Reorganization of the endoplasmic reticulum in pea leaf epidermal cells infected by the powdery mildew fungus Erysiphe pisi

During the formation of haustoria by biotrophic pathogens the synthesis of extra host plasma membrane (PM) is required to accommodate the invagination of the PM and its transition to the extrahaustorial membrane. This implicates alterations in host exocytosis, membrane trafficking and targeting. Probes for endoplasmic reticulum (ER) and whole‐cell imaging by confocal laser scanning microscopy (CLSM) have been used to examine the organization of the endoplasmic reticulum (ER) in compatible interactions between pea leaf cells and Erysiphe pisi DC ex Saint‐Amans. In uninfected epidermal cells, the dicarbocyanine dye DiOC6(3) and the ER‐specific anti‐HDEL antibody showed cortical ER was an open network of tubules interspersed with lamellar cisternae, immediately beneath the cuticle. Endoplasmic reticulum in infected cells could not be stained with DiOC6(3) but anti‐HDEL labelling revealed that the ER was no longer evenly distributed throughout the cortical cytoplasm but was concentrated in a tight network in the viciniti, of the haustorium particularly at early stages of infection. In some infected cells there was intense ER staining very close to the extrahaustorial membrane. Endoplasmic reticulum was also observed as large cisternae adjacent to the haustorial complex (HC), contrasting with tubular ER structures distal to the HC. In Western blots, anti‐HDEL labelled one major band at 70 icDa (probably the BiP homologue) in extracts from uninfected and infected epidermis, mycelia and haustoria. Additional proteins at 55 and 80 kDa were recognized in the fungal extracts. A minor protein at 50 kDa in uninfected pea is possibly upregulated in infected leaves. Overall, the results show that in a compatible pea powdery mildew interaction, the host cortical ER reorganizes around the developing HC from early to late stages of the infection process. This reorganization is likely to be involved in the exocytosis of new membrane and matrix materials which contribute to the expanding HC.