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108 CHAPTER 5

the surface interactions of hyphae, contributing to

features such as the hydrophobicity of cells lining the

internal air spaces, to prevent water-soaking of the

fruitbody tissues.

Commercial mushrooms: the exploitation
of differentiation

Mushroom production is a substantial industry. The
major cultivated “white button” mushroom Agaricus
bisporus(orA. brunnescens) had an annual global
retail value of more than $US 10 billion in 1999. But
this species accounts for only about 40% of the total
production of cultivated mushrooms. Other import-
ant species include the oyster mushroom Pleurotus
ostreatus(about 20% of total production), the Shiitake
mushroom Lentinula edodes(about 10%), and Volvariella
volvacea(5% or more).
The commercial production process for A. bisporusis
described by Flegg (1985). A mixture of composted straw
and animal dung is pasteurized and placed in wooden
trays, then inoculated with a commercially supplied
“spawn” consisting of sterilized cereal grains permeated
with hyphae of A. bisporus. The spawn is allowed
to “run” for 10–14 days so that the fungus thoroughly
colonizes the compost. Then a thin casing layer of
pasteurized, moist peat and chalk is added to the
compost surface. Over the next 18–21 days the fungus
colonizes this casing layer by producing mycelial
cords, and fruitbodies are produced on these cords. The
cropping of fruitbodies is done over a 30- to 35-day
period, because the fungus produces “flushes” of fruit-
bodies at 7- to 10-day intervals.
In terms of differentiation there are several interest-
ing features of this system. The casing layer is essen-
tial for a high fruitbody yield, and part of its role
involves the activities of pseudomonads which are
stimulated to grow in the casing layer by volatile
metabolites, including ethanol, released by A. bisporus.
In experimental conditions the role of the casing layer
can be replaced by using activated charcoal, suggest-
ing that the fungus produces autoinhibitors of fruiting
which are removed by pseudomonads in the normal
mushroom-production process. The casing layer also
provides a non-nutritive environment in which the
fungus produces mycelial cords. As we saw earlier, these
translocating organs develop in nutrient-poor conditions
and they would be necessary for channeling large
amounts of nutrients to the developing fruitbodies. The
regular periodicity of fruiting is also of interest. In com-
mercial conditions the crop must be harvested regularly
at the “button” stage to achieve this, and any delay
in harvesting until the fruitbodies have opened will
cause a corresponding delay in the next flush. Yet, most
of the fruitbody primordia are already present at the
time of the first flush, and mushrooms at the button

stage have already received all or nearly all of their

nutrients from the mycelium. So, the effect of delayed

picking must be to delay the release of other primordia

for further development. The mechanism of this is

not fully known, but the cellulase activity of the

mycelium in the compost increases markedly as

each flush of fruitbodies develops. It seems that the

expression of cellulase genes is closely linked to fruit-

ing, presumably when the mycelial sugar reserves are

depleted (Chapter 6). Removal of the existing fruitbodies

might act as a signal for a further round of mycelial

activity, providing extra nutrients for the next batch

of fruitbodies.

A final point of interest concerns the mechanism of

fruitbody expansion from the button stage to the fully

expanded mushroom. This must involve the differ-

ential expansion of tissues, and studies on this have

been done mainly with a much simpler experimental

system – the expansion of stipes (stalks) of Coprinusspp.

In field conditions these “ink-cap” toadstools elongate

very rapidly from the button stage to the mature stage.

In laboratory conditions the initially short stipes can

be severed at both ends and incubated in humid con-

ditions, when they will elongate to more than seven

times their original length within 24 hours. Most of

this increase occurs by cell expansion rather than cell

division, and it correlates with an increase in chitinase

activity, presumably for loosening of the existing

hyphal walls, and an increase in chitin synthase

activity for new wall synthesis. But, unlike the apical

growth of normal hyphae, new wall material is

inserted along the length of the existing hyphae in the

stipes. So, the wall extension is mainly by intercalary

growth. This form of growth is also found in other

rapidly extending structures, such as the sporangio-

phores of Phycomyces(Zygomycota) on herbivore dung

(Chapter 11). The rapid expansion of mushrooms

from the button stage depends on the intake of water

from the mycelium. This is probably driven by the

conversion of storage reserves into osmotically active

compounds (Chapter 7). Consistent with this, mush-

room fruitbodies contain high levels of mannitol

which accounts for 25% or more of the dry weight,

compared with only 1.5 – 4.5% in the mycelia. Horgen

& Castle (2002) discuss several molecular approaches

for improving mushroom production. Moore et al.

(1985) and Moore (1998) discuss many aspects of the

developmental biology of Basidiomycota, and of fungal

morphogenesis in general.

Cited references

Adams, T.H. (1995) Asexual sporulation in higher fungi.

In: The Growing Fungus(Gow, N.A.R. & Gadd, G.M., eds),

pp. 367–382. Chapman & Hall, London.