Specificity of cell type and timing of events associated with papilla formation and the hypersensitive reaction in leaves of Hordeum vulgare attacked by Erysiphe graminis f.sp. hordei

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Abstract
Rates of haustorium formation by Erysiphe graminis f. sp. hordei were determined for epidermal tissues of primary leaves of a near-isogenic pair of barley (Hordeum vulgare) lines, AlgR and AlgS, which differed at the Mla locus for compatibility with the fungus. Epidermal cells were divided into two classes, namely shorter or longer than 450 .mu.m. Cells near stomatal files were always short, whereas cells more distant from stomata were long on the abaxial surface, long on the adaxial surface over vascular bundles, but short on the adaxial surface when not over vascular bundles. At 24 h after inoculation, haustoria were formed in 70-80% of attacked short cells but in only 15-20% of attacked long cells. When haustoria were absent, papillae were present, so papilla-associated resistance was more frequent in long than in short cells. However, the percentage of attacked sites with papillae was the same in AlgR and AlgS, indicating that papilla-associated resistance was not related to Mla incompatibility. In AlgR, patterns of development showed that haustoria were produced before Mla incompatibility was expressed by hypersensitive cell death.