Ultrastructure of Extracellular Matrix Deposits Associated with Conidia of the Powdery Mildew Fungus Blumeria graminis F. Sp. Hordei

Abstract

Small pieces of sterile dialysis membrane were touched to sporulating colonies of the powdery mildew fungus Blumeria graminis f. sp. hordei and either cryofixed within 5 s or incubated for 30 min or 2 h before cryofixation. Following freeze substitution and processing for study with transmission electron microscopy, thin sections of conidia on dialysis membrane were examined ultrastructurally. Each conidium was surrounded by a copious deposit of an extracellular matrix (ECM) material. ECM deposits developed as soon as conidia contacted membranes and persisted for the duration of the 2-h time period of this study. While only a thin layer of ECM was present between the central portion of the underside of each conidium and the underlying membrane, ECM filled the space between the curved conidium surface and the membrane and extended halfway or more up the surface of the conidium. The ECM spread out away from the conidium onto the membrane for a distance of up to 30 μm. Strands of ECM also extended up over some conidia. Most conidia were not in direct contact with their respective ECM deposits, and some were displaced totally from their deposits, which adhered closely to membranes. The Thiery stain for polysaccharides gave little if any staining of the ECM. Gold labeling for mannose residues in the matrix using Conconavilin A was negative.