Introduction to Fungi, Third Edition

96 STRAMINIPILA: OOMYCOTA
that described for Achlya (p. 86). When two strains
of Pythium or Phytophthora were separated by a
membrane preventing hyphal contact but
permitting the exchange of diffusible metabolites,
oospores were formed by either or both
strains (Ko, 1980; Gall & Elliott, 1985). Because
the mycelia were separated by a membrane,
oospores formed by selfing, whereas in direct
contact they may form by hybridization
(Shattock et al., 1986a,b). Oospore formation can
also be induced by non-specific stimuli, such as
volatile metabolites of the unrelated fungus
Trichoderma stimulating reproduction in A2 but
not A1 strains of Phytophthora palmivora (Brasier,
1975a). This ‘Trichoderma effect’ may well have
ecological implications, since Trichoderma spp.
are very common, especially in soil. Oospore
formation may be a defence reaction against
antibiotics commonly produced by Trichoderma,
and the ‘Trichoderma effect’ may actually enhance
the survival of Phytophthora spp. in soil, since it
stimulates production of the long-lived oospore
stage even in the absence of a compatible mating
type (Brasier, 1975b). It is not known why
Trichoderma spp. do not stimulate oosporogenesis
in A1 strains.
Like Achlya, the Pythiaceae display relative
sexuality, i.e. a strain can act as male in one
pairing but as female in another. To complicate
matters further, a given strain of Phytophthora
parasitica can switch its mating type from
predominantly male to predominantly female
or vice versa, e.g. upon fungicide treatment
(Ko et al., 1986). Clearly, despite substantial
research efforts over many years the genetic
basis of sexual reproduction in the Pythiaceae
still poses numerous unresolved questions!
By analogy with the hormones oogoniol and
antheridiol of Achlya, a male strain needs to be
induced to produce the oogonium-inducing
hormone whereas female strains constitutively
produce the antheridium-inducing hormone
(Elliott, 1994). The ability of homothallic species
to stimulate sexual reproduction in heterothallic
species (Ko, 1980) indicates that these hormones
may also fulfil a morphogenetic role in homothallic
sexual reproduction. However, nothing
seems to be known as yet about the chemical
nature of these hormones.
Sterols are neither synthesized nor strictly
required by vegetatively growing Pythium or
Phytophthora spp. (Nes et al., 1979). None the less,
they are required for the formation of sexual
reproductive organs (Elliott, 1994). It seems,
therefore, that sterols especially sitosterol
and stigmasterol which are normally taken up
from the host plant are converted into as
yet unidentified steroid hormones which initiate
sexual morphogenetic events downstream of
the action of the diffusible Achlya-like hormones
(Elliott, 1994). An alternative hypothesis is that
sterols interact with an as yet unknown membrane
protein to transmit the hormonal signal
and trigger the signalling cascade leading to
sexual morphogenesis (Nes & Stafford, 1984).
In Lagenidium giganteum, a member of the
Pythiaceae parasitizing mosquito larvae (Cuda
et al., 1997), this cascade seems to be carried by
Ca 2þ and calmodulin (Kerwin & Washino, 1986).
5.3.2 Pythium
Species of Pythium grow in water and soil as
saprotrophs, but under suitable conditions, e.g.
where seedlings are grown crowded together in
poorly drained soil, they can become parasitic,
causing diseases such as pre-emergence killing,
damping off and foot rot. Damping off of cress
(Lepidium sativum) can be demonstrated by sowing
seeds densely on heavy garden soil or garden
compost which is kept liberally watered. Within
5 7 days some of the seedlings may show brown
zones at the base of the hypocotyl, and the
hypocotyl and cotyledons become water-soaked
and flaccid. In this condition the seedling
collapses. A collapsed seedling coming into
contact with other seedlings will spread the
disease (Plate 2b). The host cells separate from
each other easily due to the breakdown of the
middle lamella, probably brought about by
pectic and possibly cellulolytic enzymes secreted
by the fungus. The enzymes diffuse from their
points of secretion at the hyphal tips, so that
softening of the host tissue actually occurs ahead
of the growing mycelium. Pure culture studies
suggest that species of Pythium may also secrete
heat-stable substances which are toxic to plants.
Within the host the mycelium is coarse and