Different Classes of Resistance to Turnip Mosaic Virus in Brassica rapa

Pathotype-specific and broad-spectrum resistance to turnip mosaic virus (TuMV) have been identified in the diploid A genome brassica species Brassica rapa. The pathotype-specific resistance is effective against pathotype 1 isolates of TuMV, which are the most common in Europe. It is almost identical in its specificity to that of a mapped resistance gene (TuRB01) present in the A genome of the amphidiploid species Brassica napus. A mutant of a pathotype 1 isolate of TuMV (UK 1M) that is able to overcome TuRB01 also overcame the B. rapa resistance. This, combined with the fact that a single-nucleotide mutation in the cylindrical inclusion gene of TuMV that has been shown to induce a change from avirulence to virulence against TuRB01, had an identical effect on the B. rapa resistance, suggest that the two resistances are conditioned by the same gene. A second source of resistance in B. rapa prevented systemic spread of all TuMV isolates tested. A third source of resistance that appears to provide immunity to, or severely restrict replication of most isolates of TuMV has been characterised. This resistance source also prevented systemic spread of all TuMV isolates tested. Prior to this study, no resistance to pathotype 4 or pathotype 12 isolates of TuMV had ever been identified. For each of these three resistance sources, plant lines that are not segregating for some of the resistance phenotypes and that are presumably homozygous for the genes controlling these phenotypes have been generated. Strategies for further characterising and deploying these resistances in different Brassica species are described.