Introduction to Fungi, Third Edition

PYTHIALES
107
Fig 5.23 Schematic drawings of a zoospore of Phytophthora (not to scale). (a) Longitudinal section. (b) Transverse section of the
anterior region showing several types of vesicle, namely the water-expulsion vacuole (WEV), fingerprint vacuole (FPV), large
peripheral vesicles (LPV), small ventral vesicles (SVV), small dorsal vesicles (SDV) and peripheral cisternae (PC). Mitochondria (Mit)
with unusually lamellate cristae are also indicated. a modified from Dick (2001b); b based on the ultrastructural work of
Hardham et al.(1991).
the posterior half of the spore (Fig. 5.23a).
The nucleus is associated with the microtubular
roots of the flagella which force it into a
somewhat conical shape, the pointed end pointing
towards the kinetosome boss. Zoospores
contain several vesicular compartments. Their
positions are drawn schematically in Fig. 5.23,
and electron micrographs are provided in
Fig. 5.24. Fingerprint vacuoles, equivalent to the
dense-body vesicles of Saprolegnia and Achlya, are
defined by the lamellate structure of their
contents, presumably deposits of b-1,3-glucan
(mycolaminarin) and phosphate. Fingerprint
vacuoles are located mainly in the interior of
the zoospore and play no part in the encystment
process but are thought to provide carbon and
energy reserves during subsequent germination
of the cyst (Gubler & Hardham, 1990). In zoospores
of Phytophthora cinnamomi, there are
several kinds of peripheral vesicle which have
been distinguished morphologically (Fig. 5.23)
and by labelling with specific antibodies. When
zoospores approach a root, the groove of the
ventral surface faces the root surface, initial
contact presumably being made by the flagella.
Attachment of the zoospore is achieved by means
of a glue discharged by the synchronous fusion
of the small ventral vesicles with the ventral
plasma membrane (Hardham & Gubler, 1990).
At the same time, the small dorsal vesicles
also secrete their contents, leading to the
deposition of the first cyst wall (Figs. 5.24c,d;
Gubler & Hardham, 1988). The process of
exocytosis is complete within 2 min of receiving
the encystment trigger. In contrast, the large
peripheral vesicles do not fuse with the plasma
membrane but withdraw to the centre of the
cyst. Their contents are proteinaceous and
probably serve as reserves for the germination
process. Peripheral cisternae, ultrastructurally
distinct from the ER, line the inside of the
zoospore plasma membrane and disappear
during encystment (Hardham et al., 1991;
Hardham, 1995).