (LW), including a proliferation of the
glycoprotein reticulum (GR). G =glucan layer;
GR/P =glycoprotein reticulum embedded in pro-
tein; P =protein; C =chitin. (Based on Trinci A.P.J.
(1978) Science Progress65, 75–99.)

Fig. 3.11The dolipore septum, found in many members

of the Basidiomycota. Large deposits of glucan (G) line

the narrow central pore, and specialized perforated

membranes termed parenthosomes (P) prevent major

organelles from passing through the septal pore.