Functional analysis and RFLP‐mediated mapping of the Mlg resistance locus in barley

Summary The semi‐dominantly acting allele of the Mlg locus in barley confers a race‐specific resistance to the causal agent of powdery mildew, Erysiphe graminis f.sp. hordei . High resolution genetic mapping via RFLP analysis enabled us to identify a set of markers on chromosome 4 which are either tightly linked or co‐segregate with the resistance function. Comparison of marker distances in segregating F 2 populations originating from different genotypes revealed an unex‐pected variation of recombination frequencies in the vicinity of the locus. Based on near‐isogenic lines carrying either resistance gene Mlg or Mla12 and a cell‐type specific analysis it was demonstrated that Mlg acts at an earlier stage of fungal development than Mla12 . A time‐course analysis of the early infection process combined with gene dosage experiments provided strong evidence that attacked cells retain viability after the defence response and that hypersensitive cell death (HR) is a secondary consequence, but not causally required for Mlg ‐mediated arrest of fungal growth. It is speculated that gene dosage experiments may provide a means to separate primary from secondary defence reactions.