Transcription of the defense response genes chitinase IIb, PAL and peroxidase is induced by the barley powdery mildew fungus and is only indirectly modulated by R genes

Barley powdery mildew resistance (PM-R) genes control different infection phenotypes against avirulent strains of Blumeria graminis f. sp. hordei (Bgh). A subset of seven Pallas isolines, containing the PM-R genes Mla1, Mla12, Mlg, Mlk, MIp and mlo5, revealed fast-, intermediate- and slow-acting infection phenotypes. Scanning electron microscopy revealed the extent of Bgh development on each genotype through 72 hai. Quantitative RNA blot analysis of chitinase IIb, phenylalanine ammonia lyase and peroxidase transcription at 0-24 hai revealed similar patterns and levels of transcripts in all isolines including the susceptible parent Pallas. At 36-72 hai transcript accumulation was suppressed on the susceptible parent Pallas, where Bgh grew unimpeded. In resistant isolines transcript accumulation varied according to whether their PM-R genes were fast, intermediate or slow-acting. Transcript accumulation decreased at 36-72 hai in isolines with fast-acting PM-R genes (Mla1, Mlg, and mlo5), and this corresponded with arrested Bgh development. Transcript accumulation at 36-72 hai in isolines with intermediate or slow-acting PM-R genes (Mla12 and Mlk, Mlp) remained elevated and correlated with continued Bgh development and contact. These results suggest that defense response genes are transcriptionally activated by Bgh contact, which probably involves a general elicitor(s) from Bgh. Thus, PM-R genes appear to only modulate defense response gene transcription indirectly by limiting fungal development and contact. Fast-acting PM-R genes halt Bgh development before 24 hai, while slow-acting R genes allow Bgh development throughout 72 hai. Differences in infection phenotypes due to differing PM-R genes may be due to temporal differences in interactions between R genes and avirulence gene product(s); alternatively, slow-acting PM-R genes or required partner genes may be non-constitutive and need time to be induced.