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 Wes t European wheats 30 years ago, and is probably present in many of the ir descendants today. In varieties with a history of durable adult-pla nt resistance to yellow rust and carrying this translocation, removal of the 5BS-7BS chromosome gave adult plants which were much more susce ptible. It was suggested that this chromosome might therefore carry th e gene(s) responsible for a major part of their resistance and possibl y their durability. To test this, a series of lines was developed in w hich 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 origi n, was found to produce the resistant phenotype, indicating that backg round chromosomes were responsible for the differences between the var ieties. The resistance and durability of the resistant varieties canno t therefore be due solely to the translocated chromosome. In similar e xperiments, the 5BS and 7BS arms from varieties not carrying the trans location were substituted into a variety carrying the trans location. In each instance, the lines with the substituted arms were much more s usceptible than their recipient, confirming the major effect of the 5B S-7BS chromosome on resistance. The complete correlation between the t ranslocation and resistance and between increased susceptibility and i ts absence suggests that the gene(s) for adult-plant resistance, locat ed on the 5BS-7BS chromosome, may be closely linked to the break point . Alternatively, it may be a consequence of the close relatedness of s ome of the varieties. The presence of this gene(s) might be a factor e xplaining the prevalence of this translocation in some West European w heats.