Introduction to Fungi, Third Edition

100 STRAMINIPILA: OOMYCOTA
In some forms, e.g. P. ultimum var. ultimum,
sporangia do not release zoospores but germinate
directly by producing a germ tube.
Sporangia of P. ultimum var. ultimum may survive
in soil, whether moist or air-dry, for several
months, and are stimulated to germinate within
a few hours by sugar-containing exudates from
seed coats. The germ tubes grow very rapidly so
that a host in the vicinity may be penetrated
within 24 h (Stanghellini & Hancock, 1971). The
oospore of P. ultimum var. ultimum can germinate
either by means of a germ tube or by forming
a zoosporangium which releases zoospores
(Figs. 5.18d,e).
The zoospore
Zoospores of Pythium spp. are always of the
principal type. They can swim for several hours
in a readily recognizable manner of helical
forward movement. Donaldson and Deacon
(1993) have provided evidence that the zoospore
swimming pattern is regulated by Ca 2þ and
calmodulin; manipulations of Ca 2þ concentrations
cause aberrations such as circular, straight,
spirally skidding or irregular movement. Zoospores
of Pythium are attracted towards host
surfaces, usually roots. The Ca 2þ /calmodulin
system may be the means by which the sensing
of attractants is translated into directed movement.
It is this directed movement (taxis), i.e. the
ability to aim precisely at a suitable encystment
site, rather than the ability to move per se,
which represents the main benefit of zoospores
to their producer (Deacon & Donaldson, 1993).
Chemotaxis to root exudates is often nonhost-specific,
being mediated by amino acids
and other common metabolites ( Jones et al.,
1991). Other tactic movements also occur, such
as phototaxis, electrotaxis or negative geotaxis
(Dick, 2001a). In general, zoospores of Pythium
spp. accumulate around the root cap, root
elongation zone or sites of injury.
Once the zoospore has alighted on a suitable
surface, it encysts by shedding rather than
withdrawing its flagella, and secreting a wall
from pre-formed material. Much valuable ultrastructural
work has been carried out on the
encystment process of Phytophthora and is
discussed on pp. 102 111. The cyst of Pythium
spp. can germinate almost immediately, usually
by emitting a germ tube which can directly
penetrate the relatively soft root tissue. In P.
marinum, which is parasitic on marine red algae,
the germ tube forms a specialized infection
structure termed an appressorium (Kerwin
et al., 1992); this is also commonly formed by
leaf-infecting Phytophthora spp. The entire process
from zoospore encystment to successful penetration
is called homing sequence and may take
place in as little as 30 min (Deacon & Donaldson,
1993). If a zoospore encysts on a non-host surface,
the cyst may germinate by producing a further
principal zoospore.
Sexual reproduction
Most species of Pythium are homothallic, i.e.
oogonia and antheridia are readily formed in
cultures derived from single zoospores. However,
Fig 5.18 Oogonia and oospores of
Pythium.(a)Pythium debaryanum.
Note that there are several antheridia.
(b) Pythium mamillatum.Oogonium
showing spiny outgrowths of oogonial
wall. (c) Pythium ultimum.
(d, e) Germination of oospores of
P. ultimum (after Drechsler,1960).