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residues (Chapter 7), whereas the fungal glucans are

branched polymers, consisting mainly of β-1,3-linked

backbones with short β-1,6-linked side chains. The

Zygomycota typically have a mixture of chitin and

chitosan (a poorly acetylated or nonacetylated form

of chitin; Chapter 7), polymers of uronic acids such as

glucuronic acid, and mannoproteins. The Oomycota

(which are not true fungi) have little chitin, and

instead they have a mixture of a cellulose-like β-1,4-

linked glucan and other glucans.

Having made these points, it is important to recog-

nize that the wall composition of a fungus is not

fixed, but can change substantially at different stages

of the life cycle. This is also true for many dimorphic

fungi, which can grow as either hyphae or budding

yeast-like cells (Chapter 5).

Wall architecture

The major wall components of fungi can be thought
of as the bricks and mortar, but it was only in 1970
that we began really to understand the architecture of
the fungal wall. For this, Hunsley & Burnett developed
an elegently simple technique which they termed
enzymatic dissection. They mechanically disrupted
fungal hyphae so that only the walls remained, and then
treated the walls with combinations and sequences
of different enzymes, coupling this with electron
microscopy to detect any changes that the enzyme
treatments had caused. If, for example, the surface
appearance of the wall changed after treatment with a
particular enzyme “X,” then the substrate of “X” is likely
to be the outermost wall component. So, by using
various sequences and combinations of enzymes it
was possible to strip away the major wall components
and to see their relationships to one another. Three
fungi were used in this work, but we will take Neuro-
spora crassaas an example to illustrate the essential
features (Fig. 3.9).
In mature regions of hyphae the wall of Neurospora
was shown to have at least four concentric zones;
these are shown as separate layers in Fig. 3.9, but in
reality they grade into one another. The outermost zone
consists of amorphous glucans with predominantly
β-1,3 and β-1,6 linkages, which are degraded by the
enzyme laminarinase. Beneath this is a network of gly-
coprotein embedded in a protein matrix. Then there
is a more or less discrete layer of protein, and then an
innermost region of chitin microfibrils embedded in
protein. The total wall thickness in this case is about
125 nm. But the wall at the growing tip is thinner
(c. 50 nm) and simpler, consisting of an inner zone
of chitin embedded in protein and an outer layer of
mainly protein. So it is clear that the wall becomes
stronger and more complex behind the extending

hyphal tip, as further materials are added or as further

bonding occurs between the components.

Neurosporaseems to have an unusually complex

wall architecture, because a glycoprotein network has

not been seen in some other fungi. Nevertheless, the

general pattern of wall architecture of hyphae is fairly

consistent: the main, straight-chain microfibillar com-

ponents (chitin, or cellulose in the Oomycota) are

found predominantly in the inner region of the wall,

and they are overlaid by nonfibrillar or “matrix” com-

ponents (e.g. other glucans, proteins, and mannans) in

the outer region. However, there is substantial bond-

ing between the various components, serving to

strengthen the wall behind the apex. In particular, some

of the glucans are covalently bonded to chitin, and the

glucans are bonded together by their side chains. We

return to this topic in Chapter 4, when we consider

the mechanisms of apical growth.

The extrahyphal matrix

In addition to the main structural components of the

wall, some yeasts can have a discrete polysaccharide

capsule, and both hyphae and yeasts can be surrounded

by a more or less diffuse layer of polysaccharide or gly-

coprotein, easily removed by washing or mild chem-

ical treatment. These extracellular matrix materials can

have important roles in the interactions of fungi with

other organisms. For example, the yeast Cryptococcus

neoformansis a significant pathogen of humans; its

polysaccharide capsule masks the antigenic com-

ponents of the cell wall so that the fungus is not

engulfed by phagocytes and can proliferate in the

tissues (Casadevall 1995). In a different context, the fun-

gus Piptocephalis virginiana(Zygomycota; see Chapter 12)

parasitizes other Zygomycota such as Mucor spp. on agar

media, but does not parasitize them in liquid culture

56 CHAPTER 3

Fig. 3.9Diagram to illustrate the wall architecture in

a “mature” (subapical) region of a hypha of Neurospora

crassaas evidenced by sequential enzymatic digestion.

(a) Outermost layer of amorphous β-1,3-glucans and

β-1,6-glucans. (b) Glycoprotein reticulum embedded

in protein. (c) A more or less discrete protein layer.

(d) Chitin microfibrils embedded in protein. (e) Plasma

membrane. (Based on Hunsley & Burnett 1970.)