Sporisorium reilianum f. sp. zeae is the causal agent of maize head smut, a disease present in several regions of France. A cytological study was carried out to describe a key step of the fungal etiology, in which the mycelium invades the vegetative shoot apex. Light and transmission electron microscopy observations show that the fungus is mostly intracellular and suggest that it passes through the host cell wall by lysis and mechanical pressure. The hyphae are surrounded by an amorphous vesicle-rich layer limited by a membrane related to the host plasmalemma. The encasement can be considered as an exchange zone between the plant and the fungus. The infected host cells appear normal; therefore, the fungus seems to act like a biotrophic endophyte.

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A cytological study was carried out to describe the initial steps of infection of maize roots by the soil fungus Sporisorium reilianum f. sp. zeae. Morphogenetic changes of the fungal cells were induced in the presence of maize roots. Extensive hyphal growth led to the formation of a thick fungal layer colonising the maize root surface. This structure is original in interactions of members of the family Ustilaginaceae with plants. In the thick fungal layer, we observed fimbriae inserted into the host cell wall, suggesting a direct role of these fibrillar structures in cell adhesion and infection processes. During infection, no reaction of host cells was observed. In this way, the fungus acts as a biotrophic endophyte during the initial steps of infection.

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