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Blumeria graminis Interactions with Barley Conditioned by Different Single R Genes Demonstrate a Temporal and Spatial Relationship Between Stomatal Dysfunction and Cell Death

First author: Instituto de Agricultura Sostenible, CSIC, Alameda del Obispo, Menéndez Pidal s/n, 14080 Córdoba, Spain; second, third, fourth, fifth, eighth, and ninth authors: Institute of Biological, Environmental and Rural Sciences, Aberystwyth University, Aberystwyth, Ceredigion SY23 3EB, UK; sixth author: ADAS High Mowthorpe, Duggleby, Malton, North Yorkshire, YO17 8BP, UK; and seventh author: Department of plant Biology and Biotechnology, Faculty of Life Sciences, University of Copenhagen, Denmark.

Hypersensitive response (HR) against Blumeria graminis f. sp. hordei infection in barley (Hordeum vulgare) was associated with stomata “lock-up” leading to increased leaf water conductance (g_l). Unique spatio-temporal patterns of HR formation occurred in barley with Mla1, Mla3, or MlLa R genes challenged with B. graminis f. sp. hordei. With Mla1, a rapid HR, limited to epidermal cells, arrested fungal growth before colonies initiated secondary attacks. With Mla3, mesophyll HR preceded that in epidermal cells whose initial survival supported secondary infections. With MlLa, mesophyll survived and not all attacked epidermal cells died immediately, allowing colony growth and secondary infection until arrested. Isolines with Mla1, Mla3, or MlLa genes inoculated with B. graminis f. sp. hordei ranging from 1 to 100 conidia mm² showed abnormally high g_l during dark periods whose timing and extent correlated with those of each HR. Each isoline showed increased dark g_l with the nonpathogen B. graminis f. sp. avenae which caused a single epidermal cell HR. Guard cell autofluorescence was seen only after drying of epidermal strips and closure of stomata suggesting that locked open stomata were viable. The data link stomatal lock-up to HR associated cell death and has implications for strategies for selecting disease resistant genotypes.