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fungi might also benefit from the growth of nitrogen-
fixing bacteria in wood.
The most remarkable feature of white-rot fungi is their
ability completely to degrade lignin – they are the only
organisms known to do this. As shown in Fig. 11.22,
lignin is a complex polymer composed of three types
of phenyl-propane unit (six-carbon rings with three-
carbon side chains) bonded to one another in at least
12 different ways. If lignin were to be degraded by con-
ventional means it would require a multitude of
enzymes. Instead, lignin is degraded by an oxidative
process. The details of this are complex, but essentially
the white-rot fungi produce only a few enzymes
(lignin peroxidase, manganese peroxidase, H 2 O 2 -
generating enzymes, and laccase) and these generate
strong oxidants, which virtually “combust” the lignin
framework (Kirk & Farrell 1987).
The major enzyme that initiates ring-cleavage is
laccase, which catalyses the addition of a second
hydroxyl group to phenolic compounds. The ring can
then be opened between two adjacent carbon atoms
that bear the hydroxyl groups (Fig. 11.23). This pro-
cess occurs while the ring is still attached to the lignin
molecule. It is termed orthofission, in contrast to meta
fission which bacteria employ to cleave the phenolic
rings of pesticide molecules (where the ring is opened
at a different position – see Fig. 11.23).

FUNGAL ECOLOGY: SAPROTROPHS 233

Fig. 11.22Structural components of lignin, showing the three main phenyl-propane units and three of the main types
of bond that link them. Note that only a small part of the lignin molecule is shown.

HOOC

OH

Protocatechuic acid

Ortho fission Or Meta fission

HOOC OH

OH

COOH

COOH

Acetate

+

Succinate

HOOC

COOH

CHO

2 Pyruvate

+

Formate

Fig. 11.23Patterns of opening of aromatic rings. (Left)

During lignin breakdown, by the process of orthofission.

(Right) During the breakdown of pesticides and other xeno-

biotics, by metafission. Initially the ring is substituted

with two hydroxyl groups on adjacent carbon atoms. Then

it is opened either between these two carbons (ortho

fission) or adjacent to one of them (meta fission, which

is a plasmid-encoded function of bacteria).