| Barley, Hordeum vulgare L., responds to attempted infection by the powdery mildew fungus, Blumeria (syn. Erysiphe) graminis f. sp. hordei. There are two cellular responses, penetration resistance and hypersensitive cell death. Penetration resistance is characterized by local accumulation of autofluorogenic, phenylpropanoids in epidermal cells subtending fungal contact sites. Cell death is characterized by accumulation of phenylpropanoids throughout attacked epidermal and subtending mesophyll cells. Additionally, steady state levels of defense-related mRNAs, e.g. PR proteins, PAL and peroxidase, increase in inoculated leaves. Resistance to B. graminis hordei is determined from disease phenotype and is associated with single, resistance gene loci. I examined whether barley powdery mildew resistance genes, in the same genetic background, modulate defense responses. Five race-specific resistance genes, viz. Mla1, Mla12, Mlg, Mlk and Mlp, and one non-race-specific resistance gene, viz. mlo5 were selected. Seven lines from the near-isogenic Pallas isoline set, containing one resistance gene, and the susceptible Pallas parent were used. Isolines were considered to have fast-, intermediate- or slow-acting resistance genes based on their disease phenotypes. Inhibition of phenylpropanoid synthesis caused an increase in the frequency of susceptible interactions between fungal germlings and barley epidermal cells in all, but the mlo5 isoline. This corresponded with high frequencies of elongating hyphae and reduced local and whole cell autofluorescence responses. Accumulation of PAL, peroxidase and chitinase mRNAs was biphasic in all isolines during the first 24 hours after inoculation and corresponded with fungal penetration attempts. At 36, 48 and 72 hours response gene mRNA levels were high only in intermediate- and slow-acting isolines and coincided with continued fungal development. This suggests that the Pallas background responds to a general elicitor, possibly similar to a soluble carbohydrate elicitor in the wheat powdery mildew fungus, B. graminis tritici. Barley defense responses are indirectly modulated by resistance genes, which restrict fungal infection and/or colonization. This may explain why mRNAs of what we identified as a class II chitinase localize differently in Mla1 and mlo5 resistant lines. Chitinase mRNAs localized in the epidermis in a mlo5 penetration resistant line, and only around putative fungal attack sites in a Mla1-cell death resistant line. |
