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 wer e determined al various time intervals, 0-48 h after inoculation of le aves with Erysiphe graminis f.sp. hordei in two near-isogenic barley l ines, RISO 5678 R (RISO-R) (resistant) and RISO 5678 S (RISO-S) (susce ptible), differing at the ml-o locus for resistance. At each time inte rval, leaf tissue was fixed and stained for light microscopy of fungal development and host epidermal cell response to E. graminis. Resistan ce in the recessive ml-o line was expressed by failure of fungal germl ings to penetrate host epidermal cells fully and produce haustoria and was correlated with deposition of papillae at sites of attack. Transc ripts of the six host response genes usually accumulated in a biphasic pattern, as determined by quantitative radioanalysis of northern blot s. The initial peak of activity was observed at 4-6 h (the:time of cyt oplasmic aggregation and papilla deposition beneath tips of the primar y germ tubes). The second peak was observed at 12-15 h (the time of cy toplasmic aggregation and papilla deposition beneath the tips of the a ppressoria). The time and magnitude of peaks of cytoplasmic aggregatio n and papilla deposition was the same in RISO-R and RISO-S as were pat terns of response gene transcript accumulation. There were no pattern differences in transcript accumulation that could be directly attribut ed to differences at the Ml-o locus. The results indicate that the res ponse genes were activated and transcribed as a general defence respon se to attempted penetration by the fungus because they occur in both i solines.