Epidermal cell cytoplasmic events and response gene transcript accumulation during Erysiphe graminis attack in isogenic barley lines differing at the Ml-o locus

Patterns in the accumulation of six host response gene transcripts were determined at various time intervals, 0-48 h after inoculation of leaves with Erysiphe graminis f.sp. hordei in two near-isogenic barley lines, RISØ 5678 R (RISØ-R) (resistant) and RISØ 5678 S (RISØ-S) (susceptible), differing at the ml-o locus for resistance. At each time interval, leaf tissue was fixed and stained for light microscopy of fungal development and host epidermal cell response to E. graminis. Resistance in the recessive ml-o line was expressed by failure of fungal germlings to penetrate host epidermal cells fully and produce haustoria and was correlated with deposition of papillae at sites of attack. Transcripts of the six host response genes usually accumulated in a biphasic pattern, as determined by quantitative radioanalysis of northern blots. The initial peak of activity was observed at 4-6 h (the time of cytoplasmic aggregation and papilla deposition beneath tips of the primary germ tubes). The second peak was observed at 12 15 h (the time of cytoplasmic aggregation and papilla deposition beneath the tips of the appressoria). The time and magnitude of peaks of cytoplasmic aggregation and papilla deposition was the same in RISØ-R and RISØ-S as were patterns of response gene transcript accumulation. There were not pattern differences in transcript accumulation that could be directly attributed to differences at the Ml-o locus. The results indicate that the response genes were activated and transcribed as a general defence response to attempted penetration by the fungus because they occur in both isolines.