Proton extrusion is an essential signalling component in the HR of epidermal single cells in the barley-powdery mildew interaction

We propose a model for activation of the epidermal cell hypersensitive resp onse (HR) in the barley/powdery mildew interaction. The model suggests that the plasma membrane proton pump (H+-ATPase) of epidermal cells is activate d following penetration by an avirulent powdery mildew fungus. This will ca use an acidification of the apoplast towards the mesophyll cells, thereby a ctivating generation of H2O2 from the mesophyll, which subsequently trigger s the epidermal cell to undergo HR. The model is supported by the following data: (1) the earliest HR-related H2O2 is found in the attachment zones be tween the epidermal cell and underlying mesophyll cells; (2) scavenger trea tment reduces HR; (3) treatment of leaves with low-pH (3.5) citrate and suc cinate buffers causes more cells to undergo HR in the compatible interactio n, while treatment with the same buffers at pH 5.5 reduces the number of HR -cells in the incompatible interaction; (4) race-specific proton extrusion is observed underneath epidermal tissue detached from leaves inoculated 15 h earlier; and (5) treatment of leaves with fusicoccin, an activator of the plasma membrane H+-ATPase, increases the number of HR-cells in the compati ble interaction.