THE RELATIONSHIP BETWEEN THE EXPRESSION OF DEFENSE-RELATED GENES AND MILDEW DEVELOPMENT IN BARLEY

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