Fusarium head blight (FHB) of wheat is a crippling disease that causes severe economic losses in many of the wheat-growing regions of the world. Temporal patterns of fungus development and transcript accumulation of defense response genes were studied in Fusarium graminearum-inoculated wheat spikes within the first 48 to 76 h after inoculation (hai). Microscopy of inoculated glumes revealed that the fungus appeared to penetrate through stomata, exhibited subcuticular growth along stomatal rows, colonized glume parenchyma cells, and sporulated within 48 to 76 hai. No major differences in the timing of these events were found between Sumai 3 (resistant) and Wheaton (susceptible) genotypes. In complementary experiments, RNA was extracted from spikes at several time intervals up to 48 hai and temporal expression patterns were determined for defense response genes encoding peroxidase, PR-1, PR-2 (beta-1,3-glucanase), PR-3 (chitinase), PR-4, and PR-5 (thaumatin-like protein). In both genotypes, transcripts for the six defense response genes accumulated as early as 6 to 12 hai during F. graminearum infection and peaked at 36 to 48 hai. Greater and earlier PR-4 and PR-5 transcript accumulation was observed in Sumai 3, compared with Wheaton. Our results show that the timing of defense response gene induction is correlated with F. graminearum infection.
Rates of haustorium formation by Erysiphe graminis f.sp. hordei were determined for epidermal tissues of primary leaves of a near-isogenic pair of barley (Hordeum vulgare) lines, AlgR and AlgS, which differed at the Mla locus for compatibility with the fungus. Epidermal cells were divided into two classes, namely shorter or longer than 450 μm. Cells near stomatal files were always short, whereas cells more distant from stomata were long on the abaxial surface, long on the adaxial surface over vascular bundles, but short on the adaxial surface when not over vascular bundles. At 24 h after inoculation, haustoria were formed in 70–80% of attacked short cells but in only 15–20% of attacked long cells. When haustoria were absent, papillae were present, so papilla-associated resistance was more frequent in long than in short cells. However, the percentage of attacked sites with papillae was the same in AlgR and AlgS, indicating that papilla-associated resistance was not related to Mla incompatibility. In AlgR, patterns of development showed that haustoria were produced before Mla incompatibility was expressed by hypersensitive cell death. Key words: papilla, hypersensitive, barley, epidermis, resistance, powdery mildew.
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