Basal Host Resistance of Barley to Powdery Mildew: Connecting Quantitative Trait Loci and Candidate Genes

The basal resistance of barley to powdery mildew(Blumeria graminis f. sp. hordei) is a quantitatively inheritedtrait that is based on nonhypersensitive mechanismsof defense. A functional genomic approach indicates thatmany plant candidate genes are involved in the defenseagainst formation of fungal haustoria. It is not knownwhich of these candidate genes have allelic variation thatcontributes to the natural variation in powdery mildewresistance, because many of them may be highly conservedwithin the barley species and may act downstream of thebasal resistance reaction. Twenty-two expressed sequencetag or cDNA clone sequences that are likely to play a rolein the barley–Blumeria interaction based on transcriptionalprofiling, gene silencing, or overexpression data, aswell as mlo, Ror1, and Ror2, were mapped and consideredcandidate genes for contribution to basal resistance. Wemapped the quantitative trait loci (QTL) for powderymildew resistance in six mapping populations of barley atseedling and adult plant stages and developed an improvedhigh-density integrated genetic map containing6,990 markers for comparing QTL and candidate genepositions over mapping populations. We mapped 12 QTLat seedling stage and 13 QTL at adult plant stage, ofwhich four were in common between the two developmentalstages. Six of the candidate genes showed coincidencein their map positions with the QTL identified for basalresistance to powdery mildew. This co-localization justifiesgiving priority to those six candidate genes to validatethem as being responsible for the phenotypic effects of theQTL for basal resistance.