Broad bean responses to attempted P. striiformis f. sp . tritici ( Pst ) infection. (A–C) Inoculated broad bean leaves were stained with 3,3-diaminobenzidine (DAB) to detect production of H2O2 (reddish brown) at 24 hpi. A dome-shaped papilla (P) (A) and thickened cell walls (CW) (B) were formed in the broad bean cell walls in contact with the fungal haustorial mother cells (HMC). DAB staining was visible in the mesophyll cell wall adjacent to a HMC (A), the mesophyll and epidermal cell in contact with a substomatal vesicle (SSV) (B), the papilla (A), the encasement surrounding the haustorium (H) (C) , and an HMC of Pst (A). (D–F) Callose deposition (intensive green fluorescent) coinciding with H2O2 distribution (A–C) was detected by aniline blue staining at 24 hpi. (G) A mesophyll cell in contanct with an infection hypha (IH) at 24 hpi. (H) Bright fluorescence was observed in the cell walls of the mesophyll cell and a neighboring mesophyll cell at the same infection unit shown in (G). (I) In some infection units with formation of haustoria (H), plant cell autofluorescence, indicative of HR was detected at 24 hpi. (J) Collapsed mesophyll cell (arrow) at 72 hpi. Bar = 20 μm.