LIGHT AND ELECTRON-MICROSCOPY OF USTILAGO-MAYDIS HYPHAE IN MAIZE

Infection of maize leaves and ears by the corn smut fungus, Ustilago m aydis, was characterized microscopically. High-pressure cryofixation, followed by freeze-substitution was used to preserve sporogenous hypha e. Hyphal growth was exclusively intracellular in newly infected tissu es, but intercellular growth became more common with time, and sporula tion occurred mainly between host cells. Hyphae exhibited frequent bra nching at structures resembling clamp connections, and they were dikar yotic until immediately prior to sporogenesis. Nuclei fused early in s porogenesis, when hyphal walls were beginning to swell and gelatinize. Bundles of tubular ER were conspicuous in thin sections of hyphae fro m before the gelatinization of the hyphal walls through early stages o f sporogenesis. Enlarging spores emerged from remnants of the original hyphal walls.