The role of papillae in resistance to powdery mildew conditioned by the ml-o gene in barley. I correlative evidence

Resistance to powdery mildew conditioned by the ml-o gene in barley is expressed mainly during primary penetration by the pathogen. Since barley epidermal cells usually respond to attempted penetration by the deposition of a papilla at the encounter site, the possible association of papilla formation with ml-o resistance was examined. Information on penetration efficiency, papilla frequency and papilla diameter was obtained using coleoptiles and leaves of near-isogenic resistant and susceptible barley breeding lines. Increased papilla frequency and, usually, increased papilla diameter were associated with a much lower penetration efficiency on resistant lines. Moreover, penetration failures in the presence of papillae were more frequent on resistant than on susceptible lines. These results establish a clear coincidence between resistance to penetration and an enhanced papilla response in ml-o barley and set the stage for the companion experimental study.