Introduction to Fungi, Third Edition

PLASMODIOPHORALES
61
protein was detectable by immunohistochemical
methods only in cells containing sporulating
plasmodia. The activities of the above enzymes
might be regulated by the signalling molecule,
jasmonic acid (Grsic et al., 1999). However, these
metabolic changes were confined to a narrow
window of time, and other sources of IAA, such
as its release from IAA alanine conjugates by
the activity of amidohydrolase, are likely to
contribute (Ludwig-Müller et al., 1996). The
host pathogen interactions leading to enhanced
auxin levels in clubbed roots are therefore very
intricate.
At first, only cortical cells of the young root
are infected, but later small plasmodia can be
found in the medullary ray cells and in the
vascular cambium. Subsequently, tissues derived
from the cambium are infected as they are
formed. In large swollen roots, extensive wedgeshaped
masses of hypertrophied medullary ray
tissue may cause the xylem tissue to split. At this
stage, the root tissue shows a distinctly mottled
appearance. When the growth of the plasmodia
is complete, they are transformed into masses of
haploid resting spores. Only during the late
stages of resting spore development do the host
nuclei begin to degenerate. Eventually, the
resting spores are released into the soil as the
root tissues decay.
3.2.3 Spongospora
The life cycle of S. subterranea, the cause of
powdery scab of potato, is similar to that of
P. brassicae (Harrison et al., 1997; Hutchison &
Kawchuk, 1998). Diseased tubers show powdery
pustules at their surface, containing masses of
resting spores clumped into hollow balls. The
resting spores release anisokont zoospores which
can infect the root hairs of potato or tomato
plants. In the root hairs, plasmodia form which
develop into zoosporangia. Zoospores from such
zoosporangia are capable of infection, resulting
in a further crop of zoosporangia. Zoospores
released from the zoosporangia have also been
observed to fuse in pairs or occasionally in
groups of three to form quadri- or hexaflagellate
swarmers, but whether these represent true
sexual fusion stages is uncertain. Spongospora
nasturtii causes a disease of watercress in which
the most obvious symptom is a coiling or bending
of the roots. Zoosporangia and resting spore
balls are found in infected root cells (Fig. 3.6),
and plasmodia can migrate through the root
tissue by breaking through host cell walls
(Claxton et al., 1996; Clay & Walsh, 1997). The
encounter of two plasmodia might initiate
sexual reproduction and thus complete the life
cycle without any need for the parasite to leave
the host (Heim, 1960).
Fig 3.6 Spongospora nasturtii. Spore balls from watercress
roots with crook root disease.