Electron microscopy studies on infection of resistant (Sr6 gene) and susceptible near-isogenic wheat lines by Puccinia graminis f. sp. tritici

Intracellular hyphae, haustoria, and infected host cells in young stem rust infections of a wheat line containing resistance gene Sr6 were compared by electron microscopy with those of a near-isogenic susceptible line. Mesophyll cells of the resistant line which had undergone a rapid necrotic response to haustorial invasion were collapsed and lacked the thin layer of cytoplasm that typically bounded the vacuole of infected cells of the susceptible line. Instead, the lumen was filled with an electron-dense ground material of mixed cytoplasmic and vacuolar constituents containing organelles at various stages of disorganization. Haustoria within these cells were necrotic, as were their mother cells. Apart from a dense staining of mitochondrial membranes there was little evidence of damage to the infection hypha proximal to the septum of the haustorium mother cell. The cytoplasm of host cells adjacent to those showing necrosis was often diffuse and vesiculated. A deposit of electron-dense granular material was found between the wall of a necrotic cell and its plasmalemma, and a similar deposit was present inside the adjoining wall of a contiguous non-necrotic cell.These findings are discussed in relation to the inhibition of fungal growth that accompanies host cell necrosis in the resistant wheat line.