Biotrophic development of Sporisorium reilianum f. sp. zeae in vegetative shoot apex of maize

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.