MOLECULAR AND CELLULAR EXPRESSION OF QUANTITATIVE RESISTANCE IN BARLEY TO POWDERY MILDEW

The induction of five defence-related genes was examined in two barley cvs, P-03 and S-03, in direct relation to the growth and development of two near-isogenic Erysiphe graminis f.sp. hordei isolates, CC142 (Aa6) and CC143 (Va6). The barley cvs contained the mildew resistance gene Mla6, while the E. graminis f.sp. hordei isolates differed only in virulence to Mla6. Differences in the development of the two isolates on the two cvs were reflected in the patterns of defence gene induction. The defence genes examined included a chitinase, a peroxidase, a phenylalanine ammonia lyase, a leaf-specific thionin and a gene showing homology to the pathogenesis-related R protein from tobacco. The two isolates showed similar patterns of development up to 16 h after inoculation. At this time, a retardation in the development of the avirulent isolate, CC142, became apparent on S-03. However, inhibition of CC142 on P-03 was not seen until 48 h after inoculation. This difference in CC142 development was paralleled by an earlier, specific defence gene response to the avirulent isolate in S-03. By 16 h after inoculation higher transcript levels of the chitinase and the peroxidase genes were seen in S-03, while in P-03, a specific defence gene response to CC142 was only seen after 24 h. The earlier defence gene response, seen in S-03 to the avirulent isolate, may contribute to the earlier inhibition of CC142 development seen on this cultivar.