Introduction to Fungi, Third Edition

PYTHIALES
105
Fig 5.21 TEM images of haustoria of P. infestans. (a) Mature haustorium within a leaf cell of potato. (b) The basal region of a
haustorium.The haustorium contains fungal vacuoles (FV) and mitochondria (M) but no nuclei. However, a nucleus (NF) is located
within the intercellular hypha close to the branch point.The plant tonoplast (T), plant extrahaustorial membrane (EM),
extrahaustorial matrix (EX) and fungal wall (FW) are visible.The seemingly empty space surrounding the haustorium is the plant
vacuole (V). Both images reprinted from Coffey and Wilson (1983) by copyright permission of the National Research Council of
Canada.Original prints kindly provided by M.D.Coffey.
When water becomes available again, such
sporangia may germinate by the formation of
a vegetative hypha, or a further sporangium.
Thick-walled asexual spherical chlamydospores
have also been described for many
Phytophthora spp. and can survive in soil for
several years (Ribeiro, 1983; Erwin & Ribeiro,
1996). The morphological differences between
sporangia, chlamydospores and oospores are
illustrated in Fig. 5.22.
Once formed, mature sporangia may remain
undifferentiated for several hours or even days,
but zoospore differentiation can be induced by
suspending mature sporangia in chilled water or
soil extract. Detailed methods to trigger zoospore
release have been established for many species
(Erwin & Ribeiro, 1996). Once cold-shock has
been received, differentiation can be completed
in less than 60 min and probably involves
cAMP-mediated signalling cascades (Yoshikawa
& Masago, 1977). The processes of differentiation
of sporangial protoplasm into zoospores differ
in certain details between Phytophthora and
the Saprolegniales (see Hardham & Hyde, 1997).
For instance, in Saprolegnia the central vacuole
is prominent and its membrane as well as
the plasma membrane contribute to the plasma
membranes of the developing zoospores (p. 81).
In contrast, in Phytophthora the central vacuole
disappears from the young sporangium before
cleavage of the cytoplasm begins, and the
plasma membrane remains intact even after
zoospores have become fully differentiated. The
zoospore plasma membranes therefore mostly
originate from Golgi-derived cleavage cisternae
(Hyde et al., 1991). Detailed cytological studies