Ultrastructure of the host-pathogen interface in Arabidopsis thaliana leaves infected by the downy mildew Hyaloperonospora parasitica

Can. J. Bot. 82, 1001-1008

Mims, CW, EA Richardson, BF Holt III and JL Dangl

Summary "Transmission electron microscopy was used to examine the host-pathogen interface in Arabidopsis thaliana(L.) Heynh. leaves infected by the biotrophic downy mildew pathogen Hyaloperonospora parasitica (Pers.:Fr.) Constant.Both conventionally fixed as well as high-pressure frozen samples were examined. Excellent preservation of thehost-pathogen interface was obtained in many of our high-pressure frozen samples and provided information not availablein conventionally fixed samples. Mature haustoria of H. parasitica were distinctly pyriform in shape. A smallcollar of host cell wall material surrounded the neck of each haustorium near the host cell wall penetration site. Thepresence of callose in collars was demonstrated using immunogold labeling with a monoclonal antibody specific for(1 3)- -glucans. The body of each haustorium was ensheathed by an invaginated portion of the invaded host-cellplasma membrane known as the extrahaustorial membrane. Lying between this membrane and the haustorial wall was alayer of electron-dense material known as the extrahaustorial matrix (EHM). The EHM typically was thicker at the distalend of a haustorium than at the proximal end. The surface of the EHM covered by the extrahaustorial membranewas highly irregular in outline. Considerable vesicular activity was observed in association with the extrahaustorialmembrane."