Genetic control of immunity to Turnip mosaic virus (TuMV) pathotype 1 in Brassica rapa (Chinese cabbage).

Turnip mosaic virus (TuMV) is the major virus infecting crops of the genus Brassica worldwide. A dominant resistance gene, TuRB01b, that confers immunity to the virus isolate UK 1 (a representative pathotype 1 isolate of TuMV) on Brassica rapa was identified in the Chinese cabbage cultivar Tropical Delight. The TuRB01b locus was mapped to a 2.9-cM interval on B. rapa chromosome 6 (A6) that was flanked by RFLP markers pN101e1 and pW137e1. This mapping used a first backcross (B(1)) population segregating for the resistance gene at TuRB01b and sets of RFLP markers employed in previous mapping experiments in Brassica. Virus-plant interaction phenotypes were assayed in inbred progeny derived from B(1) individuals to allow different virus isolates to be tested. Comparative mapping confirmed that A6 of B. rapa was equivalent to chromosome 6 of Brassica napus (A6) and that the map position of TuRB01b in B. rapa could be identical to that of TuRB01 in B. napus. Detailed evaluation of plant-virus interactions showed that TuRB01 and TuRB01b had indistinguishable specificities to a range of TuMV isolates. The possibility that TuRB01 and TuRB01b represent similar or identical alleles at the same A genome resistance locus suggests that B. napus acquired TuRB01 from the B. rapa gene pool.