Barley yellow dwarf virus resistance of wheatgrass: Molecular and immunological evaluation and introgression into wheat

Barley yellow dwarf virus (BYDV) is an important pathogen of wheat (Triticum aestivum L. em Thell.), oat (Avena sativa L.) and barley (Hordeum vulgare L.), in which only tolerance and/or low levels of resistance are known. High levels of resistance have been discovered in Thinopyrum spp. (formerly Agropyron), and this resistance is primarily characterized by a reduction of virus concentration in plant tissues as demonstrated by low enzyme-linked immunosorbent assay (ELISA) values. The use of cDNA probes as diagnostic tools also proved to be a powerful technique, however, there is not an established method for quantification of viral RNA levels in plant tissue. In this work, BC$\sb2$ and BC$\sb3$ populations derived from crosses between cultivated wheat and Thinopyrum spp. were evaluated for segregation for resistance to BYDV. A BC$\sb2$ population (wheat x Th. intermedium (Host) Bark & Dewey) was developed without previous selection to study the inheritance of BYDV resistance, based on random segregation and elimination of Thinopyrum chromosomes. Frequency distribution of ELISA values suggests that two chromosomes of Th. intermedium may carry genes for resistance to BYDV. Five BC$\sb3$ segregants with 44 chromosomes were observed to be resistant to BYDV. Further, ELISA tests of wheat addition lines provided evidence that the homoelogous chromosome group 7 of Th. intermedium is responsible for a large part of the resistance, while an intermediate level of resistance is conferred by the homoeologous chromosome group 4. An attempt to characterize the resistance was made using cDNA dot-blot hybridization. The objectives were to compare ELISA and cDNA dot-blot hybridization and to establish a method for quantification of relative levels of viral RNA in plant tissues analyzed by densitometry of dot-blots. Although uniformity in the results with both techniques depend on carefully controlled infestation and sample preparation, a high correlation between ELISA values and densitometry scans was obtained (0.93). To maximize differences between susceptible and resistant reactions, one should take into consideration ELISA substrate incubation period ($\u3e$90 min), autoradiographic exposure period with intensifying screens ($\u3e$3 days) and use of denaturing conditions in sample preparation. Relative viral RNA concentrations for cvs. Abe (wheat) and Clintland 64 (oat) were ca. 100 and $\u3e$200 ng/g plant tissue, respectively. Th. ponticum has 0 to 50 pg of virus RNA/g of plant tissue. Three levels of resistance were distinguished: resistant, moderately resistant, and susceptible. Resistant lines had densitometry absorbance values of 0.25 to 0.60 and ELISA values from 0.03 to 0.30, with a range of viral RNA concentration from 0 to 5 ng/g plant tissue