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many haploid spores that are wind-dispersed. These

spores germinate to produce either amoeboid cells

termed myxamoebaeor flagellate swarmers, which

fuse in pairs, and the resulting diploid cell grows into

a plasmodium. Physarum polycephalumis perhaps the

best-known example. It has been studied intensively

by morphogeneticists because it can be maintained

on defined (bacterium-free) media. Full details of the

Myxomycota can be found in Martin & Alexopoulos

(1969).

Plasmodiophorids

Plasmodiophorids (Fig. 2.37) are obligate intracellular

parasites of plants, algae, or fungi. They grow only in

a host organism, and cannot be grown in laboratory

culture. As a group, the plasmodiophorids show no

obvious relationship to fungi or to other fungus-like

organisms, so their phylogenetic status remains

unclear (Down et al. 2002). The most important

organism of this group is Plamodiophora brassicae,

which causes the damaging clubroot disease of crucifer-

ous crops (the cabbage family). In this disease the roots

are severely deformed, with proliferation of cell divi-

sion caused by the production of phytohormones,

and this can lead to serious yield losses. A related

organism, Spongospora subterranea, causes powdery

scab of potato tubers. In addition to these, several

common plasmodiophorids, such as Polymyxa spp.,

grow as symptomless parasites in the roots of many

plants, causing little or no damage, but they can act

as vectors of some economically important plant

viruses (Chapter 10).

Significant stages in the life cycle of P. brassicaeare

shown in Fig. 2.38, although there is still doubt about

the details of some stages. The thick-walled resting

spores can persist in soil for many years, and only a

low proportion of them will germinate in any one year.

So, once this fungus is established in a site it is almost

impossible to eradicate. The resting spores eventually

germinate to release a motile, biflagellate zoospore,

which locates a host root by chemotaxis and then

encysts in a defined orientation. The cyst protoplast is

then injected into a root epidermal cell or a root hair

by means of a bullet-like structure. The protoplast

then grows into a small primary plasmodiumwithin

the infected cell, and converts into a sporangium,

which releases zoospores when the root cell dies or when

an exit tube is formed. At this stage, the zoospores are

DIVERSITY OF FUNGI 45

Fig. 2.37Plasmodiophorids. (a) Club-

root symptoms caused by Plasmodio-

phora brassicae on a cruciferous

seedling: note the grossly deformed

root system. (b) Large numbers of thick-

walled resting spores of P. brassicaein

clubroot tissue. (c,d) Clusters of thick-

walled resting spores of Polymyxa

graminisat different magnifications in

the roots of grasses. ( (a,b) Courtesy

of J.P. Braselton, Ohio University; see

http://oak.cats.ohiou.edu/~braselto/

plasmos/ )

(a) (b)

(c) (d)

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