Cytochemical aspects of fungal penetration, haustorium formation and interfacial material in rose leaves infected by Sphaerotheca pannosa var. rosae

Abstract

Fungal development and host reactions induced in rose leaf epidermal cells by the powdery mildew fungus, Sphaerotheca pannosa var. rosae , were examined on the basis of ultrastructure and of cytochemistry of chitin, pectin, and cellulose subunits. Fungal growth in the epidermis was associated with the formation of haustoria which appeared multilobed and delimited by an extrahaustorial membrane probably originating from the host plasmalemma. The extrahaustorial matrix was free of chitin as judged by the absence of labelling following incubation with a wheat germ agglutinin/ovomucoid-gold complex. Similarly, neither pectin nor cellulose was detected in this matrix, whereas these compounds were abundant in host cell walls. Various host reactions were observed at sites of fungal penetration. The formation of papillae was often associated with a restricted development of the infection peg. The most striking reaction was the occurrence of successive layers of a fibrillar material, forming a collar around the haustorial neck. Our cytochemical observations demonstrated that the collar was made of an amorphous material surrounded by fibrillar layers, the outermost ones being cellulose-rich. The possible nature and origin of this structure is discussed in relation to its possible involvement in resistance to fungal attack.