Light and electron microscopy of Ustilago maydis hyphae in maize

Infection of maize leaves and ears by the corn smut fungus, Ustilago maydis, was characterized microscopically. High-pressure cryofixation, followed by freeze-substitution, was used to preserve sporogenous hyphae. Hyphal growth was exclusively intracellular in newly infected tissues, but intercellular growth became more common with time, and sporulation occurred mainly between host cells. Hyphae exhibited frequent branching at structures resembling clamp connections, and they were dikaryotic until immediately prior to sporogenesis. Nuclei fused early in sporogenesis, when hyphal walls were beginning to swell and gelatinize. Bundles of tubular ER were conspicuous in thin sections of hyphae from before the gelatinization of the hyphal walls through early stages of sporogenesis. Enlarging spores emerged from remnants of the original hyphal walls.