Electron microscopy of the sporangium of Phytophthora capsici

Abstract

This paper reports results of a study on the ultrastructural cytology of sporangia of Phytophthora capsici Leonian with emphasis on flagellum formation, general sporangial structure during zoospore cleavage, and the presence, structure, and transition of cytoplasmic organelles and inclusions during these processes.Non-cleaving sporangial cytoplasm contains a high concentration of ribosomes, mitochondria, vacuoles, lipid inclusions, and endoplasmic reticular cisternae scattered throughout the cytoplasm. Nuclei in mature sporangia are located at the periphery of the cell, with their narrow poles aligned towards the plasma membrane. The apical papilla measures 4–5 × 10 μ, and is initiated as a fibrous third layer under the two-layered cell wall several microns from the apex. The outer wall layer surrounds the papilla while the inner wall narrows and disappears near the crown. The basal septal plug is a combination of the inner wall layer and slime substances.One of the first structural changes in the cytoplasm during zoospore cleavage is the genesis of the flagellar apparatus. Paired centrioles next to the narrow poles of the nuclei elongate to form kinetosomes which extend through the cytoplasm toward receptive axonemal vesicles. Axonemes then form in the axonemal vesicles. The terminal plate and its prisms account for the appearance of the axoneme when it forms above the terminal plate in the axonemal vesicle. The axonemal cylinder has a typical 9 + 2 morphology and the axonemal sheath is continuous with the axonemal vesicle tonoplast. The nucleus is an integral part of the flagellar apparatus and appears to be connected to the kinetid (axoneme + kinetosome) base via microtubules. Golgi dictyosomes occur in the sporangia during all stages of growth and may be responsible for elaboration of needed membranes during zoospore production. Osmiophilic droplets (liposomes), located within vacuoles, are a predominant feature of precleavage cytoplasm. These globules are probably lipid in nature. As cleavage begins, the liposomes become less opaque at the margins, and striations appear, eventually encompassing all or most of the liposomes at the time of cleavage. The liposomes then become less spherical and expand, filling the vacuoles. Electron-transparent regions eventually appear throughout the liposomes and the vacuolar membrane may disappear.Cleavage of the cytoplasm into zoospores occurs by the alignment and fusion of cleavage vesicles around individual nuclei. During this period organelles migrate to these centers. The cleavage vesicles coalesce with each other and the axonemal membranes, eventually becoming the plasma membranes of the daughter zoospores.