Teliospore Maturation in the Smut Fungus Sporisorium sorghi: An Ultrastructural Study Using Freeze Substitution Fixation

Teliospores of the smut fungus Sporisorium sorghi developed in galls on Johnson grass (Sorghum halapense) inflorescences. Pieces of gall tissue were freeze substituted and processed for study with transmission electron microscopy. This procedure yielded well-preserved spores in different stages of development and permitted detailed ultrastructural observation of the spore maturation process. Uninucleate teliospore initials developed from sporogenous hyphae of S. sorghi. Hyphal walls appeared to gelatinize during the formation of spore initials, leaving the initials embedded in a fibrillar matrix. Young teliospores became surrounded by a primary wall as they enlarged. Large and small electron dense spines developed adjacent to the plasma membrane of the spore and grew into the primary wall. As spines developed, the primary wall partially disintegrated, with remnants persisting as a sheath around the spines. Two additional wall layers were then deposited beneath the spines. The outermost layer, with which the spines appeared to be continuous, was extremely electron dense and the thicker of the two layers. The inner layer was of moderate electron density. Each teliospore contained a prominent nucleus, mitochondria, microbodies, strands of rough endoplasmic reticulum, microtubules, and multivesicular bodies. Numerous lipid droplets were also present in each spore, many closely associated with microbodies.