Introduction to Fungi, Third Edition

106 STRAMINIPILA: OOMYCOTA
Fig 5.22 Reproductive structures in Phytophthora cactorum. (a) Sporangia. (b) Chlamydospore. (c) Oospore showing the
paragynous mode of fertilization. (d) Oospore with amphigynous fertilization. (b d) to same scale.
have revealed an important role of microtubules
in organizing the distribution of nuclei
during zoospore formation (Hyde & Hardham,
1992, 1993). Cleavage of the cytoplasm of a
zoospore begins close to that end of the nucleus
which subsequently points towards the ventral
groove. At this stage, three types of vesicle which
become important during zoospore encystment
also move into their positions: large peripheral
vesicles, dorsal vesicles, and small ventral vesicles.
When the pre-formed flagella have been
inserted, the zoospores acquire their mobility
(Hardham, 1995). Zoospores are either discharged
directly through the plug after this has dissolved,
or they are transferred into a very transient
membranous vesicle which forms outside the
opened plug upon discharge and bursts one or
a few seconds later (Gisi, 1983). Since the plasma
membrane of the sporangium has not become
part of the zoospore membranes, the membranous
vesicle is probably continuous with the
plasma membrane.
Encystment of zoospores
Zoospores of Phytophthora swim for several hours,
travelling distances of a few centimetres in water
or wet soil, although they can be spread much
further by passive movement within water
currents (Newhook et al., 1981). They are
attracted chemotactically to plant roots by
non-specific root exudates such as amino acids,
host-specific substances, or the electrical field
generated by plant roots (Carlile, 1983; Deacon
& Donaldson, 1993; Tyler, 2002). No equivalent
studies seem to have been carried out for zoospores
of Phytophthora infecting leaves. The
process of zoospore encystment described below
for Phytophthora seems to apply also to Pythium
(Hardham, 1995). It is an act of regulated secretion,
i.e. the release of pre-formed contents by
synchronous fusion of vesicles with the plasma
membrane. Regulated secretion is common in
animal cells, e.g. in epithelial or neuronal
systems, but in fungi it is probably confined to
encysting zoospores.
Zoospores of Phytophthora are kidney-shaped;
both flagella arise from the kinetosome boss
protruding from within the longitudinal groove
at the ventral surface. The anterior end of the
spore is indicated externally by the straminipilous
flagellum and internally by the water
expulsion vacuole; the nucleus is located in