ULTRASTRUCTURAL COMPARISON OF INCOMPATIBLE AND COMPATIBLE INTERACTIONS IN THE BARLEY POWDERY MILDEW DISEASE

In the powdery mildew disease of barley, Erysiphe graminis f. sp. hordei forms an intimate relationship with compatible hosts, in which haustoria form in epidermal cells with no obvious detrimental effects on the host until late in the infection sequence. In incompatible interactions, by contrast, the deposition of papillae and localized host cell death have been correlated with the cessation of growth by E. g. hordei. With the advent of improved, low temperature methods of sample preparation, we felt that it was useful to reevaluate the structural details of interactions between barley and E. g. hordei by transmission electron microscopy. The haustoria that develop in susceptible barley lines appear highly metabolically active based on the occurrence of abundant endoplasmic reticulum, Golgi-like cistemae, and vesicles. In comparison, haustoria found in the resistant barley line exhibited varying signs of degradation. A striking clearing of the matrix and loss of cristae were typical early changes in the haustorial mitochondria in incompatible interactions. The absence of distinct endoplasmic reticulum and Golgi-like cisternae, the formation of vacuoles, and the occurrence of a distended sheath were characteristic of intermediate stages of haustorial degeneration. At more advanced stages of degeneration, haustoria were dominated by large vacuoles containing membrane fragments. This process of degeneration was not observed in haustoria of E. g. hordei developing in the susceptible barley line.