Résumé
Barley (
Hordeum vulgare
) has played a pivotal role in Old World agriculture since its domestication about 10,000 yr ago
1
. Barley plants carrying loss-of-function alleles (
mlo
) of the
Mlo
locus are resistant against all known isolates of the widespread powdery mildew fungus
2
. The sole
mlo
resistance allele recovered so far from a natural habitat,
mlo
-11, was originally retrieved from Ethiopian landraces and nowadays controls mildew resistance in the majority of cultivated European spring barley elite varieties
2
. Here we use haplotype analysis to show that the
mlo
-11 allele probably arose once after barley domestication. Resistance in
mlo
-11 plants is linked to a complex tandem repeat array inserted upstream of the wild-type gene. The repeat units consist of a truncated
Mlo
gene comprising 3.5 kilobases (kb) of 5′-regulatory sequence plus 1.1 kb of coding sequence. These generate aberrant transcripts that impair the accumulation of both
Mlo
wild-type transcript and protein. We exploited the meiotic instability of
mlo
-11 resistance and recovered susceptible revertants in which restoration of
Mlo
function was accompanied by excision of the repeat array. We infer
cis
-dependent perturbation of transcription machinery assembly by transcriptional interference in
mlo
-11 plants as a likely mechanism leading to disease resistance.