In situ detection of a fungal glycoprotein-elicitor in stem rust-infected susceptible and resistant wheat using immunogold electron microscopy.

Immunoelectron microscopy (IEM) was used to analyze the compatible and incompatible host-pathogen interaction between the obligate, biotroph stem rust (Puccinia graminis f.sp. tritici; Pgt) and primary leaves of wheat (Triticum aestivum L.). The investigation was focused on the subcellular localization of a fungal elicitor glycoprotein of stem rust (Pgt-elicitor). Uredospores as well as fungal infection structures of stem rust on wheat leaves were probed with a specific monoclonal antibody, in order to determine the in situ distribution pattern of the antigen. Binding to the anti-elicitor antibody was observed over the cell wall and the germ pore of germinating uredospores. Immunogold staining was found over the infection structures of stem rust within the wheat leaf tissue of both the compatible and incompatible plant-pathogen interaction. Distinct cell wall layers of the intercellular mycelium, of the haustorial mother cells, as well as of the haustoria were clearly labeled. Gold particles were also detected over the intercellular space and the extrahaustorial matrix in between the extrahaustorial membrane and the haustorial cell wall which indicated a release of elicitor molecules from the fungal cell wall. No labeling was observed over the host cell cytoplasm of the compatible and incompatible interaction, respectively. The immunocytochemical detection of elicitor epitopes over the hyphal cell walls of in vitro grown axenic cultures of P. graminis f.sp. tritici confirmed the occurrence of elicitor molecules in young hyphal material. Elicitor molecules were released by the hyphae of axenic cultures of stem rust in vitro.