Introduction to Fungi, Third Edition

108 STRAMINIPILA: OOMYCOTA
Fig 5.24 Ultrastructure of Phytophthora cinnamomi zoospores as seen with theTEM. (a) Oblique section through a zoospore.
Several kinds of vesicle are visible, as are mitochondria, the water-expulsion vacuole (arrow) and the conical nucleus with its
prominent nucleolus. (b) Fingerprint vacuoles. (c,d) Immunogold labelling of wall material located within dorsal vesicles before
(c) and in the cyst wall1min after (d) encystment of the zoospore. (a,b) reproduced from Hardham and Hyde (1997), with permission
from Elsevier; (c,d) previously unpublished work. All images kindly provided by F.Gubler and A.R. Hardham.
Zoospore encystment can be triggered by
several stimuli, e.g. contact with host cell surface
polysaccharides, change in medium composition,
or presence of root exudates. Commitment
to encystment occurs within 20 30 s of receiving
the stimulus (Paktitis et al., 1986). Complex
signalling cascades involving Ca 2þ and phospholipase
D are involved (Zhang et al., 1992), and
commitment to several future developmental
processes is made before the onset of encystment,
including the point of germ tube emergence
(Hardham & Gubler, 1990).
Zoospore cysts germinate quite rapidly after
their formation, usually by means of a germ tube
which infects the plant roots directly. In the case
of hard surfaces such as leaves, the germ tube
may form an appressorium which mediates
infection (see pp. 378 381).