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synthesize sterols from nonsterol precursors and
instead need to be supplied with sterols from the host.
Ergosterol is the primary target of several fungicides that
are used to control plant-pathogenic fungi. Ergosterol
is also a primary target of several antifungal drugs that
are used to treat human mycoses (Chapter 17).

The fungal secretory system: golgi,
endoplasmic reticulum, and vesicles

Fungi have a secretory system, consisting of the
endoplasmic reticulum (ER), the Golgi apparatus
(or Golgi equivalent), and membrane-bound vesicles.
Proteins destined for export from the cell are synthe-
sized on ribosomes attached to the ER, then enter the
ER lumen and are transported to the Golgi. During their
progressive transport through the Golgi cisternae, pro-
teins undergo various modifications, including partial
cleavage and reassembly, folding into a tertiary struc-
ture, and the addition of sugar chains (glycosylation).
Then the proteins, or glycoproteins, are packaged into
vesicles which bud from the maturing face of the
Golgi, and are transported to the plasma membrane
for secretion. This intricate postal system sorts and
delivers proteins to specific destinations, including the
enzymes (pectinases, cellulases, proteases, etc.) that
are destined for export to degrade polymers in the sur-
rounding environment.
The secretory system is involved in at least some
aspects of fungal tip growth, because glycoproteins
are only produced in the Golgi and are transported
in vesicles to the sites of wall growth (Chapter 4).
Additionally, the Golgi is important for the commer-
cial production, and release from the cell, of foreign
(heterologous) gene products (Chapter 9). The ability
of a protein to enter the ER is determined by a signal
sequence at the N-terminus, which is subsequently
removed. Without this sequence the protein will
remain in the cell.
Once again, the fungal secretory system shows
some unique properties. The typical Golgi apparatus of
animals, plants, and Oomycota consists of a stack of
membraneous cisternae (Fig. 3.12a). But fungi have a
Golgi equivalentwhich is ultrastructually quite different
from a typical Golgi – it consists of sausage-shaped
strings, beads, and loops (Fig. 3.12b).

Chitosomes

Most of the vesicles seen in hyphal tips have not been
chemically characterized, but some of the smaller
ones (microvesicles) resemble particles that have been
purified in vitroand are termed chitosomes. These par-
ticles were initially discovered by homogenizing hyphae,

removing the wall material and major membranous

organelles by centrifugation, and then subjecting the

supernatant to sucrose density centrifugation so that

its components separated out as bands at different

depths in the tubes. When one of these bands was

examined by electron microscopy it was found to con-

tain chitosomes – small spheroidal bodies, 40–70 nm

diameter, each surrounded by a “shell” about 7 nm thick

(Fig. 3.13). When the band containing these particles

was incubated in the presence of an activator (a

protease enzyme) and UDP-N-acetylglucosamine (the

sugar-nucleotide from which chitin is made), each

particle was seen to produce a coil of chitin inside it,

then the particles ruptured to release ribbon-like

chitin microfibrils, each composed of several chitin

chains. If the supernatant was first treated with digi-

tonin (a saponin which solubilizes sterol-containing

membranes) and then centrifuged in a density gradi-

ent, the chitosomes were no longer found; instead, the

addition of activator and UDP-N-acetylglucosamine

resulted in the production of chitin in a different

band corresponding to a lighter fraction of the homo-

genate. But subsequent removal of the digitonin

caused the chitosomes to reappear, indicating that they

FUNGAL STRUCTURE AND ULTRASTRUCTURE 59

Fig. 3.12(a) Magnification of part of Fig. 2.3, showing

the typical Golgi body found in plants and animals.

(b) Magnification of part of Fig. 2.4, showing mitochon-

dria (M) with plate-like cisternae typical of fungi, and the

“Golgi equivalent” consisting of sausage-shaped rings,

beads, and loops (arrowheads).

(a)

(b)