untitled

believed to fuse in pairs, and the resulting secondary

zoospores infect root cortical cells, where they develop

into large, secondary plasmodiathat will eventually

produce thick-walled resting spores. There is evidence

that meiosis occurs just before the development of rest-

ing spores, suggesting that the primary zoospores are

haploid, and therefore that the secondary zoospores and

zoosporangia are diploid.

The main damage caused by Plasmodiophoralies in

the fact that the root responds to infection of the cor-

tex by undergoing rapid cell expansion (hypertrophy)

and cell division (hyperplasia), leading to the devel-

opment of large galls. The plant nutrients diverted to

these galls severely reduce the shoot growth and yield

of crops such as cauliflower, while the galls themselves

make root crops unmarketable. The proposed explana-

tion for gall development is that glucobrassicin, a

characteristic compound in cruciferous plants, is con-

verted to the plant hormone indolylacetic acid (IAA)

by the action of an enzyme, glucosinolase (Fig. 2.39).

As a group, the plasmodiophorids are difficult to study

because they cannot be grown in laboratory culture,

and so many basic aspects of their biology remain

unclear. They show no obvious relationship to other

fungus-like organisms, so they seem to represent a

basal lineage of the early eukaryotic organisms. They

are very common, because when the roots of almost

any grass plant are cleared of protoplasm and stained

with trypan blue the roots are frequently seen to con-

tain resting spores of Polymyxa graminis(Fig. 2.37) or

46 CHAPTER 2

(c) (d)

(e)

(f)

(a) (b)

Fig. 2.38Stages in the life cycle of Plas-

modiophora brassicae, which causes clubroot

disease of cruciferous crops. (a) Uninucleate

primary zoosporesare released from germin-

ating resting spores in soil. (b) The zoospores

encyst on a root hair or root epidermal cell and

inject a protoplast into the host cell. (c) The

protoplasts grow into small primary plas-

modiaand then convert to sporangia, which

release zoospores into the soil when the root

cell dies, or when an exit tube is formed. (d) The

zoospores fuse in pairs. (e) The fused zoospores

infect root cortical cells and develop into

secondary plasmodia. (f ) At maturity the sec-

ondary plasmodia covert into resting spores

which often completely fill the cortical cells.

These spores are finally released into soil when

the roots decay. Eventually, the resting spores

germinate to release haploid zoospores which

repeat the infection cycle.

Glucobrassicin Indoleacetonitrile (active hormone?)

CH 2 C Glucosinolase

N

N OSO 3 –

S C 6 H 11 O 5

CH 2 CN

N

H

Indoleacetic acid (1 AA) (active hormone)

CH 2 COOH

N

H

Fig. 2.39Proposed conversion of glucobrassicin to the plant hormone indoleacetic acid by Plasmodiophora brassicae.

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