The control of adult-plant resistance to yellow rust by the translocated chromosome 5BS-7BS of bread wheat

The reciprocal translocation 5BL-7BL and 5BS-7BS was widespread in West European wheats 30 years ago, and is probably present in many of their descendants today. In varieties with a history of durable adult-plant resistance to yellow rust and carrying this translocation, removal of the 5BS-7BS chromosome gave adult plants which were much more susceptible. It was suggested that this chromosome might therefore carry the gene(s) responsible for a major part of their resistance and possibly their durability. To test this, a series of lines was developed in which 5BS-7BS chromosomes from both resistant and susceptible varieties were substituted into a number of the durably resistant varieties. In every case, the substituted 5BS-7BS chromosome, irrespective of origin, was found to produce the resistant phenotype, indicating that background chromosomes were responsible for the differences between the varieties. The resistance and durability of the resistant varieties cannot therefore be due solely to the translocated chromosome. In similar experiments, the 5BS and 7BS arms from varieties not carrying the translocation were substituted into a variety carrying the translocation. In each instance, the lines with the substituted arms were much more susceptible than their recipient, confirming the major effect of the 5BS-7BS chromosome on resistance. The complete correlation between the translocation and resistance and between increased susceptibility and its absence suggests that the gene(s) for adult-plant resistance, located on the 5BS-7BS chromosome, may be closely linked to the break point. Alternatively, it may be a consequence of the close relatedness of some of the varieties. The presence of this gene(s) might be a factor explaining the prevalence of this translocation in some West European wheats.