Interactions hôte–parasite lors de l'infection par Cercosporella herpotrichoides, agent du piétin-verse : morphologie du parasite et ultrastructure des parois d'hôtes sensibles et résistants

Segments of seedlings of susceptible and resistant lines of Triticineae were infected in vitro by a virulent strain of Cercosporella herpotrichoides Fron. Samples were taken 4 days after inoculation and examined using scanning electron microscopy. In susceptible lines, a strong adhesion and an important development of the mycelium occurred in contact with the coleoptile. Simultaneously, a massive sporulation was observed. Conversely, in resistant lines, the hyphal stroma remained loose and poorly developed and failed to sporulate. A cytochemical and ultrastructural study of the walls of host cells showed changes in texture and an important deposition of strongly reactive compounds. These components could not be extracted by the usual solvents of matrix constituents of both cellulosic and lignified cell walls. An enhanced synthesis of such substances could prevent the action of the parasite glycolytic enzymes and therefore stop the fungal invasion. The wall, or at least some of its components, seems to be implicated both in the recognition of the pathogen by the host and in the triggering of a response leading to sequestration of the latter and arrest in its development.