untitled

to antagonize and to overgrow other fungi on agar

plates (Chapter 12). So they probably grow in the root

cortex as secondary (opportunistic) invaders.

Fungal communities in decaying wood

In terms of both its physical and chemical properties,
wood is an exceptionally difficult substrate to degrade,
so it is largely unavailable to most fungi. Wood
consists mainly of cellulose (40 –50% dry weight),
hemicelluloses (25– 40%), and lignin (20–35%). Of
these, lignin often presents the main obstacle to wood
decay, because it is a complex aromatic polymer that
encrusts the cell walls, preventing access of enzymes
to the more easily degradable cellulose and hemicel-
luloses. Lignin is highly resistant to breakdown by
conventional enzyme systems because it is chemically
complex, variable, nonhydrolysable, and water-
insoluble. Wood also has a very low nitrogen content
(commonly a C : N ratio of about 500 : 1) and low
phosphorus content. And, it contains potentially
fungitoxic compounds, which are deposited in the
heartwood. In broad-leaved trees the toxic compounds
are usually tannins, well know for their ability to
cross-link proteins, making animal skins resistant to

decay. By contrast, conifers contain a range of phenolic

compounds such as terpenes, stilbenes, flavonoids and

tropolones. The most toxic of the tropolones are the

thujaplicinswhich act as uncouplers of oxidative

phosphorylation; they are particularly abundant in

cedarwood, making this a naturally decay-resistant

wood for high-quality garden furnishings, etc.

Despite this formidable list of obstacles, woody tissues

are degraded by fungi, and these fall into three types

according to their mode of attack on the woody cell

walls – soft-rot fungi, brown-rot fungi, and white-rot

fungi.

Soft-rot fungi

Soft-rot fungi grow on wood in damp environments.

They are the characteristic decay fungi of fence posts,

telegraph poles, wooden window frames, the timbers

of cooling towers, and wood in estuarine or marine

environments. They have a relatively simple mode of

attack on wood, illustrated in Fig. 11.18. Their hyphae

grow in the lumen of individual woody cells, usually

after entering through a “pit” (depression) in the wall.

Then they produce fine penetration branches that

grow through the thin, lignin-coated S3 layer of the

FUNGAL ECOLOGY: SAPROTROPHS 229

Fusarium

Zygomycota

Penicillium

Clonostachys

Trichoderma

Fus

Zygo

Pen

Clon

Trich

Endo

Ph

Ster

Fus

Trich

Clon

Trich

Clon

Zygo

Fus

Zygo

Pen

Clon

Trich

Endo

Ph

Ster

Fus

Trich

Clon

Zygo

Fus

Zygo

Pen

Clon

Trich

Endo

Ph

Ster

Fus

Trich

Clon

Fus

Zygo

Pen

Clon

Trich

Endo

Ph

Ster

Fus

Fus

Zygo

Pen

Clon

Trich

Endo

Ph

Ster

Fus

Zygo

Pen

Clon

Trich

Ph

Fus

Zygo

Pen

Clon

Trich

Ph

Endophyte

Phialophora

Root

surface

Outer

cortex

Inner

cortex

Vascular tissues

Progressive age/distance behind root tip

Fig. 11.17The sequence of fungal invasion of ryegrass roots, with increasing age and distance behind the extending
root tips. Phialophora graminicola(Ph), a weak parasite, initiates the invasion sequence as the root cortex starts to senesce.
Subsequent invaders include a sterile hyaline fungus (Ster), Fusarium culmorum(Fus), and then a suite of saprotrophic
fungi, including Zygomycota (Zygo), Penicillium(Pen), Clonostachys rosea(Clon), and Trichoderma(Trich). (Based on
Waid (1957) but with additional information and interpretation.)