untitled

periphery contain a glycoprotein that is thought to serve

as a protein store after encystment. These vesicles

migrate towards the center of the cell when a zoospore

encysts. The smaller dorsal vesiclescontain a differ-

ent glycoprotein which is released by exocytosis at an

early stage of encystment, and it accumulates on the

cell surface as a cyst coat. A third class of vesicles, the

ventral vesicles, are found only around the zoospore

ventral groove. They also contain a protein which is

released during encystment, but it is deposited locally

between the cyst and the surface on which encystment

has occurred, and it acts as an adhesive.

A newly encysted zoospore has no wall, only an

amorphous glycoprotein coat deposited by the dorsal

vesicles. But a true wall is synthesized beneath the cyst

coat in the first few minutes of encystment. This wall

is derived from the peripheral cisternae (membrane

lamellae) that lie immediately beneath the plasma

membrane of the motile spore (Fig. 10.15). Once the

FUNGAL SPORES, SPORE DORMANCY, AND SPORE DISPERSAL 197

Fig. 10.14(a,b) Two parallel cross-
sections through a zoospore of
Phytophthora palmivora (Oomycota).
See text for details. (Courtesy of M.S.
Fuller; from Cho & Fuller 1989.)

Fig. 10.15Close-up view of a part of the zoospore sur-

face of Pythium aphanidermatum, showing the laminate

peripheral cisternae that will subsequently become vesicles

for release of the cyst wall.