JM ANDERSON¹, DB BUCHOLTZ¹, OR CRASTA², A GREENE¹, MG FRANCKI², HC SHARMA² and HW OHM²

¹USDA-ARS, and ²Purdue University, Agronomy Department, 1150 Lilly Hall, W. Lafayette, IN 47907, USA

Background and objectives
Barley yellow dwarf virus (BYDV) is the most economically important virus pathogen of wheat as well as all other small grain cereals. However, resistance to BYDV does not exist in wheat. A wheat line (P29), in which the 7D chromosome has been substituted with a 7E chromosome from the wheatgrass Thinopyrum intermedium, was previously shown to contain some resistance to BYDV [1]. This substitution line, however, is not suitable for a breeding program as this alien chromatin also engenders agronomically unfavorable traits such as reduced yield and longer maturity date. The objectives of this study were: (i) determine the effectiveness of the resistance contained in this substitution line to infection by subgroup I and subgroup II BYDV strains, and (ii) identify and characterize resistant transiocation lines containing a reduced amount of the 7E wheatgrass chromosome.

Results and conclusions
Thinopyrum intermedium, a hexaploid wheatgrass, possesses a high level of resistance to BYDV subgroup I and subgroup II strains. A wheat line (P29), in which the 7D chromosome has been substituted with a 7E chromosome from T. intermedium, was examined for the level of resistance to subgroup I and subgroup II BYDV strains. In P29 plants inoculated with subgroup I PAV strains the titre of virus in leaf and stem tissue was reduced 42-52% when compared with the BYDV susceptible cv. Abe. P29 and Abe had the same content of PAV in roots. These results and the absence of detectable virus in inoculated T. intermedium plants indicate that the complete resistance to subgroup I possessed by the wheatgrass has not been introgressed into P29. In contrast, P29 was completely resistant throughout the plant to the subgroup II strains, NY-RPV and NY-RMV, demonstrating that the complete resistance to subgroup II in T. intermedium was incorporated into P29.

Because P29 is a substitution line containing an entire wheatgrass chromosome it is not suitable for breeding for BYDV resistance. Consequently, this line was irradiated to induce chromosomal transiocations as a means of retaining BYDV resistance while reducing the amount of alien chromatin. Following gamma irradiation and selfing, a standard phenotypic analysis approach resulted in a very low success rate of identifying BYDV resistant and susceptible translocation lines (4.0%). However, analysis of susceptible progeny of irradiated seed with a repetitive sequence, specific to the alien chromosome, quickly demonstrated a high rate of deletions in the introgressed alien chromosome (58%). A comparative RFLP analysis of susceptible lines containing alien chromatin, their resistant sister lines and other resistant lines showed that 36% of the progeny of gamma-irradiated seed had deletions in the alien chromosome. This approach of initially identifying BYDV susceptible deletions lines using an alien chromosome-specific repetitive sequence followed by comparative genome analysis of their resistant sister lines rapidly identified resistant translocation/deletion lines and localized the BYDV resistance to the distal end of 7EL. These resistant translocation lines are now being used as a germplasm source for incorporating BYDV resistance into elite wheat lines, Furthermore, the development of a unique strategy for efficiently identifying translocation lines will significantly enhance the introgression of alien genes into crop plants in the future.

References
1. Sharma HC et al., 1995. Genome 3, 406-413.