Centrifugation Studies Help to Clarify the Role of Papilla Formation in Compatible Barley Powdery Mildew Interactions

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Abstract
In compatible interactions between barley [Hordeum vulgare] and Erysiphe graminis hordei, failures of fungal penetration are commonly associated with host papillae. To determine if papillae are responsible for these failures, and if enhanced papilla formation increases the percentage of failures, papilla deposition was experimentally altered in host cells. During attempted penetrations of coleoptile cells from fungal appressoria, the living coleoptiles were centrifuged sufficiently to produce 2 distinct zones [cytoplasm-rich (CR) and cytoplasm-poor (CP)] within individual host cells. Papilla deposition occurred at interaction sites in CR zones, but not in CP zones. Using interference contrast microscopy, penetration attempts from appressoria at pairs of interaction sites, one member of each pair located over each zone, were observed. Comparisons among the zones and noncentrifuged coleoptiles were made of 2 parameters: the percentage of successful penetrations (penetration efficiency, PE); and the percentage of appressoria with pegs that failed to produce haustoria. In CP zones (where papillae did not occur) and noncentrifuged coleoptiles these papameters did not differ significantly. Papillae do not cause the penetration failures that occur normally in this host-pathogen combination. Further data are needed to substantiate this inference. In CR zones, PE was only 1/3 that in either the CP zones or in the noncentrifuged controls. In CR zones the only penetration attempts that were affected by the resistance response were apparently by those appressoria that induced papillae. A centrifugally enhanced papilla (or some mechanism linked to papilla formation) has the potential to prevent fungal ingress.