Ultrastructural aspects of encystation and cyst-germination in Phytophthora parasitica.

Encystation in Phytophthora parasitica can be divided into 3 stages. In the first, the zoospores line their peripheries with flattened vesicles and nbrillar vacuoles in preparation for encystation. In the second stage, as the zoospores round up and shed their flagella, an initial wall is produced which takes the form of the mature cyst wall in thickness, but not in density. The participation of the flattened vesicles and fibrillar vacuoles in the formation of this initial wall is suggested by the disappearance of these organelles concomitant with wall formation. The third stage involves the maturation of the cyst wall and occurs only after dictyosomes produce vesicles which move to the cyst periphery and fuse to the plasmalemma. Germ tubes are formed in direct and indirect germination and involve the evagination of the plasmalemma and cyst wall proximal to an accumulation of dictyosome-derived vesicles. These vesicles remain at the germ-tube tip as it extends. In indirect germination the germ tube stops after having attained an average length of 6 fim and the vesicles appear to fuse at the hyphal apex, thus forming a cap. Lomasomes do not appear to be cell organelles with a specific function such as wall synthesis, but rather seem to represent aggregations of excess membranous material that have formed as a result of the discharge of vesicles at the cell periphery during wall formation. When dictyosome vesicles are inhibited from forming and moving toward the cell periphery, lomasomes are not formed.