Characterisation of early transcriptional changes involving multiple signalling pathways in the Mla13 barley interaction with powdery mildew (Blumeria graminis f. sp hordei)

Suppression subtractive hybridisation was used to isolate 21 cDNAs (bmi1-bmi21) up-regulated 1-5 h post-inoculation (hpi) in a barley (Hordeum vulgare L. cv. Pallas) near-isogenic line (NIL) P11 (Mla13) challenged with either avirulent or virulent isolates of Blumeria graminis f. sp. hordei. Transcriptional changes at these time-points are crucial for the Mla-mediated hypersensitive response [W.R. Bushnell and Z. Liu (1994) Physiol Mol Plant Pathol 44:389-402]. Seven sequences were up-regulated by 1 hpi, when the pathogen has formed only the primary germ tube. Some transcripts were similar to genes with a role in regulating programmed cell death in animals, including NF kappaB and oxysterol-binding protein. Moreover, bmi7, similar to rice resistance gene Xa21, was rapidly up-regulated in both compatible and incompatible interactions, but was then down-regulated by 5 hpi in the virulent interaction. Only nine of the transcripts were up-regulated in mlo5 resistance in cv. Pallas NIL P22, confirming differential pathway induction between Mla13 and mlo5. However, eight sequences up-regulated in the Mla13 response in P11 were already highly elevated in uninoculated mlo5 mutant P22, suggesting that they may be negatively regulated by wild-type Mlo. Regulation of bmi sequences was investigated using salicylic acid, methyl jasmonate, ethylene, H2O2, abscisic acid, wounding and a glucan elicitor. No single stimulus up-regulated all genes, suggesting either combinations of these stimuli, or additional stimuli, are involved in early Mla13 and mlo5 resistances. Whereas H2O2 up- or down-regulated 17 of the transcripts detected in Northern analyses, salicylic acid stimulated only down-regulation of 5 transcripts.