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

The ml-o gene in barley conditions resistance to primary penetration by Erysiphe graminis f. sp. hordei as well as an enhanced papilla response. To determine if papillae are necessary for ml-o resistance, cytoplasmic aggregation and papilla formation Were inhibited in near-isogenic resistant-susceptible pairs of barley breeding lines by low-speed centrifugation of leaves and centrifugation and heat-shock treatments of coleoptiles. Papilla frequency was markedly reduced by both methods. Centrifugation separated papilla formation from resistance at most encounter sites, although a small increase in penetration efficiency occurred because haustoria developed at some sites where papilla formation was inhibited. Heat-shock caused a substantial increase in penetration efficiency. Here too, this increase occurred because haustoria developed at sites where papilla formation was inhibited. Taken together, these results demonstrate a strong link between papilla formation and ml-o resistance, but they do not uniformly support the hypothesis that ml-o resistance is based on papilla formation. The centrifugation results, in particular, suggest that papilla formation is not necessary for resistance.