HOST RESPONSE GENE TRANSCRIPT ACCUMULATION IN RELATION TO VISIBLE CYTOLOGICAL EVENTS DURING ERYSIPHE-GRAMINIS ATTACK IN ISOGENIC BARLEY LINES DIFFERING AT THE ML-A LOCUS

Temporal patterns of transcript accumulation of six host response genes (β-1,3 glucanase and five of unknown function), were determined for two near-isogenic barley lines (Algerian R and Algerian S), differing at the Ml-a locus for race-specific incompatibility with Erysiphe graminis f.sp. hordei. The cDNAs of these response genes were originally cloned in other laboratories. Transcript levels of the six response genes were measured using quantitative, radioanalytical analysis of Northern blots containing RNA isolated from leaves at discrete intervals from 0-48 h after inoculation. At each time interval, leaf tissue was also fixed and stained for light microscopic observations of fungal development and host epidermal cell responses to E. graminis. Response gene transcripts accumulated in biphasic patterns corresponding to periods of cytoplasmic aggregation and papilla deposition in host cells beneath primary germ tubes at 2-10 h after inoculation and again beneath appressoria at 10-21 h. This biphasic pattern was virtually the same in both isolines up to 21 h, indicating that transcript accumulation was a general, race nonspecific response to pathogen attack. From 21 to 40 h after inoculation, transcript levels tended to remain higher in the resistant than in the susceptible isoline, possibly as a consequence of hypersensitive epidermal cell death which was visible only by light microscopy and occurred at 18-30 h in the resistant isoline. © 1993 Academic Press, Inc.