The Relationship Between the Expression of Defense-Related Genes and Mildew Development in Barley. Lesley A. Boyd. Cambridge Laboratory, John Innes Centre, Colney Lane, Norwich NR4 7UJ U.K. Philip H. Smith, Rachael M. Green, and James K. M. Brown. Cambridge Laboratory, John Innes Centre, Colney Lane, Norwich NR4 7UJ U.K. MPMI 7:401-410. Accepted 22 February 1994. Copyright 1994 The American Phytopathological Society.

The induction of five defense-related genes in barley was examined in relation to the development of the fungus Erysiphe graminis f. sp. Iwrdei, the causal agent of powdery mildew. The barley cultivar Midas, carrying the resistance allele Mla6, was inoculated with one of two near-isogenic E. g. f. sp. hordei isolates, CC142 (Aa6) and CC143 (Va6). The defense genes were not induced by wounding alone, but required the presence of the growing pathogen. The defense genes were induced in response to both isolates, showing similar transcript patterns up to 24 hr after inoculation. It was only after 24 hr that a resistance response to the avirulent isolate was seen. By 30 hr after inoculation a significant difference in the rates of development of the two isolates was observed. The inhibition of the development of the avirulent isolate, CC142, paralleled the continued high levels of transcript seen for three of the five defense genes examined.