untitled

FUNGAL METABOLISM AND FUNGAL PRODUCTS 129

The main mobilizable carbohydrates are trehalose and
straight-chain sugar alcohols (polyols) such as mannitol
(Fig. 7.6) and arabitol, although ribitolis common
in the Zygomycota. The Oomycota have none of these
typical “fungal carbohydrates.” Instead their storage
compounds are lipids and soluble mycolaminarins
(β-linked polymers of glucose), and they translocate
glucose or similar sugars.
All these fungal carbohydrates can be derived from
the more familiar sugars. For example, trehalose is a
disaccharide of glucose, and mannitol is derived from
fructose by a one-step reaction involving the enzyme
polyol dehydrogenase. But a point of special interest
is that these compounds are interconvertible, and seem
to have different roles in different parts of a mycelial
network. Brownlee & Jennings (1981) investigated this
for the dry-rot fungus Serpula lacrymans (Basidio-
mycota). They devised a simple experimental system
(Fig. 7.7) in which small blocks of wood were colonized
by S. lacrymansand were then placed on a sheet of
Perspex, so that the fungus grew across the Perspex, sup-
ported by nutrients translocated from the wood blocks.
The fungus grew initially as a broad mycelial colony,
but then formed mycelial cords (Chapter 5) behind the
colony margin. Different regions of the colony could
then be sampled to determine the different proportions
of soluble carbohydrates that they contained.

The mycelial cords contained high levels of tre-

halose, but low levels of other sugars or sugar alcohols.

The mid zone of the colonies (Fig. 7.7) contained high

levels of trehalose and arabitol (a sugar alcohol), and

these high levels were maintained in the submarginal

and marginal zones of the colonies. In further experi-

ments,^14 C-labeled glucose was added to the wood

blocks, and the label was followed through different

zones of the mycelial colonies. This again confirmed

that arabitol and especially trehalose were the main

forms in which sugars and sugar alcohols are trans-

located within the mycelial system, accumulating in

the mid zone and submarginal zone. The conversion

of trehalose (a disaccharide) to arabitol near the margin

of the colony is likely to increase the osmotic poten-

tial, helping to draw water towards the colony margin

so that the fungus can grow across nutrient-free

zones.

Serpula lacrymansis well known to colonize the struc-

tural timbers in buildings. It needs water to colonize

the timbers initially, but then its rapid rate of cellulose

breakdown and the subsequent breakdown of glucose

(to CO 2 and H 2 O) generates “metabolic” water, which

is sometimes exuded as droplets on the hyphae. In

this way, the fungus can spread several meters across

brickwork or plaster, drawing the water and nutrients

forwards to colonize other timbers (Fig. 7.8).

Fig. 7.7Diagram of the soluble carbo-
hydrate contents in different regions of
a colony of Serpula lacrymansgrowing
across a sheet of Perspex from a precol-
onized wood block. Data are per cent
of dry weight of the sugars in different
zones. (Based on Brownlee & Jennings
1981.)

Sucrose, fructose, glucose 0.11 0.59 1.66 2.74

Mannitol 0 1.54 2.52 1.23

Arabitol 0.35 20.04 13.90 7.95

Trehalose 10.89 10.00 16.70 17.90

TOTAL 11.35 32.17 34.78 29.82