Zoospore formation in Phytophthora cinnamomi has been studied by electron microscopy in conjunction with immunogold labelling of selected cellular elements. Zoosporogenesis involves the compartmentalization of the mature multinucleate sporangium into a number of biflagellate, uninucleate zoospores. Before the process is initiated, the pyriform nuclei of the sporangium are regularly distributed throughout the cytoplasm. Several cellular elements, including basal bodies and dictyosomes, are found exclusively or predominantly near the narrow poles of nuclei. A second order of organization involves the nuclei of the sporangial cortex whose narrow poles point toward the wall. These patterns are critical to the sequence of changes during cleavage and in establishing the dorso-ventral axis of the zoospore. Zoosporogenesis is initiated by a cold shock, and within 20 min large clusters of cleavage vesicles appear near each narrow nuclear pole and elsewhere in the cytoplasm. Monoclonal antibody Cpw-1 labels material closely associated with the membrane of the cleavage vesicles and also binds to nearby dictyosomes which are proposed as the source of the vesicles. Between 20–35 min after induction, the large clusters of cleavage vesicles near the narrow poles of nuclei in the cortex disperse to form a plane of vesicles parallel to the wall. These vesicles are closely aligned with microtubules that fan out from the basal bodies. Between 25–50 min after induction, clusters in the sporangial interior disperse, and their vesicles align in planes that link with the plane of vesicles in the cortex. The planes of vesicles surround polyhedral domains of cytoplasm that contain one nucleus. Fusion of these vesicles brings about compartmentalization of the sporangium. The antigen labelled by Cpw-1 is closely associated with the plasma membrane of the newly formed zoospore initials. This is the first time that monoclonal antibodies have been used to identify the source and fate of cleavage vesicles in a fungal sporangium, and the results of this study suggest that aspects of the cleavage process in other Phytophthora species may need to be reassessed. © 1991, British Mycological Society. All rights reserved.
Hyde, G. J., Gubler, F., & Hardham, A. R. (1991). Ultrastructure of zoosporogenesis in Phytophthora cinnamomi. Mycological Research, 95(5), 577–591. https://doi.org/10.1016/S0953-7562(09)80072-5