Inhibition by 2-deoxy-D-glucose of callose formation, papilla deposition, and resistance to powdery mildew in an ml- o barley mutant

Resistance to powdery mildew in the ml- o mutant barley line, Riso 5678/3 ∗ Carlsberg II, is characterized by a high frequency of early formed, callose-containing papillae. Treatment of ml- o resistant barley coleoptiles with 10 −5 m 2-deoxy-D-glucose, an inhibitor of callose formation in plants, resulted in a decrease in papilla frequency from 92-41% and an increase in penetration efficiency from 15–78%. Time-course analysis showed that papilla formation was also delayed by 2-deoxy-D-glucose treatment and that sites with late-forming papillae were penetrated by the fungus whereas sites with early-forming papillae were not. These results support previous evidence that early papilla formation is the mechanism of ml- o resistance. Use of the callose-specific dye, sirofluor, showed that nearly all papillae in both the controls and the 2-deoxy-D-glucose-treated cells contained callose, suggesting that callose formation may greatly facilitate papilla deposition in this ml- o barley mutant. Papilla frequency also decreased in the susceptible barley isoline, and an increase in penetration efficiency occurred at 4 × 10 −5 m 2-deoxy-D-glucose. We infer that the mutation at the ml- o locus enhanced a constitutive, but originally ineffective, resistance mechanism, namely papilla formation.