Challenge Appressoria ofErysiphe graminisFail to Breach Preformed Papillae of a Compatible Barley Cultivar

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Abstract
To investigate the potential role of papillae in resisting fungal ingress, the ability of E. graminis f. sp. hordei to form haustoria in host cells of a compatible barley cultivar at sites of preformed papillae was tested. Oversize, preformed papillae were obtained by incubating inoculated coleoptiles on a Ca(H2PO4)2 solution from 9-24 h after inoculation. These papillae had a heterogeneous ultrastructural appearance which differed substantially from that of normal papillae in barley coleoptiles. After the induction of preformed papillae, the fungus was removed from the epidermis, and a 2nd (challenge) inoculum was applied. In 2 separate experiments, challenge appressoria located over papilla-free host wall areas had a normal percentage of haustoria, 72 or 52%; those over preformed papillae on the same 2 sets of coleoptiles had only 24 or 0%, respectively. The preformed papillae apparently permitted few or none of the appressoria located over them to form haustoria, depending on the experimental design. Appressoria located near, but not over, preformed papillae had an intermediate level of haustorium formation that may have been caused by substances diffusing from the papillae or by the papilla margins that were, presumably, too thin to be visualized by interference-contrast light microscopy. Preformed papillae apparently can prevent appressoria from producing haustoria in cells of a compatible host plant.