Ultrastructure of chlamydospores of Phytophthora cinnamomi during development and germination

Abstract

Thin sections of developing, fully expanded, and germinating chlamydospores of Phytophthora cinnamomi were studied by electron microscopy. Expanding chlamydospores had thin walls and contained nuclei, mitochondria, dictyosomes, rough endoplasmic reticulum, lipid-like bodies, peripheral vesicles, and small vacuoles with dense, spherical inclusions. Small vesicles, about 200 nm in diameter, were occasionally observed underlying the plasmalemma.In fully expanded chlamydospores, structures that are necessary for zoosporogenesis, i.e. exit pores, flagella, and cleavage vesicles, were not present. The vacuoles with dense, spherical inclusions constituted a large proportion of fully expanded chlamydospores and had usually fused to form a large central vacuole. The walls of two-week-old chlamydospores were similar in density to sporangial walls and were non-layered.Chlamydospore germination was direct, i.e. by the production of germ tubes with numerous vesicles at the tips which ruptured through the chlamydospore wall. A germination wall was formed de novo beneath the chlamydospore wall and extended with the germ tube to form the germ tube wall.The distinguishing ultrastructural characteristics between chlamydospores and sporangia, aged sporangia, oogonia, and oospores of Phytophthora are discussed.