Microscopic Study on Expression of Yr-18 Gene Related to Adult Plant Resistance in a Near-isogenic Line of Spring Wheat (Triticum aestivum L.) to the Stripe Rust (Puccinia striiformis f. sp. tritici)

In this study, isolate SR99-UA (Race 70E 128) of wheat stripe rust (Puccinia striiformis West. f . sp. tritici Eriks) was used to inf ect susceptible spring wheat (cultivar Thatcher) and its near isogenic line possessing Yr-18 adult-plant resistance gene. Samples were taken at the f lag leaf stage and observed using light, transmission and scanning electron microscopes. No major dif ferences in the inf ection process were detected prior to haustorium f ormation, but sub-stomatal vesicles appeared to be f ormed af ter penetration, slightly earlier in susceptible than in the resistant genotypes. Higher numbers of haustoria were formed at any particular time in the susceptible cultivar than in the resistant near isogenic line 2-3 days af ter inoculation. In the early stages of inf ection, host cell necrosis was not observed in the susceptible host, but was occasionally seen in the resistant isogenic line. Fourteen days af ter inoculation, the number of necrotic cells of hypersensitive response were higher on the resistant genotype than on the susceptible cultivar at this time when many of the infected mesophyll cells of the resistant host had collapsed. The f irst clearly developing pustules were usually observed 12 days af ter inoculation on the susceptible cv. Thatcher and 14 days af ter inoculation on its resistant near isogenic line. Eighteen days af ter inoculation, the mean number of pustules produced, per unit of inf ected leaf area, was high on the susceptible cultivar but on the resistant genotype the pustules were much less in number, diminutive and poorly sporulating