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Genetic control of broad-spectrum resistance to turnip mosaic virus in Brassica rapa (Chinese cabbage)

Rachel L. Rusholme^1,2,3,

¹ Agriculture and Agri-Food Canada, 107 Science Place, Saskatoon, SK S7N 0X2, Canada
² Warwick HRI, University of Warwick, Wellesbourne, Warwick CV35 9EF, UK
³ John Innes Centre, Colney Lane, Norwich NR4 7UH, UK

Correspondence
John A. Walsh
john.walsh{at}warwick.ac.uk

The Brassica rapa line RLR22 was resistant to eight diverse turnip mosaic virus (TuMV) isolates. A B. rapa genetic map based on 213 marker loci segregating in 120 first back-cross (B₁) individuals was established and aligned with the B. rapa genome reference map using some of the RFLP probes. B₁ individuals were self-pollinated to produce B₁S₁ families. The existence of two loci controlling resistance to TuMV isolate CDN 1 was established from contrasting patterns of segregation for resistance and susceptibility in the B₁S₁ families. The first gene, recessive TuMV resistance 01 (retr01), had a recessive allele for resistance, was located on the upper portion of chromosome R4 and was epistatic to the second gene. The second gene, Conditional TuMV resistance 01 (ConTR01), possessed a dominant allele for resistance and was located on the upper portion of chromosome R8. These genes also controlled resistance to TuMV isolate CZE 1 and might be sufficient to explain the broad-spectrum resistance of RLR22. The dominant resistance gene, ConTR01, was coincident with one of the three eukaryotic initiation factor 4E (eIF4E) loci of B. rapa and possibly one of the loci of eIF(iso)4E. The recessive resistance gene retr01 was apparently coincident with one of the three loci of eIF(iso)4E in the A genome of Brassica napus and therefore, by inference, in the B. rapa genome. This suggested a mode of action for the resistance that is based on denying the viral RNA access to the translation initiation complex of the plant host. The gene retr01 is the first reported example of a recessive resistance gene mapped in a Brassica species.

Present address: University of East Anglia, School of Biological Sciences, Norwich, Norfolk NR4 7TJ, UK.